Rogue wave impact on offshore structures
Authors: Clauss G¨unther , Stutz Katja , Schmittner Christian 
Published: October 2008

For the design and operation of offshore structures, heave motion, airgap, splitting forces as well as bending moments are key parameters to reduce down time and ensure safe operations. The increasing number of reported rogue waves with unexpected large wave height (Hmax/Hs20), crest height (?c/ Hmax0.6), wave steepness and group pattern (e.g. Three Sisters) suggests a reconsideration of design codes by implementing an Accidental Limit State with a return period of 10000 years. For investigating the consequences of specific extreme sea conditions numerical simulations of the seakeeping behavior, including motions and structural forces, as well as model tests have been carried out with FPSOs and semisubmersibles in a reported roguewave, the Draupner New YearWave. Both, frequency and time-domain results are presented. With frequencydomain analysis the profound data for the standard assessment of structures, concerning seakeeping behavior, operational limitations and fatigue are obtained. In addition, time-domain analysis in real rogue waves gives indispensable data on extremes, i.e. motions and structural forces. As the wave/structure interaction is analyzed in deterministic (freak) wave sequences the most critical position is evaluated by systematic simulations, and the causes of (nonlinear) structure response are revealed.

Introduction

Surviving a freak wave - what an experience. However, only scarce observations are available of such mystic disasters.

Reports on individual extreme waves in deep water mention either single high waves or several successive high waves.

Fig. 1-Rogue wave observations - Bay of Biscay (top) and Atlantic Ocean South (bottom) 1

Fig. 1 shows two exceptional and frightening events 1. Breathtaking waves have also been presented by Faulkner 2 who proposes the definition Hmax2.4Hs for abnormal wave height. From a probability analysis of rogue wave data recorded from 1994 to 1998 at North Alwyn Wolfram et al. 3 conclude that these waves are generally 50% steeper than the significant steepness, with wave heights Hmax2.3Hs. The preceding and succeeding waves have steepness values around half the significant values while their heights are around the significant height. Registrations of rogue waves are shown in Fig. 2 and 3:a giant wave (Hmax25.63m) with the crest height ?c18.5m hit the Draupner jacket platform on January 1, 19954off Yura harbor in the Japanese Sea a 13.6m wave with ?c 8.2m has been recorded in a sea state of Hs5.09m5 Exceptional waves have also been reported from the Norwegian Frigg field6 - Hs8.49m, Hmax 19.98m,? c_ 12.24m, water depth d=99.4m, as well as from the Danish Gorm Field7 - Hs_ 6.9m, Hmax17.8m, ? c= 13m, water depth d=40m. All these wave data, with Hmax/Hs2.15 and ? c/Hmax0.6, prove that rogue waves are serious events which should be considered in the design process. Although their probability is very low they are physically possible. It is a challenging question which maximum wave and crest heights can develop in a certain sea-state characterized by Hs and Tp.

In addition to the global parameters Hs and Tp the individual wave height and shape as well as its effects on a structure depend on superpositions and the interaction of wave components, i.e. on local wave characteristics.