Mooring Configurations Research Articles

Bayesian analysis of short term extreme mooring tension for a point absorber with mixture of gamma and generalised pareto distributions

Model tests have been conducted to study the performance of slack, catenary and hybrid mooring systems for a point absorber in survival sea states. The snap loads can be found in mooring tension time series of all the tested mooring configurations. The classical Weibull distribution is applied to model the extreme dynamic mooring tensions. In addition to the Weibull distribution, the Bayesian inference method with a new Markov chain Monte Carlo sampling procedure is proposed to study the short term extreme mooring tensions based on model test results. Before applying this method to analyse extreme mooring tension, its accuracy is validated. The probability densities as well as short term extreme dynamic tension estimated by a mixture of Gamma and Generalised Pareto Distributions models are compared with the two-parameter Weibull model results. In the end, an extensive discussion of about the mixture of Gamma components, peak definitions, threshold and Markov chain Monte Carlo iteration on Bayesian inference is carried out. It is validated that the mixture model proposed in this paper shows good performance in predicting the extreme mooring tensions even when the snap loads occur, while the Weibull distribution fails to give accuracy predictions.

Impacts of water depth increase on offshore floating wind turbine dynamics

This paper aims at investigating the effect of water depth increase on the global performance of a floating offshore wind turbine, with a special focus on the environmental loading effects and turbine operating status. An integrated aero-hydro-servo-elastic (AHSE) analysis was simulated in the time domain. The model was first validated against published results in terms of mooring system restoring force and platform natural frequencies. The considered water depth is between 200 and 300 m, which is the deep-water range used in the current floating offshore wind turbine (FOWT) industry. In this study, both normal operating and failure conditions were considered. Key conclusions from case studies indicated that, based on the current water depth range, platform heave motion with slack mooring configurations and mooring line top tension are more sensitive to water depth. Water depth increase influences the tower base bending force when the turbine has a high-speed shaft brake due to grid loss, but the effects are restricted to the high-frequency response range (>2 Hz) and less obvious than the influences on mooring lines.

Concept Design of a Deployable Marine Energy Testing System

AbstractMarine renewable energy has the potential to be a significant contributor to the world's energy portfolio. However, the technology is still in early stages of development, largely due to the lack of infrastructure to support in-water testing. A small number of established permanent test sites exist, but these sites are often prohibitively expensive to build and operate. The objective of this project was to develop a concept design for a small deployable marine energy test system. The solution described is a self-contained system with the capability to monitor the local environmental conditions, host and test one small wave energy converter (typically <1-kW power generation), and collect and store data from the device. The system employs a Sofar Spotter V2 buoy for environmental assessment, a triangular mooring configuration to secure the device, and a USB data acquisition device paired with a laptop computer for data collection and analysis. The system is completely housed in a portable trailer and could be deployed to any area with suitable water conditions (depth, wave environment, etc.). The concept design presented here could serve as a baseline for low-cost customized deployable systems for use by academia, industry, or government organizations.

Optimization of mooring systems in the context of an integrated design methodology

This work describes an enhanced mooring optimization procedure, oriented towards recent floating production systems (FPS) for oil & gas exploitation in ultra-deep-water scenarios, which may present a large number of risers in an asymmetric layout. Acknowledging that the risers are the key component of an FPS, the optimization procedure is associated to an integrated mooring-riser design methodology; thus, instead of simply minimizing the platform offsets and/or the costs of the mooring system itself, one of the main objectives is to obtain a mooring configuration that ensures the integrity of the risers. Other highlights of the optimization procedure include the following aspects: Enhancements in the modeling of the optimization problem (including the definition of design variables, objective function and constraints that are relevant for such actual applications); The use of the PSO optimization algorithm associated to the ε-constrained method to efficiently handle the constraints; Enhancements in the evaluation of candidate solutions, by full nonlinear time-domain dynamic Finite Element simulations with coupled models; and the implementation in a parallel computing environment to deal with the high associated computational costs. A case study considering an FPS representative of actual applications in deepwater scenarios is presented to illustrate the practical application of the optimization tool.

Study of Snap Loads for Idealized Mooring Configurations with a Buoy, Inextensible and Elastic Cable Combinations for the Multi-Float M4 Wave Energy Converter

There has been considerable modelling and wave basin validation of the multi-mode, multi-float, moored wave energy converter M4. The 6 float (2 power take off) (PTO) configuration is considered here with mooring from a buoy with light inextensible cables. Large mean mooring forces and very large peak or snap forces were measured in large waves while the rotational response about the hinges (for power take off in operational conditions) was predominantly linear. Modelling has been extended with elastic mooring cables connected directly to the base of the bow float and to the buoy. The experimental mean force is input to the linear diffraction/radiation model. The device response is effectively unchanged. The peak mooring force and tensions remain large with direct connection to the base of the bow float but are only slightly greater than the mean forces with elastic cables to the buoy, and an elastic hawser provides a further slight reduction. For the largest waves measured, the force was about 10% of the dry weight of the platform. The idealized efficient modelling may inform more detailed design while efficient methods for determining highly nonlinear mean forces remain to be established.

