Analysis and Design of Mooring and Turret Systems for Ship-shaped Floating Production Systems (FPSOs)
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- Institutt for marin teknikk 
During the past years, the requirements for mooring and station keeping systems of mobile and permanent units have become more complex. The oil and gas industry is moving into deeper and colder regions in search of new oil and gas fields, and complex station keeping systems must be designed to withstand the new environmental challenges. There are getting more and more marine operations adjacent to other installations, thus it is very important to restrict the top end motion of moored floating structures by providing sufficient line restoring forces. However, line failures are still an existing problem of moored floating structures, and thus it is of interest to study the line tensions and top end motions of floating structures in extreme weather conditions. A literature study of the different permanent and mobile keeping systems, and their hardware components is conducted in order to build a fundamental knowledge about mooring. When designing permanent station keeping systems there are three design limit states that must be satisfied, and these are the ultimate, accidental and fatigue limit state. Ultimate and accidental limit state analyzes are conducted in this thesis. Time domain simulations of a turret moored FPSO are performed in this thesis, and a literature study of the time domain analysis methods is conducted. A time domain analysis captures the environmental forces and describes the behaviour of the vessel, mooring lines and risers (if any). How well their behavior is described is dependent on the approach and there are two approaches of performing a time domain analysis: separated and coupled approach. The software Sima is used to perform the time domain analyzes, however there are two features within Sima that performs the analyzes: Simo and Riflex. A literature review of these programs and their simplifications are conducted. The FPSO's RAOs show that the most critical response is in roll with a maximum response of 9.9 deg/m. It is thus very important that the vessel is able to weathervane up against the weather such that roll motions is prevented. Sway-yaw stability calculations are performed for different distances between the center of gravity and the turret to investigate the weathervaning ability of the vessel. In this master's thesis, quasi-static and dynamic simulations of a turret moored FPSO are performed in the software Sima in ULS and ALS during co-linear and spread environments. Before performing the ULS and ALS simulations, a surge decay test is conducted to investigate if the surge natural period is the same between the system in Simo and Simo-Riflex. Also a system characteristic test is conducted to investigate if there exist any stiffness differences between Simo and Simo-Riflex. A convergence test of the line tension's standard deviation is conducted to investigate how many simulations are necessary before the standard deviation becomes stable. The number of simulations performed in the ULS and ALS analyzes are based on the convergence test. The system's responses studied in this thesis are the vessel's six degree of freedom motions, mooring line tension responses and the turret loads. The characteristic most probable maximum line tensions have been found through the Gumbel extreme distribution, and is compared with the line breaking strength according to the requirements for permanent mooring units on the Norwegian Continental Shelf. The turret design loads have been calculated, and turret uplift is investigated in ULS and ALS during co-linear and spread environmental conditions. The motion and line tension response is compared between system in Simo and in Simo-Riflex to study the difference between them.