Summary
Objective
A tanker-based FPSO can be exposed to a Hurricane, in which the wind direction is continuously changing and waves and loop currents are not collinear. For the safety of FPSO in such a survival condition, it is very important to predict accurately the extreme response and the maximum mooring tension during the storm. For non-collinear environments, it is not straightforward to find the equilibrium position of a tanker and the steady-state response with respect to it. The existing hydrodynamics computer programs based on perturbation approach are not directly applicable to this kind of application including large yaw motions. The drag force estimation on the tanker hull is usually based on empiricism, and thus needs to be calibrated against experimental data. If the hull hydrodynamics is well established, a time-domain hull/mooring/riser coupled dynamic analysis program can be used to compare the performance of various FPSO designs. In particular, it is important to compare the performance for different sets of mooring system including turret positions. A series of numerical simulations will be conducted for a particular FPSO for 3 different turret positions and various wave-wind-current conditions. The numerical results will be compared with the planned OTRC experiments. If the computer programs successfully predict FPSO motions and mooring-line tensions, they can be used repeatedly for future FPSO designs and experiments. Using the computer program, a comprehensive case study can be made in the future to develop a useful guideline for industry and MMS.
Approach
In conventional perturbation-based approach for platform-motion calculations, all the hydrodynamic quantities are calculated with respect to the mean position. If the relevant yaw angle is large, such an approach cannot be used since the wave loading can significantly change depending on wave-current headings. The turret-moored tanker-based FPSO is free to yaw with respect to the turret position and belongs to the case where the conventional perturbation approach cannot be directly applicable. To accurately calculate the wave loading at each instantaneous yaw angle, a fully-nonlinear numerical wave tank (NWT) simulation may be used. However, the NWT is still computationally very intensive and has to be further developed to be useful in practical problems. In this project, we propose an alternative numerical method, which is computationally less intensive compared to the direct NWT simulation but still useful for practical applications.
When large yaw motions occur, all the hydrodynamic quantities for various yaw angles can be calculated in advance (using a 3D boundary element code such as WAMIT or THOBEM) and stored, and then the data can be used for the wave loading calculation at each instantaneous FPSO yaw angle. For this, the wave and current loading need to be precalculated, for instance, at 5-degree interval with respect to the equilibrium position. The prediction of the equilibrium position of a FPSO in non-collinear environment is not an easy task. In particular, there exists no elegant theory for the estimation of the drag force on FPSO hulls by waves and currents with arbitrary heading angles. Therefore, the drag force calculation has to be based on empirical formulas and needs to be calibrated against well-established lab data.
Several researchers have used ship-maneuvering theory in FPSO motion simulations in time domain with many empirical hydrodynamic coefficients. However, the approach itself has many uncertainties with respect to the selection of hydrodynamic coefficients. The present method calculates all the hull hydrodynamics and wave loading as accurate as possible up to second order using 3D diffraction/radiation computer programs, such as WAMIT or THOBEM. The THOBEM is a hydrodynamics program based on higher-order boundary element methods in time domain and can calculate wave drift damping in addition to all the hydrodynamic quantities of WAMIT.
After the hydrodynamics of a FPSO hull is verified, the hull hydrodynamics program can be coupled with a existing mooring/riser dynamic analysis program WINPOST to simulate the responses of the hull/mooring/riser integrated system in the time domain. The time-domain simulations will be conducted for a particular turret-moored FPSO for three different turret positions and various environmental conditions, as planned in the proposed OTRC experimental program. The responses and stability of the FPSO for different turret positions will be analyzed through a series of numerical simulations and in comparison to experimental data.
Project Plan
Scope of Work:
The current hull hydrodynamic analysis program has to be modified to include the effects of time-dependent possibly large yaw angles. The hydrodynamic loading will be calculated for various yaw angles using WAMIT or THOBEM. The data will be stored and used in the subsequent time-domain hull/mooring/riser coupled dynamic analysis. The current coupled dynamic analysis has to be modified accordingly. A module for the prediction of static equilibrium position will be developed. The estimation of drag forces for various yaw angles has to be investigated as well. After the code is developed, the numerical results for a turret-moored FPSO will be compared with the experiment planned to be conducted in the OTRC 3D wave basin in the near future.
Anticipated Results
A software “hull/mooring/riser coupled dynamic analysis in time domain” will be fully developed to analyze the design and performance of future FPSOs. A report summarizing the comparison between numerical prediction and measurement for several turret-moored FPSO designs will be available. The behavior of hulls and mooring lines for a particular design will be demonstrated using computer graphics animation.
Related Publications
Ran, Z., Kim, M.H., and Zheng, W., “Coupled dynamic analysis of a moored spar in random waves and currents (time-domain vs. frequency-domain analysis)” Journal of Offshore Mechanics and Arctic Engineering, Vol.121, 194-200, 1999
Kim, M.H., Ward, E.G., & Haring, R., “Comparison of Numerical Models for the Capability of hull/mooring/riser coupled dynamic analysis for spars and TLPs in deep and ultra deep waters”, Proc. 11th International Offshore and Polar Engineering Conference, Stavanger, 2001
Kim, M.H., Arcandra, Y.B. Kim, “Variability of spar motion analysis against design methodologies/parameters” Proc. Offshore Mechanics and Arctic Engineering, OMAE’01, Rio De Janeiro, 2001
Kim, M.H., Arcandra, Y.B. Kim, “Variability of TLP motion analysis against design methodologies/parameters” Proc. 11th International Offshore and Polar Engineering Conference, Stavanger, 2001
Kim, M.H., Ran. Z., and Zheng, W., “Hull/mooring coupled dynamic analysis of a truss spar in time domain”, Journal of Offshore and Polar Engineering, Vol.11, No.1, 42-54, 2001