Experimental investigation on a dual chamber floating oscillating water column moored by flexible mooring systems

A series of 1:50 model tests are conducted to study the motion responses and energy harvesting performance of a novel dual chamber floating oscillating water column device moored by three flexible mooring systems in regular and irregular waves. This novel energy conversion device consists of one traditional oscillating water column chamber and one Backward Bend Duct Buoy chamber. The mooring systems, include one modified Catenary Anchor Leg Mooring concept and two compact mooring systems, designed for station keeping of this specific floating device. Strong nonlinearities of mooring tension are found, especially in the low wave period sea conditions. It is also found that the surge and yaw motion has significant differences when the device moored by the different mooring systems. The pitch and heave natural periods are derived from their Responses Amplitude Operator curves. The performance of spectral methods in mooring fatigue damage estimation depends on mooring configuration and material. It is seen that the attached clump weights affect the fatigue damage of the compact mooring rope. It has been found that the novel device shows good performance in energy harvesting within broad range of wave periods, and the influence of mooring configuration on energy conversion performance depends on the wave height.

Mooring systems with submerged buoys: influence of buoy geometry and modelling fidelity

Mooring systems often make use of submerged buoys (SBs) in order to make the moorings compliant. In this paper we present the dynamic effects of changing the buoy geometry or the buoy model fidelity on the mooring system response. Three cylindrical SBs with increasing slenderness (height/diameter) are studied for a mooring leg with two polyester ropes and a SB. The results show a large impact of SB geometry on the mooring dynamics. A larger height/diameter ratio (with preserved mass and buoyancy) is shown to be beneficial as it gives both smaller tension force magnitudes and, more importantly, avoids slack-snap occurrence in the upper cable. We further present a comparison between four numerical methods for SB dynamics: (i) a high-fidelity model using computational fluid dynamics (CFD); (ii) the Morison equation with slender body drag force approximation using numerical quadrature; (iii) the Morison equation with an independent evaluation of the fluid drag due to translation and rotation; and (iv) a translating Morison model which simulates a vertical cylinder in three degrees of freedom with no rotation. All methods are used together with a high-order finite element mooring dynamics solver. The results show that the translating method is inadequate to model this mooring configuration. The remaining three methods agree moderately well, but the Morison formulations give larger motions and higher tensions compared to the CFD results. We show that the quadrature drag model is better suited to model the drag moment on SBs than the independent model, and that the improvement increases with increasing slenderness of the buoy. The uncertainty, sensitivity and importance of the hydrodynamic coefficients of the buoy are discussed and examined by a regression analysis from the CFD data.

Wave energy converters in array configurations—Influence of interaction effects on the power performance and fatigue of mooring lines

In an array system of wave energy converters (WECs), hydrodynamic interactions between the WECs occur through the absorption, radiation, and diffraction of waves. The main objective of this study was to analyse the hydrodynamic interactions between floating point-absorbing WECs and the influence of this interaction on the power performance and fatigue life of the WECs’ mooring lines. The hydrodynamic and structural response of the WEC systems were analysed primarily using potential flow theory, the Morison equation, and continuum mechanics theory. Four 2-WEC models and two 10-WEC models, among which different separating distances and mooring configurations were considered, were investigated. The models were simulated for various environmental loading conditions. The results from each simulation were evaluated in terms of the power performance of each WEC, accumulated fatigue damage in each mooring line, and the levelised cost of energy (LCoE). The hydrodynamic interactions show a larger impact on the 10-WEC simulation models. To account for the hydrodynamic interactions in the simulations using the 10-WEC models, the fatigue damage in the mooring lines is varied at an average of 15% and a maximum of an order of magnitude difference. A similar comparison of power performance showed an approximate difference in the results of 10%. The LCoE calculation of the 10-WEC array system showed LCoE values that range from −40% to +200% relative to the simulations that did not consider the hydrodynamic interaction effects. Comparatively, the LCoE calculation of the 2-WEC model either decreased by 17% or increased by 23% when the hydrodynamic interaction was included in the simulation model.

