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.

Validation of Mooring Simulations (for Mooring Integrity Assessment) With In-Service Tension Measurements

2021 ◽  
Author(s):  
Willemijn Pauw ◽  
Remco Hageman ◽  
Joris van den Berg ◽  
Pieter Aalberts ◽  
Hironori Yamaji ◽  
...  
Keyword(s):  
Numerical Models ◽  
Weather Conditions ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Data Set ◽  
Integrity Assessment ◽  
Motion Data ◽  
Load Cells ◽  
Line Loads

Abstract Integrity of mooring system is of high importance in the offshore industry. In-service assessment of loads in the mooring lines is however very challenging. Direct monitoring of mooring line loads through load cells or inclinometers requires subsea installation work and continuous data transmission. Other solutions based on GPS and motion monitoring have been presented as solutions to overcome these limitations [1]. Monitoring solutions based on GPS and motion data provide good practical benefits, because monitoring can be conducted from accessible area. The procedure relies on accurate numerical models to model the relation between global motions and response of the mooring system. In this paper, validation of this monitoring approach for a single unit will be presented. The unit under consideration is a turret-moored unit operating in Australia. In-service measurements of motions, GPS and line tensions are available. A numerical time-domain model of the mooring system was created. This model was used to simulate mooring line tensions due to measured FPSO motions. Using the measured unit response avoids the uncertainty resulting from a prediction of the hydrodynamic response. Measurements from load cells in various mooring lines are available. These measurements were compared against the results obtained from the simulations for validation of the approach. Three different periods, comprising a total of five weeks of data, were examined in more detail. Two periods are mild weather conditions with different dominant wave directions. The third period features heavy weather conditions. In this paper, the data set and numerical model are presented. A comparison between the measured and numerically calculated mooring line forces will be presented. Differences between the calculated and measured forces are examined. This validation study has shown that in-service monitoring of mooring line loads through GPS and motion data provides a new opportunity for mooring integrity assessment with reduced monitoring system complexity.

A Direct Time Domain Simulation of Floating Structures With Mooring Lines

2011 ◽  
Author(s):  
Jingpu Chen ◽  
Dexiang Zhu
Keyword(s):  
Time Domain ◽  
Time Domain Method ◽  
Test Results ◽  
Time Domain Simulation ◽  
Mooring Lines ◽  
Restoring Force ◽  
Floating Structures ◽  
Rankine Panel Method ◽  
Domain Method ◽  
Force Characteristics

A direct time domain method is established for analyzing the interactions between the floating structures and mooring lines. The motions of floating structures are solved by a Rankine Panel method in time domain, while the static restoring force characteristics of the mooring system is taken into account. As numerical examples, the motions of two SPARs are solved by this direct time domain method. The numerical results of present method are analyzed and compared with model test results.

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.

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.

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.

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.

Hybrid Verification of a Deepwater FPSO Using Truncated Model Tests and Numerical Simulations

2021 ◽  
Author(s):  
Xiangbo Liu ◽  
Ching Theng Liong ◽  
Nitesh Kumar ◽  
Kie Hian Chua ◽  
Allan Ross Magee ◽  
...  
Keyword(s):  
Numerical Model ◽  
Numerical Modelling ◽  
Water Depth ◽  
Return Period ◽  
Line Length ◽  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Test Results ◽  
Set Up

Abstract This paper presents verification of a deep water FPSO with a semi-taut mooring system using model tests and numerical modelling commonly referred to as the hybrid method. The vessel under investigation is a FPSO of 310m in length and 47m in beam with an internal turret mooring system of 12 lines in 2000m water depth. Two configurations of the mooring systems i.e. inline and bisecting are investigated for sea-states up to 1000yr return period. A full depth mooring system has been developed for the FPSO and model tests will be carried out to verify the model. Due to limitations to the size of the model basins, the model tests will be carried out for a truncated mooring setup. Non-linear horizontal stiffness of a single mooring line and the complete mooring system with truncation is compared to that of the existing full depth mooring system. Discrepancies in the vertical forces due to truncation of line length will be discussed in the paper. A numerical model of the truncated set-up will be calibrated using model test results.

