ROGUE WAVES Impact on Ships and Offshore Structures
Total Page：16
File Type：pdf, Size：1020Kb
DNV GL STRATEGIC RESEARCH & INNOVATION POSITION PAPER 05–2015 ROGUE WAVES Impact on ships and offshore structures SAFER, SMARTER, GREENER 2 Rogue waves – impact on ships and offshore structures Acknowledgements: DNV GL would like to express its thanks to the partners of MaxWave, SEAMOCS, CresT, ShorTCresT, and, in particular, EXTREME SEAS, for their collaboration. DNV GL acknowledges the EC and the Research Council of Norway for funding research on rogue waves that was essential for reaching the conclusions presented in this position paper. Authors: Elzbieta M. Bitner-Gregersen and Odin Gramstad Images: iStock, DNV-GL Contact: [email protected] Rogue waves – impact on ships and offshore structures 3 CONTENT Executive summary 5 Introduction 6 DNV GL activities on rogue waves 10 EC MaxWave project ......................................................................................................... 10 EC Network SEAMOCS ...................................................................................................... 12 EC EXTREME SEAS project .................................................................................................. 12 JIP CresT/ShortCresT ......................................................................................................... 13 Theoretical background 14 Linear description of waves .................................................................................................. 14 Second-order description of waves ........................................................................................ 15 Higher order description of waves ......................................................................................... 16 Definition of rogue waves .................................................................................................... 18 Field observations of rogue waves .........................................................................................20 Generation mechanisms of rogue waves 24 Spatial focusing ................................................................................................................ 24 Temporal focusing .............................................................................................................26 Nonlinear focusing ............................................................................................................26 Numerical and theoretical investigation of rogue waves 28 Shape of the wave spectrum and occurrence of rogue waves ........................................................28 Crossing sea states ............................................................................................................29 Shallow water effects .......................................................................................................... 31 Wave-current interactions .................................................................................................... 31 Experimental investigations of rogue waves .............................................................................32 Statistical and spectral description of waves 34 Probability of occurrence of rogue waves 38 Warning criteria for extreme and rogue waves 40 Impact of rogue waves on loads and responses 42 Current design practice and Marine Operations 49 Conclusions and recommendations 52 References 54 4 Rogue waves – impact on ships and offshore structures Rogue waves – impact on ships and offshore structures 5 EXECUTIVE SUMMARY Rogue waves have attracted considerable attention We also discuss how the state-of-the–art knowledge in academia, the shipping and offshore industries, on rogue waves can be utilized to improve current and the media during the past two decades. design, as well as the planning and execution These waves – also called freak, abnormal, or giant of marine operations. Finally, we propose future waves – are very steep and much larger than the research activities that are needed to support the surrounding waves. Several accidents involving ships possible inclusion of rogue wave effects in ship and offshore structures have been reported due to and offshore structure design and operational such waves, and yet, as of today, rogue waves are procedures. The need for satisfactory warning not explicitly included in classification society rules criteria for rogue waves is emphasized. and offshore standards. The occurrence of rogue waves presents the shipping and offshore industry The significance of severe sea state conditions with two important questions: should these waves be may grow in some ocean regions in the future accounted for in design? If so, how best to account due to global warming. Therefore, taking rogue for them? waves into account in the design and operation of marine structures may become an important part of This position paper summarizes our latest adaptation to climate change. knowledge on rogue waves and their impacts on marine structures in general and ships in particular. We also focus on research projects in which DNV GL has participated and that relate to rogue waves and their impacts on marine structures. We hope that this summary, together with our own insights, provides further understanding on the rogue wave phenomenon. 6 Rogue waves – impact on ships and offshore structures INTRODUCTION The existence of exceptionally large waves – much important driver for research on rogue waves is higher, steeper, and more dangerous than those the substantial increase in computational power in expected for a given sea state – has always been a recent decades. This has enabled the study of rogue part of maritime folklore. For centuries, sailors have waves using numerical models describing realistic reported giant waves, often described as ’walls of ocean conditions. water’, appearing from ’nowhere’. For a long time such events were believed to be mostly anecdotal, The motivation for investigating and understanding but in recent decades these waves, often referred to of rogue waves is obvious. Large waves represent a as rogue, abnormal, freak, giant, or extreme waves, danger to ships and marine installations A number have received considerable attention, both in the of ship accidents related to rogue waves have media and in the scientific community, and also in been reported. Public and media awareness of the shipping and offshore industries. The first real the dangers associated with extreme waves has account of the rogue wave phenomenon in scientific also been increasing in recent years, largely due to literature was given by Draper (1964), while Mallory incidents of rogue waves hitting passenger ships (1974) provided initial discussions on the large and (e.g. Queen Elisabeth II in 1995, Caledonia Star and abnormal waves in the Agulhas current off the south- Bremen in 2000, Explorer, Voyager and Norwegian east coast of South Africa. However, it is primarily Dawn in 2005, Louis Majesty in 2010, and MS Marco during the last two decades that rogue waves have Polo in 2014), some of which resulted in passenger become the focus of wider attention, from industry fatalities (e.g. Louis Majesty and MS Marco Polo and the scientific community, through larger research accidents). Recent hurricanes in the Gulf of Mexico programmes, meetings, workshops, and conferences have also shown how dangerous extreme sea states all dedicated to the rogue wave phenomenon. can be for marine structures. During Hurricane Katrina, 30 oil platforms were damaged or destroyed, One important factor that has led to increased nine refineries were closed, and oil production in interest in rogue waves is the emergence of reliable the region was decimated. Such incidents have observations of such waves provided by wave highlighted the need for improvements to reduce measurements from, for example, oil platforms. the risk of wave-related incidents. In the future, One of the first, and probably best-known, reliable the significance of severe sea state conditions for recordings of a rogue wave is the so-called New Year ship traffic may even grow in some ocean regions wave, also referred to as the Draupner wave, that was because of the expected increase in the frequency recorded at the Draupner platform in the North Sea and severity of extreme weather events associated on 1st January 1995. Whereas previous recordings with global warming (Bitner-Gregersen and Toffoli, of extremely large waves were often regarded as 2015). Therefore, taking rogue waves into account in measurement errors, the validity of the Draupner the design and operation of marine structures may wave was confirmed by damage to equipment become an important part of the adaptation due to the wave hitting a platform deck. Another to climate change. Rogue waves – impact on ships and offshore structures 7 Figure 1: Photographs showing some rogue wave incidents and rough wave conditions. 8 Rogue waves – impact on ships and offshore structures In academia, the main focus has been on have been revised as a result of the EC EXTREME understanding the physics behind the rogue wave SEAS project, which was coordinated by legacy phenomenon and determining the conditions under DNV. Furthermore, it is worth mentioning that the which such waves can be expected to occur more oil company STATOIL has already introduced an frequently. This has also received attention in the internal requirement that accounts for rogue waves, marine industry because of the possible effect on in a simplified way, when designing the height of a design and marine operations. The occurrence