DEGREE PROJECT IN INDUSTRIAL ENGINEERING AND MANAGEMENT, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2021

A Conceptual Design of a Reliable Hard Docking System Docking of an autonomous underwater vehicle to the new generation A26 submarine

ELIN EKSTRÖM

ELLEN SEVERINSSON

KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT This page was left intentionally blank A Conceptual Design of a Reliable Hard Docking System

Docking of an utonomous underwater vehicle to the new generation A26 submarine

ELIN EKSTRÖM ELLEN SEVERINSSON

Master of Science Thesis TRITA­ITM­EX 2021:379 KTH Industrial Engineering and Management Machine Design SE­100 44 STOCKHOLM This page was left intentionally blank

Master of Science Thesis TRITA-ITM-EX 2019:379

A Conceptual Design of a Reliable Hard Docking System

Elin Ekström Ellen Severinsson

Godkänt Examinator Handledare

2021-06-15 Claes Tisell Roger Berg

Uppdragsgivare Kontaktperson

Swedish Defence Materiel Administration Matteo Perrone, Johan Wahren

Abstract

In year 2024 and 2025 the Royal Swedish Navy is expected to launch two new submarines with new possibilities to dock underwater vehicles. The submarines are part of the new Blekinge Class (A26) and will aid the Swedish Armed Forces and the Swedish Defense Materiel Administration (FMV) in their aim to develop and use more autonomous systems, to increase staff efficiency and to face the technological challenges of tomorrow.

This thesis was carried out at FMV, with the purpose of investigating the physical requirements put on the new submarines, when docking an autonomous underwater vehicle. These requirements were identified through an analysis of qualitative and quantitative research. The analysis resulted in ten key insights, which led to thirteen requirements.

The requirements were combined with project specific data of the AUV62 system and A26 submarine, to develop three conceptual designs of hard docking systems. The concepts were verified through analysis of material, stress and deflection, and geometric constraints.

The concepts were evaluated based on how well they were fulfilling each requirement. A hammock-alike concept was shown to have most potential in being the most reliable hard docking system. The thesis ended with concluding that its purpose had been fulfilled, followed with recommendations for continued work.

Keywords: Hard docking, AUV, submarine, A26, HMS Blekinge, HMS Skåne This page was left intentionally blank

Examensarbete TRITA-ITM-EX 2019:379

En konceptuell design av ett pålitligt hårddockningssystem

Elin Ekström Ellen Severinsson

Godkänt Examinator Handledare

2021-06-15 Claes Tisell Roger Berg

Uppdragsgivare Kontaktperson

Försvarets materielverk Matteo Perrone, Johan Wahren

Sammanfattning

Under 2024 och 2025 förväntas Svenska Marinen sjösätta två nya ubåtar, med nya förmågor gällande dockning av undervattensfordon. Ubåtarna ingår i nya Blekingeklass (A26) och är en del av Försvarsmaktens och Försvarets Materielverks (FMV) målsättning om att utveckla och använda mer autonoma system, för att öka personaleffektivitet och för att kunna möta morgondagens tekniska utmaningar.

Detta examensarbete utfördes på uppdrag av FMV, med syftet att undersöka vilka fysiska krav som ställs för att hårddocka ett autonomt undervattensfordon på de nya ubåtarna. Dessa krav identifierades genom analys av en kvalitativ och kvantitativ undersökning. Analysen uppdagade tio nyckelinsikter som gav upphov till tretton krav.

Kraven kombinerades med projektspecifik data för AUV62-systemet och ubåt A26, för att utveckla tre konceptuella designförslag av hårddockningssystem. Koncepten verifierades genom analys gällande material, spänning och utböjning, samt geometriska begränsningar.

Koncepten utvärderades baserat på hur väl de uppfyllde respektive krav. Ett hängmatteformat koncept visade sig ha störst potential för att bli ett pålitligt hårddockningssystem. Examensarbetet avslutades med att projektets syfte ansågs vara uppfyllt, följt av förslag på vidare arbete.

Nyckelord: Hårddockning, AUV, ubåt, A26, HMS Blekinge, HMS Skåne

This page was left intentionally blank Acknowledgements

We would like to express our special thanks of gratitude to our supervisors at FMV, Matteo Perrone and Johan Wahren, and our supervisor at KTH, Roger Berg.

Thank you, Matteo and Johan, for giving us the opportunity to do this wonderful project. We could not have had a better finale to our education at KTH.

Thank you, Matteo, for being so engaged in our work. We appreciate that you have thought that our learning experience is more important than any result. The balance between feedback and freedom has been perfect.

Thank you, Johan, for your understanding of our learning process and that you have given feedback to us the moment we needed it. Your enthusiasm and your help has been invaluable.

Thank you, Roger, for all of the advice you have presented to us during this project. You have provided us with many great ideas, thoughts, and tips throughout our work and we are grateful that you have taken the time to work with us.

We would also like to thank the people at Saab and the Royal Swedish Navy, who have taken their time to share their knowledge with us. Special thanks to Daniel, for telling us about life on a submarine and for your overall helpfulness. Thank you Leif and Magnus, for letting us come along on the submarine hunt.

Thank you to our opponents, for helping us making this project better with your insightful comments. Finally, we would like to express thanks to our corridor buddies at Brinellvägen 85 for making the special situation of writing a thesis during the coronavirus less lonely.

Elin Ekström and Ellen Severinsson Stockholm, June 15, 2021 This page was left intentionally blank Acronyms

AUV Autonomous Underwater Vehicle CAD Computer Aided Design CFRP Carbon Fiber Reinforce Polymers DDS Dry Deck Shelter FM tree Function and Means Tree FMV Swedish Defence Materiel Administration FPL Flexible Payload Lock RSwN Royal Swedish Navy ROV Remotely Operated Vehicle SD Saab Dynamics SK Saab Kockums SOD Special Operations Divers SwAF Swedish Armed Forces UHMWPE Ultra­high­molecular­weight polyethylene UUV Unmanned Underwater Vehicle Contents

1 Introduction 1 1.1 Background ...... 1 1.2 Problem ...... 2 1.3 Purpose and Research Questions ...... 2 1.4 Stakeholders ...... 3 1.5 Delimitations ...... 3

2 Methodology 5 2.1 Quantitative Research ...... 5 2.2 Qualitative Research ...... 6 2.3 Analysis and Synthesis ...... 6 2.4 Concept Methods ...... 7

3 Theoretical Background 8 3.1 Project A26, Blekinge Class Submarine ...... 8 3.1.1 Flexible Payload Lock ...... 8 3.2 Unmanned Underwater Vehicles ...... 9 3.2.1 Autonomous Underwater Vehicle ...... 10 3.3 The Docking Process ...... 11 3.4 Underwater Environment ...... 13 3.4.1 Environment in Baltic Sea ...... 13 3.4.2 Submarine Effect on Currents ...... 14 3.4.3 Signatures ...... 16 3.5 Submarine Positioning States ...... 16

4 Existing hard docking systems 19 4.1 Docking on a moving submarine ...... 19 4.2 Docking on a still submarine ...... 20 4.3 Free­standing docking stations ...... 20

5 Human Factors 23 5.1 Life on a Submarine ...... 23 5.2 Human Involvement ...... 23 5.3 Opinion on New Systems ...... 24 CONTENTS

6 Analysis of Docking Scenarios 26 6.1 Scenario: Submarine States and AUV Docking Direction ...... 26 6.2 Scenario: What Is a Non­Functional Docking? ...... 27 6.3 Scenario: What is an Emergency? ...... 28 6.4 Scenario: AUV Operation of Communication Requirements ...... 29

7 Factors Characterising a Reliable Hard Docking System 32 7.1 Key Insights ...... 32 7.2 System Requirements ...... 36 7.3 Specific Project Data ...... 37

8 Concept Ideation and Continuous Evaluation 39 8.1 Wide Concept Ideation ...... 39 8.2 Systematic Concept Ideation ...... 40 8.2.1 Subsystem Docking Station ...... 41 8.2.2 Subsystem Moving Frame and Static Frame ...... 44 8.3 A First Evaluation ...... 45 8.3.1 Capturing Success­Rate ...... 45 8.3.2 System Complexity ...... 46 8.3.3 Results From First Evaluation ...... 47

9 Concept Refinement 49 9.1 Concept Brugd ...... 49 9.2 Concept Valhaj ...... 51 9.3 Concept Flundra ...... 52 9.4 Electric Actuators ...... 53 9.5 Loading the Docking Systems Through the Top Hatch ...... 54 9.6 Design for Safety ...... 55 9.6.1 Abort at Any Moment ...... 55 9.6.2 A Second Safety System ...... 56 9.6.3 Distancing System ...... 56

10 Concept Realisation 57 10.1 Material Analysis ...... 57 10.2 Stress and Deflection in the Linear Guide ...... 59 10.2.1 Forces Acting on System ...... 59 10.2.2 Stress and Deflection ...... 61 10.3 Analysis of Subsystem Relationships ...... 62

11 Final Concepts and Concept Evaluation 66 11.1 Concept Brugd ...... 66 11.2 Concept Valhaj ...... 67 11.3 Concept Flundra ...... 69 11.4 Concept Evaluation ...... 71

12 Results 73

13 Discussion 76 13.1 Scope ...... 76 13.2 Implementation of Methodology ...... 76 13.3 Results ...... 77 13.4 Outside Perspective ...... 78 13.5 Future Work ...... 79

14 Conclusions 80 14.1 Final Words ...... 80

References

A Submarine States and AUV Docking Direction

B Determination of the Importance Factor

C Functions and Means Tree

D Material Analysis

E Solid Mechanics Calculations

F Evaluation Analysis 1 Introduction

In this section, the background of this thesis is presented together with problem, purpose, goal and deliverables, identified stakeholders and delimitations.

1.1 Background

In the year of 2015, the Royal Swedish Navy ordered two new submarines from the Swedish Defence Materiel Administration (FMV). The submarines will be a part of the new generation Blekinge class, known as submarine A26, and are planned to be delivered in the year 2024 and 2025 respectively. (Hellström, 2017)

At the front of the new A26 submarines, an opening with a diameter of circa 1.5 meter will be implemented, see Figure 1a. Through this opening, Unmanned Underwater Vehicles (UUVs) can exit and enter the submarine. When an UUV enters the opening, it must proceed through an open water­filled area called the forepeak before it arrives to a large tube called the Flexible Payload Lock (FPL). From the FPL, the UUV can be reached by the crew of the submarine through the torpedo room (FMV, 2020a). The location of the forepeak, the FPL and the torpedo room is illustrated in Figure 1b.

(a) An AUV exiting the A26 submarine from the (b) The location of the forepeak, FPL and FPL (FMV, 2020a) torpedo room in the A26 submarine

Figure 1: The new submarines A26 have an opening to an FPL where UUVs can be retrieved and launched

Autonomous Underwater Vehicles (AUV) are a form of an UUV, operating without a physical connection to an operator. Instead, Autonomous Underwater Vehicle (AUV)s use a computer system, manned from distance, allowing the AUV to follow a preprogrammed route. The system tells the AUV where to go and what to do. Common purposes include subsea oil

1 1 INTRODUCTION drilling, oceanic or seafloor research, military training, and intelligence gathering. (Marine Insight, 2016)

According to Hagström (2020), autonomous and unmanned platforms will, in the future, be necessary systems for the Swedish Armed Forces (SwAF), with many potential fields of applications. The platforms will add abilities regarding clearing of explosives and minimize the need for human personnel in missions considered dirty, dull, or dangerous. Furthermore, unmanned autonomous platforms minimize risks for military personnel (Hagström, 2020). The SwAF and FMV aim to develop and use more autonomous platforms, in order to increase military abilities and to face the technological challenges of tomorrow. With the usage of AUVs in the defence field on the rise, new methods for handling the AUVs will become useful.

Previously built submarines are equipped with 21” torpedo tubes in the bow, from which a Remotely Operated Vehicle (ROV) is able to recover AUVs (Siesjö, 2011). While the ROV method is considered efficient, the new FPL brings both new challenges and opportunities for docking and launching AUVs from the submarine A26.

1.2 Problem

Docking an AUV to a submarine comes with many challenges. The hard docking, which is within the scope of this project, begins when the AUV has physical contact with a docking system, aiding the retrieving process of the AUV into the FPL. During hard docking, errors are critical and the submarine is vulnerable as the hatches are open and the AUV is at risk to collide with the submarine. To decrease the vulnerability, there is a need for efficient and safe ways to hard dock AUVs.

Military technology is constantly improving all over the world and it is of great importance to constantly innovate, even if currently used technology is functional and well­developed. While the usage of the SUBROV is considered innovative and functioning today, it is important to continue developing new AUV docking methods in order to not fall behind when other nations develop new technology in the future.

1.3 Purpose and Research Questions

The overall objective of this research was to gain a deeper understanding of the hard docking process, both generally and for the specific submarine and AUV presented, and to identify challenges and possibilities with a hard docking system that could be used for future work. To

2 1 INTRODUCTION fulfil this objective, the following research questions were answered:

1. What factors characterise a reliable hard docking system?

2. What possibilities and challenges emerge from these factors and how are they reflected in the design of a hard docking system?

To accomplish answering the research questions, the final deliverables of this thesis was as following:

• A specification of requirements for a conceptual hard docking system

• Three realistic conceptual designs, fulfilling the found requirements

• An analysis of which conceptual design is most reliable, identifying possibilities and challenges

1.4 Stakeholders

The primary stakeholders in this thesis were FMV, responsible for supplying the A26 submarines to the Royal Swedish Navy (RSwN), and Saab Kockums (SK), as designers and producers of the A26. SK would most likely also be responsible for designing and manufacturing a hard docking solution for the FPL.

Primary but external stakeholders were identified as the RSwN, as they would own and staff the A26 once in use. Secondary and external stakeholders were AUV suppliers such as Saab Dynamics (SD), and material and sub­system suppliers, with some being required to be Swedish in order to enable native production for national security.

1.5 Delimitations

This project was conducted over a time period of five months. Due to COVID­19 recommendations, the thesis work was conducted mostly online with lack of physical meetings with stakeholders. Due to confidentiality within the defence sector, precise measurements were avoided. The project focused specifically on hard docking of the AUV62­system into the FPL on the A26­submarines. Smaller changes on the AUVs were allowed to be suggested.

The conceptual design presented was of an AUV garage, placed within the FPL. It was not investigated how the garage would be placed and fastened within the FPL, nor were precise dimensions for the garage presented. Instead, it was assumed that a garage with an outer

3 1 INTRODUCTION dimension smaller than the FPL dimension would be able to fit and be secured within the FPL.

It was assumed that all docking phases prior to hard docking were solved, allowing this project to focus solely on the hard docking. The solutions were made for a torpedo shaped AUV, similar to Figure 1a. Other possible AUV shapes were disregarded. This project was limited to not look at alternatives where ROVs or similar were used to guide the AUV, as such solutions already existed and were considered well­developed for submarines. Furthermore, the thesis brief included an AUV weight of 1000 kg, and the AUV length was defined as 6 meters.

4 2 METHODOLOGY

2 Methodology

This chapter describes the methodology used, describing chosen methods and how they were implemented. Methods are presented in order of implementation, and the following sections of the report present the results of each implemented method. The methodology process for this thesis was based on an iterative double diamond model, as illustrated in Figure 2.

The project consisted of four phases: discover, define, develop, and validate, going from problem to solution. Moving between these phases was an iterative process, done multiple times back and forth. The discovery phase consisted of quantitative and qualitative research. In the defining phase, the research was analyzed and synthesized in order to define the needs and wants for the upcoming solution, resulting in a specification of requirements. The result of the defining phase was seen as the answer for the first research question.

Based on the found requirements, possible solutions were ideated in the ideation phase. Nine ideas were presented, of which three were refined into fully developed concepts. The three concepts were evaluated against the specification, in a final validation phase. The result of the validation phase was seen as the answer for the second research question.

Figure 2: Iterative double diamond process

2.1 Quantitative Research

The quantitative research focused on relevant theoretical background and existing solutions. Theoretical background covered Project A26 and the abilities and demands of the submarine, AUVs, the docking process, and the underwater environment of the Baltic Sea. Information regarding submarines and AUVs was gathered by viewing public information from FMV and Saab, as they had been identified as primary stakeholders. Public information was prioritised, due to confidentiality. Information regarding the underwater environment was collected through online research.

5 2 METHODOLOGY

Existing solutions were researched and analysed in order to identify needs and solutions that could be useful for the end result. As there was no known solution to AUV docking onto a submarine, similar situations were investigated such as docking AUV to seafloor garages and ROV docking.

2.2 Qualitative Research

The qualitative research consisted of interviews with primary stakeholders and one field study along with a contextual interview with secondary stakeholders. Unstructured interviews were performed in meetings with project supervisors at weekly basis throughout the project. Semi­ structured interviews were conducted with engineers from SK regarding submarine and AUV capabilities and limitations, and with submarine personnel regarding the user perspective. Contextual interviews were performed with engineers from SD during a field study, in which the behaviour of an AUV in water was witnessed.

2.3 Analysis and Synthesis

The collected data was processed in two steps, using analysis and synthesis. Analysis was defined as the act of breaking down the data and looking at all pieces of information individually, synthesis was defined as the act of putting the information together and looking at it to understand patterns and relationships. The goal was to gain key insights and to formulate a specification of requirements.

Three scenario analyses were performed (Wikberg et al., 2015), regarding non­functional docking, possible emergencies, and preferable docking states. Insights were defined using a cluster analysis (Qualtrics, 2020), based on findings from the scenario analyses and the qualitative and quantitative research. Ten key insights were defined, formulated as important findings that could be applied either before, during, or after the docking process.

Requirements were defined based on at least one insight. The importance of the presented requirements were defined using a Weight Setting Matrix (Wikberg et al., 2015), in which two requirements were compared at a time. The most important requirement was given one point and the importance factor was defined based on which requirement had the most points, using a scale of 1­5. The weighted requirements were concluded to answer the first research question.

6 2 METHODOLOGY

2.4 Concept Methods

Concept ideation moved from roughly defined ideas to concepts, defining concepts as an idea with a defined solution to each required function and sub­function (Stier, 2019). Concept ideation began with a wide ideation phase using braindrawing (Wikberg et al., 2015), resulting in many rough ideas. The ideas were clustered based on primary technical principle and on at what time of the process the AUV locked itself to the dock. Based on these findings, the docking system was divided into three sub­systems and further concept ideation was done separately for the subsystems.

A systematic concept ideation was conducted using a Function and Means Tree (FM tree) (Hubka and Eder, 2002), defining required functions before, during, and after the hard docking process. Different means, solutions for the presented functions, were defined and combined using morphological matrices (Wikberg et al., 2015). Only the functions regarding the docking sequence were considered in the systematic concept ideation. Nine ideas were formulated for the larger sub­system, and one fully defined concept was formulated for the smaller sub­ systems. The nine ideas were evaluated based on function and complexity, resulting in three ideas being chosen for further refinement.

The chosen ideas were refined into three concepts where functions regarding the system as a whole were integrated. Refinement begun with a material analysis, identifying suitable materials for each subsystem. A geometry analysis was made for each concept, concluding how given dimensions of the FPL and the AUV affected the designs. In order to gain a higher understanding of the complex geometries, the concepts were modelled in Computer Aided Design (CAD). A force analysis regarding stress and deformation was conducted with the purpose of proving whether each concept could realistically be built. Finally, it was ensured that each concept had a solution for every presented requirement.

Concept evaluation was performed using a Weighted Criteria Matrix (Wikberg et al., 2015), in which each concept was assessed based on how well it fulfilled each requirement, using a scale of 1­3. The score was multiplied with the importance factor of the requirement and the concept with the highest score was considered to have the highest potential of being a reliable hard docking system. An analysis was performed on how difficult easy requirement was to fulfill. This analysis was concluded to answer the second research question.

7 3 THEORETICAL BACKGROUND

3 Theoretical Background

This section brings up the theoretical information found in current research and sources. Research has been collected by literature studies or given by stakeholders involved with the submarine A26.

3.1 Project A26, Blekinge Class Submarine

Project A26 aims to deliver two new submarines in the Blekinge class to the SwAF. The delivery of the first submarine is planned for year 2024 and the second is planned for 2025. The responsibility of the production is Saab Kockums, who develops, produces, verifies and delivers submarines. The A26 will be optimised for operations within shallow waters, such as the Baltic sea, but will be capable of operating world­wide. The chosen engine system allows for the submarine to be submerged for multiple weeks (Hellström, 2017).

It is highly important for the A26 to be able to operate during long time without being detected (Försvarets Materielverk, 2020a). This is done by minimising the submarine’s signature, such as radiated noise, acoustic target strength, electric signature, magnetic signature, and by increasing hydrodynamic abilities (Berg, 2021). Contrary to earlier Swedish submarines, the A26 will contain a larger tube in the front alongside the torpedo tube, referred to as the Flexible Payload Lock (FPL) (Försvarets Materielverk, 2020a).

