Deepwater Mooring Analysis: Synthetic Line
Objective: To reduce the weight of mooring lines and the footprint of a mooring system, synthetic mooring lines, such as made of polyester, are being deployed in deep water. Because the behaviors of polyester mooring lines are quite different from those of steel ones, this project intends to extend CABLE3D, a numerical code originally developed for dynamic analysis of steel mooring lines and risers, to allow for tension-dependent modulus and hysteresis loop in simulating synthetic mooring lines. The extended code will be used to simulate interaction between a FPS and its synthetic mooring-line system.
Objective: 1) Improving CABLE3D to make it more efficient in numerical simulation of mooring lines. 2) To develop a numerical scheme to allow for a direct simulation of polyester ropes when the modulus depends the on mean tension and time-dependent hysteresis loop.
Potential Impact: the extended and improved CABLE3D will be used to study dynamic interactions between a FPS and its synthetic mooring system. Further, it will be used to study the interaction between synthetic mooring lines and related innovative anchors/foundation.Approach:
1. Empirical formulation proposed by Del Vecchio (1992) for the Young’s modulus of a polyester rope is used in the simulation. The modulus depending on mean and dynamic loads is determined through iteration.
2. It is known that energy dissipation per cycle is less dependent of the frequency of cycles. To simulate polyester ropes in the time domain whose modulus depends on time-dependent amplitudes of cyclical loads, a new approach employing the Hibert transform of the deformation in a polyester rope will be incorporated in the numerical scheme. The simulated results will be compared with the approach based on the empirical formulation by Del Vecchio (1992).
3. Using the extended and improved CABLE3D to study the dynamic interactions between a FPS and its polyester mooring system and between the mooring system and its anchors/foundation.
Development of Results:
The numerical scheme for simulating synthetic mooring lines has been developed and is being used in the study and design of a moored deep-water FPS, say a SPAR.Anticipated Project Duration: 3 years (FY 2002-2005)
Plan for FY 2004-2005:
1) Develop a numerical scheme based on the Hilbert transform technique for simulating the hystersis of a polyester rope in the time domain.
2) Continue to improve numerical efficiency of CABLE3D.
3) Comparing numerical simulations with the corresponding measurements.Principal Investigator
PI: Jun Zhang.
Others: Mr. Y. Ding and Mr. Shaosong Zhang (Research Assistants)
Date: December 2005.Project Name: Deepwater Mooring Analysis: Synthetic Line
Project Number: 32518/1510C
Principal Investigators: Jun Zhang
Projected Terminated Date: Aug. 31, 2005
Project Description: In addition to the light wet-weight and taut-leg configuration, a polyester rope yields relatively large elongation under tension, possesses a nonlinear stress-strain relation and dissipates energy under cyclical loads. In this project we modify a numerical scheme previously developed for steel mooring lines so that it considers these special properties of polyester ropes and can more accurately predict the interaction between a hybrid polyester mooring system and an offshore floating structure under severe met-ocean environments. Based on the numerical simulation of hybrid polyester mooring lines, a parametric study of determining the pretensions and angle of mooring lines with the mud line is conducted for the purposes of designing an optimal hybrid polyester mooring system in deep water.
Progress:
1) A numerical code CABLE3D, originally developed for steel wires and chains, has been modified and extended to allow for large elongation of a polyester rope and the Young’s modulus depending on tensions. The extended CABLE3D considers the effects of static load as well as the amplitude of dynamic load. Since the amplitude of dynamic tension is not known in advance, an ad hoc iterative process has been developed for the numerical simulation.
2) By considering large elongation and/or energy dissipation resulting from hysteresis loops in polyester material, the longitudinal resonant vibration occurring in numerical simulation has been eliminated.3) The extended program has been integrated into COUPLE to simulate the global responses of a SPAR interacting with a polyester mooring system. Our main findings are: a) in comparison with the computation based on the assumption of neglecting the elongation in polyester ropes, the results based on our extended CABLE3D indicate a softer offset curve of a polyester mooring system, and the reduction in the restoring force increases with the increase in water depth, that is, the length a polyester rope (for example, the reduction can be as large as 10% in 6,000 ft water and 20% in 10,000 ft water); b) the effects of the mean loads on the modulus of polyester ropes are much greater than those of the dynamic loads, and c) the energy dissipation in polyester ropes under cyclic loading does not play significant roles in the responses of the Spar and tensions in the polyester mooring systems.
