HSE Health & Safety Executive
Review of issues associated with the stability of semi-submersibles
Prepared by BMT Fluid Mechanics Limited for the Health and Safety Executive 2006
RESEARCH REPORT 473 HSE Health & Safety Executive
Review of issues associated with the stability of semi-submersibles
BMT Fluid Mechanics Limited Orlando House 1 Waldegrave Road Teddington Middlesex TW11 8LZ
This review study was undertaken as part of a wider exercise to assess the need for HSE Guidance within the UK Safety Case regime. The study included a comparison between stability standards specified by key regulatory authorities and classification societies for intact and damaged semi-submersible units, a review of relevant published literature and of HSE/ Department of Energy reports, a review of past incidents involving loss of stability of semi-submersibles, and a review of issues associated with alternative uses of semi- submersible units.
A key recommendation coming out of this study is that the HSE should investigate further the practicality of reconciling traditional prescriptive stability standards with a risk-based Safety Case approach.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
HSE BOOKS © Crown copyright 2006
First published 2006
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the copyright owner.
Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]
ii EXECUTIVE SUMMARY
This review study was undertaken as part of a wider exercise to assess the need for HSE Guidance within the UK Safety Case regime. The study included a comparison between stability standards specified by key regulatory authorities and classification societies for intact and damaged semi- submersible units, a review of relevant published literature and of HSE/ Department of Energy reports, a review of past incidents involving loss of stability of semi-submersibles, and a review of issues associated with alternative uses of semi-submersible units.
A key recommendation coming out of this study is that the HSE should investigate further the practicality of reconciling traditional prescriptive stability standards with a risk-based Safety Case approach.
Intact Stability No obvious major deficiencies were found in established intact stability standards. These appear to have been successful in avoiding capsize and loss while units remained intact and watertight, and in accordance with the standards.
Key conclusions from the review study, relating to intact stability standards, were: • The intact stability standards of most regulatory authorities are outwardly similar, although there are important differences in specific requirements (eg. in values of limiting angles and the minimum value of GZ). It is not obvious how these variations affect stability margins, or levels of uniformity and consistency between different units. • Ambiguities in definitions of the most critical heel axis and between ‘free trim’ and ‘free twist’ analysis procedures have been criticised. It is important that key procedures and definitions should be understood clearly by all engaged in such work. • Large heel angles are required by certain designs in order to meet the 1.3 area ratio requirement. Large heel angles would seem to be undesirable for several reasons, including hazards caused by large items shifting, hazards to personnel, and difficulties during evacuation and escape. • There is a sound basis for retaining existing HSE minimum GM and GZ requirements, because these are likely to reduce the likelihood of abrupt changes in heel during operation, large angles of heel, steady tilt and low-frequency motions. • Certain authorities (including HSE) state that moorings should be ignored in a stability analysis. It would seem prudent, however, to require detrimental effects of moorings, risers, thrusters and similar items to be considered explicitly. • Existing standard procedures for calculating the wind heeling moment are simplified, and most such procedures do not take adequate account of the lift-induced moment. This component can be important for heeled semi-submersibles, and may be either over or under-estimated. • In circumstances where location-specific environmental standards are appropriate, careful consideration should be given to the averaging and return periods used when estimating the wind speed. • The alternative ABS/ IMO intact criteria should be treated with caution until experience has been gained. It is not known how extensively these criteria are used for semi-submersibles working on the UKCS.
Damaged Stability There are major differences between the damaged stability standards adopted by different regulatory authorities (see Appendix A). There are in particular major differences between the approaches adopted by the HSE (based on a minimum area ratio) and in the IMO’s 1989 MODU Code (based on
iii minimum values of the righting arm and stability range). Discussions leading towards convergence between these different standards should be encouraged.
The damaged stability standards in the IMO MODU Code are based on criteria originally proposed by ABS. The small range of units considered during the ABS JIP, and simplifications adopted during the model tests, are matters of concern. These criteria should be treated with caution until further experience has been gained.
Key conclusions from the review study, relating to damaged stability standards, were: • There are differences between the areas of damage and flooding specified by different authorities, the consequences of which are not known. • The level of damage that occurred in most incidents was consistent with levels envisaged by conventional damaged stability standards, and generally involved flooding of a single compartment. • The traditional 1.5m penetration depth requirement seems somewhat arbitrary, and does not take account of increases in the size and power of supply vessels over the years, the size of the unit itself or its structural loading capacity. • A number of authorities (including HSE) specify a minimum 4m wave clearance above the damaged waterplane, regardless of sea conditions, the vessel’s behaviour or the location and size of the downflooding point. The rationale for this 4m clearance seems questionable. • The use of a 50 knot wind speed in damaged stability standards has been questioned on several occasions. At issue is whether the standards should reasonably address the combination of circumstances that led to the Ocean Ranger accident.
