Ships' Deck Fittings Utilised for Towage

Day 1 ITS Paper 6 2018

MARSEILLE Organised by The ABR Company Ltd

Ships’ Deck Fittings Utilised for Towage

Capt Arie Nygh (speaker/author), SeaWays Consultants, Australia

SYNOPSIS There has been a move around the world to use high-powered escort tugs to ensure the safe passage of ships in restricted waterways. What has become apparent is that while these tugs can render significant assistance, there is an Achilles Heel – the ship’s fittings to which the towline is attached in many cases are unable to handle the forces generated by the tug (see Figures 1a- 1c). Investigation has found classification society regulations are confusing and lead to significant misunderstanding by pilots, tug masters and indeed ship owners. This potentially renders the whole exercise of escort towage a waste of time. This paper presents the issues, the underlying history of the situation, and comes up with some pragmatic guidelines to assist all concerned in making informed decisions.

INTRODUCTION In January 2017, the author was undertaking annual competency assessments on a client’s tugs operating in an Australian port. When ordered in to connect the towline, the pilot informed the tug master that the safe working load (SWL) of the ship’s bitts the tug was to connect to was 50 tonnes. It was noted that throughout the towage operation the tug master endeavoured to keep the tug’s bollard pull to a maximum of 50 tonnes, which for that tug was only three-quarters power.

The pilot service management was asked about the logic behind this procedure. The reply was that this was common in the port, and was designed to ensure there was no likelihood of damage or failure to the ship’s fittings. Mention was made at this point that the SWL of

Figures 1a, 1b, 1c: Images of damaged ship’s fittings 1 a ship’s bitts has nothing to do with the forces created mooring bitts on ships by a tug’s towline during towage operations. Rather, A classifi cation society will assign a vessel an the SWL is referencing the forces created by a ship’s equipment number (EN) based on design criteria such mooring line when connected to the wharf. as vessel dimensions, displacement and windage. These criteria are the main components that determine Because the mooring line is belayed in fi gure-of- the tidal and wind forces that will act on the vessel and eight turns around the legs of the bitts, the SWL is a must be counteracted to moor the vessel safely. The calculation based on the collapsing force of the vertical EN is then used to determine the number, length and levers created by the fi gure-of-eight turns nearing the strength of mooring lines that must be supplied onboard top of the bitt’s legs. With this in mind, it is accepted the vessel. NB: The EN is also used to determine practice in the towage industry, though little known, to the strength of anchoring and emergency towage effectively double whatever the SWL of the bitts is when arrangements (see Table 1). placing the towline at the base of one of the ship bitt’s legs. For example, if the bitts are rated at a SWL of 50 Strength requirements: mooring line tonnes, from a tug’s perspective this can be read as 50 tonnes x 2, giving the tug a SWL of 100 tonnes. IACS MBL as determined by EN OCIMF MBL as determined by Class EN Subsequently the author contacted colleagues in the Table 1: Mooring line strength requirements Brisbane Pilot Service and suggested undertaking work to investigate the subject of ships’ fi ttings utilised in Ship fi ttings are marked with a safe working load towage operations. At that stage we had no idea where (SWL). This is the maximum load that can be applied the research programme was to lead us, and the issues to a line attached to or passing around the fi tting for we were to uncover. mooring purposes. Note that the resultant load on the fi tting may exceed the load on the line (see Table 2). Joining us in this research was a small team of exemplary industry professionals: Capt Henk Hensen, author of publications such as Tug Use in Port: A Strength requirements: bitts Practical Guide and Tug Stability: A Practical Guide IACS 2 x MBL SWL = MBL to Safe Operations; Capt Brenton Winn, senior pilot OCIMF 2 x MBL at Brisbane Marine Pilots, and Gijsbert de Jong, of Table 2: Bitts’ strength requirements classifi cation society Bureau Veritas. For mooring purposes, it is assumed that the mooring DeSIgN AND CONSTRUCTION STANDARDS line is attached to the ship’s bitts in a fi gure-of-eight OF ShIPS’ BITTS AND FAIRleADS fashion. Turns are taken around each post in turn to It is important to clearly understand that ship deck secure the line. The load imposed on each post by a fi ttings are primarily provided for mooring the ship to the line attached in this manner is twice the load on the line wharf, not towage operations. Their required strength is (see Figures 2 and 3). determined by the breaking strain of the mooring lines deemed necessary by classifi cation societies. Ships’ OCIMF guidelines state that: “Belaying fi gure-of-eight mooring fi ttings and their foundations should be designed tends to pull the two posts together inducing a higher to carry at least the force imposed on the fi tting and stress in each barrel than that produced by an eye laid foundation by an attached mooring line to the wharf at around a single post.” The design load of the bitts must the line’s minimum breaking load (MBL). be twice the maximum expected load of an attached line. Remembering that the SWL of a fi tting is defi ned Gaining an understanding regarding ship deck fi ttings as the maximum load of a line attached to the fi tting, used for towage requires an understanding of ships’ then the design load of the bitts must be 2 x SWL. mooring fi ttings design regulations.

