Numerical Study on Mathieu Instability of DDMS Platform

2010 ◽  
Author(s):  
Binbin Li ◽  
Jinping Ou
Keyword(s):  
Numerical Study ◽  
Mooring Line ◽  
Early Investigation ◽  
Spar Platform ◽  
Mooring Lines ◽  
Natural Period ◽  
Ballast Tank ◽  
University Of Technology ◽  
Mathieu Instability ◽  
Decay Tests

Deep draft multi-spar (DDMS) is an innovative platform which is specially designed for deepwater drilling and production in 2009 by Center for Deepwater Engineering, Dalian University of Technology. The hard tank of DDMS is composed of four columns at corners and a novel moonpool protecting the top tension risers at center. In addition, the top tension and self-weight of rigid risers are provided by air-cans in the moonpool. At the foot of hard tank, the pontoons and horizontal bracing are used to connect the separated columns and moonpool. It is noted that two heave plates are directly integrated with the hard tank in order to reduce the heave response. The middle section consists of 4 columns of smaller diameter which connect the hard tank and ballast tank. The early investigation indicates that the global hydrodynamic and motion behavior of DDMS are similar with Spar platform, and furthermore the heave natural period is close to the half pitch natural period. Therefore the DDMS platform may have possibility to trigger the Mathieu instability which has been validated on Spar platform through the numerical and experimental method. In this paper, a coupled heave and pitch motion equations of DDMS platform are established with accounting the time-varying restoring heave and pitch restoring stiffness. A damping case matrix is generated considering the heave plate damping, mooring line damping and hull drag damping. The damping ratios are identified by free-decay tests. The nonlinear motions under the action of regular waves of different periods and heights are numerically solved by the 4th order Runge-kutta method. The calculational results reveal that the heave damping significantly influences the occurrence of pitch instability, meanwhile the damping contribution of heave plates and mooring lines also play an important role in suppressing the instability. The phenomenon of Mathieu instability is owing to the energy exchange in this paper, and the mechanism of this phenomenon is amply studied as well as 3 different ways of instability are summarized.

Numerical Study on Hydrodynamic Behavior of Deepwater Spar Platform with Different Mooring Configurations

2011 ◽  
Vol 137 ◽  
pp. 50-58
Author(s):  
Jin Wei Sun ◽  
Xiu Tao Fan ◽  
Xiao Zheng Wan ◽  
Shi Xuan Liu
Keyword(s):  
Static Analysis ◽  
Numerical Study ◽  
Coupled System ◽  
Element Model ◽  
System Structure ◽  
Mooring Line ◽  
Mooring System ◽  
Performance Study ◽  
Spar Platform ◽  
Mooring Lines

The motion performance of Spar platform and dynamic characteristics for the mooring lines under different mooring configurations have been studied both in static analysis and coupled dynamic analysis. First, 3D hydrodynamic finite element model is built and the effects of the mooring system are taken into account by giving the specified pre-tension, angle and stiffness of the mooring lines on the fairleads. And hydrodynamic analysis of Spar platform is performed by the way of utilizing potential flow theory in frequency domain in order to calculate the hydrodynamic coefficients. Then, static analysis is applied to obtain restoring stiffness curves for the mooring system, structure displacements and mooring line tensions etc.. At last, coupled time domain analysis of the motion response of Spar is conducted for the coupled system and the dynamic tensions of mooring lines are calculated. The research results can be served as a reference for the selection and the performance study for mooring systems during preliminary design.

Mooring Analysis of a Dual-Spar Floating Wind Farm with a Shared Line

2021 ◽  
pp. 1-37
Author(s):  
Guodong Liang ◽  
Zhiyu Jiang ◽  
Karl O. Merz
Keyword(s):  
Wind Turbines ◽  
Wind Farm ◽  
Wind Farms ◽  
Sensitivity Study ◽  
Design Stage ◽  
Mooring Lines ◽  
Natural Period ◽  
Preliminary Design Stage ◽  
Decay Tests ◽  
Line Design

