Development of Water Tank Test Device for Deep-Water Mooring

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.

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

29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 4 ◽

10.1115/omae2010-20190 ◽

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.

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Coupled Mooring Analysis for a Deep Water CALM Buoy

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B ◽

10.1115/omae2004-51370 ◽

2004 ◽

Cited By ~ 5

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.

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STRUCTURAL BEHAVIOUR OF FULLY COUPLED SPAR–MOORING SYSTEM UNDER EXTREME WAVE LOADING

Journal of Civil Engineering and Management ◽

10.3846/13923730.2013.801899 ◽

2014 ◽

Vol 19 (Supplement_1) ◽

pp. S69-S77 ◽

Cited By ~ 9

Author(s):

A. B. M. Saiful Islam ◽

Mohammed Jameel ◽

Suhail Ahmad ◽

Mohd Zamin Jumaat ◽

V. John Kurian

Keyword(s):

Response Time ◽

Deep Water ◽

Mooring Line ◽

Mooring System ◽

Wave Loading ◽

Extreme Wave ◽

Spar Platform ◽

Mooring Lines ◽

Time Histories ◽

Fully Coupled

Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme wave loading at severe deep sea. Morison's equation has been used to calculate the wave forces. Spar responses and mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state, it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability of a spar platform for deep water harsh and uncertain environmental conditions.

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“ASEMOS”—Advanced Semisubmersible Mooring System

Marine Technology and SNAME News ◽

10.5957/mt1.1985.22.1.36 ◽

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.

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

Volume 8A: Ocean Engineering ◽

10.1115/omae2014-23594 ◽

2014 ◽

Cited By ~ 1

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.

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Influence of a Seabed Trench on a Taut Mooring Line

Volume 1: Offshore Technology ◽

10.1115/omae2017-61472 ◽

2017 ◽

Author(s):

Lingzhi Xiong ◽

Jianmin Yang ◽

Xinliang Tian

Keyword(s):

Numerical Model ◽

Numerical Simulations ◽

Inclination Angle ◽

Significant Influence ◽

Deep Water ◽

Mooring Line ◽

Station Keeping ◽

Suction Caisson ◽

Floating Systems ◽

Suction Caissons

Taut mooring systems have become a prospective solution for the station keeping of offshore floating facilities in deep water. In such scenario, the associated anchor foundations may withstand substantial uplift loads. To maximize the holding capacity, it is common to attach the anchor chain to the suction caisson at 60 to 70 % of the embedded depth. However, new phenomenon has been observed, which may be a challenge to the integrity of mooring systems, such as the trenching of the seabed near the suction caissons of floating systems. It is unclear whether trenching will induce significant effect on the integrity of mooring systems. To evaluate the influence of the seabed trenching, where existing commercial codes have not covered, a numerical model has been developed in this study. Based on this developed model, numerical simulations have been conducted with and without considering the trenching effect at the seabed. It is found that the seabed trench has a limited influence on the tension at fairlead but has a significant influence on both the tension and the inclination angle at padeye.

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

Processes ◽

10.3390/pr9081393 ◽

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.

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

Journal of Marine Science and Engineering ◽

10.3390/jmse9020103 ◽

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.

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Five-Wave Resonances in Deep Water Gravity Waves: Integrability, Numerical Simulations and Experiments

Fluids ◽

10.3390/fluids6060205 ◽

2021 ◽

Vol 6 (6) ◽

pp. 205

Author(s):

Dan Lucas ◽

Marc Perlin ◽

Dian-Yong Liu ◽

Shane Walsh ◽

Rossen Ivanov ◽

...

Keyword(s):

Normal Form ◽

Numerical Simulations ◽

Gravity Waves ◽

Deep Water ◽

Plane Waves ◽

Surface Elevation ◽

Wave Interactions ◽

Frequency Space ◽

Target Mode ◽

The Hilbert Transform

In this work we consider the problem of finding the simplest arrangement of resonant deep-water gravity waves in one-dimensional propagation, from three perspectives: Theoretical, numerical and experimental. Theoretically this requires using a normal-form Hamiltonian that focuses on 5-wave resonances. The simplest arrangement is based on a triad of wavevectors K1+K2=K3 (satisfying specific ratios) along with their negatives, corresponding to a scenario of encountering wavepackets, amenable to experiments and numerical simulations. The normal-form equations for these encountering waves in resonance are shown to be non-integrable, but they admit an integrable reduction in a symmetric configuration. Numerical simulations of the governing equations in natural variables using pseudospectral methods require the inclusion of up to 6-wave interactions, which imposes a strong dealiasing cut-off in order to properly resolve the evolving waves. We study the resonance numerically by looking at a target mode in the base triad and showing that the energy transfer to this mode is more efficient when the system is close to satisfying the resonant conditions. We first look at encountering plane waves with base frequencies in the range 1.32–2.35 Hz and steepnesses below 0.1, and show that the time evolution of the target mode’s energy is dramatically changed at the resonance. We then look at a scenario that is closer to experiments: Encountering wavepackets in a 400-m long numerical tank, where the interaction time is reduced with respect to the plane-wave case but the resonance is still observed; by mimicking a probe measurement of surface elevation we obtain efficiencies of up to 10% in frequency space after including near-resonant contributions. Finally, we perform preliminary experiments of encountering wavepackets in a 35-m long tank, which seem to show that the resonance exists physically. The measured efficiencies via probe measurements of surface elevation are relatively small, indicating that a finer search is needed along with longer wave flumes with much larger amplitudes and lower frequency waves. A further analysis of phases generated from probe data via the analytic signal approach (using the Hilbert transform) shows a strong triad phase synchronisation at the resonance, thus providing independent experimental evidence of the resonance.

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