Other Issues Moves towards quantitative risk-based stability analysis procedures, integrated within an overall safety assessment, should be encouraged as a long-term objective, although there are likely to be major difficulties in achieving this objective in the short term. Further review of the RABL work and other developments in this area would be a useful first step.
The review of past incidents suggested that there is a relatively low risk of major damage following collision, although minor damage has been recorded. Blowout is undoubtedly a major hazard, because of fire and explosion risks, but does not seem to be a major issue for capsize. Ballast system failures, errors in operating the ballasting system, and flooding due to variety of causes, including human error, seem to be the most important issues. Storm damage and towing errors are also significant factors. The number of ballasting faults and errors is disturbing, and also the fact that many flooding incidents outside the UKCS are unexplained.
No overlap was found between incidents reported in the WOAD and HSE databases, raising possible concerns about the accuracy and completeness of these records.
iv Contents
Page EXECUTIVE SUMMARY ...... iii 1. INTRODUCTION ...... 1 2. GENERAL APPROACH ...... 3 2.1 REVIEW OF EXISTING STABILITY STANDARDS...... 3 2.2 LITERATURE REVIEW ...... 3 2.3 REVIEW OF ALTERNATIVE USES OF SEMI-SUBMERSIBLES...... 4 2.4 REVIEW OF PAST INCIDENTS ...... 4 3. BACKGROUND TO STABILITY STANDARDS ...... 5 3.1 HISTORICAL DEVELOPMENT...... 5 3.2 ALTERNATIVE APPROACHES ...... 7 4. REVIEW OF STABILITY STANDARDS AND GUIDANCE...... 11 4.1 INTACT STABILITY STANDARDS ...... 12 4.2 DAMAGED STABILITY STANDARDS ...... 14 5. ISSUES ARISING FROM THE LITERATURE REVIEW ...... 19 5.1 DISCUSSION ON INTACT STABILITY STANDARDS...... 19 5.2 DISCUSSION ON DAMAGED STABILITY STANDARDS...... 25 5.3 WIND FORCES AND HEELING MOMENTS ...... 29 5.4 EFFECTS OF MOORING LINES, RISERS AND THRUSTERS ...... 31 5.5 EFFECT OF ICING ...... 32 5.6 STEADY TILT ANGLE AND SUBHARMONIC MOTIONS...... 32 5.7 WATERTIGHT AND WEATHERTIGHT INTEGRITY...... 33 5.8 LOSS OF BUOYANCY FOLLOWING A BLOW-OUT ...... 34 5.9 VARIATIONS IN METACENTRIC HEIGHT DURING OPERATION ...... 34 5.10 STABILITY MANAGEMENT ...... 36 6. ALTERNATIVE USES OF SEMI-SUBMERSIBLES ...... 39 6.1 DRILLING UNITS ...... 39 6.2 FLOATING PRODUCTION UNITS ...... 39 6.3 ACCOMMODATION UNITS ...... 41 6.4 DIVING SUPPORT VESSELS ...... 41 6.5 PIPELAY VESSELS ...... 41 6.6 CRANE VESSELS ...... 41 6.7 OTHER APPLICATIONS...... 41 7. REVIEW OF PAST INCIDENTS ...... 43 8. DISCUSSION OF KEY ISSUES ...... 47 8.1 INTACT STABILITY ...... 47 8.2 DAMAGED STABILITY ...... 48 8.3 STABILITY MANAGEMENT ...... 49 8.4 OTHER ISSUES ...... 50 8.5 REVIEW OF PAST INCIDENTS ...... 50 9. CONCLUSIONS...... 51 9.1 INTACT STABILITY ...... 51 9.2 DAMAGED STABILITY ...... 52 9.3 OTHER ISSUES ...... 52 10. REFERENCES...... 55 ABBREVIATIONS AND NOTATION ...... 61 APPENDIX A: SUMMARY OF STABILITY STANDARDS...... 63 APPENDIX B: SUMMARY OF INCIDENT INFORMATION ...... 73 APPENDIX C: THE ALEXANDER L. KIELLAND AND OCEAN RANGER ACCIDENTS ...... 81
v vi Review of Issues Associated with the Stability of Semi-Submersibles
1. INTRODUCTION
BMT Fluid Mechanics Limited (BMT) was commissioned by the UK Health and Safety Executive (HSE) to undertake a review of issues associated with the stability of semi-submersibles which are mainly used as drilling units. This review was undertaken as part of a more general appraisal of the need to retain elements of the Department of Energy/ HSE’s Fourth Edition ‘Guidance on Design, Construction and Certification’ [1]1 within the framework of the UK Safety Case regime.