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.

scholarly journals Dynamic Response of Floating Wind Turbine Under Consideration of Dynamic Behavior of Catenary Mooring-Lines

2017 ◽  
Author(s):  
Shuangxi Guo ◽  
Yilun Li ◽  
Min Li ◽  
Weimin Chen ◽  
Yiqin Fu
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Nonlinear Dynamic ◽  
Ocean Waves ◽  
Wave Frequency ◽  
Mooring Line ◽  
Mooring Lines ◽  
Restoring Force ◽  
Dynamic Behaviors ◽  
Floating Wind Turbine

Recently, wind turbine has been developed from onshore area to offshore area because of more powerful available wind energy in ocean area and more distant and less harmful noise coming from turbine. As it is approaching toward deeper water depth, the dynamic response of the large floating wind turbine experiencing various environmental loads becomes more challenge. For examples, as the structural size gets larger, the dynamic interaction between the flexible bodies such as blades, tower and catenary mooring-lines become more profound, and the dynamic behaviors such as structural inertia and hydrodynamic force of the mooring-line get more obvious. In this paper, the dynamic response of a 5MW floating wind turbine undergoing different ocean waves is examined by our FEM approach in which the dynamic behaviors of the catenary mooring-line are involved and the integrated system including flexible multi-bodies such as blades, tower, spar platform and catenaries can be considered. Firstly, the nonlinear dynamic model of the integrated wind turbine is developed. Different from the traditional static restoring force, the dynamic restoring force is analyzed based on our 3d curved flexible beam approach where the structural curvature changes with its spatial position and the time in terms of vector equations. And, the modified finite element simulation is used to model a flexible and moving catenary of which the hydrodynamic load depending on the mooring-line’s motion is considered. Then, the nonlinear dynamic governing equations is numerically solved by using Newmark-Beta method. Based on our numerical simulations, the influences of the dynamic behaviors of the catenary mooring-line on its restoring performance are presented. The dynamic responses of the floating wind turbine, e.g. the displacement of the spar and top tower and the dynamic tension of the catenary, undergoing various ocean waves, are examined. The dynamic coupling between different spar motions, i.e. surge and pitch, are discussed too. Our numerical results show: the dynamic behaviors of mooring-line may significantly increase the top tension, particularly, the peak-trough tension gap of snap tension may be more than 9 times larger than the quasi-static result. When the wave frequency is much higher than the system, the dynamic effects of the mooring system will accelerate the decay of transient items of the dynamic response; when the wave frequency and the system frequency are close to each other, the displacement of the spar significantly reduces by around 26%. Under regular wave condition, the coupling between the surge and pitch motions are not obvious; but under extreme condition, pitch motion may get about 20% smaller than that without consideration of the coupling between the surge and pitch motions.

Research on the Arrangement of the Mooring Lines of the Thruster Assisted Mooring System

2013 ◽  
Author(s):  
Gang Zou ◽  
Lei Wang ◽  
Feng Zhang
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Main Function ◽  
Time Domain Simulation ◽  
Mooring Lines ◽  
Dynamic Positioning System ◽  
Optimum Arrangement ◽  
Line Angle ◽  
Mooring Forces ◽  
Offshore Industry

As the offshore industry is developing into deeper and deeper water, station keeping technics are becoming more and more important to the industry. Based on the dynamic positioning system, the thruster assisted mooring system (TAMS) is developed, which consisted of mooring lines and thrusters. The main function of the TAMS is to hold a structure against wind wave and current loads with its thruster and cables, which is mainly evaluated by the holding capacity of the system. The arrangement of the mooring lines (location of anchor or the mooring line angle relative to platform) will directly affect the TAMS holding capacity because of the influence of the directions of the mooring forces. So finding out an optimum arrangement of the mooring lines is essential since the performance of the TAMS depends greatly on the arrangement of the mooring lines. The TAMS of a semi-submersible platform, which is studied in this paper, consisted of eight mooring lines. By fixing the layout of the thrusters and changing the location of each mooring line for every case, the performances of the TAMS are analyzed. The platform motions, mooring line tensions and power consumptions are compared to obtain the optimum arrangement of mooring lines, and thus a thruster assisted mooring system with a better performance can be achieved. Time domain simulation is carried out in this paper to obtain the results.

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