Experimental Study on the Effects of Mooring System on Air Gap Response of Semi-Submersible Platform

2020 ◽  
Author(s):  
Xu Li ◽  
Longfei Xiao ◽  
Handi Wei ◽  
Mingyue Liu
Keyword(s):  
Model Tests ◽  
Air Gap ◽  
Mooring System ◽  
Complex Task ◽  
Ocean Engineering ◽  
Free Surface Elevation ◽  
Mooring Lines ◽  
Scaled Model ◽  
Air Gap Responses ◽  
Floating Platforms

Abstract The air gap response is crucial for the safe design and operation of large-volume floating platforms such as semi-submersible and tension leg platforms. It is a complex task to perform numerical simulation on the air gap response considering the wave free surface elevation and the motions of the floating vessel. Therefore, the prediction of air gap response still relies heavily on model tests. This paper attempts to investigate the effects of the mooring system, especially the effects of the length of mooring lines, on the air gap response of semi-submersible platform based on model tests results. The scaled model of the semi-submersible platform is supported by a symmetric mooring system composed of 8 mooring lines. A set of model tests with different length of mooring lines was performed in the State Key Laboratory of Ocean Engineering basin at Shanghai Jiao Tong University, and the air gap responses of 15 locations were measured using wave probes. The results indicate that the mooring system plays an important role in the air gap response of semi-submersible platform.

Guidelines for CFD Simulations of Spar VIM

2013 ◽  
Author(s):  
Charles Lefevre ◽  
Yiannis Constantinides ◽  
Jang Whan Kim ◽  
Mike Henneke ◽  
Robert Gordon ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Test Data ◽  
Model Test ◽  
Model Tests ◽  
Full Scale ◽  
Mooring System ◽  
Time Step ◽  
Cfd Simulations ◽  
Scaled Model ◽  
Sway Motion

Vortex-Induced Motion (VIM), which occurs as a consequence of exposure to strong current such as Loop Current eddies in the Gulf of Mexico, is one of the critical factors in the design of the mooring and riser systems for deepwater offshore structures such as Spars and multi-column Deep Draft Floaters (DDFs). The VIM response can have a significant impact on the fatigue life of mooring and riser components. In particular, Steel Catenary Risers (SCRs) suspended from the floater can be sensitive to VIM-induced fatigue at their mudline touchdown points. Industry currently relies on scaled model testing to determine VIM for design. However, scaled model tests are limited in their ability to represent VIM for the full scale structure since they are generally not able to represent the full scale Reynolds number and also cannot fully represent waves effects, nonlinear mooring system behavior or sheared and unsteady currents. The use of Computational Fluid Dynamics (CFD) to simulate VIM can more realistically represent the full scale Reynolds number, waves effects, mooring system, and ocean currents than scaled physical model tests. This paper describes a set of VIM CFD simulations for a Spar hard tank with appurtenances and their comparison against a high quality scaled model test. The test data showed considerable sensitivity to heading angle relative to the incident flow as well as to reduced velocity. The simulated VIM-induced sway motion was compared against the model test data for different reduced velocities (Vm) and Spar headings. Agreement between CFD and model test VIM-induced sway motion was within 9% over the full range of Vm and headings. Use of the Improved Delayed Detached Eddy Simulation (IDDES, Shur et al 2008) turbulence model gives the best agreement with the model test measurements. Guidelines are provided for meshing and time step/solver setting selection.

Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies

2009 ◽  
Author(s):  
Huilong Ren ◽  
Jian Zhang ◽  
Guoqing Feng ◽  
Hui Li ◽  
Chenfeng Li
Keyword(s):  
Equilibrium Position ◽  
Equations Of Motion ◽  
Offshore Structures ◽  
Hydrodynamic Performance ◽  
Coupled Analysis ◽  
Mooring System ◽  
Main Function ◽  
Ocean Engineering ◽  
Mooring Lines ◽  
Floating Structures