3.1.1 Flexible Payload Lock

The new A26 is equipped with an FPL in the bow, placed in between the torpedo tubes. The FPL is 1.5 meters in diameter and approximately 6 meters long (Söderblom, 2016), and placed slightly below the submarine body center­line. It can be entered through a hull opening in the bow and through the torpedo room. The FPL can hold a so called payload; an added system with the purpose of adding a new ability to the submarine. An example of such a payload is the Saab SUBROV, as later described in section 4.2 Docking on a still submarine and seen schematically in Figure 3 as the gray structure holding the ROV in (a). The FPL can also be configured to hold Special Operations Divers (SOD), schematically seen as (b) in Figure 3.

Torpedoes, ROVs and other payloads are loaded through the Weapon Embarkation Hatch (top hatch) into the torpedo room, using a crane and winch. The payload is lowered through the top hatch, seen as the dotted line in Figure 3, and placed in the torpedo room, seen as the gray area inside the cutout view of the submarine illustration in Figure 3. Top hatch loading requires the

8 3 THEORETICAL BACKGROUND system to have a cylindrical shape with a maximum diameter of 0.8 meters (Perrone, 2021). From the torpedo room, the payload can be loaded into torpedo tubes or into the FPL, through hatches in the torpedo room. Smaller torpedoes, such as the Swedish Torpedo 47 (Försvarets Materielverk, 2020b), can be rotated along its vertical axis inside the torpedo room, due to their shorter body and lower weight compared to larger torpedoes. Larger torpedoes and the AUV62­system cannot be rotated, due to their long bodies and heavy weight (Edvardsson and Helgesson, 2021).

Figure 3: Schematic figure of loading through the top hatch, (a) a payload of SUBROV and (b) divers

The FPL has three hatchways; one into the submarine and two outwards between two hulls. Submarines are built with an inner pressure hull, designed to withstand the water pressure, and an outer form hull, designed to increase hydrodynamic abilities. The area between the pressure hull and the form hull is referred to as the forepeak, seen as the blue area in Figure 3. In order to transport an object out of the FPL, it would have to pass through the pressure hull hatchway, move through the forepeak, then pass through the form hull hatchway (Perrone, 2021). The forepeak is equipped with mechanical gills, allowing the water inside the forepeak to flow out when surfacing. As a result, there is no flow within the FPL when the outer hatches are open and the submarine moves forward (Edvardsson and Helgesson, 2021).

3.2 Unmanned Underwater Vehicles

Underwater vehicles can be divided into two groups; manned and unmanned underwater vehicles (UUV). Unmanned Underwater Vehicle (UUV)s vary in size and shape, from only a few kilos to many tons. They never carry a crew on board and are instead controlled via

9 3 THEORETICAL BACKGROUND cables, wireless connection, or through pre­programmed systems. UUVs can be categorized into four sub­groups; ROV, Autonomous Underwater Vehicle (AUV), Gliders, and Extra Large UUVs. An ROV is maneuvered and controlled real­time, by an operator whose control system is physically connected to the ROV through a tether (Hume, 2019).

3.2.1 Autonomous Underwater Vehicle

AUVs are computer­controlled and capable of following pre­defined routes using little to no contact with a human operator. Common purposes for AUVs include survey and training operations in otherwise unreachable regions, and motivational factors for using them are their relatively low cost and high accessibility in complex environments. Typical AUVs are torpedo­ shaped (cylindrical with rounded or pointy bow), driven by a propeller mounted by the stern (Bellingham, 2009).

As AUVs are untethered, they must store energy onboard, often using batteries. With a typical shape and weight, a battery powered AUV has an endurance of roughly a day with a working speed of 3 knots (1.5 m/s). The torpedo­shaped body allows for the vehicles to be easily extended without loosing hydrodynamic abilities, such as when adding batteries or sensors. However, the shape also means that the vehicle must remain in a forward motion in order to steer and to keep depth. As a result, it becomes hard to control and maneuver at low speeds (< 2 knots) (Bellingham, 2009).

The SwAF currently use two types of AUVs, constructed by Saab. These are the AUV62­ AT (Acoustic Target) and the AUV62­MR (Mine Reconnaissance), seen in Figure 4, together referred to as the AUV62­system.

Figure 4: AUV62­AT (top) and AUV62­MR (bottom) (Berg, 2016)

10 3 THEORETICAL BACKGROUND

The AUV62­system has a modular design, enabling changes to be made to the AUVs in order to use the same vehicle for a multitude of different tasks. The basic modules, roughly illustrated in Figure 5, vary in length and as a result, the total length of the AUV62­system vary between approximately 4­7 meters (Saab, 2010). The weight also varies, from approximately 800 kg for the lighter AUV62­AT (Saab, 2020), up to 1500 kg for the heaviest configured AUV62­MR (Saab, 2010). The AUV62­system has a diameter of 533 mm (21”) (Saab, 2020).

Figure 5: AUV62 Modular design (Berg, 2016)

The nose module, seen to the left in Figure 5, functions as a crash zone and possible cargo holder. While shorter in the MR model illustrated in the figure, the module is longer in the more recent AT model. For future AUVs, it is possible to make design adjustments in the nose module as well as adding new modules, in order to support the docking process (Wahren, 2021). The AUV62­system uses only one propulsor, seen to the right in Figure 5. As a result, it has poor maneuverability; its turning radius is large (>50 m) and it is almost impossible for the AUV62 to reverse with any precision (Wahren, 2021). The mast seen in the second unit (UW­comms and Mast Module) in Figure 5, was disregarded during this thesis project.

3.3 The Docking Process

The definition of docking an AUV to a device or platform, called the dock, is the process of starting at having a free soaring AUV and ending with an AUV which is physically connected to the dock. The outline of a full docking process means the AUV must navigate to the dock vicinity, the AUV must approach the dock with an appropriate velocity and orientation, and there must be a physical linkage of the AUV and the retrieval mechanism. The linkage between the dock and the AUV creates possibilities to charge and transfer data, and enables communication between the AUV and the dock. (Bellingham, 2016)

11 3 THEORETICAL BACKGROUND

The phases identified in a complete docking sequence are seen in Figure 6 and defined as:

Remote phase ­ The AUV is positioned far away relative the submarine. In this phase, the AUV and submarine are independent from each other and the distance between the two units could be multiple kilometers. (Abrahamsson, 2019)

Recall phase ­ The AUV and the submarine begin a communication and the submarine recalls the AUV. At this stage, the vehicles are working as two separate units. The distance between the submarine and the AUV could be up to a few kilometers. (Abrahamsson, 2019)

Rendezvous phase ­ The AUV and submarine communicate in order to navigate relative to each other, rather than relative to ground. This is to avoid collision and to position for hard docking. The distance between the two vehicles are between tens of to a few hundreds of meters. (Abrahamsson, 2019)

Soft docking ­ The distance between the AUV and the submarine is a few tens of meters. The units have a close two­way communication which enables the AUV to navigate relative the submarine and to find the way into the submarine. Most of the maneuvering is done by the AUV and the phase ends with the AUV positioning itself to enable physical contact with the dock. (Abrahamsson, 2019)

Hard docking ­ The AUV and the submarine have physical contact and communication is needed only to notice if errors occur (Abrahamsson, 2019). The hard docking is finalized when the AUV is stationed in a capture mechanism inside the FPL (Bellingham, 2016).

Figure 6: A schematic view of phases of a full docking sequence

Generalizing hard docking into three steps can look as the following: (1) The AUV has physical contact, precisely enough to be captured, (2) the AUV moves between the hull entrance and the FPL, and finally (3) the AUV is secured and stationed in the FPL.

12 3 THEORETICAL BACKGROUND

3.4 Underwater Environment

The underwater environment of the Baltic Sea was researched, as the A26 submarines would mainly operate within this area. In this subsection, relevant information regarding the Baltic Sea is presented, along with how the submarine body affects currents and which signatures the submarine has while submerged.

3.4.1 Environment in Baltic Sea

The Baltic Sea is considered to be a shallow sea with a maximum depth of 459 meters and an average depth of 60 meters (NE, 2000). Ocean circulation generally emerge from wind, sea level height differences, tides, and differences in water temperature and salinity. The currents are affected by the coastline, the bottom topography, Coriolis effect, and the friction between water mass and sea bottom (SMHI, 2011).

The wind creates waves and currents by the surface. When the wind forces the surface to move, molecules in layers below the surface are forced into movement due to friction in between layers of water molecules. This phenomenon of horizontal current movements in different water layers is called the Ekman Spiral (National Ocean Service, 2013). It has been shown that a molecule layer located far down from the surface, is affected less by the wind, leading to weaker currents. Water layers located at a depth of 100 meters are not affected by the wind (National Ocean Service, 2013). In open areas in the Baltic Sea, currents are generally calm and regular due to weak tides (SMHI, 2011). The difference is only a few decimeters between high and low tide by the Coast of Sweden (Livet i Havet, 2019).

At the bottom, the topography can create local currents when water flow is forced through narrow areas (Livet i Havet, 2019). In the northern Baltic Sea, the bottom is fragmented and diverse, while the southern Baltic Sea has a flatter bottom. It is estimated that flat areas, such as plains and structural basins cover 2/3 of the seafloor. However, narrow canyons, sea holes, and deep valleys can be found in many areas (Kotilainen, 2020).

In the Baltic Sea, there are often two or more density layers present, found at depths of roughly 40­80 meters. The layers rarely mix and during warm months, there is a difference in temperature as salty cold water locates near the bottom and warm fresh water near the surface. During colder months, the layers can mix and the salt percentage and temperature can be the same down to 30 meter (Livet i Havet, 2019).

13 3 THEORETICAL BACKGROUND

An illustration of density layers is seen in Figure 7. The darker blue areas have a lower density due to higher salt percentage than the lighter blue areas (Livet i Havet, 2019). Salinity along the western Coast of Sweden is generally low, but high salinity water occasionally enters and begins to move across the seafloor. The phenomenon of salt water with high density moving like waves near the bottom is called the dead­water phenomenon. These waves often affect the movement of boats (Agullo et al., 2020).

Figure 7: The movement of density layers over hilled seafloor (Livet i Havet, 2019)

3.4.2 Submarine Effect on Currents

When the submarine moves with speed in water, it pushes the water in front of it, causing changes in current direction and velocity. The six flow­charts in Figure 8 show simulation results of water velocity around a submarine bow, moving forward (X­direction) with a speed in water of 6 knots. The submarine has an open torpedo tube, placed on the right side of the submarine and slightly below center line. A torpedo is placed within the open torpedo tube. Also seen in the Figures 8b, 8d and 8f are the mechanical gills, allowing water to exit the forepeak.

Figure 8a and Figure 8b show water flow in positive X­direction, with the submarine viewed from above (XY­plane) and from the side (XZ­plane). Both figures show that the water in front of the submarine is pushed forward, lowering flow velocity and creating a barrier of relative slow water. This barrier begins at roughly six meters from the hull, and grows stronger at three meters from the hull. Some water flows into the hull opening and exits through the gills and is relatively still inside the forepeak and the torpedo tube.

14 3 THEORETICAL BACKGROUND

(a) Flow velocity X­direction, viewed in XY­plane (b) Flow in X­direction, viewed in XZ­plane

(c) Flow in Y­direction, viewed in XY­plane (d) Flow in Y­direction, viewed in XZ­plane

(e) Flow in Z­direction, viewed in XY­plane (f) Flow in Z­direction, view in XZ­plane

Figure 8: Submarine effect on water flow, with speed over water in 6 knots (Svensson, 2021)

Figure 8c and Figure 8d shows the water flow in positive Y­direction, with the submarine viewed from above (XY­plane) and from the side (XZ­plane). The charts show that flow in Y­direction appears at the corners of the bow, with no flow in front of the submarine center line. Maximum velocity occurs closest to the hull and the flow begins at roughly 3 meters from the hull.

Figure 8e and Figure 8f, shows the water flow in positive Z­direction, with the submarine viewed from above (XY­plane) and from the side (XZ­plane). When viewing from above, the flow velocity is low and equally spread by the submarine bow. When viewing from the side, the flow velocity is highest at the corners of the bow.

15 3 THEORETICAL BACKGROUND

The analysis show that the water flow changes direction close to the bow, from its initial positive X­direction to both positive and negative flow in Y­direction and Z­direction. In the X­direction, a barrier of still water appears in front of the bow. In both Y­direction and Z­ direction, flow velocity is highest at the corners of the submarine. No matter direction, there is little to no effect on water flow at 3­4 meters from the hull.

3.4.3 Signatures

Signature management is an important factor for naval stealth, a crucial part of a submarine’s mission. Signature management is based around prediction, measurement, and mitigation of emitted signatures, which are many. Underwater radiated noise (acoustic), infra red, radar cross section, and magnetic and electric signatures can be considered the most important (Kumar, 2020).

Acoustic signatures are the most potential threat for submarines, as the noise can be detected at long distances. Primary contributors for acoustic signatures are machinery aboard the submarine and the propulsor. Infra red signatures are, amongst other uses, used for detection and attack of warships; the contrast of colours in an infra red image is detected by seekers and it is therefore important to reduce the contrast. One method of doing so is by reducing the temperature of the object to the reference temperature range, making in undetectable. Radar cross section can be used to identify a target at long range measures, using a function of target shape, frequency of observation, and aspect angle. The shape of a vessel can thus be used to reduce RCS signatures, along with material treatment. Magnetic signature consist of permanent and induced magnetic fields and is used, along with acoustics and pressure, by sea mines for target detection and detonation. Electric signatures can be detected and identified at long ranges, but has not yet been completely mastered. It can be generated as either static potential, or as alternating fields, and countermeasures include active cathodic protection and shaft grounding. Static potential is caused by corrosion, while alternating fields are caused by no­corrosive currents flowing between dissimilar metals, such as the hull and the propeller. (Kumar, 2020)

3.5 Submarine Positioning States

Three submarine positions under the water surface can be generalised as the states seen in Figure 9. The seafloor state(a) is when the submarine is resting on the seafloor with engines off. The hovering state (b) is when the submarine is floating under the surface with engines off.

16 3 THEORETICAL BACKGROUND

The forward state (c) is when the submarine is positioned under the surface with engines on, moving forward.

(a) The seafloor state (b) The hovering state (c) The forward state

Figure 9: The three states the submarine might locate in

Seafloor state Resting a submarine on the seafloor is a unique ability, made possible by the shallow waters of the Baltic Sea. Whether it is possible or not also depends on seafloor roughness; it is most suitable on a soft and even sand­bedded seafloor, as a rocky and uneven seafloor might cause damage to the submarine, caused by a rocky seafloor, shipwrecks and lost sea mines can be mistaken for rocks. As Swedish submarines mainly operate within the Baltic Sea, their hulls are specifically designed and strengthened to withstand the forces of seafloor resting (Perrone, 2021). In case of docking an UUV, the submarine is stationary and the UUV must be maneuverable. Strong currents can affect the UUV during docking, but these are generally weaker near the seafloor.

Entering the seafloor state with a submarine is time­consuming and requires the crew to be well familiar with the seafloor topography. It begins with the submarine moving closely to the seabed, lowering the speed of the submarine to zero knots over ground. The crew then lowers the bow of the submarine, as to not damage the propulsor. The submarine takes in water, adding extra weight. This allows the crew to, in a slow and controlled manner, lower the entire submarine until it rests fully on the seafloor. If the seafloor is covered in a thick salt layer, more overweight must be added or the submarine might start drifting on top of the salt layer. Once in the resting position, the submarine turns off electronic devices. This results in a quiet, still, and camouflaged submarine and hence difficult to detect (Hakkarainen, 2021).

Hovering state The hovering state means positioning the submarine beneath the surface and above the seafloor, without moving the submarine forward. In this state, the submarine can turn its engines off to lower its signatures. As the forward motion stabilizes the submarine, it is inherently unstable in the hovering state. This state is mainly preferred when the submarine must be completely

17 3 THEORETICAL BACKGROUND quiet and the seafloor is not an option (Berg, 2021).

It is possible to stabilize the submarine in the hovering state, by placing it on top of a salt layer. The salt layer can be thick enough to act as the seafloor, but can be affected by currents which slowly move the submarine. The submarine itself is also affected by currents, and reaching equilibrium with the submarine and currents can take up to two hours. As a result, the submarine might enter the hovering state in a horizontal position, but is rotated into a tilted position over the course of two hours. Once equilibrium (and therefor a tilted position) is reached, the submarine will stay in that position until the currents change. (Hakkarainen, 2021)

When docking an AUV in hovering state, it is desirable to do so with periscope depth or deeper. Periscope depth is the depth during which the submarine’s periscope is just above surface, roughly 20 meter for a typical Swedish submarine. If a salt layer can be found, it is always most desirable to rest the submarine on it. (Hakkarainen, 2021)

Forward state The submarine is most stable in the forward state, as the forwards speeds allows for better maneuverability. Marching speed is usually 2­5 knots, as higher speed requires more engine power and lower speed has a negative effect on maneuverability (Berg, 2021). The normal currents of the Baltic Sea are rarely strong enough to affect the submarine while in forward state and as a result, the submarine crew rarely measure the currents acting on the submarine (Hakkarainen, 2021).

While the submarine is not affected by natural currents, it causes currents itself by moving forward. These hydrodynamic disturbances could cause turbulence in front of the submarine bow, affecting the AUV while docking. It was concluded that these disturbances begin at roughly 3­4 meters in front of the submarine (Svensson, 2021). Docking should occur at periscope depth or deeper, where the submarine is less affected by surface waves (Hakkarainen, 2021).

18 4 EXISTING HARD DOCKING SYSTEMS

4 Existing hard docking systems

In this section, currently existing and relevant similar solutions to hard docking systems found in research is presented. Only a few solutions were found of docking systems specific for submarines and therefore inspiration was taken from similar solutions. Most of the docking stations for AUVs presented in research were free­standing docking stations often located on the seafloor. This section only presents the found solutions that aided in the design of a conceptual hard docking system.

4.1 Docking on a moving submarine

While there is no known solution for AUV docking on a moving submarine, multiple institutions and countries have investigated the issue. Sujit et al. (2011) investigated the ability to dock an AUV in to a top­mounted funnel while the submarine was moving, as seen in Figure 10a, using numerical and analytical solutions.

With the setup illustrated in Figure 10a, docking could only be achieved by having the stern facing the dock. In the simulations used to validate the docking process, the AUV moved with a speed of 2 m/s, the submarine moved with a speed of 0.5 m/s, resulting in a relative speed of 1.5 m/s. The result showed that underwater docking of an AUV on a moving docking station was successful with the navigation function used, offering a potential solution for real­world applications (Sujit et al., 2011).

(a) Simulated docking scenario (Sujit et al., (b) Active docking system (Watt et al., 2016) 2011)

Figure 10: Docking on moving submarine

When docking an UUV on a submerged and slowly moving submarine, Watt et al. (2016) suggests using an automated active docking system with the purpose of correcting transverse

19 4 EXISTING HARD DOCKING SYSTEMS relative motion between the two vehicle. In order to minimize risk for collision, it is also suggested to place the docking station on a comfortable distance from the submarine. In the study, the docking station is placed alongside the submarine, 4 meters from the hull, seen in Figure 10b. Furthermore, Watt et al. claims that docking with forward speed is beneficiary over stationary docking, as it is faster, quieter, and more stable if environmental disturbance appears (Watt et al., 2016).

4.2 Docking on a still submarine

An existing and tested solution by the RSwN is the Saab SUBROV system, seen in Figure 11. It consists of a ROV with six degrees of freedom, an operator’s console, a winch, and a power supply. An operator steers the ROV through the console, while being able to display video and sonar images. The system has multiple possible usages; one being an active docking tool for an AUV.

Docking begins by placing the AUV on the seafloor, awaiting the SUBROV. The submarine, also placed on the ocean floor, opens a torpedo tube hatch and launches the ROV, which lifts the AUV using a sling and pushes the AUV into the open torpedo tube. With the AUV docked, the SUBROV returns to its own torpedo tube. Testing of the system showed that an operator required only little training in order to recover the AUV with the SUBROV.(Siesjö, 2011)

Figure 11: ROV catching (left) and lifting (right) an AUV during testing (Siesjö, 2011)

4.3 Free­standing docking stations

A majority of found existing docking stations consisted of funnel constructions placed on the seafloor, with a similar design to the one seen in Figure 12. The most common design for stationary docking stations in research is the funnel­shaped dock (Zhang et al., 2017). Common constructions included a guiding cone followed by a tube, a clamping mechanism, guidance accessories such as beacons or communication buoys, and systems for wireless data transfer

20 4 EXISTING HARD DOCKING SYSTEMS and power charging. The AUV used in Figure 12b was 54 cm in diameter, 358 cm long, and 640 kg heavy, the tube was made out of fiberglass and was 57 cm in diameter. Initial tests were performed in a salt water tank, followed by a five­month trial outside Monterey, California. Trials proved successful, although unexpected challenges occurred with bio­fouling and wave motions (Hobson et al., 2007). Comparing the AUV diameter to outer funnel diameter in Figure 12a, it was concluded that the funnel had an opening diameter of 1.5­2 meters.