Most regulatory authorities and classification societies have stability standards for vessels and semi- submersibles operating under their jurisdiction. Whether any particular standard is mandatory or advisory depends on the responsibilities of the organisation in question, on the area of operation and on the unit’s flag status. International Maritime Organisation (IMO) MODU Code standards are not mandatory. Responsibilities for specifying and enforcing stability standards are placed on flag and coastal states. They are also delegated to the classification societies, such as the American Bureau of Shipping (ABS), Det Norske Veritas (DNV) and Lloyd’s Register of Shipping, and there have been moves towards harmonising classification society rules towards the IMO MODU standard.
The Offshore Safety Division (OSD) of the HSE is responsible to the UK Government for ensuring that risks to people from work activities in the ‘upstream’ oil and gas industries are properly controlled, when taking place in UK coastal waters. A key feature of the UK post-Cullen regulatory regime is the requirement for the owner or operator of every offshore installation to prepare a Safety Case, and submit it to OSD for formal acceptance. The Safety Case has to demonstrate that an effective management system is in place to control major accident risks and to ensure that they are ‘as low as is reasonably practical’ (ALARP). These risks specifically include those arising from loss of stability. The responsibilities of the owner or operator are interpreted in a very wide-ranging manner, and are not limited to meeting any particular prescribed standards or criteria.
The HSE’s Fourth Edition Guidance [1] specifies certain minimum standards, but these standards are not mandatory under the Safety Case Regime and represent guidance only. The HSE is now reviewing whether particular sections of the Guidance should be retained, and in particular whether there is any need to retain Section 31 of the Guidance relating to stability, watertight integrity and ballasting.
During the present review study BMT was specifically requested to: • review issues which may affect the intact and damaged stability of semi-submersibles; • identify issues which are likely to be of particular concern to the HSE and the offshore industry.
This report describes the review findings, and is structured as follows: • Sections 2 and 3 outline the general approach adopted during this review, and the historical background to existing and proposed alternative stability standards; • Section 4 reviews the standards specified by key regulatory and classification authorities for both intact and damaged semi-submersible units; • Section 5 covers additional issues arising from a review of relevant literature; • Section 6 reviews issues associated with alternative uses of semi-submersible units; • Section 7 reviews past incidents involving loss of stability of semi-submersibles; • Sections 8 and 9 summarise key issues and conclusions coming out of the review study.
1 A list of references may be found in Section 10 on page 55.
1 2 2. GENERAL APPROACH
The main tasks undertaken during this review study were as follows: • to review the stability standards of significant regulatory and classification authorities relating to semi-submersible units in both the intact and damaged conditions; • to carry out a literature review, which involved a search of publicly accessible abstracts databases, and a review of reports and documents made available by the HSE; • to review recorded incidents and casualties, following a search of the WOAD and HSE incidents databases; • to review and identify key issues, and then draw conclusions.
Views from a major semi-submersible designer and an operator were also sought to help identify problems experienced in complying with stability requirements during design or in service. These views were taken into account when identifying key issues.
Some of the issues identified during this investigation are similar to those highlighted during an earlier review study on jack-up stability [2]. In both cases conventional intact stability standards seem to have been successful in avoiding capsize, but a number of losses have occurred after damage and flooding. There are important differences between jack-ups and semi-submersibles, however, such as: • There have been many reported losses of jack-ups, but relatively few semi-submersible losses. • The causes of two major semi-submersible losses have been thoroughly investigated and are well documented. Significant factors included catastrophic structural failure in the case of the Alexander L. Kielland [3], and loss of control over the ballasting system in the case of the Ocean Ranger [4, 5, 6]. After these initiating events, subsequent flooding and capsize took place relatively quickly (see Appendix C). • Most jack-up losses occurred after minor damage and a long period of progressive flooding. Green water is a relatively common occurrence on the decks of jack-ups in severe storms, because of their relatively low freeboard. Water impact damage to items on deck proved to be a significant factor in many jack-up losses. Green water can only reach the deck of a semi-submersible, however, after the unit has already developed a large heel or trim angle.
2.1 REVIEW OF EXISTING STABILITY STANDARDS
Section 3 of the report reviews the historical development of stability standards for ships as well as semi-submersibles, including attempts to make the standards take better account of vessel and wave dynamics.
A critical review of current intact and damaged stability standards applying to semi-submersible (column-stabilised) units was then undertaken. Comparisons were made between standards specified in the HSE’s Fourth Edition Guidance, Norwegian regulations, the International Maritime Organisation MODU code, and classification society rules.
Results from this review of current stability standards for semi-submersibles in both the intact and damaged conditions are presented in Section 4, and issues of special concern are identified. Appendix A summarises the stability standards of six organisations.
2.2 LITERATURE REVIEW
A literature search was performed to identify relevant research papers and reports. These documents were obtained and reviewed, and key issues relating to the stability of semi-submersibles were identified.
