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.

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.

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.

Reliability Analysis of Mooring Lines for Deep Water Floating Systems

2012 ◽  
Author(s):  
K. Gurumurthy ◽  
Suhail Ahmad ◽  
A. S. Chitrapu
Keyword(s):  
Dynamic Analysis ◽  
Reliability Analysis ◽  
Deep Water ◽  
Reliability Assessment ◽  
Mooring Line ◽  
Analysis Method ◽  
Mooring Lines ◽  
Line Load ◽  
Coupled Dynamic Analysis ◽  
Floating Systems

Reliability analysis of mooring lines requires an accurate prediction of extreme responses for large number of sea states even for a short-term based approach. In deep water, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been shown to give more accurate results but at a higher computational expense. Therefore, efficient computational tools are required for reliability analysis of mooring lines for deep water floating systems. Enhanced decoupled dynamic analysis method, in which the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring line, is an efficient method and provides results comparable in accuracy with the fully coupled dynamic analysis procedures. This paper presents the application of enhanced de-coupled dynamic analysis method for reliability assessment of mooring lines for deep water floating systems. For reliability analysis of mooring lines, the methodology presented in Ding et al. [5] is adopted. Reliability analysis of a critically loaded mooring line for a deep water classical spar floater under extreme environmental loads is performed using environmental contour approach. Mooring line tension time histories under various storm conditions are calculated using enhanced de-coupled dynamic analysis. The uncertainty in the predicted maximum mooring line load due to different storm events, variability in met-ocean conditions and numerical models is considered. Probability of failure and the corresponding reliability index of the mooring line are calculated. The impact of variability in predicted mooring line load, line capacities and factors of safety on mooring line reliability are studied. It is seen that enhanced de-coupled dynamic analysis, which predicts the mooring line loads as accurately as coupled dynamic analysis with lesser CPU time, can be used more efficiently for reliability assessment of mooring lines for deep water floating systems.

Evaluation of Conventional Methods of Establishing Extreme Mooring Design Loads

2017 ◽  
Author(s):  
Dunja Stanisic ◽  
Michalakis Efthymiou ◽  
Mehrdad Kimiaei ◽  
Wenhua Zhao
Keyword(s):  
Time Domain ◽  
Coupled Model ◽  
Gumbel Distribution ◽  
Extreme Condition ◽  
Mooring Line ◽  
Sea State ◽  
Mooring Lines ◽  
Line Load ◽  
Design Loads ◽  
The Time Domain

A key aspect in the design of a mooring system for a floating production unit is the estimation of the extreme mooring line loads for a specified short-term sea state of typical duration equal to 3 hours. Commonly used design approaches today are based on time-domain simulations whereby each 3 hour sea state is run a number of times (typically 10–30 times) to represent the randomness of the sea. A maximum response is recorded from each simulation. Particular statistic of the maxima data (e.g. mean, most probable maximum or a percentile) is used to represent the extreme mooring load for which the lines are designed. This paper studies and assesses the accuracy of obtaining design value from a population of maxima with reference to the mooring line load of a large ship-shaped floating production vessel. A coupled model, including all mooring lines and risers, has been developed, validated and used to generate responses for 100yr extreme condition and 10,000yr survival condition. To establish an accurate benchmark against which the results are compared, the time-domain analyses (duration 3 hours) are repeated 170 times, for each sea state, to represent different random realisations of each environment. It is examined how the accuracy of predicting the design mooring line load, from a sample of response maxima, improves as the number of simulations is increased progressively from 10 through to 170. The assessment is performed across different statistics of maxima that are usually chosen to represent the design response. Besides the mooring line load, other response parameters such as heave and turret excursion, are examined in this paper. The paper examines whether the severity of the response (100yr vs 10,000yr storm) or the response variable affect the number of maxima required to achieve statistical stability. The results indicate fitting a Gumbel distribution to the maxima from about 30–40 simulations can yield results that are statistically stable and accurate and are recommended as preferred methods of estimating the design response.

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.

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

2017 ◽  
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 load and the dynamic response of a moored structure in drifting ice. The mooring lines were explicitly modelled 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 6 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 can 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 a full numerical model for the coupled analysis of a moored structure in a broken ice field with current and wave.

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.

Experimental Investigation vs Numerical Simulation of the Dynamic Response of a Moored Floating Structure to Waves

2014 ◽  
Author(s):  
Daniele Dessi ◽  
Sara Siniscalchi Minna
Keyword(s):  
Dynamical Behavior ◽  
Irregular Waves ◽  
Mooring Line ◽  
Floating Structure ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Time Histories ◽  
Actual Configuration ◽  
Proper Orthogonal ◽  
Dynamic Wave

A combined numerical/theoretical investigation of a moored floating structure response to incoming waves is presented. The floating structure consists of three bodies, equipped with fenders, joined by elastic cables. The system is also moored to the seabed with eight mooring lines. This corresponds to an actual configuration of a floating structure used as a multipurpose platform for hosting wind-turbines, aquaculture farms or wave-energy converters. The dynamic wave response is investigated with numerical simulations in regular and irregular waves, showing a good agreement with experiments in terms of time histories of pitch, heave and surge motions as well as of the mooring line forces. To highlight the dynamical behavior of this complex configuration, the proper orthogonal decomposition is used for extracting the principal modes by which the moored structure oscillates in waves giving further insights about the way waves excites the structure.

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