(b) AUV placed in dock during testing in seawater (a) Cross section drawing of docking station tank

Figure 12: Section view and photograph of funnel­type docking station (Hobson et al., 2007)

According to Wu et al. (2014), a funnel­shaped dock should have an open tube for low drag, and a cone with large area of closed faces for target recognition. Out of eight analysed dock shapes, it was concluded that an open tube with a semi­open funnel, as seen in Figure 13, was most suitable. Analysis of hydrodynamic properties revealed that symmetrical placement of holes in the funnel were preferable over unsymmetrical placement, as it lowered drag and pressure around the AUV bow. The high amount of recognisable walls made the shape of the funnel to the left in the same image the optimal choice, as it was easily detected by sonar but had almost identical drag properties as the shape of the funnel to the right (Wu et al., 2014).

Figure 13: Comparison of funnel shapes and its affect on the docking

21 4 EXISTING HARD DOCKING SYSTEMS

Using a funnel with an entry diameter of 1.2 meter and an AUV with a diameter of 0.2 meter, Zhang et al. (2017) found that the perfect posture of docking is when the longitudinal axis of the AUV coincide with the central axis of the cone before physical contact between the two. However, some deviation from a perfect alignment was deemed inevitable. It was found that when the AUV suddenly rotated 12 degrees laterally or vertically, there was a sharp change of the impact force. Impact force was further increased with funnel angle; evaluation of impact force and docking time for funnel angles of 50­90 degrees showed that a small angle was preferable to optimise the docking process (Zhang et al., 2017). Furthermore, it was concluded that docking materials with small friction and stiffness coefficients were preferable, in order to lower impact force and docking time. Recommended materials were acrylic and nylon, preferable over both steel and aluminium (Zhang et al., 2017).

In the same study, the authors mention that the lateral and vertical velocities of the AUV has a negative affect on docking, whereas an appropriate forward speed can optimise the impact process. Within the scope of the study, it showed that when a velocity increases, starting at 1.7 m/s, the force impact increases almost linearly and the time keeps constant. Due to this, the failure rate of docking and the possibility of damage will increase with greater horizontal velocity. With the used AUV in the study, an acceptable forward speed for docking was resulted to be between 0.8 and 1.5 m/s. The lateral and vertical velocities also affected the docking process (Zhang et al., 2017). With regards to docking with affecting cross currents, Wu et al. (2014) concluded that a beneficial docking method depends on the angle between the dock center line and the current. The study indicated that the angle should be less than 20 degrees (Wu et al., 2014).

22 5 HUMAN FACTORS

5 Human Factors

In this section, human interaction and the potential users’ point of view on AUV hard docking are brought up. While the purpose of the AUV is to lower the need for personnel, the docking system will require involvement of the submarine crew. It is therefor still necessary to consider how the system can be perceived as reliable so the system is designed as a system the crew want to use. As the FPL is a new concept and the A26 has not yet been launched, the human factors had to be based on speculation and experiences of previous similar systems.

5.1 Life on a Submarine

Hakkarainen (2021) presents life on a submarine as a fun but challenging environment. It is important to have a high understanding of the systems on­board the submarine. Every crew member must practice noise discipline, meaning they must move quietly through the submarine in order to avoid spreading noise into the water or to awake their sleeping crew­mate. Dropping just one metal piece can send out loud acoustic signatures that can be heard by external systems many kilometers away. (Hakkarainen, 2021)

Furthermore, Hakkarainen (2021) discusses the responsibilities of the submarine captain, mainly those regarding safety. As the crew performs many difficult and dangerous tasks, the captain must ensure the crew is never under danger for life. A captain’s decision will always center around crew and submarine safety and they will not use a system if deemed unreliable. An example of such a decision is when the captain decides to place the submarine on the seafloor; this is only done if there is enough information regarding the seafloor in question to ensure that the submarine can land safely. If there are any unknown objects, cliffs or currents, the captain will not land the submarine. (Hakkarainen, 2021)

5.2 Human Involvement

AUVs are transported using a crane truck, of the type seen in Figure 14a. Launching is done by lifting the AUV in its mass centre, using a sling, seen in Figure 14. Responsible for transport and launch are the AUV operators and the method is considered well­established. The truck allows for easy and efficient transport, as it can park in a regular harbour and only requires one operator. Top hatch loading of torpedoes is performed in the same manner, with a crane truck slowly lowering the torpedo into the top hatch (Perrone, 2021). The lifting method can also be applied aboard ships, such as the HMS Fårösund seen in Figure 14b (Lövgrena and Andersson,

23 5 HUMAN FACTORS

2009).

(a) Two AUVs transported on a crane truck (b) An AUV 62 launched from HMS Fårösund (Wahren, 2019) (Lövgrena and Andersson, 2009)

Figure 14: Transportation and launching AUV from ship

Only a limited number of crew members can be aboard the submarine at the same time. A docking system requiring personnel with unique training, such as special operation divers, would not be efficient since the idea is to always use the knowledge already existing among the staff to keep the number of people in the staff low. The submarine crew wants control over critical steps of the process, such as opening and closing hatches. The system should not require special staff or that ordinary staff must spend a lot of time training in simulators in order to use it. (Hakkarainen, 2021)

The crew prefers a Human­In­The­Loop system, where they have control over critical junctures but the automated system handles a majority of decision making and work. Preferable commands to the system would be ”start docking” and ”stop docking”, rather than, for example, controlling how many centimeters to the left the system should place itself. It is important to find a balance between an autonomous and a manual system. (Hakkarainen, 2021)

5.3 Opinion on New Systems

According to Söderblom (2016), the military benefit of the FPL can be optimised if the FPL can be opened and used while moving forward. The military efficiency of using AUVs depends on the purchased system, but advanced integrated AUV systems with the A26 has the ability to ”revolutionise submarine warfare”. (Söderblom, 2016)

With the FPL being a completely new system, it can be reasoned that some will be skeptical against it. Bruzelius (2016), claims that the submarine must be placed on the seafloor in order to flood the FPL, lowering its usage capacity. He theorizes whether submarine captains will be

24 5 HUMAN FACTORS willing to use such a new and untested system, and refers to the US Navy radio buoy. According to Bruzelius, the buoy was easy to both construct and install on the submarines, but getting the submarine captains to use it proved impossible, as the buoy proposed a small risk of getting its tether tangled in the propulsor. He argues that a similar situation could appear with the new FPL. (Bruzelius, 2016)

Hakkarainen (2021) presents a more optimistic outlook on the new FPL. When discussing what makes a new system reliable, he discusses having a back­up system to account for when the primary system fails. According to Hakkarainen, a system that fails and gets stuck can require the submarine to turn back to shore which can take up to three days and therefore ruins the current mission. Having a back­up system lowers the risk of that happening and adds a sense of security. (Hakkarainen, 2021)

When presenting new systems, Edvardsson and Helgesson (2021) claims that military customers become more receivable if all risks with the new system have been thoroughly analysed and are directly addressed. In order to avoid resistance from the customer, the designer should be clear with existing risks with the system and how those risks can be avoided. (Edvardsson and Helgesson, 2021)

25 6 ANALYSIS OF DOCKING SCENARIOS

6 Analysis of Docking Scenarios

In order to understand the hard docking process, four scenario analyses were conducted. The first scenario revolved around which submarine state and which AUV direction were most appropriate to dock in, with the goal being to understand which states and AUV directions the dock should be designed for. The second investigated scenario revolved around what a non­ functional docking means, with the goal being to understand how these situations could be avoided. The third scenario revolved around what an emergency while docking could be, with the goal being to understand how a system could be designed robust enough for these situations. The final scenario revolved around the required communication between AUV, crew, and dock, by identifying where communication is needed during an AUV docking operation.

6.1 Scenario: Submarine States and AUV Docking Direction

The different possible submarine states were analysed in order to conclude which state was most desirable during docking and how the states affected the docking abilities. All three states (seafloor, hovering, and forward) can be used by Swedish submarines, but the forward state is the most common and therefore assumed to be the most desirable state to dock in. The seafloor and hovering state can be dangerous and problematic to enter.

The AUV can approach the dock with its bow or stern first. Approaching with bow first has the advantages of improved maneuverability as it allows the AUV to drive forward, and lower risk of damaged components due to the bow already being a designated crash zone. The stern, in comparison, has multiple sensitive components and a damaged stern could severely affect the AUVs ability to perform its mission.

In this scenario analysis, the most appropriated AUV direction for each submarine state was investigated. The analysis conducted into three appropriate docking situations, these are found in Figure 15. The full analysis consisted of eight different situations and can be found in detailed in Appendix A Submarine States and AUV Docking Direction. Following in this subsection, the most appropriate docking situations can be found together with the conclusions drawn from the scenario analysis.

The scenario analysis concluded that the most preferable docking scenario was scenario (c), seen in Figure 15c, having the lowest relative velocity, enabling relaunch, and giving more time for adjusting positions. Preferable for docking in seafloor and hovering state was docking bow first, see scenario (a) in Figure 15a and (b) in Figure 15b, due to better AUV maneuverability.

26 6 ANALYSIS OF DOCKING SCENARIOS

Docking bow first sacrifice the relaunching ability, compared to docking stern first. As a result, the dock must be designed so that the AUV can enter with both directions. In order to increase relaunching abilities, the dock should be able to guide the AUV out of the FPL and the forepeak without the AUV needing to reverse.

(a) Scenario: Seafloor state, AUV direction (b) Scenario: Hovering state, AUV direction stern first, AUV reversing bow first, AUV moving forward

(c) Scenario: Forward state, AUV direction stern first, AUV moving forward

Figure 15: Submarine states with AUV direction when docking an AUV that is either moving forward or reversing

6.2 Scenario: What Is a Non­Functional Docking?

In order to define what a non­functional docking meant, possible issues were brainstormed. Following are three scenarios which were seen likely to happen, with the scenarios illustrated in Figure 16. For every non­functional docking scenario, a generic solution was defined. By integrating these solutions to the design, collision were aimed to be avoided and noise and breakage reduced.

Figure 16: Three scenarios of a non­functional docking scenario

Scenario 1: The AUV misses the dock The AUV approaches the dock but hydrodynamic disturbances, poor maneuvering, or a miscommunication, contributes to the AUV changing head of course and missing the dock.

27 6 ANALYSIS OF DOCKING SCENARIOS

An AUV missing the dock means the docking must re­start, which takes time. Furthermore, a the AUV could collide with the submarine hull.

A solution was to compensate for quick changes in course by having a dock that can be entered with some error in position, as well as having a slow and controlled docking where the AUV is certain to succeed before approaching the dock.

Scenario 2: The AUV enters the dock incorrectly The AUV enters the dock incorrectly and is positioned so that the dock cannot be brought into the FPL. If the AUV gets stuck in an incorrect position, the system has no choice but to either bring in the system despite the AUV colliding with the hull opening or to discharge the whole system including the AUV.

The solution was to have the system be brought in slowly, minimizing possible collisions with high force. The dock should be designed so that, no matter how the AUV enters the dock, it ends up in a stable position before being locked to the dock. The dock should neither lock the AUV in one specific position if that leads to just a little chance of getting an AUV stuck oriented in an undesired position.

Scenario 3: The AUV leaves the dock unexpectedly The AUV leaves the dock unexpectedly while the system is brought inside the submarine. In this case, the AUV risks getting stuck in the forepeak or in the FPL. A free­moving AUV inside the forepeak or the FPL risks causing noise and breakage. Re­starting the docking process from this position could also be difficult and time consuming. Due to these reasons, the AUV should not be able to leave the dock. The AUV should be locked to the dock or kept in an enclosure.

6.3 Scenario: What is an Emergency?

In order to define possible emergencies that could emerge, possible emergencies were brainstormed. For every emergency that placed within the scope, a generic solution was defined. The emergencies and solutions identified are presented in Table 1.

Possible emergencies were broken down into four types; high shock impact, forces acting on the system over time, high amount of noise, and something getting stuck. All of these issues were also found in the analysis of a non­functional docking, but were considered severe enough to cause an emergency.

28 6 ANALYSIS OF DOCKING SCENARIOS

Table 1: Possible emergencies with corresponding generic solution

Emergency Solution

The submarine is exposed to an explosion, The payload must be designed to withstand possibly from a naval mine or hostile high shock impact foreign vessel

The payload breaks and begins to move The docking station must be designed to uncontrollably through the FPL and withstand wear and tear from movements, forepeak reoccurring forces, and salt water

The noise caused by the docking process Add dampening elements to the construction results in the submarine being detected by foreign forces

The docking station or AUV gets stuck, Add a back upsystem for moving the making it impossible to close hatches docking station or the AUV, in case the first system fails

6.4 Scenario: AUV Operation of Communication Requirements

The following scenario investigated the need for communication between the dock, the crew, and the AUV, during the hard docking process. The AUV has finished the soft docking and is positioned near the submarine ready to touch the dock. The following needs for communication were found in the seven steps found in Figure 17.

Figure 17: Steps including the communication of the hard docking process

(1) Confirm AUV Before bringing the AUV onboard, it is important to only handle an AUV that has not been compromised with. An AUV that travels far from the submarine could theoretically be

29 6 ANALYSIS OF DOCKING SCENARIOS tampered with. The AUV must communicate to the crew that everything is fine, before it is brought aboard the submarine.

(2) Prepare system Once the AUV is confirmed, the hatches can open, the docking system is brought out and the hard docking process can start. Opening the hatches and bringing out the system makes the submarine vulnerable. The right moment to start a process should be decided by the crew so they are prepared and have the time for the docking process.

(3) Know where to dock Once the dock is outside, the AUV must know what it should look for and how it should position itself. The dock and the AUV must communicate to enable the AUV to know where to go.

(4) Position itself correctly in the dock When the AUV has entered the dock, it must position itself correctly. If a positioning fails, it must reposition itself. To know when the position is incorrect, the dock and the AUV must communicate.

(5) Lock itself correctly to the dock Once positioned correctly, the AUV can lock itself to the dock. The locking must also be conducted properly and another communication between the dock and the AUV is needed. The dock should only act when the AUV is locked.

(6) Finalise system When the AUV is positioned and locked correctly, the system can be brought into the FPL and hatches can close. The dock should communicate to the crew that everything seems fine and that the dock can go back into the FPL.

(7) Docking succeeded All three parties should be aware that hatches are closed and that docking is finished. Finalising the system can only be done when the AUV is positioned and locked correctly to the dock. If this has not occurred, the process must restart. Restarting the process at the end of the process could be complicated. An idea could be to manually steer the AUV to faster distance the AUV from the submarine and begin the process again.

To summarise, cameras and lights should be present inside the FPL and inside the forepeak, allowing visual confirmation that offers trust and reliability to the process. Visual overview should be complemented with sensors that give more precise information about where the AUV is located relative to the docking system. For each step, the crew should know what

30 6 ANALYSIS OF DOCKING SCENARIOS has happened. Once everything is inside the FPL, the crew should have the ability to decide when hatches can close. When hatches are closed and the AUV is secured inside the FPL, the hard docking process is over.

31 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

7 Factors Characterising a Reliable Hard Docking System

In this section, key insights gathered from research are presented together with resulting requirements. These requirements were found to characterise a reliable hard docking system and were used to further investigate how a design of a hard docking system could be done.

7.1 Key Insights

The most comprehensive insight found describes the high safety­thinking aboard Swedish submarines. The captain is responsible for the crew and the captain will never expose the crew to dangerous situations. Docking an AUV onto a submarine comes with many advantages, but high safety must be a central aspect in every function of the design to ensure that the crew is kept safe while docking.

In total, ten key insights were noted and these describe a collection of system and functional requirements needed or desired to have in a reliable docking system. Following in this subsection are the ten insights presented.

1. High signatures compromises submarine stealth

Stealth is a key factor in a successful submarine mission and docking must be possible without increasing the risk of being detected. The docking station should not send out any loud noise since those can be heard by external parties. Signatures must be considered for when docking occurs, but also for when docking is finished and the AUV is placed inside the FPL.

2. A time­efficient docking process occurs more spontaneously and increases the chances of being used

If the process of docking is time­efficient, the system will likely be used more often since it gives the crew more time for other activities. Factors such as if the submarine must enter a specific state to dock, the success­rate of capturing the AUV, and the level of man­steered actions play a role in how much time and effort a docking requires.

32 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

3. The underwater environment affects the system and the docking process

The underwater environment affects the system through corrosive water, pressure differences, and drag forces from water current. The system must withstand high shocks from potential sea mines or explosions in a time of conflict.

The underwater environment also affects the docking procedure due to hydrodynamic disturbances, most commonly currents, which can worsen the AUV’s maneuverability. At the same time, water flowing into the forepeak, either by currents or when the submarine moves forward, could potentially guide the AUV into the FPL.

4. The system will be loaded either through the top hatch or through the forepeak

There are two entrances into the FPL, through the top hatch and through the forepeak, seen in Figure 18a. Loading torpedoes through the top hatch is a well­established method, indicating that loading a docking system through this entry would mean less training for those involved. Loading the system through the forepeak would mostly occur underneath the surface and be a completely new method, requiring divers and special equipment.

Additionally, loading through the top hatch requires dock segments less than 1 meter in diameter. This constraint requires the dock to be assembled or unfolded inside the FPL, in order to take advantage of the FPL diameter of 1.5 meters. By loading through the forepeak, the system can take advantage of the full 1.5 meters available inside the FPL without requiring any assembly. The resulting dimension difference of the two entrances is illustrated in Figure 18b.

(a) Loading alternatives, through the top hatch (b) Loading dimension constraints or forepeak

Figure 18: The two alternative ways to load the system into the FPL

33 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

Today, there are more advantages in loading the system through the top hatch, since it is an established method. Thus, the concepts developed in this thesis were designed with the top hatch dimensions taken into considerations during loading.

5. Docking in the forward state is most desirable

Docking while the submarine is moving forward was found to be the most desirable state to dock in. The reasoning behind this was found to be that the submarine is the most stable when proceeding forward which makes it the most common state to be in. However, the shallow waters of the Baltic Sea offers a unique ability to dock with the submarine placed on the ocean floor and this ability or the ability to hover should not be neglected. The flexibility of being able to dock no matter submarine state would offer the system flexibility.

6. The AUV has limited maneuverability which affects its ability to dock

In a perfect world, the AUV can maneuver on its own into the FPL and a dock is not needed. Today, this is not the case. The AUV has a maneuverability with a large turn radius and its course can change suddenly and unexpectedly due to strong currents, caused either by natural events or by the submarine.

A critical area where the AUV must keep its direction is when entering the dock. A missed dock could result in the AUV colliding with the submarine hull and that must be avoided. Designing a dock with a high success rate for capture enables the AUV to position itself with some error and still enter the dock.

A second critical area was identified as the forepeak, in which the AUV must keep its straight path into the FPL. In the forepeak, space is limited and a multitude of different valuable components are placed. Neither the AUV nor the docking system could be allowed to touch these components, as it could result in damage, breakage or components getting stuck.

7. The crew wants control of critical junctures only

While there is a general aim within the SwAF for more autonomous systems, the submarine crew wants control over what they consider to be critical junctures, such as deciding when to start docking or when to abort docking. A semi­autonomous system was considered autonomous enough for the crew to both learn and use. The crew can manually affect parts of the docking without deciding every millimeter of the AUV journey. In order to give the crew control over critical steps of the process, two way­communication was required between the dock, the AUV, and the crew.

34 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

8. In case of an unexpected event, the docking system should not be a hazard for the crew

In order for the docking system to not be a hazard to the crew in case of a non­functional docking or an emergency, there should be procedures present to ensure safety. This insight resulted in a need for a plan A, plan B, and plan C, for handling abnormal situations, as illustrated in Figure 19 and described below.

Plan A: In case of a non­functional docking, the crew must be able to abort the docking process at any moment. Aborting the process is meant as the ability to as fast as possible close the hatches by taking measures such as requiring the AUV to leave the dock. Aborting can be used to avoid scenarios where a docking process has begun at an inconvenient moment or when the AUV has approached the dock wrong. Plan A is illustrated in Figure 19a where the AUV follows the crew’s demand.

Plan B: If the first system malfunctions, it must be possible to bring the system back in, with or without the AUV, one final time. A secondary safety system used to avoid having a broken dock in the way for doors should be used as one final chance to secure the dock and the AUV. Plan B is illustrated in Figure 19b where the yellow arrow represents a second system used when the first system is broken.

Plan C: In case of a completely broken system that cannot be brought back into the FPL with neither the first system nor with the secondary safety system, the dock and the AUV should be abandoned into the ocean so hatches can close. Plan C is illustrated in Figure 19c where the system is distanced from the submarine.

(a) Plan A: Abort docking (b) Plan B: A second safety

(c) Plan C: Remove system

Figure 19: Recovery plans in case of an unexpected event

35 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

9. While in the FPL, the AUV must be accessible for the crew

The purpose of docking an AUV onto the submarine is to charge its batteries, to transfer collected data, and to conduct maintenance. To ensure these needs, the AUV should be as accessible as possible once located inside the FPL.

10. A system enabling relaunch of the AUV can be used more than once

Being able to relaunch the AUV is a must if it should be used during long missions far away from shore. Docking an AUV into the FPL and not being able to remove it would make the FPL unusable for other activities. This is not desired.