4) A parametric study is being conducted regarding the effects of the pretension in a hybrid polyester mooring system on the global motion of a floater and maximum tension in the mooring system.
5) A case study is being conducted on the fatigue life of polyester ropes interacting with SPARs.
Reports & Publications:
Minsuk Kim, Yu Ding and Jun Zhang (2003) ‘Dynamic Simulation of Polyester Mooring Lines’, Proceedings of International symposium on deep-water mooring system, Houston, TX, 2003, p101-114.
Minsuk Kim, (2004). ‘Dynamic Simulation of the Responses of A SPAR with A Polyester Mooring System Under Severe Ocean Environment’, MS thesis, Texas A&M University.
Minsuk Kim, Yu Ding and Jun Zhang (2004) ‘Numerical Simulation of A SPAR Interacting with A Polyester Mooring System’, Proceedings of OMAE. 2004, paper No. 51222.
Yu Ding, B. Theckum purath, Jun Zhang, R. B. Gilbert, S. Dangyach and Y.J. Choi “Reliability of Mooring Systems for A Spar”, Proceedings of OMAE 2005, paper No. 67290.
Date: December 2004Project Name: Deepwater Mooring Analysis: Synthetic Line
Project Number: 32518/1510C
Principal Investigators: Jun Zhang
Estimated Completion Date: Aug. 31, 2005
Project Description: Owing to the properties of light wet-weight and high strength to weight ratios, synthetic ropes, such as polyester ropes, are replacing steel wires in a mooring system, especially deployed in deep seas. To simulate and design a polyester mooring system, it is crucial to understand its differences from a traditional steel mooring system. In addition to the light wet-weight and taut-leg configuration, a polyester rope yields relatively large elongation under tension, possesses a nonlinear stress-strain relation and dissipates energy under cyclical loads. In this project we modify a numerical scheme previously developed for steel mooring lines so that it considers these special properties of polyester ropes and can more accurately predict the interaction between a hybrid polyester mooring system and an offshore floating structure under severe met-ocean environments.
Progress:
1) A numerical code CABLE3D, originally developed for steel wires and chains, has been modified and extended to allow for large elongation of a polyester rope and the Young’s modulus depending on tensions. The extended CABLE3D considers the effects of static load as well as the amplitude of dynamic load. Since the amplitude of dynamic tension is not known in advance, an ad hoc iterative process is developed for the numerical simulation.
2) It was found that longitudinal resonant vibrations might occur in the numerical simulation of polyester ropes under large tensions and they resulted in unrealistic large (high frequency) tensions. By considering large elongation and/or energy dissipation resulting from hysteresis loops in polyester material, the longitudinal resonant vibration occurring in numerical simulation can be eliminated or suppressed.3) The extended program has been integrated into COUPLE to simulate the global responses of a SPAR interacting with a polyester mooring system. Our main findings are: a) in comparison with the computation based on the assumption of neglecting the elongation in polyester ropes, the results based on our extended CABLE3D indicate a softer offset curve for a polyester mooring system, and the reduction in the restoring force increases with the increase in water depth or the length a polyester rope (for example, the reduction can be as large as 15% in 10,000 ft water); b) the effects of the mean loads on the modulus of polyester ropes are much greater than those of the dynamic loads, and c) the energy dissipation in polyester ropes under cyclic loading does not play significant roles in the responses of the Spar and tensions in the polyester mooring systems.
4) Initiated development of a procedure to estimate the fatigue life of polyester ropes.
Reports & Publications:
Minsuk Kim, Yu Ding and Jun Zhang (2003) ‘Dynamic Simulation of Polyester Mooring Lines’, Proceedings of International symposium on deep-water mooring system, Houston, TX, 2003, p101-114.
Minsuk Kim, (2004). ‘Dynamic Simulation of the Responses of A SPAR with A Polyester Mooring System Under Severe Ocean Environment’, MS thesis, Texas A&M University.