Abstract Wind farms with shared mooring lines have the potential to reduce mooring costs. However, such wind farms may encounter complex system dynamics because adjacent wind turbines are coupled. This paper presents an analysis of the shared mooring system with a focus on the system natural periods. We first apply Irvine's method to model both the shared line and the two-segment single lines. The response surface method is proposed to replace iterations of the catenary equations of the single lines, and a realistic single line design is presented for OC3 Hywind. Then, system linearization and eigenvalue analysis are performed for the wind farm consisting of two spar floating wind turbines, one shared line, and four single lines. The obtained natural periods and natural modes are verified by numerical free decay tests. Finally, a sensitivity study is carried out to investigate the influence of mooring properties. It is found that the shared line has a significant influence on the natural periods in the surge and sway modes. The natural periods in the surge and sway modes are also most sensitive to the mooring property variations. Two sway eigenmodes are identified, and the lower sway natural period varies between 23 s and 88 s in the sensitivity study. The present analysis method can be used to identify critical natural periods at the preliminary design stage of shared mooring systems.

scholarly journals Numerical Study on Hydrodynamic Responses of Floating Rope Enclosure in Waves and Currents

2020 ◽  
Vol 8 (2) ◽  
pp. 82
Author(s):  
Hui Yang ◽  
Yun-Peng Zhao ◽  
Chun-Wei Bi ◽  
Yong Cui
Keyword(s):  
Numerical Study ◽  
Line Tension ◽  
Wave Period ◽  
Windward Side ◽  
Mooring Line ◽  
Fish Stock ◽  
Lumped Mass ◽  
Mooring Lines ◽  
Design Factors ◽  
Volume Retention

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.

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

2014 ◽  
Vol 19 (Supplement_1) ◽  
pp. S69-S77 ◽  
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.

Nonlinear Response of Coupled Integrated Spar Platform Under Severe Sea States

2012 ◽  
Author(s):  
A. B. M. Saiful Islam ◽  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
Mohd Zamin Jumaat
Keyword(s):  
Deep Water ◽  
Beam Element ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Intermediate Water ◽  
Spar Platform ◽  
Mooring Lines ◽  
Spar Platforms ◽  
Water Conditions ◽  
Wave Heights

The oil and gas industry has moved towards the offshore deep water regions due to depletion of these resources in shallow and intermediate water depths. Conventional fixed jacket type platforms and bottom supported compliant platforms have been found to be inefficient and uneconomical for exploring these resources in deep water regions. In view of deep water conditions, Spar platforms have been seen to be the most economical and suitable alternative offshore platforms. Several operational Spar platforms such as SB-1, Shell’s ESSCO, Brent Spar, Oryx Neptune Spar, Chevron Genesis Spar and Exxon’s Diana Spar in the Gulf of Mexico and North Sea have shown the effectiveness and success of such platforms in deep-ocean. In deep water conditions, the severity of sea states has substantial effects on the spar platform. The mooring lines contribute significant inertia and damping because of their longer lengths, larger sizes, and heavier weights. Precise motion investigation of platforms should consider these actions in deep waters. However, proper dynamics cannot be assessed by the commonly used decoupled quasi-static method that ignores all or part of the interaction effects between the mooring lines and platform. Coupled analysis, which includes the platform and mooring lines in a single model, is the only way to capture the damping from mooring lines in a consistent manner. In the present study, coupled analysis of integrated Spar-mooring system has been performed. Cylindrical spar hull is treated as a rigid beam element and catenary mooring line as hybrid beam element. Nonlinear dynamic responses have been evaluated under several severe sea states of dissimilar wave heights and wave periods. Damping due to mooring lines has been assessed. An automatic Newmark-β time incremental approach has been implemented to conduct the analysis in time domain. Wave induced spar hull motion in surge, heave and pitch direction along with maximum tension in mooring line has been assessed for different wave conditions with and without current in 1018 m water depth. The time histories of spar responses follow substantial alteration for larger wave heights and wave periods. Maximum tensions in mooring line are very sensitive with momentous value for extreme sea loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines.

Wind Induced Nonlinear Response of Coupled Spar Platform

2014 ◽  
Author(s):  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
A. B. M. Saiful Islam ◽  
Mohd Zamin Jumaat
Keyword(s):  
Deep Water ◽  
Oil And Gas ◽  
Wave Force ◽  
Wind Loading ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Wave Direction ◽  
Spar Platform ◽  
Mooring Lines ◽  
Spar Platforms

