• Ocean Systems Engineering
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• v.6 no.3
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• pp.265-287
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• 2016

The change of the global performance of a turret-moored FPSO (Floating Production Storage Offloading) with DP (Dynamic Positioning) control is simulated, analyzed, and compared for two different internal turret location cases; bow and midship. Both collinear and non-collinear 100-yr GOM (Gulf of Mexico) storm environments and three cases (mooring-only, with DP position control, with DP position+heading control) are considered. The horizontal trajectory, 6DOF (degree of freedom) motions, fairlead mooring and riser tension, and fuel consumptions are compared. The PID (Proportional-Integral-Derivative) controller based on LQR (linear quadratic regulator) theory and the thrust-allocation algorithm which is based on the penalty optimization theory are implemented in the fully-coupled time-domain hull-mooring-riser-DP simulation program. Both in collinear and non-collinear 100-yr WWC (wind-wave-current) environments, the advantage of mid-ship turret is demonstrated by the significant reduction in heave at the turret location due to the minimal coupling with pitch mode, which is beneficial to mooring and riser design. However, in the non-collinear WWC environment, the mid-turret case exhibits unfavorable weathervaning characteristics, which can be reduced by employing DP position and heading controls as demonstrated in the present case studies. The present study also reveals the plausible cause of the failure of mid-turret Gryphon Alpha FPSO in milder environment than its survival condition.

• Journal of Ocean Engineering and Technology
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• v.29 no.2
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• pp.143-153
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• 2015

This paper presents the results of an experimental study on the motion of an FPSO (Floating production storage and off-loading) and the characteristics of the mooring systejavascript:confirm_mark('abe', '1');m according to the turret position. Model tests of a turret-moored FPSO were carried out in the Ocean Engineering basin of KRISO. The FPSO was moored using an internal turret and catenary mooring. The models (1/60 scale) that were prepared included the FPSO, turret, and mooring lines. The experiments were conducted in irregular waves and combined environments, with waves, currents, and winds. A time-domain simulation was performed using OrcaFlex. The motion response and mooring line tension from the present calculations were compared with the results of experiments, and the agreement was fairly good. In addition, the results showed that the weathervaning stability was improved when the position of the turret was moved in the bow direction.

• Journal of the Society of Naval Architects of Korea
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• v.52 no.5
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• pp.418-424
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• 2015

This paper is about the bow structure design of the ship-typed and turret moored FPSO which is subjected to the bow-flare slamming load in harsh North Sea environments. Quad 204 FPSO project involves the redevelopment of the existing Schiehallion FPSO which is damaged by impact wave loads. Normally all offshore systems including FPSO are designed to withstand the 100 year storm I.e. the storm that happens once every hundred years at the location where the system is installed. Several incidents have revealed that impact loading is important issue for moored floating production systems. In this paper, the design impact loads are estimated considering the ship owner’s specification, measured data from model tests, requirements of the classification society rules and results of numerical simulation analyses. The impact pressure by numerical analysis is 1.8 times greater than required value by CSR adopted by IACS. Based on the selected design load, plastic design formulae allowing the local material yielding are applied for the initial scantling of the bow structure. To verify the structural integrity, FE analyses are carried out considering the local area subjected to the impact wave loads. Their results such as structural arrangement, design loads and scantlings are shown and discussed. It is found that plastic design formulae in adopting Initial design phase give sufficiently conservative results in terms of structural strength.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.1
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• pp.8-14
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• 2004

As the number of offshore structure is glowing in deep waters, there have been increased damages of it. These floating structures in offshore locations exposed to harsh environmental conditions. In recent years, there has been a slowing attention around damages on bow and deck on FPSO caused by waves in steep storm condition. This paper describes a study of the water on deck due to the dynamic behavior of a FPSO with turret mooring system. The nonlinear motions of the FPSO are simulated under external forces due to wave, current, wind, and mooring forces in the time domain. The direct integration method is employed to estimate low frequency drift wave forces. The current forces are calculated by using slow motion maneuvering equations in the horizontal plane. The coefficients of a model for wind forces are calculated from Isherwood's experimental data and the variation of wind speed is estimated by wind spectrum according to the guidelines of API-RP2A.