Life extension of ship-shaped floating production units

Abstract

In the last two decades, the number of floating, production, storage, and offloading units (FPSOs) deployed internationally has increased rapidly. Many of them are still young but have already started ageing. The objective of this thesis is to investigate life extension of ship-shaped floating production units in the petroleum activity. This will include an assessment of relevant ageing mechanisms for FPSOs, such as cracks, corrosion, load changes, deflection, and dents in the hull girder. In addition, two literature studies have been performed, one on hull structural integrity management and one on life extension practices for FPSOs. This project investigates the operator experiences on managing actual ageing mechanisms on their FPSOs. This has been supported by interviews with representatives of the operators on how these are maintained during the operation. In addition, this project reviews actual ageing mechanism data (cracks) on two FPSOs (Balder and Jotun A) operated by Vår Energi. The data has been collected and analyzed with respect to the annual number of cracks as a function of severity and time in operation, cracks in terms of structural details, cracks in terms of crack length, and distribution of cause of failure. The results show that the annual number of cracks partitioned on the two units is very uneven, were for Balder, there have been found 333 cracks, and for Jotun A, there have only been found 12 cracks. Investigations have shown that the majority of the cracks have an insignificant severity classification on both FPSOs. For Balder, most of the cracks are found in ballast tanks in the way of longitudinal side shell stiffeners connection to transverse frames and bulkheads and in the form of the weld between the side shell and the longitudinals. These cracks seem to be caused by unfavorable design of details and fatigue failure. For Jotun A, most of the cracks were found in void spaces at door frames. These cracks seem to be caused by unfavorable design of details. The review on the operator’s life extension practices of ship-shaped units addresses the extent to which the operating companies have performed life extension assessment of their FPSOs in accordance with Norwegian regulation and the standard NORSOK N-006. Results show that the companies are following this regulation and standard. The project closes with a discussion regarding suggestions for improvements in the standard NORSOK N-006 for assessments of FPSOs.