3 Initial searches revealed a large number of projects, reports and papers relating to semi-submersible stability and sea-keeping. It was not practical to carry out an exhaustive and detailed review of all these documents. Attention was therefore focussed on results from relatively recent research, or documents which were considered to be especially relevant.
The literature review found many research papers from the period immediately following the losses of the Alexander L. Kielland and Ocean Ranger, but relatively few papers after about 1990, suggesting that research interest in semi-submersible stability had waned. The literature review therefore includes many papers published before 1990, which are still relevant to the issues in question.
The Norwegian ‘Risk Assessment of Buoyancy Loss’ (RABL) project [7] arose directly out of research undertaken after the loss of the Alexander L. Kielland, and was an important step towards developing more rational, risk-based stability standards. The RABL reports were deliberately excluded from this review study, however, because they raise issues which go well beyond those of a conventional stability analysis, and it was agreed that they should therefore be reviewed separately.
The HSE undertook an initial keyword search of their Herald database, and identified a shortlist of 34 Department of Energy (DEn) or HSE projects which were considered to be especially pertinent to the issue of semi-submersible stability. After carrying out a preliminary review of project summary details, BMT considered seven projects to be of special interest. Reports from these seven selected projects were included in the literature review, and the findings were taken into account when identifying key issues. Many of these reports have not been published, however, and so are not included in the reference list.
A number of further DEn/ HSE projects had addressed relevant topics, but were not considered in detail during the present study. Other projects appeared to have little direct relevance to present objectives, and were considered no further. These included a number of reports concerned with the seakeeping behaviour of semi-submersibles rather than stability issues.
Key issues emerging from the literature review are discussed in Section 5.
2.3 REVIEW OF ALTERNATIVE USES OF SEMI-SUBMERSIBLES
Semi-submersibles have always been used for a wide variety of different purposes, such as drilling, diving support, fire-fighting, crane operations, pipe-laying and accommodation. Semi-submersibles are now being used or proposed for a number of new offshore roles, including combined drilling, workover and floating production units [8, 9]. In some circumstances the need to remain on station for long periods in severe weather may impose special demands on the unit.
Issues associated with alternative applications of semi-submersibles are discussed in Section 6.
2.4 REVIEW OF PAST INCIDENTS
A search of the HSE’s incidents database and the Worldwide Offshore Accident Databank (WOAD) revealed 36 separate incidents involving loss of stability of semi-submersibles. These included not only the well-known and well-documented losses of the Alexander L. Kielland [3] and Ocean Ranger [4, 6], but also many other less well-known incidents.
Results from this review and key issues relating to the stability of semi-submersibles are discussed in Section 7 of this report. Summary details of these incidents are listed in Appendix B, and additional details of the Alexander L. Kielland and Ocean Ranger accidents are given in Appendix C.
4 3. BACKGROUND TO STABILITY STANDARDS
Established stability standards for semi-submersibles are based on older criteria developed for naval and merchant ships, and it is instructive to look back at the history of some of these criteria. Conventional quasi-static stability standards have developed in a historical manner, and are intended to provide a practical and reasonably uniform basis for design, so that new designs are at least as safe as existing well-designed vessels. They are less concerned to represent realistic sea conditions and vessel behaviour, and more concerned with maintaining existing standards.
The apparent arbitrariness and limitations of present quasi-static stability standards have long been recognised, and the fact that no explicit dynamic formulation has yet been agreed is a measure of the difficulty of the task. The international search for a universally applicable and acceptable dynamic stability criterion has been long and controversial; indeed Kuo et al. [10] reproduced a comment made at the First International Conference on Ship Stability in 1975: that the task will take ‘another hundred years’.
The present quasi-static standards have the attraction of being explicit, and are easy to apply in practice. History suggests that they also provide an adequate margin of stability, in the sense that no past semi-submersible losses have been attributed to deficiencies in the intact stability standards. The main motivation for any change in the standards would therefore have to be either a reduction in conservatism, resulting in lower construction costs, greater uniformity of design, or else concern that future changes in design might erode the conservatism.
A major problem for researchers on stability is the way in which stability issues impinge on other aspects of design. It has been found, for example, that the design of the mooring system can affect the vessel’s stability detrimentally, and increasing the metacentric height above a certain level may cause undesirable motions in certain sea states. As noted by Vassalos et al. [11], this can easily turn stability work into ‘a long-term research programme making no contribution whatever to immediate needs.’