Parametric Study of a Taut Compliant Mooring System for a FOWT Compared to a Catenary Mooring

The catenary mooring system is a well recognized station keeping method. However, there could be economical and environmental benefits of reducing the footprint. In the last decades, more focus has been given to synthetic mooring lines and different mooring layouts to optimize the levelized cost of energy (LCOE) for offshore renevable energy converters such as wave energy converters. Therefore, this work presents a parametric study of two important parameters, namely the mooring line angle and line pretension, for a taut mooring configuration focusing on the dynamic response when applied to the TetraSpar floating foundation compared to a catenary mooring system. The work is based on experimental results conducted in the wave basin at Aalborg University (AAU) and compared to analytical stiffness calculations. In addition, a numerical model was tuned based on the main dynamics to achieve the tension response. The results showed satisfying dynamic behavior where the angle and pretension mainly influenced the surge and yaw natural periods. The motion response showed similar behavior between the chosen parameters, and larger pitch amplitudes were found compared to the catenary system.

Experimental investigation on hybrid mooring systems for wave energy converters

Abstract This study presents a comprehensive experimental assessment of three hybrid mooring systems for a heaving-buoy wave energy converter. Free decay model tests are conducted to study the natural periods in heave, pitch and surge modes of the buoy when it is moored with different mooring configurations. A series of regular and irregular wave model tests are carried out to investigate the hydrodynamic responses of the point absorber as well as mooring dynamics. The effects of wave period, incident wave height and mooring configuration are evaluated. Besides, the influence of mooring configuration on buoy motion is discussed. To study the short term extreme dynamic tension, the traditional Weibull distribution, Weibull distribution based on tail data and peaks-over-threshold method are applied. The results showed that the Weibull distribution failed to present accurate extreme dynamic tension prediction when the snap events occur frequently, while the other two methods showed good performance despite of number of snap events. Based on the results of energy production performance and extreme dynamic tension, a novel mooring line design is suggested, which helps to reduce mooring dynamic tension.

Assessment of Wind Turbine Aero-Hydro-Servo-Elastic Modelling on the Effects of Mooring Line Tension via Deep Learning

As offshore wind turbines are moving to deeper water depths, mooring systems are becoming more and more significant for floating offshore wind turbines (FOWTs). Mooring line failures could affect power generations of FOWTs and ultimately incur risk to nearby structures. Among different failure mechanics, an excessive mooring line tension is one of the most essential factors contributing to mooring failure. Even advanced sensing offers an effective way of failure detections, but it is still difficult to comprehend why failures happened. Unlike traditional parametric studies that are computational and time-intensive, this paper applies deep learning to investigate the major driven force on the mooring line tension. A number of environmental conditions are considered, ranging from cut in to cut out wind speeds. Before formatting input data into the deep learning model, a FOWT model of dynamics was simulated under pre-defined environmental conditions. Both taut and slack mooring configurations were considered in the current study. Results showed that the most loaded mooring line tension was mainly determined by the surge motion, regardless of mooring line configurations, while the blade and the tower elasticity were less significant in predicting mooring line tension.

Experimental Investigation of the Mooring System of a Wave Energy Converter in Operating and Extreme Wave Conditions

A proper design of the mooring systems for Wave Energy Converters (WECs) requires an accurate investigation of both operating and extreme wave conditions. A careful analysis of these systems is required to design a mooring configuration that ensures station keeping, reliability, maintainability, and low costs, without affecting the WEC dynamics. In this context, an experimental campaign on a 1:20 scaled prototype of the ISWEC (Inertial Sea Wave Energy Converter), focusing on the influence of the mooring layout on loads in extreme wave conditions, is presented and discussed. Two mooring configurations composed of multiple slack catenaries with sub-surface buoys, with or without clump-weights, have been designed and investigated experimentally. Tests in regular, irregular, and extreme waves for a moored model of the ISWEC device have been performed at the University of Naples Federico II. The aim is to identify a mooring solution that could guarantee both correct operation of the device and load carrying in extreme sea conditions. Pitch motion and loads in the rotational joint have been considered as indicators of the device hydrodynamic behavior and mooring configuration impact on the WEC.

Fin whale 40‐Hz calling behavior studied with an acoustic tracking array

Fin whale 40‐Hz calling behavior studied with an acoustic tracking array

Lessons learned from the monitoring of turbidity currents and guidance for future platform designs

Abstract Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (<<2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended modifications to improve design by simplifying mooring configurations, minimizing surface area and enhancing seafloor stability. Finally, we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as feet and ballast that can be ejected. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, in a wider range of settings.

