STRUCTURAL INTEGRITY ASSESSMENT OF CRITICAL COMPONENTS OF CONVERTED FPSO HULLS

摘要

Conversion of very (ultra) large crude oil carriers (VLCC,ULCC) into floating production, storage and offloading (FPSO) offshore units has become a common practice in the industry. The conversion planning is complex and demands a careful assessment of the condition of the hull to be converted into a FPSO. The fatigue assessment is especially important at certain components that will experience cyclic loads above the originally allowed in the design of the VLCC (ULCC). A through fatigue assessment of two FPSOs that are now at the final stages of conversion was carried out using current class rules. While for the majority of the hull structure the fatigue design approach used produced satisfactory and conservative results, at the cross tie beam, located at frames in the wing tanks, the fatigue assessment was considered unreliable. This was evident during the conversion stage because a large number of cracks were found in welded joints of the cross tie beams during the hull conversion inspection. A simplified finite element analysis (FEA) showed that the location where the cracks were found was subjected to high cyclic loads during the operation as an oil tanker and that these loads would be also severe during the FPSO operation. Analysis of samples taken from the damaged areas showed that they contained weld toe fatigue cracks. Further FEA was carried out taking into account the displacements from a global structural model, which modelled the hull and the top side structures, in order to estimate the hot spot stresses at the affected welded joints. The additional fatigue assessment demonstrated that the fatigue design approach used needed to be modified to firstly explain the cracks found in the component, secondly to allow for a reliable and conservative estimation of the remaining fatigue life for the cross tie beam after repair work was carried out. The resultant fatigue approach adopted demonstrated that the cross tie beam would not survive the 20 years design requirement of the project unless modifications on the local structural design were undertaken or welded joint fatigue enhancement techniques were applied to the welded joints. To resolve the problem the project decided to adopt a controlled arc welding repair of all cracked joints followed by careful inspection and application of burr machining of weld toes as a fatigue enhancement technique. This paper describes the fatigue assessment undertaken, the suggested modification to the fatigue approach for the assessment of cross tie beams, a description of the repair procedures, non-destructive testing and fatigue enhancement methods adopted by the project.