Coupled dynamic analysis between floating marine structures and flexible members such as mooring lines and risers, is a challenging work in the ocean engineering field. Coupled analysis on mooring-buoy interactions has been paid more and more concern for recent years. For floating offshore structures at sea, the motions driven by environmental loads are inevitable. The movement of mooring lines occurs due to the excitation on the top by floating structures. Meanwhile the lines restrict the buoy’s motion by forces acting on the fareleads. Positioning is the main function of mooring system, its orientation effects can’t be ignored for floating structures such as semi-submersible, FPS, and TLP, especially when the buoy’s equilibrium position shifting to another place. Similar as hydrostatic restoring forces, mooring force related with the buoy’s displacement can be transformed into mooring stiffness and can be added in the differential equations of motion, which is calculated at its equilibrium point. For linear hydrodynamic analysis in frequency domain, any physical quantity should be linear or be linearized, however mooring stiffness is nonlinear in essence, so the tangent or differential stiffness is used. Steel chains are widely used in catenary mooring system. An explicit formulation of catenary mooring stiffness is derived in this article, which consists of coupled relations between horizontal and vertical mooring forces. The effects of changing stiffness due to the shift of equilibrium position on the buoy’s hydrodynamic performance are investigated.

Behaviour of Mooring Systems for Different Line Pretensions

2014 ◽  
Vol 567 ◽  
pp. 204-209 ◽  
Author(s):  
Montasir Osman Ahmed ◽  
Anurag Yenduri ◽  
V. John Kurian
Keyword(s):  
Experimental Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Preliminary Design ◽  
Mooring Lines ◽  
Matlab Code ◽  
Heading Direction ◽  
Restoring Forces ◽  
Static Approach ◽  
Floating Platforms

Mooring lines are the most commonly used station-keeping systems for floating platforms as they are easy to install and relocate. The mooring lines are usually pre-tensioned so as to use their energy absorption to reduce the platform motions and thereby, to lower the forces in the lines. To decide on the preliminary design of the platforms, it is necessary to investigate the restoring behaviour of the mooring systems for various parameters. In this study, two different mooring configurations with and without mooring line in wave heading direction are considered for determining its behaviour for various pretensions in the lines. A MATLAB code named QSAML has been developed using quasi-static approach to compute the restoring forces of the mooring system. The code is validated with experimental tests and used in this study. It has been observed that with increase in pretension of the mooring line, restoring performance of the mooring system can be improved. The maximum permissible excursions by mooring system in the wave heading direction are found to be more for relatively lower pretension values.

Model Tests for a Floating Wind Turbine on Three Different Floaters

2014 ◽  
Vol 136 (2) ◽  
Author(s):  
Bonjun J. Koo ◽  
Andrew J. Goupee ◽  
Richard W. Kimball ◽  
Kostas F. Lambrakos
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Wind Farms ◽  
Model Tests ◽  
Experimental Comparison ◽  
Floating Platform ◽  
Scaled Model ◽  
Advantages And Disadvantages ◽  
Floating Wind Turbine ◽  
Floating Platforms

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution and, therefore, many countries build wind farms near the shore. Until now, most offshore wind farms have been built in relatively shallow water (less than 30 m) with fixed tower type wind turbines. Recently, several countries have planned to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine. The model tests described in this paper were performed at MARIN (maritime research institute netherlands) with a model setup corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the national renewable energy lab (NREL) 5 MW horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semisubmersible, and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled. The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for the calibration and improvement of the existing design analysis and performance numerical codes. An additional objective of the model tests was to establish the advantages and disadvantages among the three floating platform concepts on the basis of the test data. The paper gives details of the scaled model wind turbine and floating platforms, the setup configurations, and the instrumentation to measure motions, accelerations, and loads along with the wind turbine rpm, torque, and thrust for the three floating wind turbines. The data and data analysis results are discussed in the work of Goupee et al. (2012, “Experimental Comparison of Three Floating Wind Turbine Concepts,” OMAE 2012-83645).

Model Tests on the Parametric Resonance of the Deep Draft Semi-Submersible Under Regular and Irregular Waves

2020 ◽  
Author(s):  
Chenglin Li ◽  
Liwei Yu ◽  
Shuqing Wang ◽  
Zhe Tian ◽  
Xujie Wang
Keyword(s):  
Wave Height ◽  
Parametric Resonance ◽  
Model Tests ◽  
Irregular Waves ◽  
Mooring System ◽  
Mooring Lines ◽  
Natural Period ◽  
Beam Wave ◽  
Wave Flume ◽  
Pitch Direction

Abstract Model tests on the parametric resonance of the deep draft semi-submersible (DDS) under regular and irregular waves are conducted in the wave flume of the Ocean University of China (OUC). Model tests with and without mooring lines are conducted under various regular and irregular waves with different wave angles. From the results of the model without mooring lines, it is found that parametric resonance generally occurs when wave period is about the natural period of heave and about half of the natural period of pitch or roll. The steady amplitude of the parameter resonance is larger under larger wave height. In heading waves, parametric resonance occurs in the roll direction, while parametric resonance occurs in both roll and pitch direction for the beam wave. In irregular waves, parametric resonance is practical non-ergodic. For the model without mooring lines, parametric resonance does not occur under the constraints of the mooring system.

