A Concise Methodology for the Design of Statically-Equivalent Deep-Offshore Mooring Systems

2014 ◽  
Author(s):  
Ikpoto E. Udoh
Keyword(s):  
Degrees Of Freedom ◽  
Offshore Structures ◽  
Optimization Techniques ◽  
Dynamic Responses ◽  
Mooring Line ◽  
Model Parameters ◽  
Mooring System ◽  
Static Properties ◽  
Floating Structure ◽  
Restoring Forces

Model testing of deepwater offshore structures often requires the use of statically-equivalent deepwater mooring systems. The need for such equivalent systems arises due to the spatial limitations of wave basins in accommodating the dimensions of the direct-scaled mooring system. With the equivalent mooring system in place and connected to the model floater, the static global restoring forces and global stiffness of the prototype floating structure can be matched (to within some tolerance) by those of the model for specified offsets in the required degrees of freedom. A match in relevant static properties of the system provides the basis for comparisons of dynamic responses of the model and prototype floaters. Although some commercial programs are capable of designing equivalent mooring systems, the physics applied in these programs are protected by intellectual property, and their methodologies are generally inflexible. This paper illustrates a concise approach to the design of statically-equivalent deepwater mooring systems. With this approach, either manual or advanced optimization techniques can be applied as needed based on the complexity of the equivalent system to be designed. A simple iterative scheme is applied in solving the elastic catenary equations for the optimal static configuration of each mooring line. Discussions cover the approach as applied in developing a fit-for-purpose tool called STAMOORSYS, its validation, and its application to the design of an equivalent mooring system for a spar platform in deepwater. The spar model parameters are representative of a structure which could be tested in the Offshore Technology Research Center, College Station, Texas, USA. Results show that the method is capable of producing good design solutions using manual optimization and a genetic algorithm.

North Sea Mooring Systems: How Reliable Are They?

2014 ◽  
Author(s):  
Will Brindley ◽  
Andrew P. Comley
Keyword(s):  
Continental Shelf ◽  
North Sea ◽  
Recent Decade ◽  
Mooring Line ◽  
Mooring System ◽  
Failure Rates ◽  
Floating Structure ◽  
Strength Capacity ◽  
Failure Statistics ◽  
Overall Reliability

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system? This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system. The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

Prediction of Hydrodynamic Damping of Moored Offshore Structures Using CFD

2019 ◽  
Author(s):  
Changqing Jiang ◽  
Ould el Moctar ◽  
Thomas E. Schellin
Keyword(s):  
Offshore Structures ◽  
Least Square ◽  
Navier Stokes ◽  
Coupled Analysis ◽  
Total System ◽  
Mooring System ◽  
Restoring Force ◽  
Hydrodynamic Damping ◽  
Restoring Forces ◽  
Decay Tests

Abstract Usually, mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced phenomena or fluid-structure interactions and the associated viscous damping effects. By assuming that only the mooring system influences the restoring force characteristics, the contribution of mooring-induced damping to total system damping is neglected. This paper presents a technique to predict hydrodynamic damping of moored structures based on coupling the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We obtained hydrodynamic damping coefficients using a least-square algorithm to fit the time trace of decay tests. We analyzed a moored offshore buoy and validated our predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised the decaying oscillating buoy motions, the natural periods, and the associated linear and quadratic damping characteristics. Predicted motions, natural periods, and hydrodynamic damping generally well agreed to comparable experimental data.

scholarly journals Dynamic Response for a Submerged Floating Offshore Wind Turbine with Different Mooring Configurations

2019 ◽  
Vol 7 (4) ◽  
pp. 115 ◽  
Author(s):  
Yane Li ◽  
Conghuan Le ◽  
Hongyan Ding ◽  
Puyang Zhang ◽  
Jian Zhang
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Intermediate Water ◽  
Mooring System ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbine

