Design Of Mooring System Research Articles

The motion response and hydrodynamic performance comparisons of the new subsea suspended manifold with two mooring scenarios

The subsea manifold as the hub oil and gas transportation equipment is a critical part for the layout optimization of subsea production system and reducing the number of pipelines. The traditional subsea manifold is fixed on seabed by its foundation, so traditional manifold system is hard to be discarded and recovered although it has many advantages and is applied widely in the deep water development scenario. In this paper, a new subsea suspended manifold concept is proposed to discuss its feasibility by the hydrodynamic performance state in vertical tension and catenary mooring methods. A mathematical model is established to solve the chain tension distribution and displacement of the subsea suspended manifold. Meanwhile, the motion response of the subsea suspended manifold under different influence factors, tension distribution of the mooring chains and the flexible jumpers in two mooring schemes are simulated, respectively, through OrcaFlex software. The research results will provide an important reference for the optimization of the preliminary scenario design of the subsea suspended manifold mooring system.

Technical Issues in Design and Construction of Foundation and Mooring System for FOWT in Domestic Conditions

Technical Issues in Design and Construction of Foundation and Mooring System for FOWT in Domestic Conditions

Design and comparative analysis of alternative mooring systems for floating wind turbines in shallow water with emphasis on ultimate limit state design

Floating wind turbines represent a cost-efficient energy solution in deep water where bottom-fixed wind turbine becomes excessively expensive. However, mooring design is quite challenging for all shallow water depths including the transition water depth between bottom-fixed and floating wind turbines, in the order of 50–80 m, for which floating concepts might become more cost-effective than bottom-fixed ones. In this paper, mooring system design for floating wind turbine in shallow water are studied considering both catenary and taut mooring systems. Seven mooring concepts designed for a 5 MW semi-submersible floating wind turbine at 50 m water depth are compared with the purpose to identify solutions that are structurally reliable and economically attractive. The concepts are made of different mooring line materials (chain and synthetic fibre rope), mooring components (clump weight and buoy) and anchors (drag embedment anchor and suction anchor). Based on the latest experimental data, the nonlinear tension-dependent stiffness of synthetic fibre rope are described with an improved numerical model. Performance of the seven mooring concepts are compared with respect to mooring line characteristics, motion response amplitude operator, utilization factor considering the ultimate limit state design and cost etc. Six mooring design concepts are finally recommended for future assessment regarding the application of floating wind turbines in shallow water say 50 m and deeper.

Effect of wave inhomogeneity on fatigue damage of mooring lines of a side-anchored floating bridge

Mooring systems are important structural components of very long floating bridges. They effectively limit the transverse motions of the bridge under environmental loads. They also add viscous hydrodynamic damping to the entire system. The safe and economical design of mooring systems is thus important but also challenging especially when the wave conditions are inhomogeneous. In this paper, a computational study is carried out to investigate the responses of the mooring lines for a 4.6 km long fjord crossing floating bridge accounting for inhomogeneous wave conditions. Based on the structural responses, this study also attempts to evaluate the fatigue damage in the mooring lines by using different fatigue analysis methods. The accuracy of the spectral methods is examined by comparison with the conventional rainflow cycle counting algorithm. Numerical studies are conducted to obtain an indication of the effect of various wave inhomogeneities on the fatigue damage in the mooring lines.

Design load estimation with IFORM-based models considering long-term extreme response for mooring systems

ABSTRACT In this paper, we present an extension of the alternative environmental contour approach based on inverse first-order reliability theory in a three-dimensional model that accounts for short-term extreme response uncertainties. Subsequently, the long-term extreme tension under wave excitation loads is investigated to evaluate return levels for the design of mooring systems. Tension extreme data are derived from time-domain simulations of a floating coupled system using the peak-over-threshold method to determine short-term load distribution. A linear interpolation scheme is utilised to establish the parametric model using distribution parameters and wave data. Long-term extreme loads are estimated using a simplified discrete approach combined with Monte Carlo simulations, which helps avoid the tedious task of direct integration. The applicability of these load assessment models is demonstrated and discussed using an example of a semi-submersible platform situated at a 500-m water depth, and the results are compared with one- and two-dimensional environmental contour-based models.

