Cost-Efficient Structural Upgrade and Life Extension of North Sea Jacket Platforms With Use of Modern Reassessment Techniques
Authors: Scherf Ingar , Etterdal Birger , Monshaugen Trond 
Published: October 2008

This paper addresses the practical use of modern analysis techniques in the structural reassessment of offshore platforms, the objective of which is to keep the operation costs low whilst documenting sufficient safety for the remaining service life. For this purpose, the assessment of four jacket structures (platforms A, B, C and D) located in approximately 75 m water depth in the North Sea is presented. The platforms are exposed to seabed subsidence, and the air gap requirement is no longer met, but despite this fact, the platforms are still in service.

Based on the reassessment procedure outlined here, the platforms were strengthened by increasing the capacity of the components governing structural collapse by use of high strength steel fibre reinforced grout or installation of new braces. The grout was injected by the use of an ROV. The theoretical capacity of these components has been validated through laboratory tests, whilst the capacity of the jacket structure was determined by use of pushover analyses and nonlinear finite element analyses.

The platforms have been instrumented to verify the procedure for calculation of wave loads on the jacket. AnROV was used to install the instrumentation devices.

Wave loads on the deck structures have been calculated using a computer programme based on change in fluid momentum. This programme has been calibrated and verified through results from model tests. Another method for the computation of wave loads on deck is based on the Volume of Fluid (VOF) method, which is used to solve the incompressible Navier-Stokes (or Euler) equations.

The following elements and their interrelation are the key to the success of this method; grout reinforcement, wave load prediction on deck, reliability analyses, advanced finite element analyses, and load and response measurements. As the safety level is consistently documented within the framework of modern reliability theory, this method ensures that the minimum mitigation needed is identified. The alternatives to this approach if not used are; extensive strengthening, jacking of deck, restrictions in the operation of the platform or decommissioning of the platform.

Introduction

The oil field was developed in the early to mid 1970s using jacket structures as drilling and production platforms, designed based on Gulf of Mexico experience (early API recommended practices). The original design lives for these platforms were typically 20 years. Extended service for some of these platforms was desired because the field continues to produce at a high level.

Since subsidence of the seabed was observed in 1984, extensive structural reassessment resulting in structural modifications and strengthening has been on going.

The subsidence at the locations of platforms C and D now approaches 8m, while the subsidence at the locations of platforms A and B is somewhat less.

This paper is concerned with the structural reassessment work for the platforms A, B, C and D. The platforms have been strengthened to mitigate the effect of seabed subsidence. It is emphasized that, owing to an air gap lower than 1.5m related to the 100-year wave crest, these platforms are not manned during severe storms. Platforms C and D are located close to a large cylindrical concrete tank, and are exposed to diffraction effects.