The ability to relaunch is closely related to how the AUV has been docked. A conclusion from Section 6 Analysis of Docking Scenarios, was that there is potential in being able to dock in both AUV directions. When docking stern first, the AUV could leave by itself as its maneuverability was good enough when proceeding forward. However, when docking bow first, the AUV could make it out by itself as maneuverability while reversing was poor. The design must implement an aid to launch whenever the AUV is docked bow first, to aid its maneuverability when reversing.

7.2 System Requirements

System requirements were built based on the key insights. In Table 2, the system requirements are listed. The importance of each requirement was evaluated using a Weight Setting Matrix, with motivations found in Appendix B Determination of the Importance Factor. The scale used for importance was: 5. Extremely important 3. Important 1. Slightly important 4. Very important 2. Fairly important

The importance factors are seen in the very right column of Table 2. Seen in the two left columns are the requirement numbers, used for easier identification, and the insights that each requirement was based on.

36 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

Table 2: System requirements, presented with importance factor and source insight

Req. Insight System requirements Importance No. No. [1 to 5] 1 1 Enable a secured AUV inside the FPL 2 2 1, 3 Generate no signatures 4 3 1, 3 Withstand the physical underwater properties of an ocean 5 4 2, 5 Enable docking no matter submarine state 2 5 2, 6 Enable for multiple different AUV entrance positions 2 6 2, 7 Enable a high level of autonomous actions 1 7 4 Enable an efficient loading into the submarine 1 8 6, 8 Keep the AUV in a straight path into the FPL 3 9 7, 8 Ability to abort a docking process at any moment 5 10 8 Ability to continue docking despite breakage 3 11 8 Ability to distance docking system from submarine 3 12 9 Enable an accessible AUV while parked in the FPL 1 13 10 Enable relaunch of the AUV 1

7.3 Specific Project Data

In order to develop a conceptual design for a hard docking system, project specific data was taken into consideration regarding dimensions for the submarine and AUV used in the project. The data given in the project brief was identified as seen in Figure 20 with the values found in Table 3.

Given was AUV mass (mAUV ), diameter (DAUV ) and length (LAUV ). Also given was the top hatch diameter (DTH ) and the FPL diameter (DFPL). The FPL diameter was assumed to be equal the hull opening diameter. An approximate length for the FPL (LFPL) was also given.

37 7 FACTORS CHARACTERISING A RELIABLE HARD DOCKING SYSTEM

Figure 20: Illustration of project specific data

Table 3: Project specific data

mAUV DAUV LAUV DFP L LFP L LF [kg] [m] [m] [m] [m] [m] 1000 0.53 6 1.5 6 3

38 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

8 Concept Ideation and Continuous Evaluation

This section describes the result of chosen concept ideation and early evaluation methods. Wide concept ideation resulted in two important findings and inspired means for the FM tree. The process and results of the systematic concept ideation is also presented, followed by evaluation of systematic concept result.

The concept ideation presented in this section focused on functions used for the docking process only. Other functions, such as how the dock is loaded into the submarine, how the AUV is relaunched, or which safety aspects should be added in case of a broken system, were added in the later Section 9 Concept Refinement.

8.1 Wide Concept Ideation

Clustering of the braindrawing sketches revealed that the sketches could be grouped based on whether the AUV was locked in its movement inside (I) or outside (O) the submarine. Locking the movement inside the submarine, seen to the left in Figure 21, allows the AUV to proceed all the way into the FPL on its own. This system would need to guide the AUV so it will not end up in the forepeak. Locking the movement outside the submarine, seen to the right in Figure 21, requires a system pulling the AUV together with the system.

Figure 21: Comparison of locking outside versus inside

Further analysis of the clustering revealed that the sketches only solved the issue of capturing or holding the AUV, without solving how the dock itself moved in and out of the FPL, nor how it was placed inside the FPL. This led to dividing the system into three subsystems, allowing for ideation on each system separately. These subsystems were defined as subsystem Docking station, subsystem Static frame, and subsystem Moving frame, illustrated in Figure 22.

Subsystem Docking station is the section of the system that had physical contact with the AUV. The AUV was also included in the subsystem. Subsystem Static frame is the section of the system attaching the system to the FPL. Subsystem Movable frame is the section of the system

39 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION moving the docking station in relation to the static frame. The subsystems are together referred to as the docking system.

Figure 22: The three subsystems; docking station (blue), moving frame (red), and static frame (green)

8.2 Systematic Concept Ideation

The main function was defined as Dock the AUV, found in the top box in Figure 23, with the system input of a undocked AUV and an output of a docked AUV. The main function was divided into a technical process consisting of the following steps: (1) prepare system, (2) guide AUV through the forepeak, (3) secure the AUV,and (4) finalise system. The systematic concept ideation was conducted by the method FM tree where a function is something required for the concept to fulfill its purpose, and a mean is referred to as a solution to a function.

Figure 23: Functions building the docking process

It was concluded that step (1) Prepare system and (4) Finalize system were strictly related to the subsystems Movable frame and Static frame, while step (2) Guide through the forepeak and (3) Secure AUV were strictly related to subsystem Docking station. In this subsection, the conclusions and results from the systematic concept ideation are presented. For a full overview of the FM tree process, morphological matrix, and differentiation process, see Appendix C Functions and Means Tree.

40 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

8.2.1 Subsystem Docking Station

The subsystem Docking station was described by the technical processes (2) Guide AUV through the forepeak and (3) Secure the AUV. Guiding the AUV through the forepeak required three functions. The first function was to receive the AUV at first physical contact, with receiving meaning to atone the shock of impact that occurs when the AUV interferes with the docking station. The second required function was to compensate for the AUVs limited ability to maneuver. The third required function was to support the direction of the AUV once inside the docking station, avoiding collision inside the forepeak.

Securing the AUV inside the FPL required two functions. The first function was to stop the AUV, which should be done quietly, followed by the second function of capturing the AUV. Capturing referred to securing the AUV by locking it in place.

The corresponding means of each function of the subsystem Docking station, were many and had different strengths and weaknesses. All means, including those considered too poor to develop further is presented in Appendix C Functions and Means Tree. The resulting nine ideas, built on combinations of means from all five functions, are seen in Figure 24. Following is a description of each idea, along with main strengths and weaknesses.

V­Guide The first idea, see Figure 24a, functions by mounting a telescope on the AUV. The docking station consists of two beams, sent out at an angle, which receive the telescope and guide the AUV by the telescope touching the beams. While inside the FPL, the telescope is slowly guided into a lock mechanism, which secures the AUV. Inside the forepeak, a structure keeps the AUV from entering the rest of the forepeak.

Main strengths were identified as low weight and simple structure. Main weaknesses were identified as low support for AUV direction, with limited support in vertical direction.

Electromagnet The second idea, see Figure 24b, uses an electromagnet to lock onto the AUV, before bringing it in. The electromagnet is mounted in a way that allowed it to move slightly in the water, making it easier to find and reach the AUV. Inside the forepeak, a structure is added to hinder the AUV from rotating around the attachment point. The electromagnet also works as the way to capture the AUV inside the FPL, together with a clearance from the structure covering the forepeak.

Main strengths were identified as low weight and a controllable way to lock and unlock the

41 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

AUV. Main weaknesses were identified as magnetic signatures and instability when dragging the AUV.

(a) V­Guide (b) Electromagnet (c) Vertical pusher

(d) Vertical net (e) Gripen (f) Bathtub

(g) Hammock (h) Long funnel (i) Short funnel

Figure 24: Solutions for the technical principles Guide AUV through forepeak and Secure the AUV

Vertical pusher The third idea, see Figure 24c, is to have a plane surface made out of a material with high friction and damping qualities, such as rubber. The AUV stabilizes itself by pushing the surface with high power so the dock slowly moves into the FPL. The surface is slightly rounded and would force the AUV into a position straight enough to make it through the hull opening. Once inside the FPL, the AUV is secured by being pressed down onto a platform by a pressing form.

Main strength was identified as being easy to abort. Main weaknesses were identified as offering no support for the AUV except forwards and relying heavily on AUV motor strength.

Vertical net The fourth idea, see Figure 24d, is a plane surface made out of a net or a weave fabric with large holes. The AUV is modified with a hook which attaches to the net when in contact. When the system is brought into the FPL, the AUV follows. To compensate for the rotational movement

42 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION the AUV might make around its attachment point, there was a structure added covering the forepeak. This structure, together with the hook, secure the AUV inside the FPL.

Main strengths were identified as having a large target surface and lightweight system. Main weaknesses were identified as inaccurate capture point and the need to untangle the AUV from the net before launch.

Gripen The fifth idea is to have one or more claws, inspired by wood grapples and seen in Figure 24e, which is brought out from the hull opening. Once the AUV is positioned within the reach of the claw, it closes and is brought back into the submarine with the AUV in its grip. Once inside the FPL, the AUV is secured by the grip.

Main strengths were identified as being suitable for different AUV lengths and easy to fit trough top hatch as the dock functions by being foldable. Main weaknesses were identified as being exposed for high rotational stresses if AUV tries to rotate while being locked and containing multiple moving mechanical parts.

Bathtub The sixth idea, see Figure 24f, is to have a bathtub­shaped volume in which the AUV can land. The volume has fabric walls with large holes on every side, so that when the system is brought into the FPL, the AUV is pushed along by a wall. The wall closest the hull opening prevents the AUV from accidentally enter on its own. Inside the FPL, a roof structure was added to lock the AUV with clearance.

Main strengths were identified as having a large capture area and using a similar movement as if the AUV parks on the seafloor. Main weaknesses were identified as requiring precision landing from the AUV and requiring the dock to be longer than the AUV which means the dock is limited to a maximum AUV length.

Hammock The seventh idea, seen in Figure 24g, is similar to the bathtub but without the front wall. Without the wall, there is less demands for precise positioning of the AUV. Four telescopes are added to the AUV, which are extracted through the holes of the walls and is used to lock the AUV to the docking station.

Main strengths were identified as being lightweight and having a large capture area. Main weaknesses were identified as not being completely locked to the dock, it is relying on the telescope’s ability to find the way through holes of the net.

43 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

Long Funnel The eight idea, see Figure 24h, is a long funnel guiding and allowing the AUV to proceed by itself all the way in to the FPL where it moves into a damping stop. Damping material was be added along the way inside the funnel. The AUV locks itself by extracting a telescope once in place inside the FPL.

Main strengths were identified as having a large capture area and high support for the AUV in multiple directions. Main weaknesses were identified as complicated assembly and difficulty to reach the AUV once docked.

Short Funnel The ninth and final idea, see Figure 24i, is a shorter funnel and the AUV locks itself to the dock while still outside. The AUV moves through the funnel and hits a damping stop block, prolongs its telescope and, and is pulled in. The funnel should have damping material on the inside.

Main strengths were identified as having a large capture area and high support for the AUV in multiple directions. Main weaknesses were identified as complicated assembly and only a somewhat reachable AUV inside the FPL.

8.2.2 Subsystem Moving Frame and Static Frame

The functions required to fulfill the technical processes of (1) preparing and (4) finalizing the system were found to mirror each other. The functions were to secure/loosen the system, bring it in/out, and to stop the system once in/out.

It was concluded that it would be most effective to use the same moving mechanism for bringing the system both in and out of the FPL. The moving frame would therefore likely be based on the same principles disregarding how the docking station was shaped. These principles could be simplified into having a driving force (e.g. engine), a force transmission (e.g. gears), and a way of simplifying the movement (e.g. rails).

It was decided that the most suitable force transmission would be electric actuators. Suitable driving force would be an electric engine. The primary competitors for force transmission were electric actuators and hydraulic cylinders, with hydraulic cylinders being quieter but also more sensitive to pressure and with a risk of leakage. It was found that a hydraulic system was not available in the FPL, making electrical actuators the only realistic option (Perrone, 2021).

Furthermore, telescopic actuators were deemed a likely necessity, in order to move the docking

44 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION station far enough from the submarine hull. Assuming similar qualities in strength between electrical actuators and hydraulic cylinders, they should not be used as structural members or to stabilize structures (Gannon, 2019). Instead, it was decided to add a linear guide with a combined purposed of simplifying the movement and stabilizing the structure. In conclusion, the chosen solutions were electric telescopic actuators, an electrical engine, and linear guides.

8.3 A First Evaluation

The work of ideating concepts was iterative and a continuous evaluation was part of the process. In this section, soft evaluation is performed on whether the docking station should be foldable, how large the capturing area should be, and what level of complexity is reasonable in regards to mechanical systems.

Following the KISS­principle, instituted by the chief of US Navy Weapons Bureau in 1960 in order to increase reliability and to decrease cost of military gadgets (Dalzell, 2009), the hard docking station should make use of as simple mechanical components as possible. The purpose of this was to minimize the amount of components that could break and to minimize the consequences of breakage, in order to increase safety.

8.3.1 Capturing Success­Rate

It was concluded to be possible to design a docking station which would successfully dock the AUV, even if the AUV did not perfectly position itself against the dock. Seen in Figure 25 is a comparison of consequences of entrance position and angle for the AUV against the docking station, for Gripen and any of the funnel ideas.

As seen in Figure 25a, the AUV must enter the opened claw in a horizontal position with centre of gravity close to the claw’s capturing area. The claw must embrace the AUV from below and capture it in the mass centre to succeed. If the claw grips the AUV slightly above the center line, the AUV might be pushed away. If the claw grips the AUV in another point than the mass centre, the AUV has a greater chance of start rotating leading to high stresses in the claw. As a conclusion, the AUV must position itself very precisely for the concept to work.

In comparison, the AUV can enter a funnel in many different angles and positions, as seen in Figure 25b. As long as the angle between the AUV and funnel is smaller than 90 degrees and the mass centre is within the funnel, the AUV should be able to correct its position against the

45 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION funnel and enter, making the funnel a more forgiving docking station than the claw when it comes to dock successfully even when the AUV is positioned with some error.

(a) Claw (b) Funnel

Figure 25: Entrance position and angle comparison

8.3.2 System Complexity

Designing the docking station to be foldable was considered. Benefits of a foldable docking station would be to maximize the capturing area and at the same time easily get a system small enough to fit through the top hatch. However, with a foldable docking station results in a more complex design compared to a static docking station.

Some of the ideas presented as solutions to the docking station, such as Gripen, had to be foldable. However, in theory folding abilities could be applied to almost all ideas, such as a funnel or the hammock. The funnels could open up once outside the hull, using rotational joints with a small electric engine, as seen in Figure 26a. This would result in a funnel opening larger than the FPL diameter. A larger area could be seen as a better change of the AUV getting into the dock.

The hammock could be constructed with the same principle, folding open once outside the FPL as seen in Figure 26b, resulting in a landing area wider than the FPL diameter. This would also allow the system to, once folded in, fit through the top hatch without requiring assembly inside the FPL.

46 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

(b) Folding mechanism for a hammock, seen from (a) Folding mechanism for a funnel front

Figure 26: Folding mechanisms that could be applied to docking station ideas

The danger with folding abilities appears when one folding mechanism breaks; if the system can not return to its smaller size, it might not be possible to pull it back into the FPL. The unfolded system would get stuck outside the submarine. Even when assuming a better chance of having a higher success­rate of AUV finding its way into the dock with a larger capturing area, the risk of joints breaking is too large to be ignored. It has also been shown in earlier research that an AUV was able to succeed docking into a station with a diameter of 1.5 meter, indicating that a static docking station with the FPL dimensions was enough.

Because of these reasons, it was concluded that (1) ideas that required rotational joints did not have potential to become reliable hard docking systems, and (2) concepts that could have rotational joints would be made static instead. Thus, Gripen was not considered to be a reliable docking station due to its many moving mechanical parts. It was also decided that every other concept should be static. A static docking system results in a capturing area smaller than the hull opening and that the system must be assembled and disassembled to fit through the top hatch.

8.3.3 Results From First Evaluation

The ideas were mapped out according to their complexity and their ability to enable a high capturing success­rate, as seen in Figure 27. It was desired to have a simple solution with a high capturing success­rate. Three ideas were concluded to have the most potential: the long funnel, the short funnel, and the hammock. These ideas had potential in a high success­rate since the AUV could approach the dock with some error, without being too complex in terms of electrical or moving parts.

47 8 CONCEPT IDEATION AND CONTINUOUS EVALUATION

Figure 27: Concept comparison graph of success­rate and complexity

The three chosen ideas from this evaluation were further refined during the process of this thesis. The other solutions were not further investigated.

48 9 CONCEPT REFINEMENT

9 Concept Refinement

In this section, the three chosen concepts were refined to include all aspects of a reliable hard docking system. The concepts had defined functions enabling docking, but in this section the concepts were refined to also include the requirements for relaunch, loading, human interaction, and design for safety. In order to separate the refined concepts from the old, new names were given: the short funnel became Concept Brugd1, the long funnel became Concept Valhaj2, and the hammock became Concept Flundra3.

9.1 Concept Brugd

Concept Brugd is illustrated in Figure 28, where subsystems static frame (green), movable frame (red), and docking station (blue) are found along with the FPL (gray) and the AUV (orange). Seen in the image is an AUV equipped with two telescopes proceeding through a funnel. The AUV can move until a damping stop mechanism stops the motion.

Figure 28: Concept Brugd as prolonged and inside the FPL

The docking sequence, seen in Figure 29 happens as following: (1) the AUV enters the funnel and its telescope prolongs, and (2) the system is brought in. For this to be possible, the telescope must be longer than the funnel exit, the AUV must know when to extrude the telescope, and the dock must know when the AUV has locked itself to the dock. Proposed solution was to place sensors at the exit to the funnel, letting the AUV to know if it has proceeded the sensors and should extrude the telescopes.

1World’s second largest fish. Eats by swimming with open mouth, making it look funnel­like 2World’s largest fish. Similar to a Brugd, but larger 3A flat fish

49 9 CONCEPT REFINEMENT

Figure 29: Use of telescope during docking sequence, concept Brugd

Two precautions can be made to ensure that the AUV can dock stern first, without damage or getting stuck. Today, the AUV has rudders by the stern, which protrude relative the rest of the body. These rudders should not break nor get stuck in between structural members of the funnel. The first possible precaution was to make a funnel with small enough holes for the stern to not get stuck. However, a funnel with more material will be more affected by currents and this was not preferable. The second possible precaution was modify the AUV, by adding a protective cover over protruding parts. This modification would prevent the AUV from getting stuck in a less structured funnel and protect in case of collision. Illustrated in Figure 30a, is the modification to the AUV stern.

The system should allow the AUV to be relaunched. When relaunching, there is a possibility that the AUV has protruding parts which can get stuck in the inner funnel opening, see the top example in Figure 30b. A way around this problem is to have a conical shape of both openings of the funnel part, see the lower example.

(a) Modification on the AUV stern (b) Launch for concept Brugd

Figure 30: Subsystem Static frame (green), subsystem Movable frame (red), and subsystem Dock station (blue)

50 9 CONCEPT REFINEMENT

9.2 Concept Valhaj

Concept Valhaj is illustrated in Figure 31, where the subsystems static frame (green), movable frame (red), and docking station (blue) are found together with the FPL (gray) and the AUV (orange). Seen in the image is an AUV equipped with two telescopes proceeding through a funnel. The AUV can move the whole way into the FPL where a damping stop mechanism stops the motion.

The Valhaj concept is equipped with the same function as the Brugd concept of launching the AUV without it getting stuck by having a cone­shaped exit and to have a modification of the AUV stern.

The Valhaj concept turned out to be a challenge when it came to fitting all parts into the FPL. The concept includes two static funnels, the inner funnel always connected to the static frame, and the outer funnel which is brought in and out from the FPL. For both of these to be static, the outer funnel places around the inner funnel when inside the FPL. The design requires the outer funnel to have a cutout on the lower side, to give space for the mountings of the inner funnel. The cutout should be small enough so the AUV cannot exit through this section.

Figure 31: Concept Valhaj as prolonged and inside the FPL

Hard docking begins with the AUV entering the outer funnel and traveling to the inner funnel, see (1) in Figure 32. To ensure that the AUV enters the inner funnel smoothly, the difference in diameter of the two funnels should be small and the inner funnel should be dressed in rubber since there might be an area for collisions. There should be no sharp edges on which the AUV could get stuck. Once the AUV has passed through the whole funnel, see (2) in the same figure, the AUV prolongs a telescope to axially lock itself from moving back out. The AUV must know when to do this and which one of the two telescopes to prolong, the one close the stern or the bow. When the AUV is secured, the system is brought back into the FPL, see (3) in Figure 32.

51 9 CONCEPT REFINEMENT

Sensors must be added to this concept as well, by the funnel exit so that the AUV knows that when to extent its telescope.

Figure 32: Use of telescope during docking sequence, concept Valhaj

9.3 Concept Flundra

The concept Flundra is illustrated in Figure 33, where the subsystems static frame (green), movable frame (red), and docking station (blue) are found together with the FPL (gray) and the AUV (orange). Seen in the image is an AUV equipped with four telescopes proceeding over the Flundra platform. The docking station can move in total twelve meters from the FPL.

Figure 33: Concept Flundra as prolonged and inside the FPL

The Flundra concept relies on its telescopes and ability to make itself heavy to lock itself to the dock. In Figure 34a, the docking process is illustrated where (1) is when the AUV lands on a rubber platform by making itself as heavy as possible, (2) the AUV prolongs four telescopes through the woven net which locks it enough to follow the dock into the FPL. The net comes in two parts which will be attached at the top and the rubber platform at the bottom. This will make sure the net is tense and not affected by currents from below pushing the net upwards.