Compact floating wave energy converters arrays: Mooring loads and survivability through scale physical modelling

The current cost of electricity has been preventing the technological progress of wave energy converters. The use of compact wave energy converters arrays is seen as a technological breakthrough for reducing these costs. These array configurations aim to maximize the energy extracted per unit area of marine space and to promote the sharing of components and installation, operation and maintenance tasks. Mooring lines are fundamental components in these systems and represent a considerable portion of the project cost. Reducing the number of mooring lines through the application of arrays with inter-body connections has a high potential for cost reduction. In this paper, we present the experimental study of different configurations of a five-device array of spar-buoy oscillating-water-column wave energy converters in a wave basin, focusing on the analysis of the devices motion and the mooring line loads. The study compares the performance of a single isolated device, an array with independently-moored devices and three arrays with inter-body connections, with different levels of connectivity in the mooring arrangement. Tests were carried out for moderate and extreme wave conditions. Results show good performance for all configurations when subjected to moderate sea conditions. Under extreme sea conditions, high peak tensions were observed in the lines of all array configurations. Particularly large values were detected in the inter-body lines, caused by their full extension. Based on these results, guidelines for the design of mooring systems for compact wave energy converter arrays are presented.

Review of Wave Energy Converter and Design of Mooring System

In recent decades, the emphasis on renewable resources has grown considerably, leading to significant advances in the sector of wave energy. Nevertheless, the market cannot still be considered as commercialized, as there are still other obstacles in the mooring system for wave energy converters (WECs). The mooring system must be designed to not negatively impact the WEC’s efficiency and reduce the mooring loads. Firstly, the overview of the types of wave energy converters (WECs) are classified through operational principle, absorbing wave direction, location, and power take-off, respectively, and the power production analysis and design challenges of WECs are summarized. Then, the mooring materials, configurations, requirements, and the modeling approaches for WECs are introduced. Finally, the design of mooring systems, including the design considerations and standards, analysis models, software, current research focus, and challenges are discussed.

Design and Application of Buoy Single Point Mooring System with Electro-Optical-Mechanical (EOM) Cable

This paper presents the design of a hybrid system named the Mooring Buoys Observation System for Benthic with Electro Optical Mechanical Cable (MBOSBC) for long-term mooring buoy observation in deep oceans. MBOSBC is comprised of three main modules: a surface buoy, a benthic node, and an Electro-Optical-Mechanical (EOM) cable. The Surface buoy provides energy for the entire system, the Benthic node is used to observe scientific phenomena on the seabed, and the Electro-Optical-Mechanical (EOM) cable connects the sea surface buoy and the benthic observation node, serving as the information and power transmission link. This paper provides design criteria for a single point mooring buoy using an EOM cable. The environmental load of the buoy under varying wind, wave, and current conditions is analyzed and the mooring hydrodynamic force of EOM is calculated. After calculating the load of the EOM cable under extreme marine conditions, the hydrodynamic force needed for the system is analyzed. After physically testing the strength of the designed EOM cable, a near-shore test of MBOSBC was carried out, in order to verify that the system has the expected function.

Experimental Flow-Induced Motions of a FOWT Semi-Submersible Type (OC4 Phase II Floater)

Abstract Flow-induced motions (FIM) are an issue for floating offshore structures, such as multi-column platforms, as the phenomenon can decrease the fatigue life of the mooring, riser, and cable systems. The new concept of floating offshore wind turbines (FOWT) have a multi-column design that may be subjected to FIM. In the past, FIM was studied mainly for Oil & Gas platforms installed in deep waters. However, the FIM phenomenon of FOWT has been insufficiently explored. To rectify this, model tests were performed for the semisubmersible (SS) floating system design developed for the DeepCwind project (Offshore Code Comparison Collaboration Continuation (OC4) phase II). This paper will investigate the presence of FIM and show its importance in the design process of FOWT. Three different incidence angles of the current were tested, namely, 0, 90, and 180 deg. For each heading, 30 reduced velocities were tested, across the range 8000 < Re < 70,000. The results showed amplitudes in the transverse direction of around 70% of the diameter of the platform column, which is similar to those observed for deep-draft (DD) SS with circular columns. Note that these amplitude values are larger for a floater with a circular column, than for a platform with square columns. The results showed that as FIM occurred for this specific FOWT SS, its effect has to be considered in the mooring system and electric cable design.

A Comparison of Numerical Approaches for the Design of Mooring Systems for Wave Energy Converters

This paper analyses the numerical outcome of applying three different well-known mooring design approaches to a floating wave energy converter, moored by means of four catenary lines. The approaches include: a linearized frequency domain based on a quasistatic model of the mooring lines, a time domain approach coupled with an analytic catenary model of the mooring system, and a fully coupled non-linear time domain approach, considering lines’ drag and inertia forces. Simulations have been carried out based on a set of realistic combinations of lines pretension and linear mass, subject to extreme environmental conditions. Obtained results provide realistic cost and performance indicators, presenting a comparison in terms of total mooring mass and required footprint, as well as the design line tension and structure offset. It has been found that lines’ viscous forces influence significantly the performance of the structure with high pretensions, i.e., >1.2, while there is acceptable agreement between the modelling approaches with lower pretensions. Line tensions are significantly influenced by drag and inertia forces because of the occurrence of snap loads due to the heaving of the floater. However, the frequency domain approach provides an insight towards the optimal design of the mooring system for preliminary designs.

