Platform Mooring Research Articles

Investigation on local mooring stresses of floating offshore wind turbines considering mooring chain geometrical and material nonlinearity

As the renewable energy industry moves towards deep water, mooring chains play an irreplaceable role in position keeping. The geometrical and material nonlinearity of the mooring chains significantly affects moored platform responses and mooring responses. To investigate local mooring stress evolution under wave loads, a simulation method was proposed, combining hydrodynamic loads derived from fully coupled simulations of floating offshore wind turbines (FOWTs). The simulation method, embedding the lumped mass mooring model, was validated by comparing the experimental data from OC4 semi-submersible wind turbines. Their reasonable segment numbers and the mooring structural damping ratio were also discussed. Based on the validated simulation technology, a coupled 3D-mooring analysis was performed, replacing linear mooring stiffness suggested in FAST tutorials with nonlinear mooring stiffness calculated by axial stretching the real-sized 3D chains. The nonlinear mooring stiffness is assumed, taking into account detailed geometry, nonlinear material properties, and contact behavior between chains. The nominal diameter of the real-sized 3D-chains was determined, so that the chains had the same volume per unit length as the validated case, ensuring a similar Morison's force on the mooring line. Then, the local mooring stresses were investigated by loading the derived nonlinear fairlead tension onto both sides of real-sized 3D chains in an explicit finite element software. The evolution of local mooring stresses under design wave loads with different wave headings of 0, 30, 60° was compared and discussed. As wave direction increases, this study indicated that the amplitude of local maximum principal stress decreases and its phase leads. Additionally, its magnitude is mainly influenced by axial tension force and contact behavior.

Applicability of a Simplified Model of a 15MW Wind Turbine for a Mooring Fatigue Study

Mooring system of floating wind turbine design, as conditioned by fatigue analysis, requires time-consuming simulations. But to provide relevant solutions for the multiple coming tenders, efficient methodologies need to be developed to design quickly optimal floating platform mooring for specific metocean data and project constraints. This paper presents a comparison between a mooring fatigue assessment calculated with two standard detailed modelling of a 15MW wind turbine and a simplified Drag and Lift Model (DLM) based on global coefficients at hub often used in floating offshore wind preliminary analysis to carry out extreme analysis. The present contribution is here focused on the proper assessment of a quantitative error introduced by the simplified turbine model in the mooring fatigue assessment, and the evaluation of the missing contributions such as 3P excitation or low frequency induced by turbine control. Two different control approaches are assessed here to capture a range of coupled behaviour which have been observed on project. It has been shown that the DLM approach is accurate to carry out analysis under extreme conditions, but also gives relevant fatigue results for the most fatiguing lines. In general, the order of magnitude is consistent even for low damage. All the calculations have been made using DeepLines WindTM software.

Real-time prediction of mooring tension for semi-submersible platforms

The harsh operating environment of deepwater semi-submersible production platforms poses severe challenges to the safe operation and maintenance of their production processes. Using artificial intelligence technology to improve the operation and maintenance level of large-scale offshore engineering equipment has become an inevitable trend. Accurate mooring tension prediction for semi-submersible production platforms can effectively identify potential damage and ensure the platform's safe and efficient production operation. Compared with traditional cyclic neural networks, transformer neural networks have faster calculation speeds, better generalization abilities and scalability, and better abilities to capture long-distance dependencies. In this study, the first deepwater semi-submersible production platform in the South China Sea, “DeepSea One,” was selected as the research object, and an improved novel transformer neural network architecture was proposed for the mooring tension prediction of semi-submersible production platforms using actual long-term mooring tension data obtained during the platform's production process. The proposed method was compared with the commonly used long short-term memory neural network model, and the comparison proved the effectiveness of the proposed method in predicting the dynamic tension of semi-submersible production platform mooring. In addition, the proposed method is also applicable for the prediction of other time series data in the field of shipbuilding and offshore engineering, which is of certain value for generalization.

Study on the Vibration Characteristics of Marine Riser Based on Flume Experiment and Numerical Simulation

This study investigated vortex-induced vibrations of marine risers in Ocean Thermal Energy Conversion systems. Flume experiments were conducted under two conditions: Condition 1 was with a fixed riser bottom, and Condition 2 was with a fixed bottom on a mooring platform. The cross-flow acceleration of the riser was measured at different current velocities, and corresponding vibration responses were analyzed. Numerical simulations based on the flume experiments were employed to validate the reliability of the simulation method. Results from the flume experiments revealed vortex-induced resonance in Condition 1 when the flow velocity approached the riser’s natural frequency. In Condition 2, similar vibration responses were observed, with maximum acceleration occurring during flow velocity-induced vortex-induced vibrations. However, at higher flow velocities, the acceleration response showed a decrease followed by an increase, indicating the excitation of higher-order modes. The numerical simulations matched the flume experiments in Condition 1. In Condition 2, while the acceleration response and frequency agreed during vortex-induced resonance, discrepancies arose in the flow velocity that caused vortex-induced vibrations compared to the flume experiments. This study demonstrated the accuracy of numerical simulations in reflecting vortex-induced vibrations of risers, providing a foundation for further research on complex riser systems.

