Implementation of a Visco-Elastic Model Into Slender Rod Theory for Deepwater Polyester Mooring Line

2014 ◽  
Author(s):  
Gang Ma ◽  
Liping Sun ◽  
Hongwei Wang
Keyword(s):  
Nonlinear Material ◽  
Mooring Line ◽  
Elastic Model ◽  
Mooring System ◽  
Restoring Force ◽  
Large Rotation ◽  
Rod Theory ◽  
Excitation Period ◽  
Strain Stress ◽  
Large Stretch

Polyester mooring line is gradually becoming popular in deepwater engineering because of its lightweight property. In catenary mooring system, the horizontal restoring force comes from the gravity effect of the catenary mooring line which occupies the payload of the platform. Thus, the synthetic mooring line overcomes the drawback together with the taut mooring system which utilizes the axial elasticity to offer restoring force. The synthetic mooring line may only be one seventh weight of the steel mooring line and has low tensile stiffness which leads to a large stretch, and nonlinear material characteristics such as visco-elasticity. These two terms need to improve the slender rod theory which is proposed by Garrett for no stretched lines and improved by Paulling and Webster for small stretched lines. In this topic, a method for large stretched slender rod theory is introduced with the finite element method to deal with the problems of the large rotation and the large deformation. And then a linear visco-elastic model in the stretch-tension relation is utilized to simulate the properties of polyester line such as creep, strain-stress hysteresis and excitation period-dependent stiffness. Finally, an implementation method for integrating the visco-elastic model into slender rod theory is proposed with the numerical method and corroborated by a specified case which has the analytical results.

scholarly journals Dynamic Stability and Protection Design of a Submarined Floater Platform Avoiding Typhoon Wave Impact

2021 ◽  
Vol 9 (9) ◽  
pp. 977
Author(s):  
Shueei-Muh Lin ◽  
Yang-Yih Chen
Keyword(s):  
Dynamic Stability ◽  
Ocean Current ◽  
Mooring Line ◽  
Elastic Model ◽  
Mooring System ◽  
Floating Platform ◽  
Wave Impact ◽  
Diving Depth ◽  
Typhoon Wave ◽  
Typhoon Waves

This research proposes the design of a mooring system that allows the floating platform to stably dive deep enough to prevent damage induced by typhoon waves. The design principle of the mechanism is that the submarined floating platform with negative buoyancy is connected to a pontoon with positive buoyancy. The diving depth of the floating platform is determined by the rope length. If the static equilibrium of the two forces is satisfied, the diving depth will be kept. If the diving depth of the floating platform is enough, the platform will not be directly damaged by the wave impact. In reality, the system will be greatly subjected to the typhoon wave and the ocean current. The stability of the system and the dynamic tension of the rope must be significantly concerned. In this study, the linear elastic model of the mooring system composed of a floater platform, towed parachute, pontoon, traction rope, and mooring foundation is derived. The theoretical solution of the static and dynamic stability analysis of the mooring system is proposed. The dynamic behaviors of the floating platform and pontoon, and the tension of the rope under the effects of waves and ocean currents, are investigated. It is discovered that the buffer spring helps reduce the tension of the rope. The proposed protection procedure can avoid the damage of the floating platform and the mooring line, due to Typhoon wave impact.

Optimal Design Methodology for Multi-Component Mooring Systems in Deep Water

2010 ◽  
Author(s):  
Long Yu ◽  
Jiahua Tan
Keyword(s):  
Optimal Design ◽  
Deep Water ◽  
Design Methodology ◽  
Element Model ◽  
Gas Production ◽  
Mooring Line ◽  
Mooring System ◽  
Offshore Platforms ◽  
Mooring Lines ◽  
Restoring Force