The paper discusses the effects of mooring configurations on the dynamic response of a submerged floating offshore wind turbine (SFOWT) for intermediate water depths. A coupled dynamic model of a wind turbine-tower-floating platform-mooring system is established, and the dynamic response of the platform, tensions in mooring lines, and bending moment at the tower base and blade root under four different mooring configurations are checked. A well-stabilized configuration (i.e., four vertical lines and 12 diagonal lines with an inclination angle of 30°) is selected to study the coupled dynamic responses of SFOWT with broken mooring lines, and in order to keep the safety of SFOWT under extreme sea-states, the pretension of the vertical mooring line has to increase from 1800–2780 kN. Results show that the optimized mooring system can provide larger restoring force, and the SFOWT has a smaller movement response under extreme sea-states; when the mooring lines in the upwind wave direction are broken, an increased motion response of the platform will be caused. However, there is no slack in the remaining mooring lines, and the SFOWT still has enough stability.

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.

Numerical Investigation on Interaction Between a Semi-Submersible Platform and its Mooring System

2015 ◽  
Author(s):  
Yuanchuan Liu ◽  
Yao Peng ◽  
Decheng Wan
Keyword(s):  
Mooring Line ◽  
Mooring System ◽  
Floating Structure ◽  
Mooring Lines ◽  
Flow Solver ◽  
Floating Structures ◽  
Marine Renewable Energy ◽  
Motion Responses ◽  
Increasing Demand ◽  
Energy Engineering

With the increasing demand of floating structures in offshore, coastal and marine renewable energy engineering, the interaction between the mooring system and floating structure becomes more and more important. In this paper, motion responses of a semi-submersible platform with mooring system under regular wave conditions are investigated numerically by a viscous flow solver naoe-FOAM-SJTU based on the open source toolbox OpenFOAM. Influence of the mooring system on the platform motion responses is evaluated in two different ways. Investigations are covered for analysis methods adopted for solving mooring lines and the length of each part of a multi-component mooring line. Several important conclusions are drawn.

Disconnectable Mooring System for Ice Class Floaters

2009 ◽  
Author(s):  
John Murray ◽  
Apurva Gupta ◽  
Foo Kok Seng ◽  
Asbjorn Mortensen ◽  
Wong Toh Tung
Keyword(s):  
Oil And Gas ◽  
Offshore Structures ◽  
The Arctic ◽  
Normal Operation ◽  
Release Mechanism ◽  
Mooring System ◽  
Offshore Platforms ◽  
Floating Structure ◽  
Arctic Regions ◽  
External Assistance

Designs for offshore structures that will be deployed in arctic regions have to consider ice forces, which can be the governing design load. In shallow-water areas (< ∼100 m), fixed bottom caisson designs and artificial islands usually offer the best solutions. In deeper water, however, moored floating systems such as the ship-shaped Floating Production Unit (FPU), the Spar and the Single Column Floater (SCF™) are practical solutions. Even in shallower water where earthquakes are a threat, a moored floater can be a better option because of its ability to avoid seismic effects of the quake due to its suspension in the water above the sea floor. It is estimated that about 30% of the remaining oil and gas reserves lie in the arctic regions of the world. Special care has to be taken to design offshore platforms to survive these extreme arctic environments. Along with severe environmental conditions, there is a possibility of icebergs and multi-year ice sheets as well. A practical solution is to remove the floating structure in case of the expected loads on the structure exceed the maximum design loads. The platform can be brought back to location after the extreme environmental condition has passed. This paper describes a design for a disconnectable mooring system that comprises of external fairleads, submerged chain locker boxes and diverter units along with quick release mechanism and a single handling winch for each group of lines. The system works in tandem with the rig’s ROV and an anchor handling tugboat when connecting the system, while disconnection is automatic without any external assistance other than an emergency release signal. The paper provides the rationale for selecting this configuration based on its strength, redundancy requirements, quick disconnectable ability, without interfering with normal operation and presents a range of applications for this design.

scholarly journals Model of Bio-Colonisation on Mooring Lines: Updating Strategy Based on a Static Qualifying Sea State for Floating Wind Turbines

2020 ◽  
Vol 8 (2) ◽  
pp. 108
Author(s):  
Benjamin Decurey ◽  
Franck Schoefs ◽  
Anne-Laure Barillé ◽  
Thomas Soulard
Keyword(s):  
Spatial Model ◽  
Expert Knowledge ◽  
Conditional Entropy ◽  
Stochastic Modelling ◽  
Life Time ◽  
Offshore Structures ◽  
Mooring Line ◽  
Model Parameters ◽  
Mooring Lines ◽  
Virtual Sensing