Mooring System Design and Verification for a Floating Vertical Axis Wind Turbine

The aim of this study is to investigate the technical feasibility of an innovative vertical axis floating wind turbine concept with the main focus on the design and verification of the mooring system. The study is developed through iterative processes in order to identify the optimum design for the new floating wind turbine concept. The Ultimate Limit State (ULS) criteria have been considered to verify the integrity of the mooring system in the extreme environmental conditions with a 50-year return period. For this purpose, time domain dynamic analysis has been performed using the commercial software OrcaFlex [Orcina website, OrcaFlex software, https://www.orcina.com/ ]. Although the analysis is carried out for a specific site deemed suitable for the project, the results can be used as an input for any future application in other locations. The present study is intended to be a proof of concept with a proposed scientific framework for optimization of the mooring system which is considered to be a crucial part in the design of floating wind turbines due to their complex dynamic behavior.

The effect of rope termination on the performance of polyester mooring ropes for marine applications

Termination of heavy-duty polyester mooring ropes has long been an issue of concern in the marine renewable energy applications. In this study, experimental investigations were conducted on two different rope materials to investigate and compare the performance of filament, yarn, strand, sub-rope, 44 mm rope and 120 mm rope. The experiment setup consisted of 5 kN Instron Machine for testing filament, yarn and strand, 500 kN Denison Machine for sub-rope, 1500 kN and 30 MN tensile equipment for testing 44 mm and 120 mm ropes respectively. At constant test conditions, extensive experiments were carried out to examine the effects of various termination configuration on rope performance. The terminations used included Parafil socket (Viking 7 socket), splice and another novel termination called Stress Relief Socket. The results show that the use of stress relief socket has led to increased tensile performance of existing ropes by up to 13% for Akzo ropes. Considering the results of the tensile tests, the Stress Relief Socket has proved to have a significant advantage for replacement of existing methods of termination e.g. the splice. It should also be noted that the reproduction of the socket termination is more consistent than that of other methods of termination. The experimental outcomes can instruct future optimal mooring system design and marine renewable energy performances.

Mooring System Design for the 10MW Triple Spar Floating Wind Turbine at a 180 m Sea Depth Location

This works presents a methodology for the design of a mooring system for the Triple Spar floating wind turbine that supports the INNWIND 10MW wind turbine. The mooring system keeps the platform at the desired location and avoids the drift caused by wind, currents and nonlinear hydrodynamics. A semi-taut mooring system configuration, combining steel chain and polyester is chosen to reduce the cost. The basic configuration is defined using static equations and provides a smooth relationship between the restoring force and the displacement, to prevent snap loads. A dynamic analysis for the environmental conditions of the Gulf of Maine is performed to verify the performance of the design. The tensions on the lines are well below the maximum breaking limit for the different materials. Nevertheless, the chain weight can not be decreased as it provides the required mooring system stiffness. It has been verified that the polyester segment does not contact the seabed that could potentially damage it. The dynamic analysis also shows that the anchors do not experience vertical forces and the lines do not impact the platform heave plates. A complete load analysis must be performed to fully validate the proposed design.

A shallow water mooring system design methodology combining NSGA-II with the vessel-mooring coupled model

In this paper, a mooring system design methodology combining the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) with the vessel-mooring coupled model is first proposed. In the proposed methodology, the multi-objective optimization problem for mooring system design which involves several design variables, objective functions and constraints is established and solved by the combination of the NSGA-II algorithm and the vessel-mooring coupled model, and the initial feasible range of design variables is narrowed in advance by combining the NSGA-II with the quasi-static mooring system model. A case study is presented to demonstrated the proposed methodology, in which a catenary mooring system is designed from scratch for a semi-submersible very large floating structure (VLFS) module. It can be concluded from the obtained results that, owing to the combination of the NSGA-II algorithm and the vessel-mooring coupled model, the accurate dynamic responses of vessel and mooring systems can be used as the objective functions; thus, the proposed methodology is able to attain an optimal mooring system considering more practical design requirements, such as the vessel motion response, the safety factor of the mooring system, the length of mooring line lay down part and mooring system layout.

