To account for the viscous effects of damping devices, for instance, bilge keels or bilge boxes, on the motions of ships and offshore structures, Morison’s equation is often adopted as an empirical but practical approach in the design process. In order to combine the standard engineering panel method with the drag term in Morison’s equation, and remain in the frequency domain, the drag term has to be linearized based on, for instance, stochastic linearization. In this paper, the stochastic linearization scheme is implemented in an in-house code and verified through the comparison with the DNV GL software WADAM. The model test results of a large cylindrical FPSO with bilge box are used to calibrate the drag coefficients in the Morison’s equation. When the linearized drag forces are included, heave motion RAOs correspond better to the model test results. However, the predicted natural periods of heave motions are seen to be smaller than those obtained from model tests. It is suspected that the viscous flow separation around the bilge box increases the added mass of the unit beyond what is predicted by potential flow alone. Discussions are made on the effect of viscous added mass on the heave natural period. It is quite common to only include the damping effects in the motion analysis for large offshore structures and ignore the contribution of the viscous effects on the excitation force. For the considered cylindrical FPSO, this paper demonstrates that the viscous excitation force can be important in survival conditions.
The present paper presents a fatigue life prediction method for chains subjected to tension and Out-of-Plane-Bending (OPB). The investigation was carried out on a high strength mooring chain segment with a diameter of 165 mm and a steel quality R5. Such chains are used in mooring systems for large Floating Production Storage and Offloading (FPSO) units in harsh weather condition. Due to the fact that the mooring chains are pre-tensioned the wave induced displacements will introduce secondary bending effects about the weak axis in the chain links, particularly in the bilge hang-off area. In such chains both conventional tension fatigue and fatigue damage accumulation due to Out-of-Plane-Bending (OPB) have to be analyzed. Results from full scale behavior tests, Finite Element Analyses and a case study with simulation of in-service loading are included in the present study. Finally, fatigue life predictions and an operational strategy are presented for a case study for a floater in the Barents Sea. With a target service life of 30 years, the endurance with respect to fatigue damage is a major design criterion for the mooring chain in this case.
To address the challenges of Gulf of Mexico (GoM) Lower Tertiary exploitation, alternative FPSO concepts are studied by the industry. This paper documents Phase 2 of a UDW RPSEA study which had as its main objective the validation of a production concept based on a permanently moored cylindrical FPSO designed by Sevan Marine with steel catenary risers (SCR). The present work focuses on model testing of the cylindrical FPSO hull design, performed at the Offshore Technology Research Center (OTRC) at Texas A&M University in College Station, in May 2016. The experiments were performed at a scale of 1:64. The model was subjected to the Central GoM Hurricane environmental conditions. The process of analysis includes extreme hurricane waves, wind, and currents, combined with ultra deep water, to demonstrate the feasibility of the concept in the GoM. This combination implicates complex hydrodynamic behavior of the hull, mooring and riser system. Thus, the model test provides better understanding of the new concept performance. Model test results, including motions and mooring loads, are compared with numerical predictions for a series of test cases. The importance of viscous effects on motions of the cylindrical unit and of the green water effect is discussed. Finally, to assess the feasibility of the hull and riser system, the numerical results of a hybrid approach, full scale, coupled riser analysis are presented, as based on measured hull motions and environmental conditions. This is finally followed by a discussion of the implications of technical and regulatory aspects of the FPSO concept. Following the project criteria, the present system based on SCR configuration seems feasible; however optimization of hull and mooring system is necessary to improve green water and viscous effects on hull motions in extreme current. The paper represents one sub-scope of the RPSEA study "Low Cost Flexible Production System for Remote Ultra-Deepwater Gulf of Mexico Field Development.", Project No. 10121-4404-03.
This paper presents a method to account for nonlinear hydrostatic restoring force/moments on vessels of arbitrary hull shape. The method calculates the time-dependent hydrostatic load on a vessel based on the direct numerical integration of the hydrostatic pressure over the instantaneous “wetted” portion of the vessel’s hull surface. The 6-DOF equation of motion with the nonlinear hydrostatic load (replacing the linear hydrostatic load) is solved. The effects of the hydrostatic nonlinearity are investigated for a few practical hull shapes (including a cylindrical SPAR, a conical BUOY, and a VLCC-converted FPSO). The nonlinear hydrostatic loads are compared with the corresponding linear ones and the importance of the hydrostatic nonlinearity and couplings on the vessel motions is assessed.
This paper presents the applications of an efficient hybrid time-domain simulation model for predicting moored Sevan-floater motions in irregular waves and finite water depth. The irregular incident waves are modeled by the extended Boussinesq equations, which can capture wave-wave interactions and the low-frequency long waves accurately in finite and shallow water depth. By imposing the incident wave kinematics on the surface of the floater, a panel model based on Rankine source method is applied for the calculation of wave forces and corresponding floater motions. The contributions from low-frequency components in incident waves as well as their diffraction effects are included in the wave force calculations. Validation of the irregular waves simulated by the present numerical model are performed against experimental data. Then, the simulated moored floater motions are compared with model test results and results based on Newman’s approximation. The general good agreements with experimental results demonstrate the present model can be used as an alternative for this problem while Newman’s approximation shows non-conservative results.
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