Bio-colonisation affects the ageing of materials and the behaviour of offshore structures. Mooring systems and umbilicals belong to the family of slender bodies which are components sensitive to bio-colonisation because of a change of dynamic behaviour due to shape, roughness and mass modifications. However, this stochastic process in time and space is hard to predict. The purpose is then twofold: first, to provide a stochastic spatial model of the bio-colonisation on a mooring line; second, to show that in some defined environmental conditions, such as low wave height, low wind and current velocities, the monitoring of mooring lines tension can help to assess and reduce uncertainty on this model. Therefore, a comprehensive stochastic modelling based on mussels colonisation was carried out using on-site videotapes, experimental campaigns and expert knowledge. We studied the efficiency of a virtual sensing network using this model and a conditional entropy metric. It is first shown that the spatial model fits well with experimental data, and second that a denser medium accuracy sensor network is to be preferred to a single high accuracy fairlead sensor to reduce the uncertainty on the model parameters. It is then worth updating bio-colonisation on mooring lines during the life-time of a floating wind turbine.

Wave-free motions of isolated bodies and the existence of motion-trapped modes

2007 ◽  
Vol 584 ◽  
pp. 225-234 ◽  
Author(s):  
D. V. EVANS ◽  
R. PORTER
Keyword(s):  
Harmonic Oscillation ◽  
Finite Depth ◽  
Finite Energy ◽  
Mooring Line ◽  
Floating Structure ◽  
Local Flow ◽  
Trapped Mode ◽  
Time Harmonic ◽  
Restoring Forces ◽  
Mooring Forces

A motion trapping structure can be defined as a freely floating structure under natural or externally applied restoring forces, on or below the free surface of a heavy fluid extending to infinity in at least one direction, which generates a persistent local time-harmonic oscillation of the fluid of finite energy at a particular frequency, due to its own motion at that frequency. Such an oscillation is termed a motion-trapped mode. In this paper it is shown, using accurate numerical computations, that a submerged circular cylinder making forced time-harmonic two-dimensional heave or sway motions of small amplitude in a fluid of either finite or infinite depth, can create a local flow field in which no waves radiate to infinity at particular frequencies and depths of submergence of the cylinder. By tethering such a buoyant cylinder to the bottom of a fluid of finite depth, using a vertical inelastic mooring line, it is shown, by suitable choice of buoyancy and length of tether, how the cylinder, moving freely under its mooring forces, can operate as a motion trapping structure. Such a cylinder would, if displaced from its equilibrium position and released, ultimately oscillate indefinitely at the trapped mode frequency. This simple geometry is the first example of a submerged isolated motion trapping structure free to move under its natural mooring forces.

Optimization of Mooring System for Multi-Arrayed Tidal Turbines in a Strong Current Area

2014 ◽  
Author(s):  
Yu-ho Rho ◽  
Chul-hee Jo ◽  
Do-youb Kim
Keyword(s):  
Power Systems ◽  
Tidal Current ◽  
Mooring Line ◽  
Limited Area ◽  
Mooring System ◽  
Ocean Energy ◽  
Floating Structure ◽  
Energy Potential ◽  
Mooring Lines ◽  
Floating Structures

Recently, focus has been placed on ocean energy resources as environmental concerns regarding the exploitation of hydrocarbons are increasing. The enormous energy potential in tidal current power fields has been exploited by installing floating tidal current power turbines. Due to other floating structures operating within a limited area, stationkeeping is needed in order to keep the motions of the floating structures within permissible limits. In this study, methods for selecting and optimizing the mooring system for floating tidal current power systems in shallow water are investigated. The mooring system provides restoring forces and moments on a floating structure, pulling the structure back toward its equilibrium position. Typically, the mooring lines are segmented in order to optimize the force and displacement characteristics known as the mooring line characteristics. The mooring system concepts investigated in this study include the distributed mass, clump weight, and buoyancy element mooring systems.

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.

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