The oil and gas exploration has moved from shallow water to much deeper water far off the continental shelf. Spar platforms under deep water conditions are found to be the most economical and efficient type of offshore platform. Several Spar platforms installed in the Gulf of Mexico and North Sea proves its suitability for deep water exploration. Accurate prediction of motions of a Spar hull is very important for the integrity and associated costs of the riser/mooring line. The most common approach for solving the dynamics of Spar platform is to employ a decoupled quasi-static method, which ignores all or part of the interaction effects between the platform, mooring lines and risers. Coupled analysis, which includes the mooring lines, risers and platform in a single model, is the only way to capture the damping from mooring lines and risers in a consistent manner. The present coupling is capable in matching the forces, displacement, velocities and acceleration for mooring line with Spar hull at the fairlead position and riser with Spar hull at the riser keel connection. It can handle possible significant nonlinearities. The output from such analyses will be platform motions as well as a detailed mooring line and riser responses. In actual field problems hydrodynamic and aerodynamic loads act simultaneously on Spar platform, mooring lines and risers. In finite element model, the entire structure acts as a continuum. This model can handle all nonlinearities, loading and boundary conditions. The selected configuration of Spar platform is analysed under wave force together with wind loading and its structural response behaviour in steady state is studied. An automatic Newmark-β time incremental approach in ABAQUS/AQUA environment has been implemented to conduct the analysis in time domain. The wind force acting on the exposed part of the platform encompasses mean and fluctuating wind components. The frontal region includes the topside assembly and the spar hull portion above the sea level. High degree of nonlinearities makes the solutions convergence sensitive and it requires large number of iterations, at each time station. Spar responses in surge, heave and pitch along with top tension in moorings are computed. The coupled Spar experiences significant lateral shift along wave direction due to wind loading. Increase in standard deviation shows the participation of wind loading giving higher fluctuations. The CML tension increases for wind loading but the extent of the tension fluctuations under wind loading is not much due to high pretension of mooring line.

Experimental and Numerical Study on Large Truncation of Deepwater Mooring Line

2009 ◽  
Author(s):  
Yihua Su ◽  
Jianmin Yang ◽  
Longfei Xiao ◽  
Gang Chen
Keyword(s):  
Dynamic Response ◽  
Water Depth ◽  
Numerical Study ◽  
Unit Length ◽  
Low Frequency ◽  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Basin

Modeling the deepwater mooring system in present available basin using standard Froude scaling at an acceptable scale presents new challenges. A prospective method is to truncate the full-depth mooring lines and find an equivalent truncated mooring system that can reproduce both static and dynamic response of the full-depth mooring system, but large truncation arise if the water depth where the deepwater platform located is very deep or the available water depth of the basin is shallow. A Cell-Truss Spar operated in 1500m water depth is calibrated in a wave basin with 4m water depth. Large truncation arises even though a small model scale 1:100 is chosen. A series of truncated mooring lines are designed and investigated through numerical simulations, single line model tests and coupled wave basin model tests. It is found that dynamic response of the truncated mooring line can be enlarged by using larger diameter and mass per unit length in air. Although the truncated mooring line with clump presents a “taut” shape, its dynamic characteristics is dominated by the geometry stiffness and it underestimates dynamic response of the full-depth mooring line, even induces high-frequency dynamic response. There are still two obstacles in realizing dynamic similarity for the largely truncated mooring system: lower mean value of the top tension of upstream mooring lines, and smaller low-frequency mooring-induced damping.

scholarly journals Numerical Study of a Moored Structure in Moving Broken Ice Driven by Current and Wave

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Biao Su ◽  
Karl Gunnar Aarsæther ◽  
David Kristiansen
Keyword(s):  
Numerical Model ◽  
Numerical Study ◽  
Torsional Stiffness ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Broken Ice ◽  
Ice Field ◽  
Drifting Ice

This paper presents a numerical model intended to simulate the mooring force and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modeled by using a generic cable model with a set of constraint equations providing desired structural properties such as the axial, bending, and torsional stiffness. The six degrees-of-freedom (DOF) rigid body motions of the structure were simulated by considering its interactions with the mooring lines and the drifting ice. In this simulation, a fragmented ice field of broken ice pieces could be considered under the effects of current and wave. The ice–ice and ice–structure interaction forces were calculated based on a viscoelastic-plastic rheological model. The hydrodynamic forces acting on the floating structure, mooring line, and drifting ice were simplified and calculated appropriately. The present study, in general, demonstrates the potential of developing an integrated numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.