Reference throughout this paper is made to the following documents:

• Requirement Concerning Mooring, Anchoring and Towing, International Association of Classifi cation Societies (IACS), 2017 • Mooring Equipment Guidelines, 3rd ed, Oil Companies International Marine Forum (OCIMF), 2008

This paper does not include detailed consideration of safety factors when discussing design or breaking loads. In general terms, the safety factor allowed is around 15-25 per cent. Figure 2: SWL when belaying a mooring line

2 Figure 4: Prefabricated deck fi ttings

SWL of the associated bitts, but caution should be exercised, as this is not always the case.

Towing The load calculation on the fairlead is the same, irrespective of whether the line is being used for mooring or towing (notwithstanding the additional dynamic loads that may occur on a towing line) (see Figure 3: Figure-of-eight belaying a mooring line to Table 3, Figures 5a and 5b). the wharf Strength requirement: fairlead Bitts are an attachment to the vessel and are often provided ‘off the shelf’. Manufacturers produce bitts Dependent on wrap Up to 2 x angle, as shown on the complying with relevant international or national industry IACS standards (ie, ISO 13795, DIN, JMSA), Type-tested MBL Mooring Arrangement to demonstrate compliance to strength requirements. Plan. SWL = MBL (Note that the standards of design differ between the OCIMF 2 x MBL Worst case: wrap angle various industry standards.) 180 degrees MBL = Minimum Breaking Load Fairleads on ships Table 3: Fairlead strength requirements A fairlead redirects a line passing through it from the wharf or tug to the ship’s bitts. The angle of the line inboard of the fairlead is determined by the layout of the fi ttings on the deck that the line may pass around or be secured to. These working angles are shown on the ship’s mooring arrangement plan.

Outboard of the fairlead, the line will pass to a bollard on a quay to moor the ship, or, in the case of towing, will be secured to a tug. The angle of the line is variable, depending on the freeboard of the ship and the position of the shore bollard or positioning of the tug assisting the ship. The external angle is likely to have a horizontal and vertical component, and, as it cannot be Figure 5a: Line force is doubled due to the angle determined, a worst-case scenario should be assumed around the fairlead (ie, ≈90 degrees).

The load on the fairlead will depend on the load on and angle of the line passing through it (the wrap angle). The worst-case scenario is where the wrap angle is 180 degrees, whereby the maximum load on the fairlead and its foundation structures will be twice the load on a line passing through the lead.

Fairleads are an attachment to the vessel and are also often provided ‘off the shelf’ and made to a Type- tested design (Figure 4). It is common to see the Figure 5b: Line force is doubled due to the angle marked SWL of the fairleads being the same as the around the fairlead 3 Towage fittings on ships The ship deck fittings will also routinely be used to secure a towline for harbour towage, and in some ports for escort towage, either in the ship’s bow or through its centre lead aft (CLA). Unlike the calculation for mooring line strength, the rules do not specify the forces that may be required on the towline to safely manoeuvre the vessel. IACS requires the fittings to meet the“intended maximum towing load (eg, tug’s ‘static’ bollard pull) as indicated on the towing and mooring arrangements plan” (Table 4).