Minsuk Kim, Yu Ding and Jun Zhang (2004) ‘Numerical Simulation of A SPAR Interacting with A Polyester Mooring System’, Proceedings of OMAE. 2004.Return to top
Date: June 2004
Project Name: Deepwater Mooring Analysis: Synthetic Lines
Project Number: 32518/1510C Industry Funded
Principal Investigators: Jun Zhang
Estimated Completion Date: September 2005
Project Description: Synthetic ropes, such as polyester ropes, possess the properties of light wet-weight and high strength to weight ratios, which make them an appealing replacement of steel wires, especially in a mooring system deployed in deep seas. The restoring force of a polyester rope results from the stretching while the weight of a steel chain or wire is the main source of tension. To simulate and design a polyester mooring system, it is crucial to understand the differences from of a traditional steel mooring system. In addition to the light wet-weight and taut-leg configuration, a polyester rope yields relatively large elongation under tension, possesses a nonlinear stress-strain relation, and dissipates energy under cyclical loads. In this project a numerical scheme previously developed for steel mooring lines is being modified so that it considers these special properties of polyester ropes and can more accurately predict the interaction between a polyester mooring system and an offshore floating structure under severe met-ocean environments.
Progress: The numerical code CABLE3D, originally developed for steel wires and chains, has been modified and extended to allow for the Young’s modulus varying as a function of tensions. The extended CABLE3D considers the effects of static load as well as the amplitude of dynamic load. Since the amplitude of dynamic tension is not known in advance, the simulation is accomplished through an iterative process. In addition, the extended program considers large elongation of a polyester rope.
The extended program has been integrated into COUPLE6D to simulate the global responses of a Spar with a polyester mooring system. A number of simulations have been completed, and the main findings include:
a) The consideration of elongation in polyester ropes may result in a softer offset curve of a polyester mooring system and about a reduction in restoring force by about 10 percent
b) The inclusion of elongation of polyester rope may effectively eliminate resonant longitudinal vibrations in a coupled dynamic simulation (although they can also be eliminated by including the energy dissipation in polyester ropes)
c) The effects of the mean loads on the modulus of polyester ropes are much greater than those of the dynamic loads and the energy dissipation in polyester ropes under cyclic loading does not play significant roles in the responses of the Spar or its mooring line tensions.Reports & Publications:
Minsuk Kim, Yu Ding and Jun Zhang (2003) ‘Dynamic Simulation of Polyester Mooring Lines’, Proceedings of International symposium on deep-water mooring system, Houston, TX, 2003, p101-114.
Minsuk Kim, (2004). ‘Dynamic Simulation of the Responses of A SPAR with A Polyester Mooring System Under Severe Ocean Environment’, MS thesis, Texas A&M University.
Minsuk Kim, Yu Ding and Jun Zhang (2004) ‘Numerical Simulation of A SPAR Interacting with A Polyester Mooring System’, Proceedings of OMAE. 2004.
Return to top
Date: December 2003
Project Name: Deepwater Mooring Analysis: Synthetic Line
TEES Project Number: 32518-1510C Industry Funded
Principal Investigators: Jun Zhang
Estimated Completion Date: August 2005
Project Description:
When offshore floating structures are deployed in deep water, the weight of their mooring lines becomes a critical issue. To reduce the weight of mooring lines and the footprint of a mooring system, synthetic mooring lines, such as made of polyester, are being deployed in deep water. Because the behavior of polyester mooring lines are quite different from those of steel ones, this study is extending CABLE3D, a numerical code originally developed for dynamic analysis of steel mooring lines and risers, to allow for large elongation, tension-dependent modulus and hysteresis loop in simulating synthetic mooring lines. The extended code will be used to simulate the interaction between a FPS and its synthetic mooring-line system.
Progress:
Based on the existing studies on the Young’s modulus of a polyester rope as a function of static and dynamic tensions, the numerical code CABLE3D has been revised to allow for the Young’s modulus varying as a function of tensions.
