Experience With Fatigue Analysis and Inspection Results in the North Sea
Authors: Green M.B. 
Published: December 2007

Green, M.B. Occidental Petroleum (Caledonia) Ltd.

The occurrence of fatigue damage is evaluated against theory for two North Sea platforms. Details are given of the fatigue analysis for each platform and the subsequent scope and methods of inspection. Observed defects are compared on a statistical basis with the probability level suggested by API RP2A.

Introduction

One criterion for the selection of areas on a jacket requiring routine inspection is the calculated fatigue life of the nodes. Current knowledge and computing capabilities have advanced significantly since the first generation of North Sea platforms were designed and installed. There are, therefore, nodes on existing platforms with calculated fatigue lives less than would be accepted in the generation of structures now being designed. Strengthening of these areas probably would be extremely costly and may not be the best technical solution. Occidental has been operating two similar North Sea platforms for about 9 years. Cracks were discovered during routine inspection 3 years after installation. The cause of cracking was fatigue. All significant defects now have been repaired. Current fatigue analyses for the two platforms, however, do suggest the potential for future fatigue damage in several other areas. potential for future fatigue damage in several other areas.

Background

The Piper and Claymore fields are about 100 miles [161 km] northeast of Aberdeen in Blocks 15/17 and 14/19 in the U.K. sector of the North Sea. The Piper field was discovered in Jan. 1973 followed by Claymore in May 1974. The Piper Platform is a 36-slot drilling and production facility in 474-ft [144.475-m] water depth. It was launched in June 1975 and initial production began in Dec. 1976. Complete details of the installation are given in Ref. 1. The Claymore Platform is a 36-slot drilling and production facility located in 360-ft [108.728-m] water depth. The jacket is conceptually similar to the Piper structure except for the removal of two bays of framing to accommodate the different water depth. The jacket was launched in June 1976 and first production was in Nov. 1977. All offshore installations in the U.K. sector of the North Sea are subject to Statutory Instrument (SI) No. 289 Offshore Installations, "The Offshore Installations (Construction and Survey) Regulations 1974," which came into effect on May 1, 1974. This regulation requires that a certificate of fitness be obtained through a certifying authority, attesting to 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. Updated certificates are issued on a regular 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. Occidental developed the initial strategy of items to be inspected, their priority, and the standard of inspection for the under-water priority, and the standard of inspection for the under-water portions of the Piper and Claymore Platforms by using the original portions of the Piper and Claymore Platforms by using the original design analyses for guidance. This strategy was reviewed with the certifying authority and was deemed to satisfy the major survey requirements of SI 289. Major surveys are required every 5 years and must include some detailed node inspection. Additional annual surveys also are carried out on a more general basis, with some detailed node inspection as necessary. For logistical purposes such as bed space and offshore equipment availability, most operators also undertake a proportion of the nondestructive testing (NDT) work required for the major survey annually, so that the scope of work for the 5-year survey is completed by an accumulation of the annual survey results at the end of each 5-year period. Implementation of our long-term inspection strategy began in 1977. However, it was not until 1979 that the first close visual inspection with prior cleaning (F-type) of jacket nodes began. Cracks in the –50-ft [ –15.420-m] level conductor framing were first discovered on Piper, followed a few weeks later by the discovery of cracks on Claymore in the –40-ft [ –12.192-m] conductor framing in similar areas. The discovery of these defects in platforms that had been installed only 3 to 4 years earlier led toan extension of the original inspection program,major reanalysis of both structures to ascertain the cause of the failures, andsetting up a test program to determine stress concentration factors for various joint geometries,

since fatigue was believed to be a contributing factor. This was to confirm existing parametric formulas where appropriate and, in the case of overlapping joints or joints in the large-diameter bottle legs, to obtain stress concentration factor (SCF) data, which were otherwise unavailable.

JPT

P. 661