Benefits of Applying Response-Based Design Methods to Deepwater FPSOs

Abstract

Abstract This paper presents key findings from the Response-Based Design Applied to FPSOs JIP. Case studies were performedon a turret-moored FPSO West of Shetland and at a deepwater (2,000m) location in the Gulf of Mexico, having first calibrated the numerical model against measured test data for BP's Schiehallion FPSO. Both case studies showed that there is potential for reducing maximum design excursions and line tensions by about 20% if response-based analysis methods are adopted rather than the traditional deterministic approach. Other key benefits from the response-based approach are the ability to quantify probabilities of exceeding design loads/ excursions, and therefore risks, and a systematic approach to selecting design combinations of wind, waves, current, and their associated directions. The response-based approach represents actual levels of coincidence and co linearity that occur between simultaneous wind, wave and currentconditions, without assuming particular design combinations. Comparisons were made between two independent analysis procedures (long-term simulation/ extreme value analysis, and FORM / SORM reliability methods) in order to assess their relative advantages. Similar results were obtained from both procedures, giving confidence in the findings. A system reliability study showed that high levels of reliability were maintained if the analysis was performed using traditional API or Lloyd's safety factors, and reliability was still satisfactory when proposed DNV factors were used. Introduction and Methodology Response-based methods aim to design a structure to withstand combinations of critical N-year return period responses, rather than responses in a combination of N-year environmental conditions. The design is therefore based on parameters that are of direct engineering significance, rather than on secondary metocean parameters. The ‘traditional’ approach to offshore structure design (i.e. calculating deterministic loads due to concurrent and collinear 100-year return period wind, wave and current conditions) often contains an element of conservatism. Using data from the Southern North Sea, for example, Prior-Jones et al. [1] showed that the N-year load on a vertical pile was 25% lower than that computed using a combination of N-year wind, wave and current values. Response and load-based design methods have become established in the design of fixed structures [2, 3], and have also been applied to compliant towers, tension leg platforms, jack-ups, semi-submersibles and other offshore systems [4 to 10]. Preliminary investigations [11, 12] suggested that similar techniques may in principle be applied to FPSO mooringdesign, although there are a number of difficulties specific to these systems, such as their sensitivity to wind, wave and current directions, the weathervaning behaviour of turretmooredvessels, and their sensitivity to wave steepness as well as wave height. The ‘Response-Based Design Applied to FPSOs’ JIP addressed all of these issues, and aimed to provide a technique that is both rational and practical, and which in principle can be applied to other forms of offshore production systems.