Long term effectiveness of life extension methodologies applied to offshore structures

• The extent and causes of fatigue cracking in a 29-year old platform were examined.
• Various methods for weld improvement repair and upgrading were employed.
• An extensive data base of inspection results up to 2014 was accumulated.
• Probabilistic fracture mechanics based analyses were used to evaluate the retrofitting program.
• Reliability analyses indicated that actual life extension was longer than anticipated.

This paper documents developments in fatigue cracking of a floating platform that required repair and upgrading during 1999–2000. The Veslefrikk B (VFB) platform was built in 1985 for drilling exploration but was converted to a production platform in 1989. Shortly thereafter extensive fatigue cracking was discovered and several repairs were made. However, extensive fatigue cracking continued and a retrofitting program was implemented, using various types of grinding and peening techniques. In 1999 the platform was temporarily decommissioned and dry-docked for a comprehensive repair and upgrading program, following the International Institute of welding (IIW) guidelines for weld improvement methods [1], this was completed in only four months. However, after a few more years of service fatigue damage again necessitated new repairs. The majority of cracks occurred in the hull skin plates and caused water leakage. It is noteworthy that cracking this time occurred only in areas of the structure that were left untreated in the first retrofitting program due to low levels of stress from FE analyses in these areas. The paper describes the original repair and strengthening program, and the types of subsequent fatigue damage that necessitated new repairs. The recent life extension program has resulted in the safe operation of the platform for an estimated additional period of 20 years.