Dynamic Response of Coupled Mooring Lines and Floating Wave Energy Devices in Waves / Currents

Various global studies have shown that ocean waves energy have large potential in renewable energy sector. Their role within renewable energy gets high priority in the future by the government of United Kingdom. The principle concept of wave energy is when wave energy is converted into potential energy by the wave energy devices to generate electricity. An understanding of the dynamic response of the devices and mooring lines is important for this paper. This paper deals with the analysis of the various effects that influence the different design of wave energy converter devices. The mooring design idea is also analyzed to show which mooring layout is suitable to fulfill the requirement. The design of mooring configuration also influence how wave power is extracted and how such system are operated and maintained. The effects investigated in this paper are regular and irregular waves, motion @ six degrees of freedom, maximum and minimum mooring tension, different waves direction, wave current, energy and power take off.

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.

Experimental Study of a Moored Floating Oscillating Water Column Wave-Energy Converter and of a Moored Cubic Box

This paper describes experimental research on a floating moored Oscillating Water Column (OWC)-type Wave-Energy Converter (WEC) carried out in the wave flume of the Coastal Engineering Research Group of Ghent University. This research has been introduced to cover the existing data scarcity and knowledge gaps regarding response of moored floating OWC WECs. The obtained data will be available in the future for the validation of nonlinear numerical models. The experiment focuses on the assessment of the nonlinear motion and mooring-line response of a 1:25 floating moored OWC WEC model to regular waves. The OWC WEC model motion has 6 degrees of freedom and is limited by a symmetrical 4-point mooring system. The model is composed of a chamber with an orifice on top of it to simulate the power-take-off (PTO) system and the associated damping of the motion of the OWC WEC model. In the first place, the motion response in waves of the moored floating OWC WEC model is investigated and the water surface elevation in the OWC WEC chamber is measured. Secondly, two different mooring-line materials (iron chains and nylon ropes) are tested and the corresponding OWC WEC model motions and mooring-line tensions are measured. The performance of these two materials is similar in small-amplitude waves but different in large wave-amplitude conditions. Thirdly, the influence of different PTO conditions is investigated by varying the diameter of the top orifice of the OWC WEC model. The results show that the PTO damping does not affect the OWC WEC motion but has an impact on the water surface elevation inside the OWC chamber. In addition, an unbalanced mooring configuration is discussed. Finally, the obtained data for a moored cubic model in waves are presented, which is a benchmarking case for future validation purposes.

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

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.

Numerical design study of multipoint mooring systems for the floating wave energy converter in deep water with a sloping bottom

The use of a floating wave energy converter (WEC) in the deep water zones around islands is increasing, but conventional mooring configurations often do not provide suitable station keeping options. This paper focus on the assessment of a suitable mooring design for floating WECs in deep water locations, with a sloping seabed. The mooring study considers a 50-year return period to assess the survival rate for three mooring designs i) catenary mooring systems, ii) synthetic cable (polyester) taut mooring system, and iii) suspended anchoring point mooring system. Furthermore, the variations of bathymetry conditions around the oceanic islands are considered, as well as factors directly influencing these deep-water mooring installations (e.g. anchor uplift forces, weight of mooring system). The coupled motion dynamics of the WEC and the mooring system are studied using a potential theory in a time-domain model, and the power production is also considered. The Chinese 100 kW Sharp Eagle WEC [22] is used as the reference design. The deployment site has a water depth of 200–700 m and the angle of the slope ranges from 30° to 90°. The paper is investigating the effects of (i) the features of the seabed condition; (ii) the mooring line material, size and layout; and (iii) the installation and maintenance costs. The outcomes of the studies for the three different mooring designs are compared and an additional fatigue analysis has been implemented to obtain fatigue damage criteria for the mixed catenary mooring system. A cost analysis has been implemented, and recommendations provided discussing the most suitable mooring design configuration.

Mooring Experimental Study of Motion Response for Pendulum Wave Energy Converters

Wave Energy Converters (WECs) is the type of Ocean energy need continual motion while operation for producing steady electricity. The mooring configuration as the answer for making sustainable WECs during operational life on the rough sea. The authors are researching trimaran pontoon of pendulum WECs type with anchors and four cables of mooring. The purpose of this research to determine the heave, pitch, and roll response from the pontoon. In this research, there are variations of wave height and wave periods. Experiment located at hydraulics laboratory in regular wave condition. The scale-up of the pontoon is (1:10) compared to real dimension. IMU (Inertial Measurement Unit) sensor used in this experiment to detect pontoon motion. Pontoon has the optimum condition at wave periods at 0.6s and height 0.4 m. Heave motion best displacement is 0.372m, Roll motion value is 1.425 degree and Pitch is 4.6164 degree. The pontoon needs fairlead for placed the mooring cables.