Strength requirements: towline IACS Intended maximum towing load OCIMF Not specified Table 4: Towline strength requirements

The ship has no control over the power of tugs, or Figure 6: TOW SWL designed for tug towline forces the breaking strain of the towline provided. The power applied by the tug may have to be limited if the bollard pull/load on the towline is likely to exceed the load towline can be controlled by limiting the power applied limits of the ship’s fittings. In calculating towing loads, by the tug, in some circumstances (dynamic loads, it is assumed that the towline is secured to the bitts by transverse arrest, indirect towing) the load on the line passing an eye over a single post of the ship’s bitts. can be excessive, comfortably surpassing the rated bollard pull of the tug. In these cases, the basic IACS It is generally accepted that the strength of the ship’s design principle requiring the line to be the weakest link fittings and foundation structures required for mooring in the system may be negated, resulting in overloading purposes will provide sufficient strength for towage of ship fittings or underpinning structure before the purposes. A problem arises as the loads applied on towline MBL is reached. some parts of the system during towage are different to the loads applied for mooring. This is primarily due Furthermore, it also must be acknowledged that to the different methods of attaching a mooring line many ships were built years ago when no one had (figure-of-eight) and a towline (single eye) to the bitts, envisaged harbour tugs of 70 or 80 tonnes BP, or plus the various towline angles, both in the horizontal high-performance escort tugs with hydrodynamic keels/ and vertical planes, that a tug in a dynamic environment skegs that can produce twice the tug’s rated bollard pull can generate. This is explained in more detail below. (commonly ≈120 tonnes BP).

Ship fittings can be marked with a safe towing load For towing purposes, it is assumed that the towline (TOW) that pertains to the forces relating to a tug. This is is attached to the bitts by a single eye placed over one the maximum load that can be applied to a line attached of the posts. The load imposed on the bitts by a line to or passing around the fitting for towage purposes. attached in this manner is equal to the load on the line. Note that this marking is rarely seen in practice, and As the design load of the bitts is 2 x SWL, the load on a frankly is too simplistic to be useful (Figure 6). line attached by a single eye can also be 2 x SWL (see Figure 7). Furthermore, there is a lack of awareness among many tug masters and pilots of how the tug’s towline forces on the ship’s deck fittings can be multiplied many times over by a combination of towline angles and the operating mode the tug is using (direct, indirect, combination arrest, transverse arrest, etc).

As an example, in a worst-case scenario it is possible that there can be a 600 per cent multiple factor applied to the tug’s rated (published) bollard pull, meaning a 60-tonne BP tug can produced a 360-tonne force on to the ship’s deck fittings. (Note: the above is not including spike (shock) loadings caused by rough tug driving or sea state.)

Consequently, the bollard pull of a tug and the MBL of a towline supplied from a tug may significantly exceed the SWL of the ship’s fittings. While the load on the Figure 7: Force on post of bitts 4 Foundation structure of deck Basic understanding for fairleads fittings on ships Any force on the fairlead is transferred to the attachment and foundation structure. The foundation Basic understanding for bitts structure must be able to withstand the loads imposed The loads imparted on to the bitts by the line are on the fairleads at various wrap angles. Strengthening withstood by the structure of the bitts, and are not required in way of the fairleads will normally be transferred to the foundation structure. The bitts can be determined by calculation and incorporated in the considered as a single box mounted to the deck, with an vessel construction at time of building. external force acting on the box equal to the load on the line. The external force is transferred through the box to Summary of system loads and the foundation structure. The load that the foundation strength requirements structure must withstand is equal to the load on the line. In the above reference, the strength of a ship’s deck The foundation structure is an integral part of the ship’s fittings is relevant to the MBL of the mooring in use. design. Strengthening required in way of the bitts will Where commonly a ship’s mooring line may be in the normally be determined by calculation, and incorporated order of 150 tonnes MBL, a modern high-performance in the vessel construction at time of building (see Figure 8 and Table 5). tug can have towlines with an MBL greater than 300 tonnes. Hence the IACS and OCIMF reference data above is not appropriate for towage operations (see Tables 6 and 7).

Investigation results

Key points • We found the rules for design and construction for ship’s deck fittings from one industry entity to another to be not only confusing, but in some cases contradictory. Even industry experts we consulted Figure 8: Force on the bitts’ foundation were at times confused by them.