A major difficulty with introducing explicit dynamic terms into stability standards has been lack of understanding of the physical processes, and therefore of the mathematical equations, and lack of agreement about the critical factors leading to capsize. Much previous research seems to have been driven by the beliefs of individual researchers about the mechanisms that are important, and (equally important) about those that may be neglected. Undoubtedly the capsize process is highly complex and non-linear, and different processes may be at work in different circumstances. All this has led to controversy and uncertainty about how to incorporate these features into design. A measure of the controversy surrounding this whole topic may be gained from the discussion following a paper by Vassalos [12]. Henrickson [13] has also summarised some of the criticisms levelled against certain types of stability standards.
Practical considerations and past experience suggest that improved standards are only likely to gain acceptance on a step-by-step basis, through a process of enhancing existing standards, rather than through some radically new approach.
3.1 HISTORICAL DEVELOPMENT
Henrickson [13], Bird and Morrall [14] have provided useful summaries of the history of methods used to assess stability, and of stability research up to the mid-1980s. An excellent review of stability theory, methods used to assess stability, comparisons between the standards of different regulatory bodies, and special considerations for different types of vessel, may be found in Chapter 4 of the recent CMPT Floating Structures guide [15]. Mills et al. [16] also provided an excellent review of the regulatory position as it stood in 1991, and Martinovich and Praught [17] reviewed the development of stability standards from the viewpoint of a semi-submersible designer.
Established methods for assessing stability may be categorised [14] as follows: a) static stability methods, such as the Rahola [18] criterion, based simply on minimum righting moment requirements,
5 b) moment balance requirements, which, together with: c) energy balance requirements, are included in all known current semi-submersible codes, as well as in the Sarchin and Goldberg [19] criteria for naval ships, and the International Maritime Organisation’s (IMO) ‘weather criterion’ [20] for merchant ships,
A number of alternative stability standards have emerged in recent years, based on considerations of vessel dynamics and risk, mainly in the aftermath of the Ocean Ranger and Alexander L. Kielland accidents. These included the Norwegian MOPS [21] and RABL [7] projects, and the PRESS project [22] in the UK. The other major recent event has been the development of alternative intact and damaged stability criteria during Joint Industry Projects managed by the American Bureau of Shipping. Further information about the ABS projects and the resulting criteria may be found in Sections 5.1.3 and 5.2.3.
3.1.1 Static Righting Moment Methods
Simple righting moment requirements have a fairly obvious physical interpretation, and require no difficult decisions about wind and wave loading parameters. They give no indication of safety margins, however, and are likely to be inappropriate if vessel forms and dimensions are changed from those originally envisaged, or for vessels with large wind areas. Henrickson [13] presented a detailed critique of such methods.
3.1.2 Moment and Energy Balance Requirements
Simple righting moment criteria have now been largely superseded by procedures based on moment and energy balance. Moment balance methods take account of the steady wind heeling moment and righting moment, together with other static forces such as weights lifted over the side, or tow-lines. These methods take account of the energy imparted to the vessel by a sudden wind gust, causing it to heel down-wind from an initial angle. Energy balance methods involve comparing areas under the wind heeling moment and righting moment curves. The energy balance method is essentially static in its approach, normally considering only the total work done by a steady wind moment as the vessel heels, and ignoring any pre-existing motions, wave-induced roll, damping and changes in the righting moment associated with the wave. These dynamic effects are taken into account through overall safety factors, such as the 1.3 factor applied to the area ratio for semi-submersibles.
Most existing intact stability standards, for all types of vessels including semi-submersibles, specify moment and energy balance requirements, together with a minimum range of angles over which the vessel is stable. Some also specify a minimum value of GM, thus retaining one feature of earlier righting moment criteria.
3.1.3 Stability Philosophies
Stability standards for both ships and semi-submersibles are based on a two-tier approach: • intact stability requirements, designed to ensure that the unit will withstand all expected environmental conditions when in its normal operating or survival condition, and while it remains undamaged and watertight; • damaged stability requirements, designed to ensure that the unit will not capsize in foreseeable environmental conditions, after undergoing a limited amount of damage or flooding, and will be capable of returning to the upright condition.
Two alternative approaches are normally adopted when defining damage: damage to any one compartment at any draught, or waterline damage, including breaching of internal watertight divisions between compartments. Mills et al. [16] noted that both approaches have their strengths and weaknesses. An offshore unit designed to meet the any one compartment standard cannot necessarily be guaranteed to meet the waterline damaged standard, and vice versa. The Norwegian Maritime Directorate (NMD) was the first regulatory authority to consider both damage scenarios. Both
6 scenarios are now considered (in one form or another) by most regulatory authorities, as discussed in Section 4.2.