Coupled Mooring Analysis and Large Scale Model Tests on a Deepwater Calm Buoy in Mild Wave Conditions

2002 ◽  
Author(s):  
T. H. J. Bunnik ◽  
G. de Boer ◽  
J. L. Cozijn ◽  
J. van der Cammen ◽  
E. van Haaften ◽  
...  
Keyword(s):  
Model Tests ◽  
Irregular Waves ◽  
Scale Model ◽  
Mooring Line ◽  
Wave Forces ◽  
Mooring System ◽  
Mooring Lines ◽  
Pitch Moment ◽  
Numerical Tool ◽  
Decay Tests

This paper describes a series of model tests aimed at gaining insight in the tension variations in the export risers and mooring lines of a CALM buoy. The test result were therefore not only analysed carefully, but were also used as input and to validate a numerical tool that computes the coupled motions of the buoy and its mooring system. The tests were carried out at a model scale of 1 to 20. Captive tests in regular and irregular waves were carried out to investigate non-linearities in the wave forces on the buoy for example from the presence of the skirt. Decay tests were carried out to determine the damping of the buoy’s motions and to obtain the natural periods. Finally, tests in irregular waves were carried out. The dynamics of the mooring system and the resulting damping have a significant effect on the buoy’s motions. A numerical tool has been developed that combines the wave-frequency buoy motions with the dynamical behaviour of the mooring system. The motions of the buoy are computed with a linearised equation of motion. The non-linear motions of the mooring system are computed simultaneously and interact with the buoy’s motions. In this paper, a comparison is shown between the measurements and the simulations. Firstly, the wave forces obtained with a linear diffraction computation with a simplified skirt are compared with the measured wave forces. Secondly, the numerical modelling of the mooring system is checked by comparing line tensions when the buoy moves with the motion as measured in an irregular wave test. Thirdly, the decay tests are simulated to investigate the correctness of the applied viscous damping values. Finally, simulations of a test in irregular waves are shown to validate the entire integrated concept. The results show that: 1. The wave-exciting surge and heave forces can be predicted well with linear diffraction theory. However, differences between the measured and computed pitch moment are found, caused by a simplified modelling of the skirt and the shortcomings of the diffraction model. 2. To predict the tension variations in the mooring lines and risers (and estimate fatigue) it is essential that mooring line dynamics are taken into account. 3. The heave motions of the buoy are predicted well. 4. The surge motions of the buoy are predicted reasonably well. 5. The pitch motions are wrongly predicted.

Effect of Seabed Trenching on Holding Capacity of Suction Anchors in Deepwater Gulf of Guinea Clays: A Numerical Study

2019 ◽  
Author(s):  
Pablo Castillo Garcia ◽  
Regis Wallerand ◽  
Dinh Hong Doan
Keyword(s):  
Numerical Study ◽  
Mooring Line ◽  
Residual Soil ◽  
Gulf Of Guinea ◽  
Mooring Lines ◽  
Actual Case ◽  
Line Load ◽  
Centrifuge Testing ◽  
Plaxis 3D ◽  
Observation Results

Abstract In recent years, there has been an increasing issue of seabed trenches developing around mooring lines attached to suction anchors in West Africa, likely related to relatively large motions of the mooring lines. These trenches are recognised by means of numerical modelling and of centrifuge testing program as significant concern to the in-place holding capacity, the integrity of the anchor and the safety of the floating facility. Conservative trench geometries and complete absence of soil and of soil resistance in front of the anchor above the padeye depth (in the loading direction) were essential assumptions due to lack of observation. Results suggested that a reduction in holding capacity of 20% to 40% for typical mooring line load angles of 30° to 45° can be expected and that the presence of the trench does not affect the portion of holding capacity developed by passive suction at the pile tip. Nevertheless, recent survey data has evidenced a wedge of soil remaining within trenches in front of suction anchors leading to a new problem-solving approach. Consequently, this paper describes a Plaxis 3D Finite Element modelling in a parametric study as a reliable tool for the assessment of the holding capacity of the suction anchors in presence of seabed trench taking into account the presence of residual soil within the trench in front of the anchor pile. Analyses were performed under 30° inclined loading (actual case of taut deep water mooring systems) with various scenarios of trench shapes, padeye depths and anchor geometries in very soft highly plastic Gulf of Guinea clays. Thus, the consideration of the residual wedge of soil in front of the anchor leads to a much less reduction in holding capacity compared to initial studies done with too pessimistic assumptions on the final trench shape. In addition, this study aims at looking at changes in the pile geometry, including the position of the attachment point, to compensate the unavoidable formation of a trench for taut moorings.

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