Strength requirements: bitt foundation structure IACS MBL = SWL bitts Sufficient to meet mooring load OCIMF 2 x MBL Sufficient strength to accommodate line attached to bitt by single eye over one post at a load of 2 x SWL Table 5: Bitt foundation strength requirements

Loads on system Figure-of-eight Single eye Doubling load on single eye (mooring) (towage) Line T T 2T Fitting and Up to 2 x T (depending on Up to 2 x T (depending Up to 4 x T (depending on Fairlead foundation wrap angle) on wrap angle) wrap angle)

Fitting 2 x T T 2 x T Bitts Foundation T T 2 x T Table 6: Summary of system loads

Strength requirements IACS OCIMF Towing Line MBL MBL Up to 2 x MBL dependent on 2 x MBL Fitting and Fairlead wrap angle (worst case wrap angle To meet expected foundation (Marked SWL = MBL) 180o) maximum towing load 2 x MBL Bitts Fitting 2 x MBL (Marked SWL = MBL) Foundation MBL 2 x MBL Table 7: Summary of strength requirements 5 • Regarding the foundation structures and attachment to the vessel of the fittings, it should be understood these are not integral parts of the ship’s structure, and must be attached to the ship. The strength of the attachment and underlying structure are an important part of the system but this is often underestimated during the design and construction of the ship.

Age matters • For ships constructed prior to 2007, there is no guarantee that class rules adequately cover the design of the ship’s deck fittings. • For ships constructed prior to 2012, there is no guarantee that class rules adequately cover the underpinning deck structure the ship’s deck fittings are connected to. Figure 9: Force on CLA Findings pertaining to ship’s bitts failure, and deck fitting failure before hull or foundation and leads failure. During towage operations this is usually far from While it is all well and good to focus on the SWL of the the reality (see Figure 10, opposite). ship’s bitts the tug’s towline is connected to, what must also be taken into account is the ship’s fairleads that Towline angles the tug’s towline runs through. It is important for a tug To further complicate things, the angle of the tug’s master to know when to focus on the SWL of one over towline from the horizontal applies a significant the other. multiplication factor to the forces the tug is creating into its towline and on to the ship’s deck fittings. As an As pointed out above, we can double the SWL of a example, with the tug’s towline angled up to the ship at set of ship’s bitts by placing the eye of the tug’s towline 60 degrees from the horizonal, there is a multiplication at the base of one leg of the ship’s bitts, but this does factor of x2 (200 per cent) into the towline and a factor not apply to the ship’s fairleads, be they roller, cotton of x1.8 (180 per cent) on to the ship’s fittings. reel or panama types. At a 75 degree towline angle, this multiplication factor Generally for tugs working in a push/pull mode on increases to x3.8 force into the towline and a x3.3 the ship’s sides or directly astern when centre lead aft factor on to the ship’s deck fittings. For example, a tug (CLA), there is not a significant force being brought to producing 85 tonnes BP into the water with a vertical bear on the ship’s fairleads. Often what force is created towline angle of 70 degrees can have 248 tonnes force is in fact a downward force that tends to press the in its towline and 223 tonnes force on to the ship’s deck ship’s fairlead into the deck, rather than trying to pull it fittings(see Table 8). off its connection to the ship’s deck foundation.