Field test study and quasi-static analysis of global characteristics and survivability of a wave energy converter test platform mooring system

This study presents a catenary spread mooring system design of a mobile ocean test berth, named the Ocean Sentinel (OS), and developed by the Northwest National Marine Renewable Energy Center to facilitate the ocean testing of wave energy converters (WECs). The original OS mooring design (which was similar to a conventional WEC point absorber mooring system) was evaluated through a field data analysis using a quasi-static approach. The field data analysis on the measurements of mooring tension and environmental conditions collected during an ocean test of the OS is based on a quasi-static analysis of the analytical catenary equations of mooring chains. Both global and survivability characteristics of the mooring system were evaluated. The global characteristics include the influence of the OS excursion on mooring tension, the directional control of the OS, and the environmental contributions of waves, current, and wind. The relationship between the dynamic mooring tension and the environmental conditions of waves and current was quantified through an empirical equation. It was found that the contribution of current to the mooring tension oscillation increased as the current velocity increased and was significant at high current velocity. The survivability characteristics include the anchor movability and strength capacities of mooring lines. Because anchor movement occurred near the end of the field test, a new systematic procedure of designing a mooring system with adequate anchor resistance was developed and applied to improve the OS mooring system design.

Numerical Study of Mobilized Friction along Embedded Catenary Mooring Chains

Understanding the soil resistance along an embedded anchor chain is imperative for efficient and economic design of an overall mooring system as it determines the magnitude and direction of the load at the padeye of the anchor. The tensioning process of an embedded chain for catenary moorings was modeled using a coupled Eulerian–Lagrangian (CEL) finite-element approach simulating the large deformations of the chain as it cuts through the soil to form an inverse catenary. The analyses reveal that the configuration of the embedded chain and the relationship between tension and chain angle at the padeye show excellent agreement with previously published analytical predictions. However, the ratio of the tension at the padeye to that at the mudline obtained from CEL is significantly higher than the theoretical values, mainly due to partial mobilization of the frictional soil resistance along the length of the chain. The CEL results indicate that the partial mobilization is a result of the combined-loading effect during failure of the soil around the embedded chain as it cuts through the seabed, in contrast with the conventional assumption that the ultimate frictional and normal soil resistances are mobilized simultaneously. A new design approach is proposed for calculating the local equivalent coefficient of friction based on the yield locus for a deeply embedded chain and the normality rule.

Design optimization of mooring system: An application to a vessel-shaped offshore fish farm

Design optimization of mooring systems of offshore floating structures is a challenging task, partly because of the large number of design variables, complicated design constraints, nonlinear system behavior, and time-consuming numerical simulations. For engineering designs, efficient yet accurate approaches are needed. This paper proposes an integrated optimization methodology for design of mooring systems. The methodology integrates the design of experiments, screening analysis, time-domain simulations, and a metamodel-based optimization procedure. To demonstrate the methodology, the mooring system of a vessel-shaped offshore fish farm was designed considering the ultimate limit state. The fully-coupled numerical model includes a floater, flexible fish cages and a single-point mooring system. The Kriging metamodels were applied as surrogates for the responses of time-domain simulations. The optimal solutions were found by exploring the design space using a gradient-based search algorithm. Validations were performed on the metamodels and the global optimal solutions. The proposed methodology is also applicable to design optimization of other marine structures.

Mooring System Optimisation and Effect of Different Line Design Variables on Motions of Truss Spar Platforms in Intact and Damaged Conditions

This paper presents the effect of mooring diameters, fairlead slopes and pretensions on the dynamic responses of a truss spar platform in intact and damaged line conditions. The platform is modelled as a rigid body with three degrees-of-freedom and its motions are analysed in time-domain using the implicit Newmark Beta technique. The mooring restoring force-excursion relationship is evaluated using quasi-static approach. MATLAB codes DATSpar and QSAML, are developed to compute the dynamic responses of truss spar platform and to determine the mooring system stiffness. To eliminate the conventional trial and error approach in the mooring system design, a numerical tool is also developed and described in this paper for optimising the mooring configuration. It has a graphical user interface and includes regrouping particle swarm optimisation technique combined with DATSpar and QSAML. A case study of truss spar platform with ten mooring lines is analysed using this numerical tool. The results show that optimum mooring system design benefits the oil and gas industry to economise the project cost in terms of material, weight, structural load onto the platform as well as manpower requirements. This tool is useful especially for the preliminary design of truss spar platforms and its mooring system.

Investigations on mooring system design for a floating platform in shallow water

Investigations on mooring system design for a floating platform in shallow water

Review of mooring design for floating wave energy converters

A review of some representative floating wave energy converter (WEC) projects in the world, including the wave energy capturing technology, development history, main dimensions and tested sea sites is presented. The design essentials of WEC mooring system are discussed and a mooring system design procedure for WEC is proposed, which includes the introduction of related design codes and mooring analysis methods. In the last part, a comprehensive review of mooring system design in the current WECs projects is conducted. Different mooring systems and mooring materials are introduced and discussed. Based on the discussions, recommendations are made for the suitable mooring design according to the WEC's dimensions and working principles. It is shown that the elastic synthetic rope has great potential in the application of WEC mooring system, and the hybrid mooring system could be a good solution for WEC station keeping problem.