Hydrodynamic Modelling of Wave-In-Deck Forces on Offshore Platform Decks
Authors: Grønbech J. , Sterndorff M.J. , Grigorian H. , Jacobsen V. 
Published: October 2008

The present paper concerns a hydrodynamic model developed for the determination of the wave induced loading on the cellar deck of the Ekofisk platforms.

The objective of the work was to establish a hydrodynamic software tool that could be used to generate the load input for the structural reassessments of the Ekofisk Platforms.

The hydrodynamic model/software tool calculates the wave forces using a load recipe based on the concept of change of fluid momentum taking into account the additional effects from buoyancy and drag. Due to the presence of the great Ekofisk Barrier the model was developed to include wave kinematics procedures for both free and diffracted wave fields.

For verification of the model, computed forces have been compared with measured forces from physical model tests with Ekofisk 2/4C and Ekofisk 2/4B. Good agreement between measured and computed forces was found.

Until recently the wave-in-deck loading has only been studied analytically and/or experimentally. Numerical modeling has, however, undergone rapid developments and in conjunction with the now available computer power, the use of computational fluid dynamics (CFD) is today feasible for many areas. Wave-in-deck loading is an area where the CFD technique has a unique applicability in the future. This issue is addressed at the end of the paper.

Introduction

The present paper is part 7 of seven companion papers, which together present the comprehensive assessments of the effects of seabed subsidence on the Ekofisk jackets, see Refs. 1-6.

The wave-induced force on offshore platform decks has become an important issue in reassessments and requalification of old platforms.

Due to seafloor subsidence the cellar deck of an offshore platform may be subject to partial or even full inundation during severe storms. This obviously affects the structural integrity of the platform and is thus a very important issue of concern. Even small inundation may generate a significant contribution to the overall forces on a platform.

Several operators have undertaken major physical test programs to address this complex issue and a number of approaches for calculating the wave induced deck loads have been proposed (see e.g. Refs. 7-15).

The wave-in-deck loading process is very complex and indeed very difficult to model with traditional analytical tools. In fact, it is also very difficult to assess the loading very accurately in model tests carried out in a wave basin due to the comparatively small model scale ratios that introduces inaccuracies in the measurements. Phenomena such as structural shielding and blocking effects are also very difficult to take into account.

In the following the Wave-in-deck model developed by DHI is presented and subsequently physical model test data and computed data are compared.

The DHIWID Model

The software tool (DHIWID) developed by DHI Water & Environment (DHI) is a numerical time domain model capable of computing load time series on partially and fully inundated offshore platform decks.