Instrumentation of Ekofisk Platforms
Authors: Kanter Per Arne , Scherf Ingar , Pettersen Bjørn , Osnes Jørund , Grigorian Henrik , Yu Wei Chi , Christensen Øyvind Arne 
Published: October 2008

As a part of the comprehensive structural reassessment of the Ekofisk jacket platforms operated by Phillips Petroleum Company Norway (PPCoN), several instrumentation projects were initiated in 1991. Simultaneous measurements of platform response and environmental data were carried out for four jacket platforms through the subsequent three winter seasons. Two of these platforms were located very close to the Ekofisk Barrier, which is a concrete cylinder with a diameter of 138m installed in 1989 to protect the Ekofisk 2/4Tank.

The knowledge obtained from these measurements has been of invaluable significance in the structural assessments and strengthening of the Ekofisk jacket platforms.

This paper presents the instrumentation set-up, the measurement devices and the main results from the instrumentation of the jacket platform 2/4A.

2/4A is located in an un-disturbed wave field; i.e. the waves and current are not affected by diffraction due to the Ekofisk Barrier. Axial stresses in several braces measured in storm sea states were compared to stresses predicted by various wave load procedures. Further, maximum measured deck displacements and water particle velocities were compared to theoretical predictions. Simultaneous measurements of environmental data and response haveprovided increased knowledge with respect to extreme storm loads for jacket structures located in moderate water depths, i.e. approximately 70m.

Introduction

This paper is part 5 of 7 companion papers, which together present a comprehensive assessment of the effect of seabed subsidence on the Ekofisk jackets, see refs. 1 through 6. Seabed subsidence at the Ekofisk field has raised the need to strengthen many of the jacket structures.

Fig. 1 presents an overview of the platforms at the Ekofisk Complex. The platforms that have been instrumented are indicated.

Platform 2/4A was monitored with respect to response and environmental data during the winter season 1993/94.

The two platforms 2/4R and 2/4P were instrumented to verify the wave load procedure proposed for the diffracted wave field in the vicinity of the Ekofisk Barrier. 2/4R, which is exposed to severe storms from the north-northwest sector, was instrumented during the winter seasons 1991/92 and 1992/93, while 2/4P was instrumented during the winter season 1992/93. 2/4P is located at the leeward side of the Ekofisk Barrier for the most severe storms. It was of particular interest to investigate how MacCamy & Fuchs linear wave diffraction theory captures the diffraction effects at the leeward side as compared to the ability of this theory to represent the diffraction effects at the side directly exposed to the extreme waves. Dependent on the wave direction, the wave load enhancement could be as large as 60% due to wave diffraction. The instrumentation of 2/4R and 2/4P is not the subject of this paper.

The main objective with the Ekofisk instrumentation project was to determine the most appropriate jacket wave load recipe for extreme storm analyses, and to provide uncertainty in jacket wave loads to adopt in the structural reliability analyses.

Further, the instrumentation set-up was planned such that fatigue loads can be assessed.