Numerical Simulations and Truncation Design of a Deepwater Internal Turret FPSO Before Model Test

2010 ◽  
Author(s):  
Hongwei Wang ◽  
Yong Luo ◽  
Xiaoming Cheng
Keyword(s):  
Numerical Simulations ◽  
Model Test ◽  
Time Domain ◽  
Model Tests ◽  
Test Model ◽  
Mooring System ◽  
Static Characteristics ◽  
Mooring Lines ◽  
Horizontal Dimension ◽  
The Time Domain

Model tests of an internal turret moored deepwater FPSO will be conducted in the deepwater offshore basin recently completed in Harbin Engineering University. Considering the limitations of the basin facilities and to reduce the complexity of fixing risers on the basin bottom, the 13 risers are simplified and combined to 4 risers. In addition, the horizontal dimension of the basin is not able to accommodate mooring system at the test model scale. Therefore, certain mooring lines have to be truncated. Two truncation methods are proposed according to the similitude of static characteristics of the mooring system. The first method only truncates the portion of the line that lies on the seabed and the second method truncates the suspended line portion. Numerical simulations have been carried out in the time domain with mooring lines coupled with FPSO. The calculated results are found to agree well with those of the prototype mooring system. Both truncation methods are found to be robust and reliable which provide a valuable reference for the forthcoming model tests.

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.

A Study on Mooring Systems of an Offshore Floating Platform for Different Configurations

2014 ◽  
Vol 567 ◽  
pp. 197-203
Author(s):  
Anurag Yenduri ◽  
Montasir Osman Ahmed ◽  
V. John Kurian
Keyword(s):  
Static Analysis ◽  
Experimental Tests ◽  
Mooring System ◽  
Preliminary Design ◽  
Floating Platform ◽  
Mooring Lines ◽  
Matlab Code ◽  
Restoring Forces ◽  
Floating Platforms

Moored systemsof floating platforms typically have mooring lines in groups,attached to hull of the platform in different configurations.Study on therestoring behaviour of these systemsfor variousparameters of mooring lines gives an insight to decide on the preliminary design of platforms.This paper includesparametric study on the behaviour of mooring systems for various symmetric, asymmetric configurations of the lines. To compute the restoring forces of the mooring system,quasi-static analysis has been adopted and a MATLAB code named QSAML has been developed, validated with experimental tests and used for the parametric study.It has been observed that, the restoring capability of mooring system reduces whenthe lines are placed away from the wave heading.For any wave heading, the variation in restoring behaviour of mooring system with symmetric configurations is insignificant up to relatively small excursions of the platform.

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.

Model Tests for a Floating Windturbine on Three Different Floaters

2012 ◽  
Author(s):  
Bonjun Koo ◽  
Andrew J. Goupee ◽  
Kostas Lambrakos ◽  
Richard W. Kimball
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Wind Farms ◽  
Model Tests ◽  
Floating Platform ◽  
Scaled Model ◽  
Advantages And Disadvantages ◽  
Near Shore ◽  
Set Up ◽  
Floating Platforms

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution, and therefore many countries build wind farms near shore. Up to now, most of offshore wind farms have been built in relatively shallow water (less than 30m) with fixed tower type wind turbines. Recently, several countries plan to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine. The model tests described in this paper were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled. The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for calibration and improvement of existing design analysis and performance numerical codes. An additional objective of the model tests was to establish advantages and disadvantages among the three floating platform concepts on the basis of test data. The paper gives details of the scaled model wind turbine and floating platforms, the set-up configurations, and the instrumentation to measure motions, accelerations and loads as well as wind turbine rpm, torque and thrust for the three floating wind turbines. The data and data analysis results are the subject of another paper in this conference [1].

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