1999-2001 OTRC Project: Dynamic Analysis Tool for Moored Tanker-based FPSOs Including Large Yaw Motions
OBJECTIVES:
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.ANTICIPATED PROJECT DURATION: 2 years
PROJECT PLAN FOR 1999/2000:
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.
PROJECT PLAN FOR FUTURE YEARS (2000/2001):
SCOPE OF WORK: Conduct a comprehensive parametric study for a turret-moored FPSO. Select a model spread-mooring FPSO, and study its performance for various design parameters. Develop a useful guideline for industry and MMS. Extend the postprocessor to analyze polyester mooring line. Continue to validate and refine the FPSO-analysis program.
ANTICIPATED RESULTS: A report of parametric study, useful guideline for FPSO design, an updated FPSO-analysis computer program which is planned to be licensed through OTRC in the future.
PRINCIPAL INVESTIGATORS & OTHERS INVOLVED: M. H. Kim (P.I.), as well as Graduate Students and Researchers involved in relevant OTRC-experiment program
OTRC PROJECT STATUS REPORT
Date: December 1, 2001
Project Name: Development of Coupled Dynamic Analysis Tools for FPSOs Including Large Yaw Motions: FPSO & Shuttle Tanker Offloading
Task Order: 16181 Project Number: 5887G and continued under 5888G
Principal Investigator: “Joseph” M. H. Kim
Estimated Completion Date: Oct. 2002
Project Description:
During the past several years, PI has developed a powerful software called WINPOST for hull/mooring/riser nonlinear coupled dynamic analysis in winds, waves, and currents in time domain. The program has been extensively tested and verified against experimental results for various kinds of classic and truss spars and TLPs. Last year, the WINPOST was designated by DeepStar Offshore Industry Consortium to produce benchmark simulation results for spars and TLPs for 3 different water depths. Recently, the WINPOST has been successfully extended to a newer version to include the effects of large yaw motions of turret-moored FPSOs. The preliminary results tested for a DeepStar FPSO look very promising. More extensive comparison as well as calibration/sensitivity study will be made in the coming months against MARIN’s and OTRC’s FPSO experiments. The WINPOST program will also be extended to solve FPSO & shuttle tanker interactions and stability. The two-body interaction results will be compared with OTRC’s FPSO/shuttle-tanker experiment. We also plan to develop an algorithm for DP-assisted FPSO simulations.
Progress:
In case of turret-moored FPSO, the yaw response can be large and the wind, wave, and current forces can change significantly depending on the instantaneous yaw angle. The hydrodynamic coefficients, such as added mass and radiation damping, may also change as the yaw angle changes. Therefore, for reasonable simulations of turret-moored FPSOs with large yaw motions, the wave, wind, and current forces need to be precalculated for each yaw-angle interval. The yaw-angle dependent wind and current forces on FPSO can, for example, be based on existing empirical data, such as OCIMF (Oil Company International Marine Forum) data.
So far, most of the proposed tasks are completed and the following features are now added to the WINPOST for turret-moored FPSOs
· Inclusion of yaw-angle dependent current forces read from table, such as OCIMF data.
· Inclusion of yaw-angle dependent wind drag coefficients read from table, such as OCIMF data, combined with wind dynamic loading calculation module.
· Inclusion of new modules for wave-frequency, sum- and difference-frequency second-order wave forces (also other necessary hydrodynamic coefficients) precalculated and tabulated for a series of yaw-angle intervals.
· Inclusion of empirical hull viscous drag as plate modelIn addition, the mooring analysis program has been extended to a new version, which can include the effects of large elongation and nonlinear stress-strain relationship. The new computer program was applied to the analysis of polyester lines and the results look reasonable and very promising.
As a case study, we have conducted the hull/mooring/riser coupled simulations for the Deepstar 3000-ft turret-moored FPSO with 12 chain-wire-chain mooring lines and 13 steel catenary risers, and the preliminary results look very reasonable.
Reports and Publications:
Ph.D. Thesis: Ran, Z., “Coupled dynamic analysis of floating structures in waves and currents”, December, 2000
Ph.D. Thesis: Arcandra Tahar, “Hull/mooring/riser coupled dynamic analysis with polyester lines” (August, 2001)
M.S. Thesis: Zheng, W., “Coupled dynamic analysis of a truss spar”, August, 2000.
Refereed Journal Papers:
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
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
Ran, Z. and Kim, M.H., “Nonlinear coupled responses of a tethered spar platform in waves” Journal of Offshore and Polar Engineering, Vol.7, No.2, 111-118, 1997
Refereed Conference Papers:
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
Papers In Preparation: Hull/mooring/riser coupled dynamic analysis of a turret-moored FPSO in winds, waves, and currents (will be submitted to J. of Ship Research)
Hull/mooring/riser coupled dynamic analysis with polyester lines (will be submitted to J. of ASCE Engineering Mechanics)