The positions of the telescopes are seen in the illustration at the bottom in Figure 34a. There are two on each side, where two are near the bow and two near the stern. For this to happen,

52 9 CONCEPT REFINEMENT the AUV must know when to prolong the telescopes, and the holes of the net must be large enough for the telescopes to always go through. The net must also be tense and strong enough to work as an axial lock. When the system is brought in, see (3) in the same figure, the AUV must follow.

Since the Flundra platform take up the whole length of the FPL, it is important that the AUV lands with its whole body on the dock without extruding sections, see Figure 34b. If the dock is brought into the FPL with an extruding AUV, the FPL hatch will not be able to close. Sensors can be placed on the top of the dock to send information to the crew if the AUV is completely on the dock and ready to be brought inside.

(a) Use of telescope during hard docking sequence, for concept Flundra

(b) Positioning onto the platform when locking itself to the dock

Figure 34: The AUV locking itself to the dock and its position on the dock when brought inside

9.4 Electric Actuators

The telescopic electric actuators should not take any loads from the sides, therefore it is important the linear guides can handle these forces. The actuators must be attached unlocked with a possibility to rotate freely in case of a bending system.

The actuators will bring in and out the system which means they must handle the drag force acting on the system when the submarine is moving forward. There is also a possibility that

53 9 CONCEPT REFINEMENT the AUV will collide with the system from the front and this is also forces the actuators must handle. However, due to a supposed low friction in the linear guides, the weight of the dock the arms must bring in and out, is negligible. Research failed to reveal a suitable actuator available on the market, but indicated that one in theory could be built based on the technology existing today (Berg, 2021).

9.5 Loading the Docking Systems Through the Top Hatch

Loading the system through the top hatch was found to be desired, requiring the system to be no larger than the top hatch diameter. Since it was decided that the system should be simple and avoid having movable and foldable parts, the system was designed to be assembled and disassembled inside the submarine.

The linear guide together with the electrical actuators are sensitive and if they bend or are assembled wrongly, they might not work. The best choice would be to make these parts, the subsystem static frame and moving frame, one already assembled unit. Thus, the combined linear guide and actuators cannot take up more space than a diameter of 800 mm.

Flundra consist of many unique and relatively small components, compared to the Brugd and Valhaj. This will likely result in a more complicated assembly, in which components must be joined by bolts and nuts. For Brugd and Valhaj, the funnels are about 1.5 meters in diameters which will not fit through the top hatch and must therefore be assembled. The funnels of Brugd and Valhaj can make use of symmetry, making assembly quicker and easier. However, the bottom sections of the funnels are mounted on the moving frame and cannot be exchanged for the top sections. Components that are similar but not entirely the same make a poor design for assembly. To simplify assembly, each part should be clearly marked where to go and which section of the funnel it is. Marking where every part is attached to another should be applied to every concept.

Every part of the funnels, must be locked in all directions to two other parts, an example is found in Figure 35. The example shows that every part locks itself planar with a guide, and surface­to­surface with a clamping mechanism. As the scope of this thesis did not include detailed design, the presented solution was considered detailed enough to be conceptual.

In between every moving part, there should be rubber or other material enabling damping. Connections will move and having metal to metal surfaces touch is not good for keeping low signatures.

54 9 CONCEPT REFINEMENT

Figure 35: An example of assembling a funnel

9.6 Design for Safety

Multiple requirements revolved around safety, with three requirements (req. 9­11) focusing on what to do in case of an unplanned event. These requirements were approached in the design as described in this subsection. The solutions presented for each of these requirements is one solution out of many possible. As the requirements refer to very important aspects of a reliable docking system, there should be future work within this field. For this scope, the requirements were given one solution, each decided by the benefits presented in this subsection.

9.6.1 Abort at Any Moment

Having the ability to abort a docking at any moment is ranked as one of the two most important requirements, being the first action when something unexpected happens. Aborting a docking refers to the ability to quickly cancel the docking, either by finishing docking or demanding the AUV to leave. The goal is to be able to close all hatches as fast as possible. A docking system that can enable closing the hatches very fast at any moment, is also a docking system enabling aborting the docking at any moment.

As a docking process takes time and includes many steps, it will at some point be more beneficial to demand the AUV to leave than to finish docking. The crew must know when this moment occurs so that the right precautions are made. Enabling the crew to observe the process through video would let them know when it is better to demand the AUV to leave and when the docking should continue. This means the AUV must also be programmed to interrupt a current measure to follow the commands of the crew. A well functioning two­way communication must be included in the system so the system follows the crew in real time.

55 9 CONCEPT REFINEMENT

9.6.2 A Second Safety System

The linear guide must handle high shocks and never break. If it breaks, it will be very difficult to push out or pull in the system. To achieve this, the linear guide should be dimensioned with a high safety factor in regards to stress and deformation.

The moving frame was given two electric actuators, both strong enough to pull the system back in on their own. The purpose of this was to be able to have a usable moving frame, even if one actuator malfunctions. If both actuators break, another motor can work as safety by attaching as a wire only used for bringing in the system, one final time.

9.6.3 Distancing System

Distancing the system, including the AUV, should be done if the docking must instantly be cancelled and there is no time to either finish docking or demanding the AUV to leave. Distancing is also relevant when the system is broken or the AUV is stuck in a position making it impossible to bring in the system, so that hatches cannot close. As this is a safety aspect enabling the submarine to instantly leave an area, the main priority is not to save the docking system nor the AUV. The main priority of this function is to save the crew.

First of all, it was concluded that distancing the system was a function that, in itself, required thorough research and concept ideation. With the time limit present, one solution was given to all concepts as an example to make sure the final concepts included such function due to its importance of a reliable docking system.

As all concepts move on a linear guide, this motion can be used to remove the dock using compressed air or water pressure cannon. It can be assumed that the parts of the system that are always inside the submarine will be more difficult to distance, as these parts must make it through the hull opening. For this reason, a system possible to move far away from the FPL on its linear guides, might have a better chance to be removed from the submarine. However, more research, tests and ideation must occur regarding this in future work.

Having a good ability to observe the docking will also aid the process of knowing when to distance and when it is acceptable to dock. Thus, being able to identify that it is the correct AUV and that no dangerous components have been added, was found to be a crucial ability.

56 10 CONCEPT REALISATION

10 Concept Realisation

Each concept was analysed with the goal of understanding whether they were realisable and to understand strengths and challenges. The following aspects were looked at: The height of the whole system must fit in the FPL diameter. The linear guide must handle its own weight and the added weight of the dock and the AUV. The total deflection of the linear guide should not be so it collides with the hull opening. The height of the linear guide and its position will determine how much room there is left for the dock while the system is located inside the FPL.

The realisation was mainly based on a worst case scenario using Flundra’s length of linear guide as it is the longest. The idea was not to receive detailed geometry but to understand if the concepts are possible and worth continue developing. Therefore, stress and deflection were analysed.

10.1 Material Analysis

A material analysis was conducted with the purpose of finding suitable materials, focusing on materials for the docking station and moving frame. It was assumed that the material chosen for the moving frame would also be suitable for the static frame. A brief overview of available materials was given through Granta Design and deeper research was conducted on the materials identified as suitable through Granta (2020). A shorter analysis of material demands and identified suitable materials will be presented in this subsection. The full material research can be found in Appendix D Material Analysis.

The materials used must conform to the harsh underwater environment and be of marine grade. Material requirements regarding subsea suitability include (1) Low density, in order to reach buoyancy neutrality, (2) High compression strength, in order to withstand subsea pressure, (3) High corrosion strength, with excellent resistance to salt water. As a result of possible large forces acting on the system along with high demands for safety, material requirements also include (4) High yield strength and (5) High tensile strength. Realistically, (6) High stiffness, is also desirable as large elastic deformations could hinder linear movements required by the system. As the defence sector must be able to operate within times of international conflicts, materials should be produced within the nation in order to secure supply security. Furthermore, galvanic corrosion caused by mixed materials should be taken into consideration.

The brief overview revealed stainless steel, titanium, or titanium alloys as suitable materials for the static and moving frame, as well as carbon fiber reinforced polymers for the docking

57 10 CONCEPT REALISATION station (Granta, 2020). Further research revealed Divinycell HCP as suitable for floating elements (Diab Group, 2020), usable if the moving frame proved too heavy and already used in submarine applications (Berg, 2021), and Ultra­high­molecular­weight polyethylene (UHMWPE) as another suitable material for the docking station (Wahren, 2021).

Material chosen for the moving frame was Ti 6Al­4V, a Grade 5 titanium. Relevant material data for further analysis is presented in Table 4, with Ti 6AL­4V density (ρ), Young’s modulus

(E), and yield strength (σs). Grade 5 accounts for 50 percent of total titanium used, making it the most common titanium alloy. It has high strength, relatively low density compared to stainless steel, high corrosion resistance, and good formability (Supra Alloys, 2021). Titanium alloys have previously been used in Russian submarines, chosen for its high specific strength, corrosion resistance, and lack of magnetism (Krylov et al., 2000).

Table 4: Material data for Grade 5 Titanium. Ti 6Al­4V

ρ E σs [kg/m3] [GPa] [MPa] 4430 114 932,5

The docking station, being located furthest away from the static frame will act as a cantilever on the moving frame and must be able to support an impact from the AUV, resulting in high demands regarding low density and and high tensile strength. A suitable carbon fiber would be high­tensile carbon fiber, with a possible tensile strength over 3 GPa. Carbon fiber is, along titanium, considered to have a high Strength to Weight Ratio. The material is corrosion resistant and chemically stable, and has good fatigue resistance. It is, however, brittle and can result in galvanic corrosion in fittings, which can be reduced by careful installation (Bhatt and Goe, 2017). Alternatively, a composite of both UHMWPE and carbon fiber can be used for better mechanical properties. In such case, the hybrid way is the most important factor to affect mechanical and thermal properties, with the most favourable hybrid way in regards to maximum impact strength being UHMWPE fiber content at 43 w% to carbon fiber (Lu et al., 2006).

With the complexity of composite materials and dynamic forces combined with limited time, it was concluded that no further material or stress analysis would be performed on the docking station. Instead, it was estimated that an UHMWPE carbon fiber composite would be realistic.

The static frame, having full support from its mountings inside the FPL, has low requirements

58 10 CONCEPT REALISATION regarding material compared to the moving frame and docking station. Most important is resistance against corrosion and high strength, making stainless steel a suitable material due to its strength and corrosion resistance, combined with low cost in comparison to titanium.

10.2 Stress and Deflection in the Linear Guide

The docking stations of all concepts were mounted on linear guides. As a crucial step to ensure that the presented concepts were realisable, an analysis was performed on the linear guides regarding forces, stress, and deflection. The analysis was used to define a possible height and width of the linear guides, ensuring that the system could fit inside the FPL. The goal was to see if the concepts had potential to continue developing. As this thesis only regarded conceptual design, simplifications were made.

The majority of loads acting on the system will be from above, mostly due to the weight of the system and the AUV. I­beams are known for high strength­to­weight ratio regarding bending and it was therefore assumed that an I­beam would be suitable as the linear guide. For simplification, it was assumed that the entire linear guide could be seen as an I­beam, see Figure 36. The two sections were assumed to be one with the same length.

In this subsection, the force and stress analysis is summarized. A full analysis can be found in Appendix E Solid Mechanics Calculations.

Figure 36: Simplified shape of linear guide (bottom), compared to a more realistic shape (top)

10.2.1 Forces Acting on System

It was concluded that the Flundra concept, being placed furthest away from the static frame, would result in the largest external forces acting on the system. Flundra will extrude the longest as its entire docking station must be placed outside the hull, compared to the funnels which do

59 10 CONCEPT REALISATION not have the same requirement for extrusion. Dimensioning for the longest extruded linear guide worked as the worst case.

Forces acting on the I­beam were identified, as seen in Figure 37b, with flange width (B), flange height (h), web thickness (b), and web height (H). In order to keep consistency with the fluid analysis presented in Section 3.4.2 Submarine Effect on Currents, a coordinate system was placed as seen in the figure.

(a) Cross section of I beam (b) Forces acting on the simplified linear guide

Figure 37: Forces acting on the simplified linear guide and its cross section

Relevant forces were the gravitational force (FG), the buoyancy force (FB) and drag force acting from the side (FDy) resulting from ocean currents of six knots. Drag force from the front was neglected, as it was assumed that the electric actuator would not be heavily affected by these. The vertical forces from the docking station were summarized into one vertical force acting on the moving frame (FEx), consisting of the gravitational and buoyancy force of the docking station, vertical drag force from water flowing in three knots, and gravitational force from the AUV. The weight of the docking station was estimated to circa 250 kg in order to perform a stress and strain analysis. This estimation was possible to make due to the iterative process where the concepts were made in CAD and a weight was received. The forces required for each bending moment was calculated as seen in Appendix E Solid Mechanics Calculations, where the full force analysis is presented.

Table 5: Chosen dimensions for I­beam used as linear guide

Lbeam H h B b [m] [mm] [mm] [mm] [mm] 9 110 40 230 100

The maximum bending moments, found in point 0, around the Y­axis (M0y) and around the

60 10 CONCEPT REALISATION

Z­axis (M0z) were calculated in accordance with Equation 1.

  1 M0y = Lbeam(FG + 2FEX − FB) 2 (1)  M0z = LbeamFDy

10.2.2 Stress and Deflection

A safety factor of stress (η) for the I beam was chosen according to the Pugsley Method of safety factors (Pugsley, 1951) as η = 4.24. The safety factor was used to calculate allowed maximum stress (σallow), found in Table 6 and based on material data presented in Table 4.

For the stress analysis, a worst case scenario was applied where the total mass of the AUV was assumed acting on the system without buoyancy. While the AUV is buoyancy neutral in water, it was concluded that the system must be able to handle the maximum weight of the AUV in order to be considered reliable. Maximum bending stress for bending around the Y­ axis (σy) and around the Z­axis (σz), are found in Table 6 along with maximum allowed stress. As seen from the calculated stresses, the real stress with the chosen cross section was less than the allowed stress. It was concluded that the simplified I­beam handles the forces acting on the system.

Table 6: Stresses when bending around y and z axis. AUV weight underwater is 1000 kg

σallow σy σz [MPa] [MPa] [MPa] 198.5849 180.2806 81.8842

When looking at the deflection of the I­beam, there are two points of interest. The first point is at the very end of the I­beam. where the maximum deflection (δmax) occurs. The second point is by the hull opening, where the deflection (δhull) should not be so large that it touches the hull. This deflection determines the position of the beam, it should be positioned high enough so that the hull is not touched. These points are seen in Figure 38.

61 10 CONCEPT REALISATION

Figure 38: Caption

The deflection in these points were analysed in Y­direction (δmax,y, δhull,y) and in Z­direction

(δmax,z, δhull,z). The worst case scenario was assumed to be when the I­beam is as long as possible (Lbeam=9 m) and when the AUV has the same weight as on land (mAUV =1000 kg). The AUV will most likely be close to weightless underwater but for these calculations, the weight on land was used to compensate for those times the AUV moves into the dock with speed.

The calculations gave the deflections shown in Table 7. The calculations showed that the I­ beam must be placed at least 6 cm above the hull opening and make sure there is place for a 3 cm deflection on each sides. The maximum deflections are 43 cm in Y­direction and 20 cm in Z­ direction. However, as an AUV weighting 1000 kg with currents of 6 knots in Y­directionwould happen rarely together, the deflection would most likely never be more than this. The maximum deflection could be a part of a shorter deflection of many in a resonance. Future calculations of resonance should be made. The deflection depends on added loads on the system, which means that decreasing the weight of the system could be done by adding float elements with the material earlier mentioned in Section 10.1 Material Analysis. It was concluded that floating materials was not necessary at this step, but a possible method for optimizing beam dimensions in the future. Table 7: Deflection of the I­beam by the hull and at the end

δhull,y δhull,z δmax,y δmax,z [mm] [mm] [mm] [mm] 30 60 204 430

10.3 Analysis of Subsystem Relationships

The funnel lengths (Lfunnel) and shapes of Brugd and Valhaj were heavily affected by linear guide length (Lguide) and height (Hguide), as seen in Figure 39. Linear guide height was

62 10 CONCEPT REALISATION determined through the stress and deflection analysis, allowing for the linear guide length and funnel shape to be adjusted. It was found that an increased linear guide length forced the funnel to be either shorter, asymmetric, or both shorter and asymmetric, as seen in Figure 39.

The Valhaj funnel consisted of a tube section and a cone section. Increasing linear guide length forced the cone section to be either shorter or asymmetric, illustrated in Figure 39b. A shorter cone resulted in a longer tube, but had no effect on total funnel length which would still be as long as the FPL.

(a) Relationships for concept Brugd (b) Relationships for concept Valhaj

Figure 39: Relationships between funnel, linear guide, and linear height

Two types of asymmetrical funnel shapes were identified. The front and end of the funnel could align, bending the bottom walls of the funnels. Alternatively, the walls of the funnel could be kept straight, forcing the center line of the larger opening to place below the center line of the smaller opening. Assuming that the AUV enters the funnel perfectly centered, unaligned openings would force the AUV to change direction, inevitably hitting the dock. Furthermore,

63 10 CONCEPT REALISATION

Wu et al. (2014) suggested that a symmetric funnel was preferable due to lower pressure around AUV and lower drag. It was therefor concluded that a symmetric funnel was preferable and that if a symmetric funnel proved unacceptable for other reasons, an asymmetric funnel with aligned openings was the second best choice. Furthermore, Wu et al. (2014) suggested making a funnel with a small angle, in order to reduce reaction forces and docking time. With a fixed outer and inner diameter, the angle was dependent on funnel length, making a long funnel desirable. However, it was found that the angles of the funnels were already smaller than the angles analysed by Wu et al. (2014), indicating that resulting funnel angle would be suitable even if not prioritized.

Watt et al. (2016) recommended using a docking system placed on a distance from the submarine, and the fluid analysis of a forward moving submarine revealed that placing the docking station 3 meters in front of the submarine removed the most severe effect from the submarine induced currents (Svensson, 2021). Increasing linear guide length allowed for increased distancing of docking station from hull, concluding that linear guide length should be made as large as possible. It was concluded that the linear guides should be long enough to allow a distance of 3 meters between fully extended funnel and submarine hull, resulting in a minimum stroke length of 6 meters. The funnel should be no longer than what is possible while keeping it symmetric, prioritizing symmetry over angle as long as the angle is less than 50°.

The different dock geometry of concept Flundra allowed for the linear guides to be the same length as the FPL. With linear guide length already maximized, dock width (B) should be made as large as possible in order to maximize capture area. It was found that the width was closely related to the height (H), in turn affected by the height of the beam (Hbeam), seen in Figure 40.

Figure 40: Relationship between dock width, height, and beam height

It was concluded that the beam height should be made as small as possible, in order to fit a

64 10 CONCEPT REALISATION deeper docking station. Desirable was to keep both docking station width and height as large as possible, while beam height only needed to be large enough to support the stresses applied. With minimum beam height calculated as H + 2h = 190mm, and linear guide stroke length defined, the concepts were modelled in CAD in order to investigate possible docking station dimensions.

The CAD­modelling revealed that Brugd had a possible funnel angle of 20°, an opening diameter of 1.4 m and was possible to extrude 4.5 m from the hull. The funnel became somewhat asymmetric, with a curved lower wall but with aligning front and end openings. Valhaj had the same possible funnel angle and opening diameter, but a shorter extrusion length of 3 meters from the hull, as a result of the larger funnel taking up space inside the FPL. The larger funnel also became asymmetric, in the same manner as the funnel in Brugd. The Flundra concept did not have a funnel taking up space inside the FPL, meaning the linear guide could be the longest with an extrusion length of 9 meters. The dock width became 1.3 meter wide and dock height became 0.5 meter.

These dimensions should be seen as an example of a possible design and can, in future work, be chosen differently. As this thesis was about conceptual design, the CAD models were used to investigate if the concepts were possible to geometrically exist. The results of the CAD models showed that all concepts were possible to fit inside the FPL and that all concepts were able to extend its dock minimum 3 meters from the hull opening. The combined analyses of materials, forces, stress and deflection, and geometrical constraints showed that all concepts could be considered realistic, although requiring further work. As precise dimensions were considered irrelevant, the dimensions presented in this section served no other purpose than proving realizability.

65 11 FINAL CONCEPTS AND CONCEPT EVALUATION

11 Final Concepts and Concept Evaluation

In the following section, the three refined concepts are presented with all included functions highlighted. The concepts are thereafter evaluated based on the weighted requirements. The concept with the highest score, according to the Weighted Criteria Matrix, was assumed to be the most suitable concept for a hard docking system.

11.1 Concept Brugd

The concept Brugd is illustrated in Figure 41. The system guides the AUV through a rubber coated funnel. Furthest out on the funnel, lights, sensors, or other equipment needed for communication between the dock and AUV can be placed. Once through the funnel, the AUV collides and stops by hitting a damping stop and a telescope locks the AUV inside the dock. When the system is brought in by the electrical actuators, the AUV follows along.

