Design Wave Conditions in the Diffracted Field around the Ekofisk Barrier
Authors: Sterndorff M.J. , Dahl C.S. , Grigorian H. , Yu W.C. 
Published: October 2008

The present paper presents the extensive analyses performed in order to establish design wave conditions for platforms in the diffracted field around the Ekofisk Barrier Wall. The paper summarises the work performed to establish the free field design wave criteria based on data from field measurements with different sensor types supplemented with numerical hindcasts. A rational procedure for transformation of the free field design wave criteria to the diffracted field around the Ekofisk Barrier is also presented. Linear diffraction calculations supplemented with model test results were employed to transform the wave exceedance probability functions for eight directional sectors from the free field to the diffracted field, and new omni-directional design wave conditions were established by combination of the transformed directional wave exceedance probability functions. The procedure has been used to determine design seastates, design individual wave heights, and design wave crest elevations for all the platforms around the Barrier. A few examples will be presented in the paper.

Introduction

The present paper is part 2 of seven companion papers, which together present the comprehensive assessments of the effects of seabed subsidence on the Ekofisk jackets, see Refs. 1-6.

At the Ekofisk Field in the North Sea, continuous registration of environmental data has taken place since 1980. Over the years operator Phillips Petroleum Company Norway has put a considerable effort into optimising the design criteria based on detailed knowledge of the conditions at the field. Special emphasis has been on the analysis of the wave conditions, which have become critical due to platform subsidence.

In 1989 a Barrier Wall with an external diameter of 138 m was installed for protection of the Ekofisk 2/4 Tank. In order to arrive at design wave conditions for the central Ekofisk Complex it is necessary to take into account the diffraction of the waves by this large diameter cylindrical wall. An overview of the Ekofisk platforms is given in Fig. 1.

The present paper presents the analyses that have been performed in order to establish design wave conditions for platforms in the diffracted field around the Ekofisk Barrier Wall. Free field design wave criteria were established based on data from field measurements by the Ekofisk Data Acquisition System supplemented with numerical hindcasts. A rational procedure for transformation of the free field design wave criteria to the diffracted field around the Ekofisk Barrier was also developed. The procedure employed linear diffraction theory to transform the wave exceedance probability functions for eight directional sectors from the free field to the diffracted field and new omni-directional design wave conditions were established by combination of the transformed wave exeedance probability functions. The procedure was used to establish directional and omni-directional design significant wave heights, design individual wave heights, design crest heights, and design crest elevations for all the platforms and bridges around the Ekofisk Barrier Wall.