Multi-component mooring systems, one of the crucial equipments of offshore platforms, play an important role in deep water oil&gas production because of relative low cost and light weight. A single mooring line can be constructed by combination of wire ropes, chains, fiber ropes, buoys and connectors etc. and provide adequate restoring force at fairlead point of platforms. Although the static and dynamic analyzing approaches for a determined multi-component system have been studied already, it is still hard to design and predetermine an appropriate mooring system that can satisfy the codes with multi-component lines. Referred to the conventional mooring system design method, this paper brings out an optimal design methodology for multi-component mooring systems. According to quasi-static method, at extreme offset position of the platform, an optimization model for designing the multi-component mooring line with biggest tension in deep water has been provided. Then, with the aid of design wave method and morison equation, a finite element model has been used to calculate mooring line dynamics at each fairlead point in time domain. The nonlinear interaction of mooring lines and seabed has also been investigated. Heave and surge of the platform have also been considered. Both 2D and 3D mooring system models have been built to search the interference of the lines and directional influence of environment loads like current and wave. The paper applied this set of analyzing methods and processes into a deep water semisubmersible serving at South China Sea. Compared with the results calculated by other software, the methodology mentioned in the paper got similar result with less weight and bigger restoring force.

“ASEMOS”—Advanced Semisubmersible Mooring System

1985 ◽  
Vol 22 (01) ◽  
pp. 36-49
Author(s):  
Christian V. Wolff
Keyword(s):  
Heavy Chain ◽  
Load Capacity ◽  
Wire Rope ◽  
Center Of Gravity ◽  
Mooring Line ◽  
Lower Grade ◽  
Mooring System ◽  
Restoring Force ◽  
Improved Stability ◽  
Drilling Rigs

The least efficient, yet most popular mooring system on semisubmersibles is the conventional chain-only type. These perform well in shallow to moderate depths, but as water depth increases, so do horizontal excursions of the rig. System efficiency also decreases because more and more of the strength of the chain is required to support its own weight and less is available to provide horizontal restoring forces. This paper describes an advanced mooring system that offers several distinct advantages over the usual alternatives: more elasticity in the mooring line at shallow depths; maximum strength and restoring force at the vessel; lower center of gravity and improved stability; increased variable deck-load capacity; and protection of the winches from wind and sea. One feature that makes the "ASEMOS" system unique is the placement of the mooring winches within the pontoons (Figs. 1 and 2). This location not only protects the winches from the elements, but results in a much lower center of gravity than that of other comparably sized semisubmersible drilling rigs. Another key feature is the use of oversized very heavy chain between the end of the wire rope mooring line and the anchors. This heavy chain provides extra elasticity in the mooring line catenary in shallow depths, and shortens the length of wire rope required to achieve maximum anchor-holding power in deep water. The greater breaking strength of the oversized chain permits the use of a lower-grade steel and avoids the high cost and fatigue problems commonly encountered in other systems.

Coupled Mooring Analysis for a Deep Water CALM Buoy

2004 ◽  
Author(s):  
J. L. Cozijn ◽  
T. H. J. Bunnik
Keyword(s):  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Test Results ◽  
Mooring Lines ◽  
Restoring Force ◽  
Small Water ◽  
Force Components ◽  
Force Characteristics ◽  
Line Loads

The effect of the mooring loads on floator motions can be significant for small water plane are floaters like CALM buoys. Not only does the mooring system contribute to the static restoring force components, but the dynamic behaviour of the mooring lines also affects the inertia and damping of the moored CALM buoy. The results from model tests with a moored CALM buoy were compared with the results from two series of time-domain computer simulations. First, fully dynamic coupled simulations were carried out, in which the interaction between the floater motions and the dynamic mooring line loads was modelled for all 6 modes of motion. Second, quasi-static simulations were carried out, in which only the (non-linear) static restoring force characteristics of the mooring system were taken into account. The comparison of results from the simulations and the model tests clearly indicates that the fully dynamic coupled simulations show a much better correspondence with the model test results than the quasi-static simulations. It is concluded that for the simulation of the behavior of a moored CALM buoy in waves a fully dynamic coupled mooring analysis is essential.