The situation changes significantly when the tug’s towline forces are at an obtuse angle to the fairleads, which in turn can create a tearing/ripping sideways force on to the fairleads (in this case, the CLA). The worst case scenario is when the towline is near horizonal to the water and at an angle of near 90 degrees to the ship’s fairlead, and the angle from where the towline enters the fairlead (in this case CLA) to where it leads to the ship’s bitts is greater than 90 degrees. This combination can create a destructive force on to the ship’s fairlead (CLA) of up to ≈2x the towline force the tug is creating (Figure 9). Table 8: Force multiples due to towline angles General design strength principle of deck fittings A possible worst case As a general principle, the ship’s mooring fittings and A tug out square to the ship is producing (say) 65 foundations should be designed to carry the force tonnes BP into the water with a vertical towline angle of imposed by an attached mooring line. The requirements 75 degrees, hence producing 251 tonnes towline force concerning the strength of the ship’s mooring fittings are and 217 tonnes force on to the ship’s fittings. Now lead based on the principle of rope failure before deck fitting the towline through the ship’s fairlead and back to the 6 Figure 10: Towline force on ship’s fittings ship’s bitts at an angle of near 180 degrees and there is While we are giving a worst case scenario here to another multiple factor of x2, hence the 217 tonnes now make the point, it is fair to say that this scenario is becomes a 434 tonne force on to the ship’s fittings! theoretically possible, and should the planets ever align in this way then something in the system will fail, If the 85-tonne BP tug is a high performance escort whether it be the tug’s winch brake slipping, the towline tug (say, a RAstar85) with a keel/skeg that creates parting, or (more likely) the ship’s deck fittings being hydrodynamic lift, it can produce up to x2 its rated ripped from the deck. bollard pull: 170 tonnes BP. Needless to say, this is not the real cost. Generally, Hence, in broad terms: tugs only operate at maximum power due to the ship being in a situation that demands it. When there is • 85 tonnes BP x 2 = 170 tonne towline force, created an equipment failure, there is little to no time for the due to indirect type towage assist pilot to save the ship from being involved in a serious incident, due to not having the tug generating the • 170 tonnes x 2.92 = 496 tonnes into the towline due required forces. to a towline angle up to the ship of 70 degrees • 496 tonnes x 2 = 992 tonnes on to the fairlead, due Active escort operations to the towline angle around the fairlead being ≈180 While the issue of ships’ deck fittings being adequate degrees for general harbour towage is serious and indeed real, (Note that the actual steering created by the tug on the issue goes to another level when it comes to active to the ship is still only 170 tonnes.) escort towage operations. Modern high performance 7 Table 9: Force created by rudder angles escort tugs are sophisticated designs with rated Basically, the rudder design is best described as an bollard pulls commonly of 80-100 tonnes. They are aeroplane wing that, instead of being horizontal in the able to generate huge towline forces via their specially air, is vertical in the water. As with a plane accelerating designed keels or skegs. down the runway, the faster the ship goes, the more lift (or steering force) the rudder creates. This is required There are good reasons to require escort tugs to be because the faster the ship goes the more steering able to produce these high tonnages. For example: force is required to control and steer it. These forces are further exaggerated by environmental infl uences such • When a ship of, say, 100,000dwt is steaming ahead as narrow waterways and low underkeel clearances. at 8 knots, its rudder can produce steering forces in the order of ≈70 tonnes steering force. Modern high performance escort tugs (ASD, ATD and • Increase the same ship’s speed to 10 knots, and its VSP designs) have either keels or skegs of a similar rudder can produce ≈100 tonnes steering force. design to the ship’s rudder, so also create hydrodynamic • Now consider a CapeSize or VLCC-type ship of lift. Consequently, the faster the ship steams ahead, the 200,000dwt. At 8 knots, its rudder produces ≈100 greater towline forces the escort tug can create. This is tonnes steering force, and at 10 knots this increases needed in order to overpower the ship’s steering forces to ≈150 tonnes (see Table 9). in the event of a rudder failure (Figure 11).

The reason for this is that a ship’s rudder is designed When a ship is in a narrow waterway such as a to create hydrodynamic lift. What does this mean? shipping channel, and/or there is low under keel