After the Alexander L. Kielland accident the NMD adopted a three-tier approach. The first two tiers were the established intact and damaged stability philosophies, and the third was a requirement that the unit should withstand loss of buoyancy from either the whole or a major part of one column, but without any requirement to return to the upright position. The objective in this case was to allow the crew time to evacuate the unit. This requirement was expressed in terms of providing a maximum angle of heel after a large loss of righting moment, and a minimum level of reserve buoyancy above the damaged waterline. The concept of providing some level of reserve buoyancy, beyond that necessary to meet basic code requirements, has since been widely accepted. The NMD’s proposals have not been adopted internationally, however, the IMO preferring an alternative form of reserve stability requirement. The reserve buoyancy concept is discussed further in Section 5.2.4.
The MOPS project [21] considered alternative philosophies which might be adopted when developing future stability standards. While recommending no immediate changes to existing Norwegian regulations, MOPS recommended a move away from rigid prescriptive regulations to simple statements of aim, supported by non-mandatory guidelines. Such moves were considered likely to increase the overall safety of units, while encouraging stability to be integrated within an overall safety assessment. This general philosophy has now been embraced by various regulatory authorities, including the HSE.
3.2 ALTERNATIVE APPROACHES
3.2.1 Weather Criteria
The Sarchin and Goldberg [19] criteria and the related IMO ‘weather criterion’ [20] attempt to place stability standards on a more physical and rational basis, by including an explicit dynamic term representing the effects of waves and wind gusting. This dynamic term is represented by a quasi-static adjustment to the unit’s initial angle of heel. Conventional stability criteria effectively assume that the initial angle of heel is zero.
Sarchin and Goldberg’s criteria were first adopted by the US Navy and subsequently by the US Coast Guard. The vessel is assumed to be initially heeled in the up-wind direction at an angle of 25o relative to the static wind heel angle. This 25 o angle represents the maximum likely roll response in waves. The criteria include an area ratio requirement with a safety factor of 1.4. It should be noted that this 1.4 factor was applied to the net areas between the righting and heeling moment curves, rather than the sums of areas used in most other standards.
Sarchin and Goldberg’s criteria also include a moment balance requirement, and separate criteria for lifting weights over the side, for crowding of passengers against the side, for high speed turns and icing.
The IMO ‘weather criterion’ [20] is intended to allow for rolling motions due to waves, and involves a complex formula based on the ratio of vessel breadth to length, its block coefficient, the ratio of bilge keel area to transverse area, the vessel’s roll period, a bilge keel factor, and the height of the centre of gravity. The basis of this formula is unclear, and its complexity makes its effects difficult to assess. It is understood that this criterion was only agreed after several years of discussions, and is a compromise between Russian and Japanese proposals. It is limited to merchant ships greater than a certain size and of conventional hull form, and is inappropriate, in its present form, for application to semi- submersibles.
Kuo et al [10] and Vassalos [12] put forward an alternative form of ‘weather criterion’. Their analysis allows the righting moment parameter GZ to vary with time, in order to represent changes in waterplane area as waves pass the vessel. These variations may cause a sudden loss of stability, and had been found to be important for small ships. These variations are likely to be less significant for semi-submersibles.
7 Henrickson [13] noted various criticisms of the ‘weather criterion’ approach. The inherent assumption, that capsize follows a large quasi-static roll in the up-wind direction, is known to be false for ships; capsize seems to occur in a more random manner [23]. Numata [24] noted that model tests on semi- submersibles had shown no tendency to capsize in extreme conditions, even when the area ratio was less than 1.0. Numata’s criticisms were directed mainly at traditional calculation procedures used to estimate the wind heeling moment, however, rather than at the stability standards themselves.
Various alternative ‘weather criteria’ have been proposed. Bush and Ahilan [25], for example, proposed an analysis procedure for jack-ups which is similar to the Sarchin and Goldberg [19] method, but the fixed 25o angle was replaced by a maximum roll angle based on results from relevant model tests and numerical calculations.
This approach has the merit of being applicable to any type of unit, rather than specific hull forms for which sets of empirical formulae have been derived, and could in principle be applied to semi- submersibles. It requires an estimate to be made of the unit’s maximum roll angle, however, and an associated choice has to be made about the design sea state. Numata [24] reported that the ABS rules committee did in fact consider a ‘weather criterion’ for semi-submersibles, similar to that developed by Sarchin and Goldberg, but was reluctant at that time to specify any particular correlation between wind and waves, and felt that there was insufficient data on actual semi-submersible motions to establish roll excursions appropriate to different configurations. The proposal was therefore withdrawn, although the rules at that time did allow for alternative criteria based on a documented and rational approach.
Vassalos et al. [11] put forward a ‘weather criterion’ based on the energy balance equation, which took account of the unit’s dynamic response. They proposed that the net area under the energy curve should be positive over an extreme half-cycle of motion, and presented a sample calculation based on an Aker H-3 design. A difficulty with applying this criterion in practice is in defining an appropriate half-cycle of response.
