Strengthening of Ekofisk Platforms to Ensure Continued and Safe Operation
Authors: Etterdal Birger , Scherf Ingar , Grigorian Henrik 
Published: October 2008

This paper addresses the use of modern analysis techniques in the structural reassessment of offshore platforms, with the objective to keep the operation costs low whilst documenting sufficient safety for the remaining service life. The assessment, modifications and strengthening of four jacket structures located in approximately 75 m water depth at the Ekofisk Field are presented. The platforms are exposed to seabed subsidence.

As subsidence develops, the probability of having wave-indeck events increases. Tools for calculation of wave-in-deck loads were therefore developed and verified by large-scale model tests. Wave-in-deck loads were then calculated in terms of increasing wave height, wave approach direction and seabed subsidence.

The capacity of the jacket structures was defined as the base-shear level at collapse. This system capacity was determined by pushover analyses duly accounting for all relevant failure modes: deck leg failure, brace buckling, joint failure, and pile/soil failure. Through these pushover analyses, critical components were identified and strengthened by use of high strength grout.

Advanced non-linear finite element analyses were performed for components where adequate capacity could not be determined based on code requirements. Such analyses were done both for "as-welded" and grout reinforced components.

Structural reliability analyses were then used to determine the extent of required strengthening to achieve an acceptable annual failure probability and to determine when the various mitigations needed to be implemented offshore.

Introduction

The comprehensive structural reassessment work for the subsided Ekofisk platforms was initiated to document that the structures can be safely operated until decommissioning, and if not, design and perform the modifications and strengthening required to achieve this goal.

This paper is part 4 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 have raised the need to strengthen many of the jacket structures.

As the air gap requirement is not longer met, the safety of the platforms was documented through the use of reliability analysis, i.e. the so-called Level III in safety checking of structures. The reliability analysis methodology and criteria, as well as the establishment of target reliabilities, are presented in Part 3 of this paper series, see Ref. 3.

The paper series address the reassessment with respect to extreme storm loads. The documentation of sufficient fatigue strength is not covered here.

Description of jacket structures

The project addressed here involved strengthening of four jacket structures at the Ekofisk field, i.e., 2/4A, 2/4B, 2/4P and 2/4R.

Two of these platforms, 2/4P and 2/4R are located very close to the cylindrical large diameter (138 m) protective wall installed around the Ekofisk Tank (2/4T) in 1989. The wave field around this tank is significantly affected by wave diffraction. The wave load on the 2/4R platform is increased by approximately 60% for storm waves from the North-West sector.