RR458- Review of Issues Associated with the Stability of Semi-Submersibles
Total Page:16
File Type:pdf, Size:1020Kb
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