The alternative stability criteria, proposed by the American Bureau of Shipping [26], also represent a form of weather criterion, based on the dynamic responses of semi-submersibles in extreme sea conditions. These criteria were developed after carrying out systematic numerical simulations and physical model tests on several generic series of semi-submersibles, and will be discussed in detail in Section 4.1.1.
3.2.2 Risk-Based Procedures
Established stability standards have been criticised on the grounds that it is difficult to relate them to levels of safety and reliability. Caldwell and Yang [27] outlined a methodology for a risk-based approach to stability analysis. A risk-based approach is attractive because it is consistent with the ‘safety case’ methodology. The level of risk that is regarded as acceptable varies between different industries and circumstances, and can only be judged in relation to other comparable hazards. Caldwell and Yang suggested that a target annual risk of capsize should be no greater than 10 -5,and the annual risk of death due to capsize should be no greater than 10 -4. These risks were considered to be comparable with the accident rate in the home, and better than the average annual risk from car travel.
Further recent research on the development of risk-based capsize standards has been reviewed by Kobylinski [28, 29].
A risk-based methodology for assessing the stability of semi-submersibles was developed during the RABL (Risk Assessment of Buoyancy Loss) project [7] in the aftermath of the Alexander L. Kielland accident.
The RABL project concluded that collisions are the most likely cause of damage, and estimated that the probability of severe damage to a semi-submersible following a collision is between 10-2 and 10-3 per year. In terms of risk of loss, collisions were followed by burning and blow-out, although the probabilities associated with these events were very vessel-dependent. The next event in terms of importance was ballast control failure. The probability of this type of failure decreased, however, with
8 the appearance of semi-submersibles designs in which ballast pumps are located at both ends of each pontoon. The fourth most probable causes were considered to be fatigue and fracture.
Despite these conclusions, it is disturbing that the only two semi-submersibles losses which resulted in major loss of life (the Alexander L. Kielland and Ocean Ranger) were associated with events ranked low in terms of risk significance. The incident review (see Section 7) also suggested a somewhat different order of importance.
Risk-based procedures will only be practical if they are based on a relatively small number of well- defined scenarios and environmental conditions. The MOPS and RABL projects made significant progress in identifying such scenarios, but the results were criticised (in an unpublished review for the HSE) for being too general, and the recommendations too subjective. A risk-based approach also needs be closely tied into an overall risk evaluation, which should consider the circumstances of past losses and the risks associated with combinations of events. All of this will add to the complexity and difficulty of carrying out such an analysis.
A quantitative risk-based approach is nonetheless a worthwhile long-term objective, in order to improve the basis for assessing priorities, for placing stability analysis within a overall safety assessment framework, and for reducing overall risks. Quantifying risk allows the most likely routes to failure to be identified, and overcomes preconceived or intuitive notions of risk. This type of approach can also help to identify the joint probability of two events, one of which affects the other. One example is damage in severe weather leading to mooring line failure, and causing an additional heeling moment through tripping. A broad risk-based approach should also help to identify other implications of severe weather, such as delays and difficulties in evacuation and rescue after loss of stability.
Quantitative risk assessment techniques have not yet become established as a practical, routine alternative for assessing the stability of a semi-submersible, and are unlikely to become practical in the short term. The present review study is concerned primarily with established techniques, used by the industry today. It was therefore agreed that a detailed review of the RABL work and other risk-based procedures should remain outside the scope of the present study.
3.2.3 Model Testing and Numerical Simulations
Systematic numerical simulations and model tests undoubtedly have a role in helping to develop and enhance stability standards. Systematic numerical simulations have been used, for example, in conjunction with model tests, to help develop survivability criteria for naval vessels [30]. The Norwegian MOPS project [21] compared predictions from a numerical simulation model with model test measurements, and found that it was possible to carry out reliable motion simulations of heavily listed, as well as intact, units. Similar conclusions were reached during the ABS JIP [26], where numerical simulations and model tests were used to develop alternative ABS criteria, which are discussed in Sections 5.1.3 and 5.2.3.
Model tests have the merit of including most of the relevant physical processes, such as vessel and wave dynamics, large-amplitude waves and roll motions, water on deck, and can also be made to represent effects of flooding.
Numerical simulations are analogous to model tests, and can include a large number of complex non- linear and coupling phenomena. There are a number of difficulties, however, in applying such techniques to capsize. The equations only represent physical processes which are understood well enough to be expressed in mathematical and numerical terms. The dynamics of capsize are often poorly understood, and the confidence attached to the resulting simulations will therefore be limited.
There are also practical difficulties in incorporating requirements for numerical simulations into standard stability assessment standards. It would be difficult, for example, to define an appropriate level of complexity for such simulations, bearing in mind that these simulations might have to be performed using standard design-office computers.
