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.
A deep surface-piercing wedge-ended hull model was towed through still water. Measurements of the surface wave pattern confirmed earlier findings for ship models, that the measured bow-wave cusp line often lies well forward of the position predicted by thin-ship theory, and that this shift increases with bow water-line angle and with decreasing model speed. Two possible explanations are considered here in terms of changes of wave phase speed with wave convection and steepness. Calculations based on a transformation method due to Guilloton predict more realistic wave profiles than linear theory, but account for less than half the observed shift. Some tentative conclusions are drawn.The singularity in the Green's function double integral is removed by an improved method, which simplifies the numerical integration. The new integrand decays within one oscillation.
This paper describes key findings from Phase 1 of the Deepwater Installation of Subsea Hardware (DISH) JIP. The objective of DISH Phase 1 was to identify key gaps in the offshore industry's technology for installing subsea hardware in water depths beyond 2,000m, by comparing present-day capabilities of the installation industry with likely deepwater installation requirements of oil and gas operators over the next 10 years. Technology gaps were identified by interviewing engineering and installation contractors, oil and gas operators and specialist suppliers; by carrying out a literature review study; and by holding a Phase 1 Mid-Flight Workshop to identify and prioritise the key gaps. The results were further refined before finalising the Phase 2 work programme. A review of the capabilities of wire rope lifting systems showed that self-weight will make conventional wire rope systems inefficient for water depths in the range 2,000m to 3,500m, and impractical on most installation vessels. The industry will therefore have to turn increasingly to deepwater fibre rope deployment systems. Key challenges are to establish the industry's confidence in fibre rope deployment systems, and to provide key information about the engineering properties of man-made fibre ropes and of the loading on such systems. Lack of knowledge of fibre rope behaviour was considered to be a fundamental, show stopper', which will inhibit the adoption of fibre rope deployment systems for ultra-deep water installation. DISH Phase 1 is now completed, and Phase 2 was launched in January 2002, based on the priority technology gaps and challenges identified during Phase 1. Background The DISH JIP was instigated following a pan-industry Workshop held in November 2000. At that time hydrocarbon developments were being planned in water depths close to 2,000m, and deeper fields were already being considered. Speakers from major installation contractors and BP believed, however, that established techniques for lowering and installing heavy items on the sea floor may either prove impractical or uneconomic in water depths beyond 2,000m, such as in emerging areas of the Gulf of Mexico. Furthermore, the deepest fields so far developed have been in relatively benign ocean environments, and the installation methods used to date are not necessarily transferable to harsher environments. A number of technical advances will therefore be needed to make some deepwater developments economic and practical. These issues were discussed further in a paper presented at a SNAME Workshop in February 2001 [1], which summarised possible technology gaps in the areas of lifting and lowering technology, load control and positioning, metocean effects and weather window requirements. General Approach DISH Phase 1 aimed to identify key technology gaps by comparing present-day capabilities of the installation industry with likely deepwater installation requirements of oil and gas operators over the next 10 years. The goal of achieving a common understanding spanning operators, contractors and suppliers worldwide, across the whole industry, was considered to be particularly important.
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