The extended program considers the effects of static load as well as the amplitude of dynamic load on the Young’s modulus in simulating the interaction between a SPAR and its mooring system. Since the amplitude of dynamic tension is not known in advance, the simulation is accomplished through an iterative process. At the first simulation, the modulus is determined based solely on the mean tension load. The average dynamic tension amplitude based on the first simulation is used to adjust the modulus in the second simulation.
The extended program considers large elongation of a polyester rope.
Longitudinal resonant vibrations may result in extremely large tensions in simulating polyester ropes under large tensions after a long period of time, say three hours. To overcome the longitudinal resonant vibration, energy damping resulting from hysteresis loops in polyester material is introduced in numerical simulation. Preliminary results show that longitudinal resonant vibrations can be suppressed.
The extended program is now being integrated into COUPLE6D to simulate the responses of a SPAR interacting with a polyester mooring system under severe ocean environment.
Reports & Publications:
Minsuk Kim, Yu Ding and Jun Zhang (2003) ‘Dynamic Simulation of Polyester Mooring Lines’, Proceedings of International symposium on deep-water mooring system, Houston, TX, 2003, p101-114.
Minsuk Kim, ‘Dynamic Simulation of the Responses of A SPAR with A Polyester Mooring System Under Severe Ocean Environment’, MS thesis, Texas A&M University, (to be defended in Jan. 2004).
Date: June, 2003Project Name: Deepwater Mooring Analysis: Synthetic Line
Project Number: 32518/1510C Industry Funded
Principal Investigators: Jun Zhang
Estimated Completion Date: 8/ 31/05
Project Description:
When offshore floating structures are deployed in deeper and deeper water, the weight of their mooring lines becomes a critical issue. To reduce the weight of mooring lines and the footprint of a mooring system, synthetic mooring lines, such as made of polyester, are being deployed in deep seas. Because the behaviors of polyester mooring lines are quite different from those of steel ones, this project intends to extend CABLE3D, a numerical code originally developed for dynamic analysis of steel mooring lines and risers, to allow for tension-dependent modulus in simulating synthetic mooring lines. The extended code will be used to simulate interaction between a FPS and its synthetic mooring-line system.
Progress:
1) Based on the existing studies on the Young’s modulus of a polyester rope as a function of static and dynamic tensions, the numerical code CABLE3D is revised to allow for the Young’s modulus varying as a function of tensions.
2) The revised program considers the effects of static load as well as the amplitude of dynamic load on the Young’s modulus in simulating the interaction between a SPAR and its mooring system. Since the amplitude of dynamic tension is not known in advance, the simulation is accomplished through an iterative process. At the first simulation, the modulus is determined based solely on the mean tension load. The average dynamic tension amplitude based on the first simulation is used to adjust the modulus in the second simulation.
Reports & Publications:
Minsuk Kim, Yu Ding and Jun Zhang (2003) ‘Dynamic Simulation of Polyester Mooring Lines’, to be submitted to International symposium on deep-water mooring system.
Date: Nov. 16, 2002Project Name: Deepwater Mooring Analysis: Synthetic Line
Project Number: 32518/1510C
Principal Investigators: Jun Zhang
Estimated Completion Date: (Starting Sept.1, 2002) Aug. 31, 2004
Project Description: When offshore floating structures are deployed in deeper and deeper water, the weight of their mooring lines becomes a critical issue. To reduce the weight of mooring lines and the footprint of a mooring system, synthetic mooring lines, such as made of polyester, are being deployed in deep seas. Considering that the behaviors of polyester mooring lines are quite different from those of steel ones, this project intends to extend CABLE3D, a numerical code originally developed for dynamic analysis of steel mooring lines and risers, to allow for tension-dependent modulus in simulating synthetic mooring lines. The extended code will be used to simulate interaction between an FPS and its synthetic mooring-line system.
Progress:
1) Based on the existing studies on the Young’s modulus of a polyester rope as a function of static and dynamic tensions, a simple but effective empirical formulation for simulation the Young’s modulus is being derived.
2) The numerical code CABLE3D is being revised to allow for the Young’s modulus varying as a function of tensions.Reports & Publications:
A conference paper based on the project will be submitted next year.