Improved design procedures to avoid fatigue cracks and fractures in FPSO vessels have recently been completed by DNV and a consortium of industry participants. The design recommendations are incorporated in the DNV guidelines and tools and will be available to designers and construction yards. This will lead to more reliable and cost effective FPSOs.
FPSOs (Floating Production Storage and Offloading vessels) are used globally in remote locations. Both newbuildings and conversions have to avoid costly production stops by enforcing stricter requirements as to a reliable fatigue design than those which have previously been normal for trading ships.
It is vital to design and fabricate according to procedures that are thoroughly calibrated against reliable data, as fatigue capacity is highly sensitive to the parameters involved. A slight mismatch between the calculated stress when designed, and actual stress at the location when operating, can reduce lifetime dramatically. Results from tests in this project show that a 10% mismatch of stress can reduce the actual fatigue lifetime by 1/3.
The improved design procedures are a result of a 5-year dedicated effort coordinated by DNV and performed in cooperation with oil companies, designers, operators, shipyards, universities and classification societies. Altogether 25 industry participants have supported the project since its initiation in 1998.
The history of structures damaged by fatigue is long and includes numerous tragedies. There are examples of fatigue cracks in all types of structures - in towers, bridges, ships and offshore installations. The Alexander Kielland accident in the North Sea in 1980, where 123 people were killed, is one of the worst known to be caused by fatigue.
The fatigue testing has been conducted at the large shipyards in Korea and in the test laboratories at DNV’s head office in Oslo, Norway. Numerical analyses have been performed to calibrate the design procedures with the test data.
In total, approximately 30 papers from the project have been published at conferences and in journals. 14 papers summarizing the project’s main findings will be shared with the industry at the OMAE special FPSO conference in Houston from August 31st to September 2nd 2004.