Figure 41: Functions and components of concept Brugd

Inside the FPL, see Figure 42, the Brugd concept enables an AUV with one half covered by the funnel and one half easily reachable. The funnel and the telescope prevent the AUV from moving around in the FPL. Once emptying the FPL from water, manually secured lashing straps provides an extra security.

Figure 42: AUV docked in FPL, concept Brugd

66 11 FINAL CONCEPTS AND CONCEPT EVALUATION

The system is equipped with two electric actuators, allowing for one actuator to break while still being able to bring the system back in. If both actuators break, a safety wire is connected to the dock and can, if needed, pull the system back into the FPL one final time. If the system is broken so it cannot be brought in, the safety wire is cut, the actuators are detached and the system can be removed using a pneumatic system. The dock will fall out and can be left behind.

The system was divided into smaller segments in order to fit through the top hatch, the segments seen in Figure 43. Seen from left to right, the segments are as following: the funnel, divided into six segments, the capture mechanism, onto which the funnel is mounted, the linear guide and electric actuators, and the modified AUV. Missing in the figure, is the static frame. The funnel segments are assembled using guides and clamps. The linear guide and electric actuators never disassembled, but delivered as one unit.

Figure 43: Segments for loading, concept Brugd

The system places certain requirements on the AUV. A telescope unit must be added and be built strong enough to pull the AUV while in water. Two­way communication must be present between AUV and dock, used to let the AUV know when to extract the telescope, when to turn off its engine, and it it needs to adjust its position. The propulsor unit must be modified to withstand light collision with the funnel, without breaking or getting stuck. The surface of the AUV must also be modified to avoid small components getting stuck in the funnel openings.

11.2 Concept Valhaj

Concept Valhaj is illustrated in Figure 44. The system guides the AUV through a rubber coated funnel. Furthest out on the funnel, lights, sensors, or other equipment needed for communication between the dock and AUV can be placed. Once through the funnel and inside

67 11 FINAL CONCEPTS AND CONCEPT EVALUATION the FPL, the AUV collides and stops by hitting a damping stop, and a telescope locks the AUV in place. With the AUV secured, the system is brought in by the electrical actuators.

Figure 44: Functions and components of concept Valhaj

Inside the FPL, see Figure 45, the AUV is covered by both the inner funnel and the outer funnel. This prevents the AUV from moving around in the FPL and it can be further secured using manually fastened lashing straps, once the FPL has been drained. The funnels have large open segments, allowing the crew to reach the AUV.

Figure 45: AUV docked in FPL, concept Valhaj

Valhaj uses the same safety mechanisms used in Brugd, with double electric actuators, wire cable, and pneumatic removal system. The longer funnel of Valhaj likely requires a stronger system, in order to remove a larger volume and higher mass.

Just like Brugd, Valhaj was divided into smaller segments in order to fit through the top hatch, seen in Figure 46. The outer funnel was divided into six segments, assembled using guides and clamps. The linear guide and electric actuators was kept as one segment, and the inner funnel with static frame was kept was one segment.

68 11 FINAL CONCEPTS AND CONCEPT EVALUATION

Figure 46: Segments for loading, concept Valhaj

Valhaj placed similar demands on the AUV as Brugd, mainly regarding adjustment of propulsor unit to withstand collision, addition of telescope unit, and two­way communication between dock and AUV. With the step from outer funnel to inner funnel, Valhaj required the surface of the AUV to be flat enough to pass through the step without damaging a component or getting stuck.

11.3 Concept Flundra

Concept Flundra is illustrated in Figure 47. The system uses a hammock­shaped landing area to enable soft landing, consisting of fabric net and a rubber coated bottom. On the top of the the dock, lights, sensors, or other equipment needed for a communication between the dock and AUV can be placed. When landing, a wall on the docking station blocks the AUV from entering the hull opening alone. When the system is brought in by the electrical actuators with a linear guide providing the movement, four telescopes are prolonged from the AUV through the holes of the net. The telescopes make sure the AUV will follow the dock into the submarine. Flundra uses four telescope, opposed to one in Valhaj and Brugd, as the open docking station leaves the AUV with too much space to move around if not properly secured.

Figure 47: Functions and components of concept Flundra

69 11 FINAL CONCEPTS AND CONCEPT EVALUATION

Inside the FPL, see Figure 48, Flundra enables an easily reachable AUV.There is a top structure, stopping the AUV from moving around freely inside the FPL. The telescopes also hold the AUV in place in every direction. Once the FPL has been drained from water, manually fastened lashing straps provides extra security.

Figure 48: AUV docked in FPL, concept Flundra

Using the same linear guide and double actuators as Brugd and Valhaj, Flundra can be brought in even if one actuator malfunctions. Should the AUV telescopes malfunction and be unable to extent, the AUVs ability to make itself heavier can be used to bring the AUV in. The system for removing Flundra works the same as for Brugd and Valhaj, but unique for this concept is that the whole docking system is brought outside the submarine, with nothing except the linear guide being able to collide with the hull opening.

Flundra is loaded using multiple segments, seen in Figure 49. As previously, the linear guide and actuators are loaded as one unit. The docking station is loaded in five segments, which must be assembled inside the FPL. The static frame is loaded in two segments; top and bottom.

Figure 49: Segments for loading, concept Flundra

Flundra requires the net and the AUV telescopes to be designed so that the AUV gets axially locked, to keep the AUV from sliding off. The four telescopes requires two telescope units to

70 11 FINAL CONCEPTS AND CONCEPT EVALUATION be added to the AUV. Flundra adds the same demands for two­way communication as Brugd and Valhaj.

11.4 Concept Evaluation

Concepts were evaluated using a Weighted Criteria Matrix, using the weighted requirements as criteria, as seen in Table 8. Each concept were rewarded a point of 1­3, depending on how well it fulfilled a requirement, using a scale of (1) Somewhat fulfills, (2) Fulfills well, (3) Fulfills completely. It was concluded that estimation of how well a concept fulfilled a requirement could not be fully evaluated for multiple requirements. Instead, estimations were made of how much potential a concept was considered to have in regards to fulfilling the requirement. Maximum available points were 99 points. As seen in Table 8, the Flundra scored highest with 81 points, followed by the Brugd with 79 points, and Valhaj with 62 points. Full reasoning behind each score is found in Appendix F Evaluation Analysis. Below follows an analysis of the Flundra concept and its received scores.

Table 8: Weighted Criteria Matrix

The Flundra concept received a higher score than the other concepts in six out of the thirteen requirements. It showed the possibility to generate low signatures by letting the AUV land on damping materials, whereas the other two concepts were designed to let the AUV collide with the structure often.

71 11 FINAL CONCEPTS AND CONCEPT EVALUATION

Flundra was estimated to have better chances in working autonomously as the AUV must only control its position horizontally to stay above the dock. When the position has been stable for a while, it can lower in height, until hitting the dock. The funnel concepts, in comparison, requires the AUV to regulate its position both vertically and horizontally to keep position within the funnel.

Comparing all three concepts, Flundra showed to consist of fewer parts and more unique parts. This enabled a more efficient assembly and therefore a more efficient loading into the submarine, as a system with few and clearly different components are easier to assemble than a system with many and almost identical components.

Flundra showed to have a better possibility to quicker abort a docking. The AUV is, during docking, mostly resting on an open platform extruded far from the submarine. Brugd and Valhaj have a dock surrounding the AUV and are located inside or somewhat inside the submarine, complicating the aborting act. Due to the open top, the Flundra concept also scored high on the ability of relaunching the AUV, as the open top allows the AUV to easily leave.

The Flundra concept was considered to have the most possibilities of being removed from the submarine since the dock emerge the longest from the FPL. The Brugd and Valhaj concepts have a funnel taking up space inside the FPL so the linear guides cannot be as long as the ones in the Flundra concept.

The Flundra received less score than the two other concepts in three of the thirteen requirements. Having a longer linear guide was considered to be good when it came to distancing the system from the FPL, but a longer linear guide makes the system vulnerable for high stresses. With the full docking station placed outside the submarine, it is also more exposed to drag forces from currents leaving high bending stresses on the linear guide. It also leaves both linear guide and actuators with more area available for breakage, lowering the ability to continue if a system breaks.

When comparing the concepts’ ability to dock no matter submarine state, the Flundra received less points than the funnel concepts. When docking in hovering or seafloor state, Flundra relies on the AUV’s ability to adjust in height, offering no support if the AUV places itself too far above the docking station. The funnel concepts do not have the same problem as the AUV can enter the dock in a larger range of height. Docking with the high relative velocity that comes with hovering and seafloor states, generates higher signatures and higher risk for unsuccessful docking regardless of concept.

72 12 RESULTS

12 Results

The goal of this thesis was to answer two research questions. These answers are presented here.

(1) The following factors characterise a reliable hard docking system:

• The system must withstand the physical underwater properties of an ocean, meaning the system is built specifically for the underwater environment to avoid breakage.

• The system must enable aborting a docking process at any moment, meaning the crew decides when and where it is appropriate to dock and the AUV should listen to the crew first

• The system must generate no signatures, meaning the docking process should be quiet enough to keep the submarine stealth during usage

• The system must enable the ability to continue docking when first system breaks, meaning it should be possible to bring in the system with or without the AUV one final time

• The system must enable the ability to distance the docking system from the submarine, meaning it should be possible to get rid of a non­functional system completely

• The system must enable docking no matter submarine state, meaning a docking can occur while the submarine is moving forward, hovering, or laying on the seafloor

• The system must enable for multiple different entrance positions for the AUV, meaning there is a high capture success­rate even when the AUV enters the dock with an error in angle or position

• The system must enable a secured AUV inside the FPL, meaning the AUV cannot move around in the FPL once all hatches have closed

• The system must enable keeping the AUV in a straight path into the FPL, meaning the AUV must never get stuck or move freely inside the forepeak

• The system must enable a high level of autonomous actions, meaning the docking can occur with minimum training

• The system must enable relaunching the AUV, meaning the AUV can be sent out again after it has been docked

73 12 RESULTS

• The system must enable an accessible AUV while parked in the FPL, meaning the crew can do maintenance, charge and download data with little problem

• The system must enable an efficient loading of the system into the submarine, meaning the system is manageable on land and a loading of the system is simple and does not mean extra training

(2) The following possibilities and challenges emerged from these factors and they were reflected in the following design of a hard docking system:

It was found that the Flundra concept, see Figure 50, were considered the most reliable system over the Brugd and Valhaj concepts. Below follows a summary of found possibilities and challenges with Flundra.

Figure 50: The Flundra concept which was considered the most reliable hard docking system

Possibilities with the Flundra concept

• Sending out low signatures by letting the AUV land on damping material and only collide with the dock once

• A high level of autonomous actions as the top of the dock is open and the position must only be controlled horizontally as the dock will stop the AUV motion vertically

• The concept consists of few unique parts which indicates to an efficient loading into the submarine

• The AUV has a great chance to leave the dock easily which is a possibility both for aborting a docking and when relaunching

74 12 RESULTS

• The long linear guides enable a possibility of removing the dock completely from the dock

Challenges with the Flundra concept

• The ability of docking when hovering or while the submarine is on the seafloor is limited as if the AUV does not stop properly above the dock, there can be a collision in the hull

• The long linear guide makes the dock vulnerable to bending stresses

75 13 DISCUSSION

13 Discussion

Following is a discussion regarding the implementation of chosen methods and the reliability of the result, a discussion regarding the functionality of the chosen concept, and a discussion regarding needs and requirements that were found to be outside the scope of this project.

13.1 Scope

The objective of this thesis was to gain a deeper understanding of hard docking. The chosen research questions focused on reliability and implementation of requirements. We believe that the objective has been fulfilled and that the chosen research questions were satisfactory in fulfilling the objective. As safety was found to be the most important factor to take into consideration, we are satisfied with focusing our research questions around reliability. We believe that the findings of this thesis can be used for future work, regarding design and construction of a hard docking system.

Early on in the project, one interviewee discussed similarities and differences in this thesis, compared to his own thesis. He gave the advice to ”limit, limit, and limit”. The delimitations present in this thesis were made to allow full focus on one step in the complex docking process. Certain delimitations were made in order to avoid confidentiality and we performed this project with an unofficial goal of using as little confidential information as possible, due to the publication requirements of the thesis. Overall, we are satisfied with the delimitations made and believe that they added depth to the project.

13.2 Implementation of Methodology

The methods used in this thesis were chosen at the beginning of the project, as part of the planning process. As expected, findings from research and concept ideation resulted in a few changes of methodology. This was seen as an important part of the iterative and flexible design process and we are satisfied with the chosen methods.

Quantitative research was thorough and covered all subjects of interest in this project. Reliable sources were found for each topic and we believe that the overall reliability of the quantitative research is high. Qualitative research consisted of one contextual interview, three semi­ structured interviews, and reoccurring unstructured interviews with project supervisors who were concluded to be highly competent within the subject. In total, eight people were interviewed, some in multiple instances. All interview subjects were found to be highly

76 13 DISCUSSION competent in the subject, but the low amount of interviewees can be seen as a weakness in this thesis. Finding suitable interviewees proved difficult, for multiple reasons. The submarine and AUV business is limited in size and surrounded by confidentiality, resulting in few suitable interviewee candidates. Accessibility to the suitable candidates was also low, due to COVID­19 restrictions and as a result of submarine crew having no outside contact while on deployment. However, this thesis only applies conceptual design and should be seen as a first step in an iterative process of design and construction of a hard docking system. Therefore, we believe that the qualitative research performed can be considered enough within our scope.

A possible result of the limited qualitative research is misjudged importance factors of the requirements. The importance factors were calculated multiple times, compared to the researched findings and key insights, and calculated again. Faulty importance factors would have affected the results and could have lead to a poor recommendation of which concept to continue working on. It is our hope and belief that our recommendation is correct and useful for future work. Recommendation for which steps to take in the continued process are presented below.

13.3 Results

One argument for automatically locking the AUV inside the FPL, was that draining the FPL is a resource heavy activity and it is therefore desirable to secure the AUV without draining the FPL. An automatic locking system was considered difficult as it would require moving mechanical parts in a subsea environment, at risk for corrosion and breakage caused by the harsh surroundings. Furthermore, the ability to launch does not only depend on how the AUV is docked, but also on ability to charge and transfer data. This requires the FPL to be drained, and for personnel to enter the FPL and manually reach the AUV. It was therefore concluded that a system which offers only simple automatic locking was good enough, as the crew would eventually want to drain the FPL and physically access the AUV. Manual security, the lashing straps, could then be fastened without a risk of injury.

Flundra has automatic locking in terms of four telescopic arms mounted on the AUV. The necessity of the telescopic arms can be further discussed, as it is not certain that four arms are necessary. At first, it was thought that Flundra could pull in the AUV while relying only on gravitational force and friction. This was discarded due to the buoyancy neutrality of the AUV and its low ability to add weight, resulting in the telescopic arms. The benefit versus difficulty of the telescopic arms should be further investigated in future work, along with other possible

77 13 DISCUSSION solutions for the specific function.

A few functions were not completely solved for, such as distancing the system or minimizing signatures, and should be further investigated in future work. It is our belief that these functions are complex and involve high risks, and are therefore deserving of thorough investigation, ideation, and validation. This was not possible within the time frame of this thesis, and focus was instead placed on requirements considered reachable within the time frame.

13.4 Outside Perspective

An outside perspective is applied in this section, discussing relevant topics that were found to be outside the scope of this project. With an established method for top hatch loading, it was concluded that this was the most efficient loading method today. However, the FPL is a completely new design that has not yet been used. As a result, an established method for forepeak loading does not exist but could be developed once the submarines are launched. The hardships of forepeak loading can today only be discussed and deserves proper evaluation once the submarines have been launched.

In a perfect world, the AUV can maneuver on its own into the FPL, without requiring assistance. With perfect maneuverability, the risk of a malfunctioning docking system would be eliminated as only a static garage would be needed. This not a possibility today and as seen in the quantitative research, few established solutions exist for AUV docking. However, with AUV technology constantly improving, this could be a possible solution in the future. Other possible solutions include eliminating the hard docking, making the AUV follow the submarine in a prolonged soft docking sequence while transferring data. Battery charging would likely not be possible, but could be solved by having a separate charging station placed on the seafloor. The submarine could drive pass the home area of the AUV, transfer desired information, and leave. A third possible solution that was outside the scope of this project, would be to build a docking station outside the FPL. One could built something similar to the Dry Deck Shelter (DDS), adding a backpack to the submarine in which the AUV could transfer data and charge its batteries without any human contact.

The recommended concept, Flundra, would theoretically also be suitable for other AUV shapes and dimensions. An AUV shorter than six meters would, without issues, fit into the presented docking station shape, and a longer AUV would fit as long as it fits inside the FPL. A non­round AUV could fit, as long as it is narrow enough.

78 13 DISCUSSION

13.5 Future Work

As a next step towards having a reliable hard docking system, detailed design should be made for Flundra. The detailed design should include technical documentation, stress and deformation analysis for the entire system, and a design for assembly analysis. The stress and deformation analysis should include an analysis regarding fatigue failure and system oscillation. With a detailed design set, a model of small scale should be built and tested, in order to evaluate basic functions. If basics functions are concluded to be satisfactory, a full­sized model should be built and tested. Doing so will reveal actual signatures, user friendliness, and demands for AUV maneuverability.

Certain requirements needs more work, as they could not be fully solved for. Once an assembly analysis has been performed, loading of the system should be further investigated. Preferably, the whole system should be loaded as either one or two loading units, which can be loaded through the top hatch. This likely requires some sort of outer frame, that holds all the segments together. Ability to continue docking despite breakage and ability to distance the system also requires thorough future work, as they are central safety aspects in docking. An automatic locking system should be investigated, eliminating the need to drain and enter the FPL.

The system should also be optimized for soft docking. How this is done depends on the soft docking system present, and during this thesis, no satisfactory soft docking system was revealed. Once present, it can be concluded what specific communication units should be placed on the docking station.

79 14 CONCLUSIONS

14 Conclusions

The aim this research was to gain a deeper understanding of the hard docking process. Based on the analysis presented, it was concluded that thirteen requirements are put on a hard docking system in order to consider it reliable. The requirements range in importance and in complexity; some were considered easy to fulfill and some were found to be very difficult to fulfill. No requirements were considered impossible and the most important conclusion drawn is that designing and constructing a reliable hard docking system is possible. Furthermore, it was concluded that hard docking is a complex process and that the results of this thesis should be seen as a pilot study for AUV docking on the A26 submarines.

14.1 Final Words

In the words of David Ogilvy, ”When people aren’t having fun, they seldom produce good work”. We hope that this thesis shows how much fun we have had during this project, and that the results produced can aid in future design and construction of an AUV docking system. The opportunities of advanced AUV technology are many and we are proud to have performed our thesis in such an interesting subject.

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The different possible submarine states were analysed in order to conclude which state was most desirable during docking and how the states affected the docking abilities. Below follows an analysis of how the AUV should be positioned for each state. The AUV can approach the dock with its bow or stern first. Combining the submarine states with the AUV direction results in the eight different scenarios found in found in Figure 1.

(a) Scenario (1): Seafloor state, AUV direction (b) Scenario (2): Seafloor state, AUV direction stern first, AUV reversing stern first, AUV reversing

(c) Scenario (3): Hovering state, AUV (d) Scenario (4): Hovering state, AUV direction bow first, AUV moving forward direction stern first, AUV reversing

(e) Scenario (5): Forward state, AUV direction (f) Scenario (6): Forward state, AUV direction bow first, AUV moving forward stern first, AUV reversing

(g) Scenario (7): Forward state, AUV direction (h) Scenario (8): Forward state, AUV direction bow first, AUV reversing stern first, AUV moving forward

Figure 1: Submarine states with AUV direction when docking an AUV that is either moving forward or reversing

The appropriateness of these scenarios were analysed by looking at their chance of providing a docking with good AUV maneuverability and a low relative velocity between the two vehicles. A low relative velocity results in a low impact force in case of collision, avoiding high signatures and breakage. It was also important to consider the entering position of the AUV,as it affects the relaunching abilities as the AUV cannot be rotated inside the submarine. Entering the dock with

A:1(2) A SUBMARINE STATES AND AUV DOCKING DIRECTION the bow first would complicate relaunch, due to poor maneuverability while reversing.

Docking with bow first during seafloor or hovering state, seen as scenario (1) in Figure 1a and as scenario (3) in Figure 1c, would result in a relative velocity of 2 knots and fair maneuverability for the AUV. Hovering state would cause a slightly higher demand for maneuverability, as the submarine might not be entirely still.

Docking stern first in the same states, see scenario (2) in Figure 1b and scenario (4) in Figure 1d, would result in a relative velocity of less than 2 knots and very poor maneuverability due to the AUV reversing. The risk of the AUV missing the dock would increase, and scenario (2) and (4) were therefore considered inappropriate. It was concluded that docking while in seafloor or hovering state should be done bow first.

