Abstract

Numerical simulations of a conventional tension leg platform (TLP) were generated to investigate its response to wave-in-deck-loads under extreme wave conditions. The model was setup as a fixed body and as a floating body connected to a fixed boundary by flexible mooring cables. The overset mesh technique was utilised to account for large platform motions in the floating body CFD simulations. The validity of the CFD simulations was extensively studied using 1:125 model-scale experiments. Special attention was given to ensure the accuracy of the simulated wave profile, global and local loads on the fixed structure as well as motions and global loads on the moored floating structure. The global horizontal wave impact loads and motions were found to agree well between simulations and experiments.

Introduction

When a large wave (abnormal wave event) impacts the deck of an offshore structure, significant wave-in-deck and slamming loads occur. These slam events could generate major global and local loads which can cause structural damage to the deck, generating large forces in the mooring lines and risers and adversely affect the motions of floating structure such as Tension Leg Platforms (TLPs) and Semisubmersibles. The problem of wave-in-deck impact on a floating platform can, therefore, be quite complicated because of the contributions of many parameters such as the platform offset, set-down and tendon dynamics (API, 2010).

The simplest way to investigate wave-in-deck impact problems is a simplified rigid model of the deck structure idealised as a flat plate or as a box-shape (Abdussamie et al., 2014a, Abdussamie et al., 2014b, Abdussamie et al., 2016b, Baarholm, 2009, Bhat, 1994, Scharnke and Hennig, 2015). Current design practices (API, 2007, DNV, 2010, ISO, 2007) recommend a number of theoretical approaches such as the global/silhouette approach "simplified loading model" (API, 2007) and a detailed component approach, e.g., the momentum method (Kaplan et al., 1995) to evaluate the wave-in-deck loads of fixed platforms. Since such engineering approaches rely on the potential flow theory to calculate the change of fluid momentum during the wave impact, using wave kinematics of a non-disturbed wave field, the effects of diffraction and entrapped air are neglected. Scharnke et al. (2014) found that the recommended simplified loading model (API, 2007, DNV, 2010) underestimates the measured horizontal wave-in-deck loads on a fixed deck of jacket platform in both regular and irregular wave tests. Even though the simplified loading model used wave kinematics obtained by Stokes fifth order wave theory, the underestimation of the loads was severe, particularly in irregular waves (Scharnke et al., 2014). The momentum method was also found to underestimate the magnitude of the wave-in-deck forces on a fixed horizontal deck subjected to unidirectional regular waves (Abdussamie et al., 2014a, Abdussamie et al., 2014c).

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