Development of Water Tank Test Device for Deep-Water Mooring

2018 ◽  
Author(s):  
Go Oishi ◽  
Hiroshi Yamaguchi ◽  
Kiyoshi Shimada ◽  
Kouichi Kayajima
Keyword(s):  
Numerical Simulations ◽  
Deep Water ◽  
Mooring Line ◽  
Mooring System ◽  
Water Tank ◽  
Restoring Force ◽  
Small Water ◽  
Tank Test ◽  
The Law ◽  
Inertia Forces

When conducting model tests in a water tank, available model sizes and wave conditions are determined for each tank, depending on measurement accuracy and tank specifications. For deep-water mooring of a floater, a mooring extent in model scale is presumably over 10 meters in depth, making it difficult to be conducted in small-sized tanks without mooring line truncation. The purpose of the research is to develop a device, which could be used as deep-water mooring system in small-sized tanks. Although the law of geometrical similarity is compelled to quit because of the line truncation, the law of mechanical similarity can be maintained by keeping the same restoring, damping and inertia characteristics as those of the full-scale mooring system obtained by numerical simulations. The mooring device consists of a cylinder, a piston, an orifice, springs, pulleys and weights. A spring attached to the mooring line is to generate required restoring force. The orifice, together with the piston, is to generate required damping forces. Inertia forces are generated by the motions of hanged weights, also by the motion of the fluid inside the cylinder. Even negative inertia forces can be given by adjusting natural frequencies of the weight-spring system. With all these examined elements, the mooring device works like the full-depth mooring system. Particulars of the elements of the device have been determined by numerical simulations of the floater moored in the full-depth condition. It has been confirmed that the mooring device behaves as expected in comparison with forced oscillation tests, where prescribed motions were given to the floater-side end point of the mooring line. A tank test has been conducted of a floater with a turret multipoint-moored with the devices and has been satisfactorily compared with numerical simulations of the full-depth system. With the present research it is verified that the mooring device can well simulate actual deep-water mooring system, which makes it possible for small water tanks to deal with deep water mooring experiments.

The Analysis of Mooring Systems of a Drillship

2009 ◽  
Author(s):  
Gang Ma ◽  
Liping Sun ◽  
Hongwei Wang
Keyword(s):  
Dynamic Analysis ◽  
Coordinate System ◽  
Time Integration ◽  
Static Problem ◽  
Mooring Line ◽  
Global Coordinate System ◽  
Mooring System ◽  
Elastic Rod ◽  
Mooring Lines ◽  
Rod Theory

In this paper the mooring system of a drill ship is analyzed which is designed for South China Sea in 1500 meters depth. The analyses of the mooring lines have been developed based on the theories dealing with slender structure and cables, so the elastic rod theory is used. Elastic rod theory is developed for the analysis of line dynamics. The governing equations of mooring lines and risers are treated in the global coordinate system without transforming the coordinate system. The hydrodynamic forces on the lines together with the strain and the stress of the structures caused by geometric nonlinearity are considered. The model of the rods allows for a small elongation, and permits large deflections and finite rotations. The rods are of elasticity and arbitrary configuration, with kind of loads and tension variation along its length acting on it, including the motion of rod, hydrodynamic force resulted from the external fluid and gravity. The fluid in the riser is considered for riser analysis, and the support of the sea bottom is presented for mooring line analysis. Finite element method is used to discrete mooring lines and risers, and lines dynamic analysis is executed with time integration method. A program is developed, and its validation is checked by comparison of numerical results to exact solutions for a nonlinear, static problem. Both static analysis and dynamic analysis of the whole system are done to ensure the mooring system of the drilling ship on 1500m depth can successfully applied.

scholarly journals Platform Motions and Mooring System Coupled Solver for a Moored Floating Platform in a Wave