In forward state, docking can be done in four different ways. The AUV can either move towards the submarine, as seen in scenario (5) and (6) in Figure 1e and Figure 1f, or move away from the submarine as seen in scenario (7) and (8) in Figure 1g and Figure 1h. Having the AUV move towards the submarine would result in a relative velocity over 4 knot, twice the velocity presented in scenario (1) and (3), and was therefore considered inappropriate. Due to poor reversing maneuverability, scenario (7) was also considered to be inappropriate. It was concluded that scenario (8), with both vehicles moving forward, was preferable for docking in the forward state. It would enable a relative velocity of less than 1 knot, enable relaunch, and give the crew and the AUV more time for adjusting positions before starting the hard docking.

To summarize, the most preferable docking scenario was scenario (8), having the lowest relative velocity, enabling relaunch, and giving more time for adjusting positions. Preferable for docking in seafloor and hovering state was docking bow first, scenario (1) and (3), due to better AUV maneuverability. Docking bow first sacrifice the relaunching ability, compared to docking stern first. As a result, the dock must be designed so that the AUV can enter with both directions. In order to increase relaunching abilities, the dock should be able to guide the AUV out of the FPL and the forepeak without the AUV needing to reverse.

A:2(2) B DETERMINATION OF THE IMPORTANCE FACTOR

B Determination of the Importance Factor

In the following appendix, the requirements seen in Table 1 are given an importance through the Weighted Criteria Method. The final importance factor for every requirement is found in the right column in the same table.

Table 1: System requirements, presented with importance factor and source insight

Req. Insight System requirements Importance No. No. [1 to 5] 1 1 Enable a secured AUV inside the FPL 2 2 1, 3 Generate no signatures 4 3 1, 3 Withstand the physical underwater properties of an ocean 5 4 2, 5 Enable docking no matter submarine state 2 5 2, 6 Enable for multiple different AUV entrance positions 2 6 2, 7 Enable a high level of autonomous actions 1 7 4 Enable an efficient loading into the submarine 1 8 6, 8 Keep the AUV in a straight path into the FPL 3 9 7, 8 Ability to abort a docking process at any moment 5 10 8 Ability to continue docking despite breakage 3 11 8 Ability to distance docking system from submarine 3 12 9 Enable an accessible AUV while parked in the FPL 1 13 10 Enable relaunch of the AUV 1

To be able to grade each requirement they must be compared to each other. If one requirement was more important than another, the most important requirement was received 1 point and the other received 0. If two requirements were equally important, both received 0.5 points.

Presented in the following subsections are the comparisons between requirements. Each subsection compares a specific requirement towards the other requirements. As a result, subsections will get shorter and shorter as less comparisons remain unmentioned. The motivation for the comparison is considering the other requirement than the specific requirement for each subsection.

B.1 Comparison of No. 1. Enable a secured AUV inside the FPL

Following is the comparison of requirement No. 1, valued against requirement No.:

B:1(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(2) Generate no signatures is more important, as a system with high signatures will not be used at all

(3) Withstand the physical underwater properties of an ocean is more important, as a system that is unreliable in currents and in corrosive water will not be used

(4) Enable docking no matter submarine state has equal importance. There is no strong relationship between these requirements

(5) Enable for multiple different AUV entrance positions is more important as a dock providing a successful docking often will lower the chances of an AUV missing the dock and potentially collides with the hull. The water inside the FPL is calmer and collisions would not occur with the same power.

(6) Enable a high level of autonomous actions is more important. If the system is easy to use and in need of little training, the system would be easier to use. If the system compromises with locking the AUV inside the FPL to easy docking, an easy docking would be preferred.

(7) Enable an efficient loading into the submarine is less important. It would still be worth loading a system inefficiently if the system is reliable to use, such as having a secured AUV.

(8) Keep the AUV in a straight path into the FPL has equal importance, since either the AUV moves around inside the FPL or it moves around inside the forepeak. Neither is desired.

(9) Ability to abort a docking process at any moment is more important, enabling the crew to decide when and how to dock is more important than having an AUV moving inside the FPL.

(10) Ability to continue docking despite breakage has equal importance as breakage on the securing mechanism can result in the crew not wanting to continue docking, making the two requirements closely linked

(11) Ability to distance docking system from submarine is more important. Without a distancing function the dock can end up in a position where hatches cannot close. Closing hatches is more important than having a secured AUV inside the FPL

(12) Enable an accessible AUV while parked in the FPL has equal importance. The AUV must be accessible and when it is the AUV must be secured

B:2(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(13) Enable relaunch of the AUV is less important, as the crew would rather secure the AUV once properly, than do it poorly multiple times

B.2 Comparison of No. 2. Generate no signatures

Following is the comparison of requirement No. 2, valued against requirement No.:

(3) Withstand the physical underwater properties of an ocean has equal importance. The system must always handle the underwater environment, but waiting for a perfect docking environment is fine if that means the docking is quiet.

(4) Enable docking no matter submarine state is less important. Docking quietly in one state is more important than loud in all states.

(5) Enable for multiple different entrance positions for the AUV is less important. It is more important to dock quietly with many tries rather than having a high successful rate with a loud process.

(6) Use a high level of autonomous actions is less important. Parts of the process could be manually if it can be kept quiet.

(7) Enable an efficient loading of the payload into the submarine is less important. Docking quietly is preferred over being able to load the system into the submarine efficiently

(8) Keep the AUV in a straight path into the FPL has equal importance since they go hand in hand. By keeping the AUV in a straight path, the AUV will not get stuck in the forepeak and might cause signatures. The function that keeps the AUV in a straight path must be designed for low signatures.

(9) Ability to abort a docking process at any moment is more important. It is assumed that when the crew wants to abort the docking, it is extremely important to do so and it might be a moment when it is required to abort a docking even if signatures are generated.

(10) Ability to continue docking when first system breaks is less important, as the continued docking will not be used during emergencies when signatures no longer matter as much.

(11) Ability to distance the docking system from the submarine is more important. It is assumed that when the crew wants to distance the system, it is extremely important to do so and in that moment signatures are not as important.

B:3(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(12) Enable a reachable AUV while the AUV is parked in the FPL is less important. If the system generates signatures it will not be used and there would be no AUV to access.

(13) Enable launch is less important. Docking once quietly is preferred over being able to dock multiple times when generating signatures.

B.3 Comparison of No. 3. Withstand the physical underwater properties of an ocean

Following is the comparison of requirement No. 3, valued against requirement No.:

(4) Enable docking no matter submarine state is less important. Having a system withstanding currents and corrosive water in one state is better than a system not withstanding the underwater environment in all states.

(5) Enable for multiple different AUV entrance positions is less important. Having a system withstanding currents and corrosive water is preferable over a system docking with a high success capturing rate.

(6) Enable a high level of autonomous actions is less important. The process can be somewhat steered manually as long as the system can withstand currents and the corrosive underwater environment.

(7) Enable an efficient loading into the submarine is less important. The loading process can be somewhat inefficient as long as the system withstands currents and the corrosive underwater environment.

(8) Keep the AUV in a straight path into the FPL is less important. A system not withstanding the underwater environment can decrease the ability of being used as breakage increases. A system with much possibility of breaking will be unsafe to use even if it leads the AUV through the forepeak.

(9) Ability to abort a docking process at any moment is less important as a system not withstanding the underwater environment is too unreliable to be used at all

(10) Ability to continue docking despite breakage is less important as a system not withstanding the underwater environment is too unreliable to be used at all

(11) Ability to distance docking system from submarine is less important as a system not withstanding the underwater environment is too unreliable to be used at all

B:4(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(12) Enable an accessible AUV while parked in the FPL is less important as a system not withstanding the underwater environment is too unreliable to be used at all

(13) Enable relaunch of the AUV is less important as a system not withstanding the underwater environment is too unreliable to be used at all

B.4 Comparison of No. 4. Enable docking no matter submarine state

Following is the comparison of requirement No. 4, valued against requirement No.:

(5) Enable for multiple different AUV entrance positions has equal importance since there is no clear correlation between the two requirements.

( 6) Enable a high level of autonomous actions has equal importance as a somewhat manual process is okay if it enables docking anywhere, but a completely autonomous process is okay if it enables docking in one state. A balance between the two is desired.

(7) Enable an efficient loading into the submarine has equal importance since there is no clear correlation between the two requirements.

(8) Keep the AUV in a straight path into the FPL has equal importance since there is no clear correlation between the two requirements.

(9) Ability to abort a docking process at any moment is more important as it is a main aspect of safety. If aborting only works in one state, that state would be preferred over the other

(10) Ability to continue docking despite breakage is more important as it is a main aspect of safety. If continuing the process only works in one state, that state would be preferred over the other

(11) Ability to distance docking system from submarine is more important. If distancing only works in one state, that state would be preferred over the other

(12) Enable an accessible AUV while parked in the FPL is less important as accessibility is mainly required for relaunch and future AUV advances can result in improved accessibility later on

(13) Enable relaunch of the AUV is less important as the crew would rather have one very successful docking, than dock many times but with complications

B:5(10) B DETERMINATION OF THE IMPORTANCE FACTOR

B.5 Comparison of No. 5. Enable for multiple different entrance positions

Following is the comparison of requirement No. 5, valued against requirement No.:

(6) Enable a high level of autonomous actions has equal importance since if the docking occurs with some manual steering the docking should succeed easily on first try. However, when having a high level of autonomous actions it would be fine to not succeed docking at first try

(7) Enable an efficient loading into the submarine has equal importance, as the multiple entrance positions are allowed to result in slightly more complicated mechanical systems and therefore slightly less efficient loading, but only to a limit. There must be balance between the two requirements.

(8) Keep the AUV in a straight path into the FPL is more important as it is more desirable to avoid having an AUV stuck in the forepeak than to dock multiple times with a lower success­rate in capturing the AUV

(9) Ability to abort a docking process at any moment is more important as aborting can avoid dangerous situations and that should not be compromised to successfully capture the AUV at first try

(10) Ability to continue docking despite breakage is more important as aborting can avoid dangerous situations and that should not be compromised to successfully capture the AUV at first try

(11) Ability to distance docking system from submarine is more important as it is a central aspect of safety and is more important than being able to capture the AUV in the dock on first try

(12) Enable an accessible AUV while parked in the FPL is less important as capturing the AUV on few tries and faster close hatches is more important than being able access the AUV easily

(13) Enable relaunch of the AUV is less important as capturing the AUV on few tries, to some degree, and faster close hatches is more important than being able to relaunch the AUV

B:6(10) B DETERMINATION OF THE IMPORTANCE FACTOR

B.6 Comparison of No. 6. Enable a high level of autonomous actions

Following is the comparison of requirement No. 6, valued against requirement No.:

(7) Enable an efficient loading into the submarine has equal importance as the requirements are connected. A complicated autonomous system can require a less efficient loading, but only to a limit.

(8) Keep the AUV in a straight path into the FPL is more important as keeping the AUV in a straight path makes docking safer and a somewhat manual process can be allowed

(9) Ability to abort a docking process at any moment is more important as aborting the docking process is a central safety aspect and a somewhat manual process can be allowed if aborting works

(10) Ability to continue docking despite breakage is more important as the ability to continue despite breakage is a central safety aspect and a somewhat manual process can be allowed if continuing works

(11) Ability to distance docking system from submarine: More important, as the ability to distance the system is a central safety aspect and a somewhat manual process can be allowed if distancing works

(12) Enable an accessible AUV while parked in the FPL is more important as complicated autonomous system should not block accessibility to the AUV

(13) Enable relaunch of the AUV has equal importance as autonomous actions are likely required to relaunch, but cannot be made too complicated

B.7 Comparison of No. 7. Enable an efficient loading into the submarine

Following is the comparison of requirement No. 7, valued against requirement No.:

(8) Keep the AUV in a straight path into the FPL is more important as keeping the AUV in a straight path makes docking safer

(9) Ability to abort a docking process at any moment is more important as aborting the docking process is a central safety aspect

(10) Ability to continue docking despite breakage is more important as the ability to continue despite breakage is a central safety aspect

B:7(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(11) Ability to distance docking system from submarine is more important as the ability to distance the system is a central safety aspect

(12) Enable an accessible AUV while parked in the FPL has equal importance as the requirements are connected. Improved accessibility is, to some extent, worth a more complicated loading and assembly, but to a limit.

(13) Enable relaunch of the AUV is more important as it was concluded that a more complicated assembly would be worth it for the ability to relaunch

B.8 Comparison of No. 8. Keep the AUV in a straight path into the FPL

Following is the comparison of requirement No. 8, valued against requirement No.:

(9) Ability to abort a docking process at any moment is more important as aborting the docking process is a central safety aspect that must always be available

(10) Ability to continue docking despite breakage has equal importance, as ability to continue is a central safety aspect but keeping the AUV in a straight path minimizes the risk for breakage, making the two requirements closely linked

(11) Ability to distance docking system from submarine is less important as keeping the AUV in a straight path can prevent the need to distance the system

(12) Enable an accessible AUV while parked in the FPL is less important as keeping the AUV in a straight path makes docking safer

(13) Enable relaunch of the AUV is less important as keeping the AUV in a straight path makes docking safer

B.9 Comparison of No. 9. Ability to abort a docking process at any moment

Following is the comparison of requirement No. 9, valued against requirement No.:

(10) Ability to continue docking despite breakage is less important, as aborting the docking process can prevent breakage

(11) Ability to distance docking system from submarine is less important as aborting the docking process can prevent the need to distance the system

B:8(10) B DETERMINATION OF THE IMPORTANCE FACTOR

(12) Enable an accessible AUV while parked in the FPL is less important as aborting docking is a crucial safety aspect while accessibility is not

(13) Enable relaunch of the AUV is less important as aborting docking is a crucial safety aspect while relaunching is not

B.10 Comparison of No. 10. Ability to continue docking despite breakage

Following is the comparison of requirement No. 10, valued against requirement No.:

(11) Ability to distance docking system from submarine is less important as a successful docking despite breakage is preferable over distancing the dock completely. Saving the docking system, and the AUV, is better than removing completely.

(12) Enable an accessible AUV while parked in the FPL is less important. Docking despite breakage is a crucial safety aspect while accessibility is not.

(13) Enable relaunch of the AUV is less important as docking despite breakage is a crucial safety aspect while relaunching is not

B.11 Comparison of No. 11. Ability to distance docking system from submarine

Following is the comparison of requirement No. 11, valued against requirement No.:

(12) Enable an accessible AUV while parked in the FPL is less important, as distancing is a crucial safety aspect while accessibility is not

(13) Enable relaunch of the AUV is less important, as distancing is a crucial safety aspect while relaunching is not

B.12 Comparison of No. 12. Enable an accessible AUV while parked in the FPL

Following is the comparison of requirement No. 12, valued against requirement No.:

(13) Enable relaunch of the AUV is less important as there is no need to relaunch if the AUV cannot be reached for data transfer and battery charging.

B:9(10) B DETERMINATION OF THE IMPORTANCE FACTOR

B.13 Result

The comparisons were filled into the Weighted Criteria Matrix as seen in Figure 1, and each requirement received a grade depending on its importance.

Figure 1: Weighted Criteria Matrix

B:10(10) C FUNCTIONS AND MEANS TREE

C Functions and Means Tree

The method used to systematically ideate concepts was the Function­Means Tree (Hubka and Eder, 2002). The systematic process conducted during this thesis is presented in this appendix. The main function was defined as Dock the AUV, see Figure 1, with Undocked AUV as input and Docked AUV as output. The main functioned involved a four step technical process of (1) Prepare system, (2) Guide AUV through forepeak, (3) Secure AUV, (4) Finalize system.

Figure 1: Main function, function and sub­functions building up the problem of hard docking an AUV

Following is an analysis of each function mentioned in Figure 1, presenting the means ideated for each function and a discussion regarding combinations and choice of means. A mean is referred to a solution to a function.

C.1 Functions: Preparing and finalising the system

In the following subsection, the processes preparing system and finalising system are discussed. The first mentioned happens first in the sequence of hard docking, meaning the docking system is unclasped, brought out of the FPL and then stopped. The second mentioned occurs last in the sequence after a docking, meaning the system is brought in, stopped in the FPL and then secured. Upon deeper analysis of the processes, it was found that they included the same 3 functions only mirrored: (1) Release/secure system, (2) Bring system out/in, (3) Stop system.

C.1.1 Subfunction: Unclasp and secure system

The functions of unclasp system and secure system centres around either allowing the docking station to move or to hinder its movement. These same mean that secures the system at the end of the docking sequence is also the one that must be released at the beginning.

The means were divided into three parts, see Figure 2. The first mean is holding the system attached (blue), one energy source (green) and one transmission of movement (purple).

C:1(7) C FUNCTIONS AND MEANS TREE

Figure 2: Means solving the function of unclasping and securing the system inside the FPL

C.1.2 Subfunction: Bring out and in the system

The sub­functions of bring out system and bring in system are the functions of enabling a movement of the movable subsystems. These sub­functions have the same means since the system bringing out the system at the beginning of docking also can be used to bring it in at the end.

The means were divided into three parts, see Figure 3. The first mean is a transmission of movement (purple), the second an energy source (green) and the third a way to enable motion (blue).

Figure 3: Means solving the function of bringing out and in the system

C.1.3 Subfunction: Stop the system

The sub­functions of stopping the system means ending the movement of bringing the system in or out. The break was supposed to be smoothly. These sub­functions have the same means since it is the same system that is stopped both times.

The means were divided into two, see Figure 4. The first mean is a source of energy (green) and the second is damping (orange).

C:2(7) C FUNCTIONS AND MEANS TREE

Figure 4: Means solving the function of stopping system

C.2 Function: Guide the AUV through the hull opening

The function of guiding the AUV through the hull opening consist of three subfunctions: Recieve the AUV, which means handling the first contact between the system and the AUV, compensate for poor maneuvering, which means increasing the chances of a successful docking even though the AUV maneuvers poorly, and finally support the AUV direction, which means the system should make sure the AUV does not exit the desired path during the sequence.

C.2.1 Subfunction: Recieve the AUV

The subfunction Recieve the AUV means the system must handle the first contact between the system and the AUV which might be a collision. To avoid generating acoustics, the system should recieve the AUV smoothly.

The means were divided into two, see Figure 5. The subfunction was created by first choosing a degree of freedom, either multiple freedoms or one, in which the damping should be directed and then choose what the first surface receiving the AUV will be like, alone or together, with a modification to the AUV.

C:3(7) C FUNCTIONS AND MEANS TREE

Figure 5: Means solving the function of receiving the AUV

C.2.2 Subfunction: Compensate for poor maneuvering

The subfunction Compensate for poor maneuvering means the system must handle a maneuvering of the AUV that is not perfect. Currents, waves and AUV ability to maneuver will affect how good it can enter a docking station.

The means were divided into two, see Figure 6. The subfunction was created by first choosing if the function compensating for poor maneuverability should be larger than the FPL diameter or if it should be less or the same diameter. Second mean determines the look of the function if it is either the dock or a modification of the AUV.

Figure 6: Means solving the function of receiving the AUV

C:4(7) C FUNCTIONS AND MEANS TREE

C.2.3 Subfunction: Support AUV direction

The subfunction Support AUV direction means the system must make sure the AUV does not exit the desired path, especially in the forepeak. The means were divided into two, see Figure 7. The subfunction was created by first choosing if the function compensating for poor maneuverability should be larger than the FPL diameter or if it should be less or the same diameter. Second mean determines the look of the dock or a modification of the AUV.

Figure 7: Means solving the function of supporting the direction of the AUV

C.3 Function: Secure the AUV

The function of securing the AUV in the dock system consists of two subfunctions: First the stop the AUV which must happen smoothly, and second capture the AUV, which means the system must secure the AUV.

C.3.1 Subfunction: Stop the AUV

The subfunction Stop the AUV means the system must make sure the AUV can stop without generating signatures. The means were divided into one, see Figure 8. The subfunction was referring to the AUV stopping by turning off its motor or being slowed down and held still by a damping mean such as moving towards a damping hydraulic piston break or a thick rubber.

C:5(7) C FUNCTIONS AND MEANS TREE

Figure 8: Means solving the function of stopping the AUV

C.3.2 Subfunction: Capture the AUV

The subfunction Capture the AUV means the system must make sure the AUV cannot move once in the dock.

The means were divided into three, see Figure 9. The subfunction was created by first choosing what should be in contact with AUV and hold it still, what this function should be powered by and which type of moving transmission was needed.

Figure 9: Means solving the function of capturing the AUV

C:6(7) C FUNCTIONS AND MEANS TREE

C.4 From functions and means into concepts

In the following Figure 10 the means presented resulted in nine concepts.

Figure 10: Connecting at least one mean to every function resulting in nine concepts

C:7(7) D MATERIAL ANALYSIS

D Material Analysis

Material analysis was performed in two steps, in order to identify suitable materials for the docking station and the linear guide. The purpose of the analysis was to gain a higher understanding of the realizability of the presented concepts. The first step of the material analysis consisted of a brief analysis, performed in Level 2 database of Granta Design Limited (2020). Materials with excellent resistance to salt water were compared with regards to Young’s modulus and density, focusing on materials with low density less and high Young’s modulus. Chosen materials were titanium alloys, stainless steel, and Carbon Fiber Reinforce Polymers (CFRP). A deeper analysis was performed on the materials presented through the shallow analysis.

