Experience With Fatigue Analysis and Inspection Results in the North Sea
Authors: Green M.B. 
Published: October 2008

This paper describes a comparison of observed defects on two similar North Sea jackets to theoretical fatigue lives.

In order to appreciate the limitations of the study, brief details are given of the fatigue analysis of each platform and the scope and method of inspection used in finding the defects.

The observed defects are compared on a statistical basis with the probability level suggested by API RP 2A.

The implications on the long term inspection strategy for these platforms are then considered.

INTRODUCTION

One criterion for the selection of areas on a jacket requiring routine inspection is the theoretical fatigue life of the nodes. Current knowledge and computing capabilities have significantly advanced since the first generation of North Sea platforms were designed and installed. There are, therefore, nodes on existing platforms with theoretical fatigue lives less than would be accepted in the current generation of structures now being designed. Strengthening of these areas would probably be extremely costly and may not be the best technical solution.

Occidental have been operating two similar North Sea platforms for approximately seven years. Cracks were discovered during routine inspection 3 years after installation. The cause of cracking was fatigue. All significant defects have now been repaired. Current fatigue analyses for the two platforms do, however, suggest the potential for future fatigue damage in several other areas.

BACKGROUND

The Piper and Claymore fields are situated some 100 miles north east of Aberdeen in Blocks 15/17 and 14/19 in the UK sector of the North Sea. The piper field was discovered in January 1973 followed by Claymore in May 1974.

The Piper platform is a 36 slot drilling and production facility located in 474 ft water depth. It was launched in June 1975 and initial production was achieved in December 1976. Complete details of the installation are given in a paper by Duvivier and Henstock (1).

The Claymore platform is a 36 slot drilling and production facility located in 360 ft water depth. The jacket is conceptually similar to the Piper structure except for the removal of 2 bays of framing to accommodate the different water depth. The jacket was launched in June 1976 and first production was achieved in November 1977.

All offshore installations in the U.K. Sector of the North Sea are subject to Statutory Instrument No. 289 Offshore Installations, " The Offshore Installations (Construction and Survey)Regulations 1974" which came into effect on 1 May 1974. This regulation requires that a Certificate of Fitness be obtained through a Certifying Authority, certifying a platform's structural integrity, as well as other aspects of safety. These certificates cover the initial design, fabrication, and installation as well as the continued operation of the platform with updated certificates being issued on a regular periodic basis throughout the life of the platform.

The continued integrity of the platforms is demonstrated by the results from routine inspection of the structure with repairs being undertaken whenever damage is detected that may affect the safety or load carrying capacity of the platform.