Hydrodynamic Aspects of Turret-Moored FPSOs
TL;DR: The paper reviews the literature on some of the hydrodynamic aspects concerning motion response of turret-moored FPSOs and considers special consideration for the representation of realistic non-collinear environments.
Abstract: As oil and gas exploration moves toward deeper, harsher waters, and marginal fields, the issues concerning the associated floating production storage and offloading ships (FPSOs) get more complex. Traditionally, at the early stage of the design of FPSOs, subsystem design is individually taken up and subsystem interactions are accounted for through an uncoupled approach, as against a more rigorous fully coupled approach. In fully coupled approach, the system components and mutual interactions are coupled concurrently. Issues related to mooring system of FPSOs are some of the key aspects needing close attention. Approaches for determining the peak behavior belonging to turret-moored FPSOs to random wind, seawater flow, and wave forces for a specified life are still evolving. In the case of weathervaning FPSOs, special consideration is essential for the representation of realistic non-collinear environments. Also, choice of location of turret inside hull is an important design decision, as turret position influences motions. Similarly, another aspect requiring attention is the development of resonant sloshing due to excitation owing to external waves. Also combined piston modes happen within the turret and in the volume between turret and moon pool walls, which is an important hydrodynamic phenomenon. The flow parting via chain table openings heavily damps the piston mode. The paper reviews the literature on some of these hydrodynamic aspects concerning motion response of turret-moored FPSOs.
TL;DR: In this paper , the optimal configuration of the mooring lines and the location of the turret were determined to identify the preferred location of a turret, and the responses of the structure to combined co-directional and misaligned wind and wave loads were computed.
Abstract: Abstract A multi-unit floating offshore wind turbine concept, the wind-tracing floating offshore wind turbine, is introduced. In this concept, the floating structure is a triangular platform that hosts three 5 MW wind turbines and is moored to the seabed with a turret-bearing mooring system. This mooring system allows the structure to rotate about the turret such that the total yaw moment by the environmental load on the turret is minimized. In this study, the optimum properties of the mooring lines and the location of the turret are determined. To identify the preferred location of the turret, the responses of the structure to combined co-directional and misaligned wind and wave loads are computed. The motions of the structure are obtained with a frequency-domain numerical model integrated with structural finite-element method for hydroelastic and aeroelastic analyses. The hydrodynamic and aerodynamic loads are obtained by wave diffraction theory and steady blade element momentum method, respectively. Finally, with the optimum configuration of the mooring system, the motion and aero- and hydroelastic responses of the fully flexible wind-tracing floating offshore wind turbines to combined waves and wind loads are determined and discussed.
•20 Jul 1982
TL;DR: In this article, the authors considered rectangular moonpools of large horizontal dimensions and determined the natural modes of oscillation of the inner free surfaces under the assumption of infinite water depth and infinite length and beam of the barges that contain the moonpool.
Abstract: So-called ‘moonpools’ are vertical openings through the deck and hull of ships or barges, used for marine and offshore operations, such as pipe laying or recovery of divers. In the present study rectangular moonpools of large horizontal dimensions are considered. The natural modes of oscillation of the inner free surfaces are determined, under the assumption of infinite water depth and infinite length and beam of the barges that contain the moonpools. The problem is treated in two and three dimensions, via linearized potential flow theory. Results are given for the natural frequencies and the associated shapes of the free surface, for wide ranges of the geometric parameters. Simple quasi-analytical approximations are derived that yield the natural frequencies. The most striking result is that the natural frequencies of the longitudinal sloshing modes increase without bounds when both the draught and the width decrease to zero, the length of the moonpool being kept constant. As a corollary the problem of waves travelling in a channel through a rigid ice sheet is addressed and their dispersion equation is derived. The same behaviour is obtained: the waves travel increasingly faster as both the draught and the width of the channel are reduced.
TL;DR: In this article, a vessel/mooring/riser coupled dynamic analysis program in time domain is developed for the global motion simulation of a turret-moored, tanker based FPSO designed for 6000-ft water depth.
Abstract: A vessel/mooring/riser coupled dynamic analysis program in time domain is developed for the global motion simulation of a turret-moored, tanker based FPSO designed for 6000-ft water depth. The vessel and line dynamics are solved simultaneously in a combined matrix for the given environmental and boundary conditions. The vessel global motions and mooring tension are tested at the OTRC wave basin for the non-parallel wind–wave–current 100-year hurricane condition in the Gulf of Mexico. The same case is also numerically simulated using the developed coupled dynamic analysis program. The numerical results are compared with the OTRC 1:60 model-testing results with truncated mooring system. The system's stiffness and line tension as well as natural periods and damping obtained from the OTRC measurement reasonably match with numerically simulated static-offset and free-decay tests. The numerically predicted global vessel motions are also in good agreement with the measurements. It is underscored that the dynamic mooring tension can be underestimated when truncated mooring system is used.
TL;DR: In this article, a turret-moored ship operating in 150 m, 330 m and 2000 m water depth is compared with a coupled analysis procedure where floater motions and mooring and riser dynamics are calculated simultaneously.
Abstract: Traditional global response analyses of moored floating structures are calculated in two separated steps: calculations of floater motions; and dynamic response analysis of moorings and risers using the top end motions estimated in the first step. Typical shortcomings in the traditional separated approach are neglection or simplification of current forces and low frequency damping contribution from moorings and risers. In a coupled analysis procedure where floater motions and mooring and riser dynamics are calculated simultaneously, these drawbacks are avoided. Motions and mooring line tensions from model tests and simulations using coupled and separated analysis procedures are compared. Illustrations are given by extensive case studies of a turret-moored ship operating in 150 m, 330 m and 2000 m water depth. The main conclusions are that the traditional separated approach may be severely inaccurate, especially for floating structures operating in deep waters. Coupled analysis should be applied for deep water concepts, at least as a check of important design cases. The agreement between model test results and results from coupled analysis is very good.
TL;DR: In this article, both time and frequency domain models of a coupled vessel/riser/mooring system are developed, which each incorporate both first and second order motions on the vessel, and it is shown that the frequency domain approach yields very good predictions of the system response when benchmarked against the time domain analysis.
Abstract: The dynamic analysis of a deepwater floating structure is complicated by the fact that there can be significant coupling between the dynamics of the floating vessel and the attached risers and mooring lines. Furthermore, there are significant nonlinear effects, such as geometric nonlinearities, drag forces, and second order (slow drift) forces on the vessel, and for this reason the governing equations of motion are normally solved in the time domain. This approach is computationally intensive, and the aim of the present work is to develop and validate a more efficient linearized frequency domain approach. To this end, both time and frequency domain models of a coupled vessel/riser/mooring system are developed, which each incorporate both first and second order motions. It is shown that the frequency domain approach yields very good predictions of the system response when benchmarked against the time domain analysis, and the reasons for this are discussed. It is found that the linearization scheme employed for the drag forces on the risers and mooring lines yields a very good estimate of the resulting contribution to slow drift damping.
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