D.1 Docking station material

Zhang et al. (2017) identified acrylic and nylon as suitable materials for a funnel­shaped docking station, due to low friction and stiffness coefficients, and recommended against the use of metals such as steel and aluminium. It was concluded that the higher Young’s modulus of carbon fiber made it a more suitable material. The deeper analysis revealed Ultra­high­molecular­weight polyethylene (UHMWPE) as further suitable material for the docking station (Wahren, 2021). Below follows a deeper analysis of each found material.

CFRP refers to fiber­reinforced composite material using carbon fibers as its primary structural component, along with a thermosetting resin, commonly polyester, epoxy, or vinyl ester. The material is considered strong, with high strength and stiffness per weight unit, compared to glass fiber or metals. It is estimated that a CFRP structure weighs 1/5 of a steel structure with the same strength, and weighs 2/3 of an aluminium structure with the same strength. Furthermore, CFRP are considered expensive, with cost depending on carbon fiber grade, fiber tow size, and current market conditions (Johnson, 2019). The material is also electrically conductive and can facilitate galvanic corrosion in fittings, although CFRP itself is corrosion resistant and chemically stable. The fibers form strong covalent bonds in each layer, allowing propagation of cracks. If bent, the fibers fail at very low strain, making the material brittle (Bhatt and Goe, 2017).

UHMWPE is a subset of polyethylene with a high strength to weight ratio, up to 8­15 times higher than for steel. It is 40% stronger than aramids, has high resistance to stress and cracking, and strong chemical resistance. UHMWPE is also 15 times more resistant to abrasion than carbon steel and has a low coefficient of friction as well as low density (Shippee, 2017).

D:1(3) D MATERIAL ANALYSIS

Lu et al. (2006) analysed the properties of UHMWPE fiber/carbon fiber hybrid composites, concluding that the hybrid way had the most effect on mechanical and thermal properties. Furthermore, they concluded that bending strength, compressive strength, and interlaminar shear strength increased with a higher carbon fiber content, with maximum shear strength (423,3 KJ/m2) at 43 wt% UHMWPE fiber content to 57 wt% carbon fiber (Lu et al., 2006). It was concluded that the increased strength would be desirable for the docking station, along with the previously mentioned properties of UHMWPE, making a UHMWPE fiber/carbon fiber hybrid composite the most suitable material choice for the docking station.

D.2 Linear guide material

Primary concerns for linear guide was stress and deformation; it had to withstand the forces present without breakage, plastic deformation, or large elastic deformation. The shallow analysis presented stainless steel and titanium alloys as suitable materials.

Stainless steels are iron­based alloys containing minimum 10,5% chromium and can be categorized into five families: (1) Austenitic, (2) Ferritic, (3) Martensitic, (4) Duplex, and (5) Precipitation Hardening. Austenitic stainless steel is considered most corrosion resistant, due to highest chromium levels. It is also the most commonly used stainless steel, making up around 50% of the stainless steel used today, with good market availability (Ryerson, 2018). It has excellent weldability and good formability. For comparison purposes, SS4404 was chosen for analysis, as it is a high­alloy steel used in aggressive environments with high demands for corrosion resistance (Outokumpu, 2016).

Pure grade titanium was concluded to not have high enough corrosion resistance. Instead, titanium grade 7 and grade 5 were identified as suitable materials. Grade 7 is the most corrosion resistant grade and has excellent weldability. Grade 5 (Ti 6Al­4V) is the most commonly used titanium grade, accounting for 50% of total titanium usage and commonly used in marine applications. It has high strength, light weight, and high corrosion resistance (Supra Alloys, 2021).

Comparing the material data presented for SS4404, titanium grade 5, and titanium grade 7, presented in table 1, it was concluded that grade 5 was preferable over grade 7. Assuming that both materials have a similar corrosion resistance, grade 5 has somewhat lower density and significantly higher yield strength. Furthermore, grade 5 has significantly lower density than SS4404, along with almost three times the yield strength and compression strength. The Young’s modulus is lower, but it was concluded that elastic deformation was the least critical

D:2(3) D MATERIAL ANALYSIS deformation. Therefor, titanium grade 5 was chosen as suitable material for the linear guide. If the density still proved too high, it was suggested to add a floating element, such as Divinycell HCP30 (Diab Group, 2020).

D.3 Summary and material data

Ultimately, five material groups were analyzed: stainless steel, titanium alloys, CFRP, UHMWPE fiber, and aramid fiber. A light analysis regarding floating elements was performed. One type of stainless steel was further analysed, along with two types of titanium alloys. A hybrid composite of 43 wt% UHMWPE fiber and 57 wt% carbon fiber was chosen for the docking station, due to low weight, high strength, high stiffness, and low friction. Titanium grade 5 was chosen for the linear guide, due to low weight, high strength, and high corrosion resistance. Material data for all analysed materials are found in Table 1, presenting density

(ρ), Young’s modulus (E), yield strength (σs), ultimate tensile strength (σb), and compression strength (σc).

Table 1: Material data, average values (Granta Design Limited, 2020)

ρ E σs σb σc Material/Quality [kg/m3] [GPa] [MPa] [MPa] [MPa] Stainless steel 7740 200 698,5 907,5 726 SS4404 7645 196 283,5 608 283,5 Titanium Alloys 4610 115 895 976,5 915 Titanium Grade 5 4430 114 842 932,5 842 Titanium Grade 7 4515 102,5 323 422,5 190 UHMWPE 940 0,906 24,5 43,45 29,4 CFRP 1550 109,5 800 800 640 Epoxy/Aramid fiber 1380 70 800 800 640 Divinycell HCP30 200 0,0031 ­ ­ ­

D:3(3) E SOLID MECHANICS CALCULATIONS

E Solid Mechanics Calculations

An analysis was performed on the forces acting on the system and resulting stress and deflection in the moving frame. The purpose of the analysis was to conclude whether the system would be realisable, by proving that the structural component of the system (the linear guide in the moving frame) could be of realistic dimensions, that would fit into the FPL. It was concluded that the hanger, being placed furthest away from the static frame, would result in the largest external forces acting on the system. Thus, calculations were based on the hanger concept as an assumed worst case scenario.

An I­beam was chosen as the most suitable shape for the linear guide, seen in Figure 1a. The dimensions of the I­beam were calculated in an iterative process, by calculating stress and deflection for different dimensions until the results were concluded satisfactory. Final dimensions for flange width (B), flange height (h), web thickness (b) and web height (H) are presented in Table 1. Beam length (Lbeam) was decided by how far out the concept had to reach in order to place its entire docking station outside the submarine hull, also presented in

Table 1, along with the density (ρbeam) chosen through the material analysis in Appendix D Material Analysis . The forces acting on the moving frame were identified and are presented in

Figure 1b. Relevant forces where the gravitational force of the beam (FG), the buoyancy force

(FB) of the beam, acting at the centre of mass (Lbeam/2 from point 0), as well as horizontal drag force (FDy) and vertical external forces (FEx). The vertical external force consisted of vertical drag force (FDz), docking station force of gravity (FDS), and the gravitational force of the AUV (FAUV ). Drag forces were results of the submarine’s velocity over water, either from ocean currents or from the submarine driving forwards and upwards.

(a) Cross section of I beam (b) Forces acting on the simplified linear guide

Figure 1: Forces acting on the simplified linear guide and its cross section

E:1(7) E SOLID MECHANICS CALCULATIONS

Table 1: Input values for masses of hydraulic cylinders, AUV, dock, length and density of I beam, density of water, cross section dimensions of I beam

Lbeam H h B b ρbeam [m] [mm] [mm] [mm] [mm] [kg/m3] 9 110 40 230 100 4430

The bending moment in point 0 around the Y­axis (M0y) and around the Z­axis (M0z) are found with Equation 2.

  1 M0y = Lbeam(FG + 2FEX − FB) 2 (2)  M0z = LbeamFDy

Forces required to calculate the bending moment around the Y­axis were all forces acting vertically (FG, FB, and FEx), calculated according to Equation 3. Buoyancy and gravitational forces of the beam were calculated with the beam volume (Vbeam) and beam dimensions, the gravitational constant (g), and the density of either the beam or of water (ρH2O = 1000kg/m3).

  F = V ρ g  B beam H2O F = V ρ g (3)  G beam beam  FEX = FAUV + FDS + FDz

The gravitational forces of the AUV and of the docking station were calculated using AUV weight (mAUV = 1000kg) and docking station weight (mDS = 250kg), according to Equation 4. The docking station weight was estimated using CAD modelling.

  FAUV = mAUV g  (4) FDS = mDSg

Two drag force scenarios were calculated. Scenario 1 included horizontal currents with a velocity (vCy) of 6 knots. Scenario 2 included a vertical current flowing downwards, with a velocity (vCz) of 3 knots. Both scenarios disregarded lifting forces. The docking station shape was simplified in order to estimate drag coefficients. For the horizontal force (FDy), it was assumed that the side of the hanger could be simplified into a rectangle, as seen in Figure 2a,

E:2(7) E SOLID MECHANICS CALCULATIONS

resulting in a drag coefficient CDy = 1, 21 (Engineers Edge, 2017). For the vertical force (FDz), it was assumed that the hanger shape could be simplified into a half pipe, as seen in Figure 2b, resulting in a drag coefficient CDz = 2, 30 (Engineers Edge, 2017).

(a) Simplified shape when horizontal (b) Simplified shape when vertical current current

Figure 2: Simplified shapes, used to estimate drag coefficients (Engineers Edge, 2017)

The rectangle shape, in Figure 2a, has a length LD and height dr. The half pipe, in Figure 2b, shares the same length LD and has a diameter dhp. Dimensions, current velocities, and drag coefficients are presented below in Table 2.

vCy vCz CDy CDz LD dr dhp [knt] [knt] [mm] [mm] [mm] 6 3 1.21 2.30 6000 750 750

Table 2: Current velocities, drag coefficients, and shape dimensions

The drag forces were calculated as seen in Equation 5.

  1 2 FDz = CDzρH2OAD,rvH2O 2 (5)  1 F = C ρ A v2 Dy 2 Dy H2O D,hp H2O

Where drag exposed areas for the rectangle (AD,r) and for the half pipe (AD,hp) were calculated according to Equation 6. It was assumed that both areas were 2/3 open, as a result of using materials such as net.

  1  AD,r = dD,rLd 3 (6)  1 dD,r A = π L D,hp 3 2 d

The calculated bending moments and forces are presented in Table 3.

E:3(7) E SOLID MECHANICS CALCULATIONS

Table 3: Output values from force analysis

M0y M0z FG FB FAUV FDS FDy FDz [Nm] [Nm] [N] [N] [N] [N] [N] [N] 218 070 69 170 10 232 2309,70 7856 569,56 8646.2 12 908

E.1 Stresses

A safety factor of bending stress (ηs) for the I­beam was chosen according to the Pugsley

Method of safety factor (Pugsley, 1951), according to Equation 7 where the factors ηs1 and

ηs2 are determined according to Figure 3.

ηs = ηs1ηs2 (7)

The parameters used to find the safety factor ηs1 were the following: A. the quality of materials, workmanship, maintenance, and inspection, B. the control over the applied load, C. the accuracy of stress, analysis, experimental data, or experience with similar designs. The parameters could be chosen as very good (vg), good (g), fair (f), or poor (p). The parameters used to find the safety factor ηs2 were the following: D. danger to personnel, E. economic impact. The parameters could be chosen as very serious (vs), serious (s), not serious (ns). The parameters were chosen as the following:

• A = g : there is a high quality of materials, workmanship, maintenance, and inspection in military designs. However, the water environment of the Baltic Sea is corrosive and the design must be carefully determined to avoid any breakage at all. The parameter A was therefore set to good as it cannot be certain in this stage to have an excellent quality at all times.

• B = p : the control over applied load is poor since currents, vehicle velocities with drag forces from the water, or exterior threats such as mine explosions can contribute to a load that is unexpected. The parameter B was therefore set to poor.

• C = f : since the environment can be unexpected and the submarines are not built yet, no experimental data or experience with similar designs. However, simulations over currents around the submarine was used in this project on which stress analysis was conducted on. The parameter C was therefore set to fair as there is still a lot of uncertainty but with stress analysis based on some simulated data.

E:4(7) E SOLID MECHANICS CALCULATIONS

• D = vs : this parameter was set to very serious as the danger to the crew is very high when using the system if safety has not been considered

• E = vs : the economic impact is also set to very serious as the systems will be designed with high standard and will therefore be expensive. However, there should be an aim within the military to spend as little money as possible.

With the parameters A, B, and C, and Figure 3a, the ηs1 = 2.65. With the parameters D and E with Figure 3b, the ηs2 = 1.6. The final safety factor becomes ηs = 4.24.

(b) Matrix for determining the safety factor ηs2

(a) Matrix for determining the safety factor ηs1 Figure 3: Matrices for determination of safety factor (Engineers Edge, 2017)

The allowed maximum bending stress (σallow) was determined by Equation 8, using the material yield strength (σmax = 842MP a) and the calculated safety factor. Yield strength was decided in Appendix D Material Analysis.

σ σ = max (8) allow η

Continuing, the real yield strength (sigmareal) that appears in the beam must be less or equal to the allowed yield strength, as described in Equation 9. The real yield strength was determined

E:5(7) E SOLID MECHANICS CALCULATIONS

by taking the bending moment (M0y or M0z) and its corresponding section modulus (Wb).

M0 σallow ≥ σreal = (9) Wb

The section modulus for bending around the Y­axis and Z­axis were found in accordance to the

Equations 10a and 10b. To determine the section modulus, the moment of inertia (Iy and Iz) and the distance from the center of mass to the largest distance to an edge of the cross section in H B either the Y­direction (y) or the Z­direction (z). The distances were z = 2 +h and y = 2 .

I W = y (10a) by z

I W = z (10b) bz y

The moment of inertia in both directions were found according to the Equations 11a and 11b. The variables b, B, h, and H, were the dimensions describing the cross section area of the I beam earlier mentioned and with values found in table 1.

b(h + H)3 hB(h + H)2 I = + (11a) y 12 2

B3h b3H I = + (11b) z 6 12

The stresses given from bending the I­beam around the Y­ and Z­axis in point 0 are found in Table 4. None of the real stresses were larger than the allowed yield strength, showing that the system can handle its own weight and the assumed external loads.

Table 4: Stresses when bending around Y­ and Z­axis

σallow σreal,y σreal,z (MPa) (MPa) (MPa) 198.5849 180.2806 81.8842

E.2 Maximum Deflection and Deflection at Hull Opening

There were two interesting points of deflection; the maximum deflection at the very end of the I­beam and the maximum deflection occurring at the hull opening. First, the maximum

E:6(7) E SOLID MECHANICS CALCULATIONS deflection of the I­beam should not be too large since it might hinder the system to be brought inside the FPL. Second, the deflection by the hull opening must be less than the placement of the linear guide so that the system does not collides with the hull.

The deflection of a console beam can be found according to the Euler–Bernoulli beam theory, illustrated with two examples in Figure 4. The deflection δQ has an evenly spread load

Q visualised in Figure 4a, and the deflection δP has a load P in one point visualised in Figure 4b.

(a) Evenly spread load Q with deflection δQ (b) Point load P with deflection δP Figure 4: Elementary bending cases

The deflection of the beam when a load Q is applied evenly over the whole length is described with Equation 12. It is known that ξ = x , where x is a length from the end of the beam to Lbeam a point where the deflection is of interest, and α + β = 1.

QL2 δ (ξ) = beam (ξ4 − 4ξ + 3) (12) Q 24EI

The deflection occurring when the load is a point load P and ξ = 0, with other words at the end of the beam (x = 0), and α = 0, is found with Equation 13a

PL3 δ (α) = beam β (13a) P,max 3EI

The deflection occurring when the load P acts at the end of the beam (α = 0) and the deflection is desired where ξ = 6m, with other words by the hull opening, is found with Equation 13b.

Lbeam−Lforepeak The value of ξ is given by x = , and Lforepeak = 3m. Lbeam

PL2 δ (ξ > α) = beam (β2 − (ξ − α)2) (13b) P,hull 3EI

E:7(7) E SOLID MECHANICS CALCULATIONS

Maximum Deflection The maximum deflection when bending around the Y­axis is determined by Equation 14. The applied load Q is given by Q = FG − FB and the load P is given by P = FEX .

δmax,y = δQ + δP,max (14)

The maximum deflection when bending around the Z­axis is determined by Equation 15. The applied load P is given by P = FDy.

δmax,z = δP,max (15)

Deflection at Hull Opening Deflection at hull opening when bending around the Y­axis is determined by Equation 14. The applied load Q is given by Q = FG − FB and the load P is given by P = FEX .

δhully = δQ + δP,hull (16)

Deflection at hull opening when bending around the Z­axis is determined by Equation 17. The applied load P is given by P = FDz.

δhullz = δP,hull (17)

Table 5: Calculated deflections

δhull,y δhull,z δmax,y δmax,z [mm] [mm] [mm] [mm] 30 60 204 430

An I­beam with the cross section area given by H = 110mm, h = 40mm, b = 100mm, and B = 230mm can handle the surrounding loads when it comes to stress and deflection with the made assumptions.

E:8(7) F EVALUATION ANALYSIS

F Evaluation Analysis

(1) Enable a secured AUV inside the FPL Both concepts Brugd and Valhaj used their docking stations to automatically lock the AUV in place, using the narrow ends of respective funnel. Flundra required a roof on the static frame, which still leaved room for the AUV to move around slightly.

(2) Generate no signatures Brugd and Valhaj are constructed so that the AUV must collide with the docking station in order to enter it, unavoidably causing sound and signatures. It was estimated that the AUV could land more quietly on Flundra, by slowly sinking onto the docking station. (3) Withstand the physical underwater properties of an ocean Flundra, being the concept that moved the furthest out from the hull, scored lower as it was exposed to more drag and higher cantilever forces, than Brugd and Valhaj. All concepts were built with water acidity and current in consideration.

(4) Enable docking no matter submarine state It was concluded that Flundra was most suitable for docking with forward motion compared to other docking states, with low relative velocity. Docking with higher relative velocity resulted in the AUV having to stop quickly once in correct position. With Brugd and Valhaj, the AUV could continue to push forward and be guided into a stop by the funnels, making them more suitable for different submarine states.

(5) Enable for multiple different AUV entrance positions Flundra demanded the AUV to place itself correctly in height, while the funnels in Brugd and Valhaj helped correct the AUV position in all directions, resulting in them scoring higher.

(6) Enable a high level of autonomous actions Brugd and Valhaj requires the AUV to find correct height position and to hold it while docking, resulting in more complicated autonomous actions. Flundra only requires the AUV to find correct position in two directions before slowly lowering itself and automatically finding correct height position, resulting in a higher score for autonomous actions.

(7) Enable an efficient loading into the submarine Both Brugd and Valhaj required their docking stations do be assembled from minimum of six components, making loading inefficient and therefor possibly unreliable. The docking station of Flundra had less loose components and was less dependent on mountings to work correctly.

F:1(2) F EVALUATION ANALYSIS

(8) Keep the AUV in a straight path into the FPL Brugd, scoring highest, locks the AUV outside the hull and pulls it in with a highly controlled movements. Flundra also pulls the AUV in, but has no upper lock and could not stop the AUV if it pushes upwards, resulting in it scoring lowest. Valhaj locks the AUV in all directions, but allows the AUV to drive itself in, resulting in a less controlled motion during which the AUV could crash into the funnel multiple times, giving it a medium score.

(9) Ability to abort a docking process at any moment Brugd locks the AUV from axial movement early in the docking process, requiring the AUV to retract the telescope in order to abort docking. Flundra also locks the AUV through telescopes, but the softer hanger and open top makes it easier for the AUV to leave. The submarine could dive or the AUV rise, allowing it to abort docking without even retracting the telescopes, earning Flundra a high score. Valhaj does not lock the AUV, but it must travel through the long funnel in order to abort once hard docking has begun. The AUV must stop its forward motion and drive out, causing turbulence and high signatures, earning Valhaj a medium score.

(10) Ability to continue docking despite breakage All concepts had the same ability to continue docking with regards to the moving frame, but different abilities with regards to docking station. It was concluded that Valhaj, with its long travel distance for the AUV inside the funnel, could break in a way that gets the AUV stuck inside the forepeak. A breakage in the docking station was considered less critical for Brugd and Flundra, although still hazardous.

(11) Ability to distance docking system from submarine Flundra was estimated to be easiest to distance as its docking station was placed furthest from the hull. Brugd, having part of the docking station inside the hull while docking, scored medium and Valhaj, having its long funnel stretching from within the FPL to outside the hull, was considered most difficult to distance.

(12) Enable an accessible AUV while parked in the FPL The long funnel of Valhaj covers the AUV once parked in the FPL, making it less accessible. Flundra covers parts of the AUV with the static roof, still leaves the AUV fairly accessible. Brugd, with its short funnel, leaves the AUV most accessible, received the highest score.

(13) Enable relaunch of the AUV Both Brugd and Flundra could assist the AUV during launch by pushing it out of the FPL, an ability that Valhaj lacked. In worst case, Valhaj required the AUV to drive backwards out of the funnel, complicating the launch.

F:2(2) TRITA ITM-EX 2021:379

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