Abstract

The application of subsidence monitoring and mitigation efforts combined with the conversion to remote operation have contributed to safely prolonging the operating lifetime of the best producing platforms in the Ekofisk field in the Norwegian North Sea.

This is the first paper in a series of seven (7) papers, which summarize the main efforts done to mitigate the effects of subsidence. The paper presents the background for the reassessment work, the tasks involved and summarize the main results thereof.

The paper series address the update of environmental criteria and the practical 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. For this purpose, the assessment of four jacket structures at the Ekofisk Field is presented.

Based on the reassessment procedures presented 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 a 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 nonlinear finite element and pushover analyses.

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

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

The following elements and their interrelation are the key to the success of the work to extend the lifetime of the Ekofisk platforms; subsidence monitoring, remote platform operation, extreme wave warning procedure, update of environmental criteria, wave-in-deck load prediction, reliability analyses, advanced finite element analyses, measurements of environmental data, wave kinematics and platform response and finally strengthening of the platforms.

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 Ekofisk Field is the oldest producing Norwegian oil and gas field. Ekofisk was first discovered in 1969, and the first producing platforms were installed in the early 1970's. Since that time Ekofisk has been one of the most successful hydrocarbon fields in the area, and has produced more than 1.6 billion barrels of oil.

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