The main disadvantages of both model tests and numerical simulations are the lack of any explicit procedures for selecting relevant test conditions, and the need to carry out a very large number of test
9 runs in order to investigate the stability boundaries and statistics of capsize. This approach is therefore likely to be costly.
10 4. REVIEW OF STABILITY STANDARDS AND GUIDANCE
Most regulatory authorities and classification societies have stability standards for vessels and semi- submersibles operating under their jurisdiction. There are a number of important differences between these standards, however, which are highlighted below. These differences are likely to affect the design, operation and inherent stability of a given unit, but it is not easy to draw general conclusions about how these differences affect safety margins, or levels of consistency and uniformity between different units.
No attempt was made during this review study to carry out an exhaustive review of all applicable stability standards, and no attempt was made to look retrospectively at earlier stability standards which still apply to many older units. The objective was simply to identify key areas of similarity and difference between the HSE’s Fourth Edition Guidance [1] and the current stability standards of five significant organisations, which are known to have been especially active in developing and applying standards for mobile and offshore units: • The Norwegian Maritime Directorate (NMD) [31], • The Canadian Coast Guard (CCG) [32], • The International Maritime Organisation (IMO) [33, 34], • The American Bureau of Shipping (ABS) [35, 26], • Det Norske Veritas (DNV) [36].
Lloyd’s Register of Shipping [37] specifies no particular stability standard. Section 1.3.1, Part 1, Chapter 2 of the Rules states that: • The Rules do not cover certain technical characteristics, such as stability, trim, hull vibration, etc., but the Committee is willing to advise on such matters although it cannot assume responsibility for them.
The stability standards of the US Coast Guard (USCG) [38] were also reviewed briefly at a late stage in the project. These standards seem to be less stringent than others studied, and are not included in Appendix A. USCG intact stability criteria require that the area ratio should be greater than 1.3, the metacentric height (GM) should be greater than 50mm over the full range of draughts, and the righting arm (GZ) should be positive between upright and the second intercept angle. There are no limitations on either the static heel or second intercept angles. USCG damaged stability criteria state that the final equilibrium waterline in operating and storm conditions should be below the lowest downflooding point. There is no minimum area ratio requirement, nor any limitations on the static heel angle or stability range. Wind speeds are identical to those specified by other authorities.
Appendix A summarises the stability standards of the HSE, NMD, CCG, IMO, ABS, and DNV in tabular form. These tables show key aspects of the standards only, as they stand at the time of writing, and as interpreted by the present authors. There are many subtle differences between the standards of different authorities, the implications of which would only become apparent in relation to a specific design. Reference should be made to the relevant organisation to obtain an authoritative interpretation, and to obtain full and up-to-date details.
This comment applies particularly to weathertight and watertight integrity requirements, which are very complex and detailed. Table A2 summarises only those integrity requirements which have a direct bearing on meeting stability standards.
Tables A8 and A9 summarise requirements relating to KG limit curves. Certain organisations (e.g. HSE, NMD, CCG and DNV) have specific and detailed requirements, whereas other bodies (e.g. IMO and ABS) outline general objectives only.
All authorities permit alternative stability standards to be considered. Alternative criteria developed during the ABS JIP will be discussed at some length in Sections 5.1.3 and 5.2.3, firstly because the ABS criteria represent a major new departure from established methods, secondly because they have
11 been adopted as either primary or alternative standards by the IMO, and thirdly because of interest expressed through earlier review studies commissioned by the HSE.
Authorities willing to consider alternative standards make statements similar to Paragraph 3.3.3 of the IMO MODU Code [34]: that alternative standards may be considered provided an equivalent level of safety is maintained, and if they can be demonstrated to afford adequate positive initial stability. It may prove difficult, complex and expensive in practice, however, to meet these conditions. In order to demonstrate the acceptability of such standards the IMO states that the following issues should be taken into account: • environmental conditions representing realistic winds (including gusts) and waves appropriate for worldwide service in various modes of operation; • dynamic responses of the unit, including the results from wind tunnel tests, wave basin tests and non-linear simulations, where appropriate; • potential for flooding, taking into account dynamic responses in a seaway; • susceptibility to capsize, considering both the mean wind speed and maximum dynamic response; • an adequate safety margin to allow for uncertainties.
4.1 INTACT STABILITY STANDARDS
Established stability standards for all types of vessels, including semi-submersibles, are quasi-static in character. It has nonetheless been recognised for some years that the actual performance of a semi- submersible in severe conditions depends to a large extent on its dynamic behaviour. Conventional stability standards take no explicit account of wave conditions, or of the response of the vessel to waves. They compare the steady wind heeling moment curve with the hydrostatic righting moment curve, both curves being plotted as functions of the vessel’s heel angle about its ‘most critical axis’.
Righting moment arm
Area C Area A Wind heeling moment Moment
Area B