Extreme Wave Effects on Deepwater Floating Structures

Abstract Extreme waves can lead to damage to floating offshore structures as a result of airgap problems, greenwater on the deck or slamming to the hull. As the physics of these problems are different, there is no single way of identifying and characterizing extreme waves. As part of the investigations into the effect of extreme waves on deepwater floating structures, this paper focuses on the challenges of the numerical prediction of platform response due to extreme waves. This will be done by using an improved Volume Of Fluid (iVOF) method. Two case studies are presented, which both required specific extensions of the methodology. First green water simulations on a FPSO are discussed, requiring the coupling of a linear diffraction code to the iVOF method as part of a domain decomposition. Second the dynamic response of a TLP to an extreme wave is studied, requiring the integrated analysis of the wave loading and platform response. Introduction Hurricanes Ivan, Katrina and Rita in the Gulf of Mexico showed the importance of extreme waves for all types of offshore structures [1-5]. Extreme waves can lead to damage to floating offshore structures as a result of airgap problems, greenwater on the deck or slamming to the hull. As the physics of these problems are different, there is no single way of identifying and characterizing extreme waves: - For airgap problems, the crest ampltidue of the waves is the most important aspect. Therefore, it is important to know the ratio Ac/Hs between Crest Amplitude (Ac) and Significant wave height (Hs). In [6] the detailed analysis of an observed extreme wave in a model basin is described. It was shown that even in a wave with moderate steepness (Hs=11.9 m, Tp=15.3 s), generated with a random phase model, extreme wave crests can be observed. The measured wave had a Ac/Hs ratio of 1.59. As a pilot study into the understanding of the occurrence of this type of extreme waves, [6] describes the spatial development of this wave in the basin, see Figure 1. Figure 1: Spatial development of a wave in a model basin (from [6]) (Available in full paper) It should be noted ofcourse that even for fixed platforms the crest amplitude is not the only factor, the structure itself can clearly enhance the local wave elevation [7]. - For floating structures such as TLPs, Semis and Spars the wave loading and response is even more complex, as was shown in recent tests on the Snorre TLP [8]. The dynamic response of the platform to the wave impact can be the main factor in the survivability of the platform. Figure 2: Airgap tests on the Snorre TLP (from [8]) (available in full paper) In [8] the important observation was made that extreme wave and airgap problems belong to the group of (what was called) ‘badly behaved’ problems. A ‘badly behaved’ problem is a problem with a step or discontinuity in the response. For a fixed platform or TLP it can for instance occur that the deck is not hit in the 100 year wave as the maximum wave crest just passes below the deck.