Figure 11: Hydrodynamic lift rudders 8 clearance, the towline forces required from the tug Given that a ship of ≈200,000dwt can produce >100 increase quite dramatically. Furthermore, in the event of tonnes steering force via its rudder, it can be seen a failure of the ship’s rudder these forces from the tug there is an immediate issue, given the escort tug must must be applied very quickly to avoid the ship sheering produce considerably more counter force via its towline off course. In this type of scenario we are talking about running through the ship’s fairleads to its bitts (see a counter steering force from the escort tug being Figure 12). applied within ≈30 seconds. With modern tug designs, high performance synthetic towlines and elite training of Reality check the tug’s crew, this is now all possible. But the Achilles The reality, when holistically reviewing this topic, is Heel we have only recently come to understand fully is that most of these types of ships when transiting a the ship’s deck fittings. narrow waterway cannot in the event of a rudder failure likely be saved by their escort tugs, given the steering Inadequency of ship’s deck fittings forces required being accentuated by the physics of Earlier in this paper we alluded to there being potential towline angles, regardless of the question marks over issues with ship’s deck fittings on ships older than the quality of the ship’s deck fittings and underpinning 2007, due to them not necessarily being covered support structure. by class rules, and ships older than 2012 due to the underpinning deck structure not necessarily being covered by class. What to do? Having gained an understanding of the issues, In addition to this, in many cases the actual SeaWays has been involved in extensive live escort SWL rating of ship’s deck fittings utilised in towage trials with one of its clients, their port authority and the operations has proven to be of serious concern. As port’s pilot service. The results of the first stage of these an example, our research into CapeSize bulkers has trials supported the findings of our research. shown more than half of these ships have deck fittings that are required to be used in escort towage of 50-80 Of course, the next question is: what to do? The tonnes SWL. This is assuming that their SWL ratings whole point of investing in high performance escort are actuals, taking into account the points we have tugs, as well as an extensive training programme for raised within this paper. pilots and tug masters, is to to ensure the operational

Figure 12: Rudder forces on a CapeSize or VLCC-type ship 9 Figure 13: Dual escort towage integrity of the shipping channel. A channel blockage each tug could easily produce, say, 80 tonnes towline due to a ship running aground in the port where we force (80 tonnes x 2 = 160 tonnes required steering conducted our trials would cause an estimated cost force) spread over two sets of ship’s fittings. running into billions of dollars, due to lost trade over the likely period before the channel could be cleared. Dual escort towage also allows the pilots to have numerous initial response scenarios and ongoing Interim fix – dual towage (T2) control over the ship, as well as redundancy in the case The second stage of our trials involved undertaking dual of mechanical failure of a tug or its towline (Figure 13). escort towage operations. This concept, also known as T2, was originally developed by Capt Greg Brooks from Naturally, this type of activity comes at a significant Towage Solutions. It involves using two escort tugs cost, though when considering the cost of a channel connected either side of a ship’s transom. blockage it can be argued that it is a small investment. Note that it is important to understand that while there Using dual escort towage in this manner can have the can be towline forces in the hundreds of tonnes, this is benefit of doubling the overall steering forces the tugs not related to the actual steering force the tug is creating can create, but this was not our motivation. Instead, on to the ship. The actual steering force is the same we wanted to be able to generate the required steering as the force the tug is generating into the water (its forces without the risk of the ship’s deck fittings failing. bollard pull), not into its towline. There are other towage Hence, while (in this case) one of the Robert Allan Ltd techniques that can also be utilised, depending of the RAstar85 escort tugs we were using in our trials could circumstance and attending tug, but this is beyond the produce upwards of 160 tonnes towline force, using two scope of this presentation, and will have to wait for when of these tugs in a dual escort configuration meant that we have another opportunity to present to industry. 10 Long-term fix Pragmatic information guide In our opinion, now that the research group has To assist industry, particularly tug masters and pilots, undertaken this work, there is an onus, indeed SeaWays has developed a diagram that provides a responsibility, on ship owners, designers and insight into the issues identified in this paper and some classification societies to ensure that all newbuilds have practical ‘rule of thumb’ operational guidelines. The deck fittings, particularly in the bow and stern of ships, challenge was to condense a significant amount of that are of a rating to be able to sustain the forces an research material into a one-page diagram that can escort tug has to generate in order to save the ship in be understood by mariners whose first language is a rudder steering failure-type emergency. These deck not necessarily English. Hopefully we have had some fitting ratings must also acknowledge the physical success with this. forces in play due to towline angles. The diagram (see Figure 14, overleaf) is a free gift The client involved in our escort and dual escort to the industry from SeaWays. It has been produced trials has placed its charterers on notice that they have in an A3 landscape format ready for printing, and can two years to ensure their ships have adequately rated be obtained either here at the ITS 2018 conference fairleads and bitts, so that in an emergency the escort from our SeaWays desk in the exhibition hall or by tug can produce the required towline steering force to going to our websites www.seaways.net.au or www. overpower the ship’s rudder lock. seawaysglobal.com and downloading it.

11 Figure 14: Operational guidelines 12