Structural Reliability Assessment of Ekofisk Jackets Under Extreme Loading
Authors: Mark Mark , Etterdal Birger , Grigorian Henrik 
Published: October 2008

Seabed subsidence increases the probability that the crests of extreme waves will impact platform deck structures. In the years since significant subsidence at Ekofisk was discovered in early 1980's, there have been on-going reassessments of the Ekofisk platforms. Over this period, the approach to reassessment has matured, both due to a better understanding of the problems particular to Ekofisk, and due to development of tools and methods for reassessment in the offshore industry in general.

The paper describes the reliability and uncertainty models, and the development of target safety levels, expressed as annual failure probabilities. The theory for estimating the wave loads on deck, and ultimate load capacity by nonlinear pushover analyses, are summarized. The reliability analysis methods which have been applied are described, and example results are given.

As the safety level is consistently documented within the framework of modern reliability theory, the method ensures that the minimum mitigation necessary is identified. Had such an approach not been applied, the alternative mitigation measures required would likely have been: extensive strengthening, jacking of decks, restrictions in the operation of the platforms, or decommissioning of the platforms.

Introduction

The present paper describes the reassessment methodology and philosophy developed and applied to four Ekofisk jackets. This paper is part 3 of 7 companion papers which together present a comprehensive assessment of the effect of seabed subsidence on the Ekofisk jackets, Refs. 1 to 6.

The Ekofisk Field, operated by Phillips Petroleum Company Norway, was discovered in 1969. It is the oldest Norwegian oil and gas field, with the first producing platforms installed in the early 1970's. The field is expected to remain economically viable beyond 2020.

Subsidence at the Ekofisk field was first recognized in the early 1980's. The seabed was subsiding as the reservoir pressure fell due to the ongoing hydrocarbon production. Currently, the total subsidence at the Ekofisk Center is approximately 8 m. Water and gas injection have played significant roles in the control of the seabed subsidence and in addition, by contributing to reservoir pressure maintenance, has enhanced recovery. The current water injection rate is approximately 700,000 barrels per day, and subsidence rates, which have been up to 40 cm per year, have recently declined and stabilized in the range 7 to 14 cm per year.

Since subsidence of the seabed was first observed, there has been continuous and extensive structural reassessment resulting in structural modifications and strengthening. The decks of six of the jacket platforms at the Ekofisk Complex were elevated 6 m in 1987. To protect the Ekofisk Tank 2/4T, the Ekofisk Barrier Wall, a protective concrete cylinder with diameter 138 m, was installed in 1989. The challenges and solutions of the subsidence problem are addressed by Ã?m in Ref. 7.

Subsidence and the continuing high production from the field resulted in the Ekofisk II redevelopment project, consisting of a new drilling/production platform 2/4X in 1996, and a new process and transport platform 2/4J in 1997-98.