Processes ◽  
2021 ◽  
Vol 9 (8) ◽  
pp. 1393
Author(s):  
Sang Chul Lee ◽  
Seongjin Song ◽  
Sunho Park
Keyword(s):  
System Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Offshore Platforms ◽  
Floating Platform ◽  
Lumped Mass ◽  
Time Step ◽  
Restoring Force ◽  
Line Model ◽  
Platform Motions

In advance of building moored floating offshore platforms, in recent years, there has been a greater demand for two-way coupled simulations between a motion solver based on the viscous flow theory and a mooring line model, including cable dynamics. This paper introduces open-source libraries such as MoorDyn (the lumped-mass mooring line model) and OpenFOAM (the computational fluid dynamics libraries). It describes the methods by which they can be coupled bi-directionally. In each time step, the platform motions calculated by OpenFOAM are transferred to MoorDyn as the boundary conditions for the mooring system analysis. In contrast, MoorDyn calculates the restoring force and moment due to the mooring system and transfers them to OpenFOAM. The restoring force and moment act on the platform as the external force and moment for the platform motions in the next time step. The static tension and profile of the mooring system, dynamic tension of the mooring system, and free decay motions of the floating buoy in the still water were simulated to check the accuracy of OpenFOAM and MoorDyn. The coupled solver was used to produce simulations of the moored decay motions of the floating buoy in the still water and the moored motions with the Stokes 5th order wave. All simulation results were compared and showed good agreement with the numerical solution and experiment results. In addition, the characteristics of each solver were investigated.

scholarly journals Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

2021 ◽  
Vol 9 (2) ◽  
pp. 103
Author(s):  
Dongsheng Qiao ◽  
Binbin Li ◽  
Jun Yan ◽  
Yu Qin ◽  
Haizhi Liang ◽  
...  
Keyword(s):  
Service Condition ◽  
Mooring Line ◽  
Mooring System ◽  
Transient Effects ◽  
Floating Platform ◽  
Transient Responses ◽  
Mooring Lines ◽  
Performance Deterioration ◽  
Steady State Responses ◽  
Influence Process

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

Application of a Viscoelastic Model for Polyester Mooring

2010 ◽  
Author(s):  
J. W. Kim ◽  
J. H. Kyoung ◽  
A. Sablok
Keyword(s):  
Material Properties ◽  
Viscoelastic Model ◽  
Practical Method ◽  
Time Dependent ◽  
Mooring Line ◽  
New Model ◽  
Global Performance ◽  
Excitation Period ◽  
Linear Viscoelastic ◽  
Floating Platforms

A new practical method to simulate time-dependent material properties of polyester mooring line is proposed. The time-dependent material properties of polyester rope are modeled with a standard linear solid (SLS) model, which is one of the simplest forms of a linear viscoelastic model. The viscoelastic model simulates most of the mechanical properties of polyester rope such as creep, strain-stress hysteresis and excitation period-dependent stiffness. The strain rate-stress relation of the SLS model has been re-formulated to a stretch-tension relation, which is more suitable for implementation into global performance and mooring analyses tools for floating platforms. The new model has been implemented to a time-domain global performance analysis software and applied to simulate motion of a spar platform with chain-polyester-chain mooring system. The new model provides accurate platform offset without any approximation on the mean environmental load and can simulate the transient effect due to the loss of a mooring line during storm conditions, which has not been possible to simulate using existing dual-stiffness models.

North Sea Mooring Systems: How Reliable Are They?

2014 ◽  
Author(s):  
Will Brindley ◽  
Andrew P. Comley
Keyword(s):  
Continental Shelf ◽  
North Sea ◽  
Recent Decade ◽  
Mooring Line ◽  
Mooring System ◽  
Failure Rates ◽  
Floating Structure ◽  
Strength Capacity ◽  
Failure Statistics ◽  
Overall Reliability

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system? This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system. The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

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