Mechanical model and characteristics of deep-water drilling riser-wellhead system under internal solitary waves

Internal solitary waves with a huge amount of energy easily trigger the large dynamic responses of riser-wellhead system and threaten its structural safety. However, previous studies have only focused on the dynamic response of the riser under internal solitary waves. The riser may experience excessive traction from the platform, especially from the mooring platform, in response to the arrival of internal solitary waves. The bottom of the riser connects to the wellhead system, which in turn exerts a reaction force on the riser. To address this problem, a coupled dynamic model of deep-water drilling mooring platform-riser-wellhead system under internal solitary waves is developed in this paper. A dynamic response analysis method based on the fourth-order Runge-Kutta method and finite element method is also proposed for the mooring platform-riser-wellhead system. A dynamical solver for the coupled system is then developed using MATLAB. The dynamic response characteristics of the riser-wellhead system under internal solitary waves are calculated. Results show that the displacement and bending moment of the system initially increases and then decreases along with the propagation of internal solitary waves, and finally reach equilibrium position. The displacement and bending moment reach their peak before the trough of internal solitary waves passes through the riser-wellhead system. The dynamic responses of the riser-wellhead system under the influence of internal solitary wave loads are much larger than those without the effect of internal solitary wave loads. The riser system experiences shearing loads at the interface of internal solitary waves, which trigger a step-like bending moment variation. The bending moment of the conductor under the mudline is greatly increased by the internal solitary waves.

Coupled Translational–Rotational Stability Analysis of a Submersible Ocean Current Converter Platform Mooring System under Typhoon Wave

This study proposes a mathematical model for the coupled translational–rotational motions of a mooring system for an ocean energy converter working under a typhoon wave impact. The ocean energy convertor comprises two turbine generators and an integration structure. The configuration of the turbine blade and the floating platform is designed. The two turbine blades rotate reversely at the same rotating speed for rotational balance. If the current velocity is 1.6 m/s and the tip speed ratio is 3.5, the power generation is approximately 400 kW. In the translational and rotational motions of elements under ocean velocity, the hydrodynamic parameters in the fluid–structure interaction are studied. Initially, the hydrodynamic forces and moments on the converter and the platform are calculated and further utilized in obtaining the hydrodynamic damping and stiffness parameters. The 18 degrees of freedom governing equations of the mooring system are derived. The solution method of the governing equations is utilized to determine the component’s motion and the ropes’ dynamic tensions. In the mooring system, the converter is mounted under a water surface at some safe depth so that it can remain undamaged and stably generate electricity under typhoon wave impact and water pressure. It is theoretically verified that the translational and angular displacements of the converter can be kept small under the large wave impact. In other words, the water pressure on the converter cannot exceed the predicted value. The relative flow velocity of the convertor to the current is kept fixed such that the power efficiency of convertor can be maintained as high. In addition, the dynamic tension of the rope is far less than its breaking strength.

A deep-water in-situ power generation system based on chain-driven hydrokinetic turbine

Harnessing hydrokinetic energy from deep water to increase the operation periods of autonomous underwater vehicles and underwater moored platforms has received considerable attention in recent years. Hydrokinetic turbines should have a reliable startup performance for deployment in deep water with a prevailing average current speed of 0.5 m/s. In the present study, a deep-water in-situ power generation system based on a novel chain-driven hydrokinetic turbine is proposed for such low-speed current application. A numerical simulation was performed on the proposed turbine to investigate its hydrodynamic performance at different initial setting positions and tip speed ratios. No static dead band exists in the operation of the turbine, indicating a superior self-starting advantage over traditional drag-type turbines. Multi-objective optimization was performed on the proposed turbine to improve its power coefficient and startup performance. Compared with the initial design (Cp = 0.234, Ct,s = 0.520), the Cp and Ct,s in the Pareto front range from 0.103 to 0.325 and 0.565 to 0.712, respectively. All-process simulation was conducted based on Simulink to estimate the power supply capacity of a deep-water in-situ power generation system using the optimal chain-driven hydrokinetic turbine. The result shows that this system can generate the electricity at 218.45 Wh/day in the deep-water location with an average current speed of 0.334 m/s, providing enough energy to the power units of autonomous underwater vehicles.

Comparative dynamic analysis of SCR and LWR in the coupled platform–mooring–riser system under internal solitary wave conditions

As a frequently occurring and widely distributed environmental load, an internal solitary wave (ISW) always appears with a large amplitude and may result in the instability of deepwater structures and offshore accidents. In this study, a self-developed program was established to describe the dynamic behavior of a coupled platform–mooring–riser system. The response of the platform is based on the rigid body dynamics theory, that of the risers/mooring lines is obtained by the lump mass method, and the coupled effect between them is achieved by a semi-coupled analysis. Two types of riser configurations, steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWRs), in the coupled system were compared under only ISWs and combined excitation of ISWs and random waves. The numerical results were proven to be reliable through validation against previous literature. The results show that different riser configurations have little influence on the platform response or mooring top tension. Both riser configurations exhibit a second-order deformation mode along the riser length and a half-wave shape with time. Additionally, a mutation in the horizontal tension, shear force, and bending moment occurs near the ISW interface. Compared with SCRs, LWRs always produce larger motion characteristics and responses with time differences owing to their complex configuration.

Performance Analysis of the Effects of Microgrooved Surface on A Marine Current Turbine with Underwater Mooring Platform

Performance Analysis of the Effects of Microgrooved Surface on A Marine Current Turbine with Underwater Mooring Platform

Effect of the multi-segment mooring system by buoy and clump weights on the dynamic motions of the floating platform

In this paper, two hybrid mooring system is proposed to improve the performance of the dynamic motions of the floating platform. The concepts are a multi-segment arrangement of the catenary and taut mooring lines, including buoy and clump weight. Two mooring concepts named MSM-BSW and MSM-BMW were designed for a 5 MW OC4-DeepCwind semi-submersible floating wind turbine. These models' mooring forces and platform response were compared to the main model (OC4) under regular and irregular wave conditions. The dynamic response of the platform is performed utilizing both Morrison theory and three-dimensional radiation/diffraction theory based on the boundary element method (BEM) in ANSYS-AQWA software. Adding buoy, increase the horizontal mooring line angle at the fairlead, resultantly on one hand's mooring stiffness in surge and pitch increased so the MSM-BSW model could reduce surge and pitch motions. On the other hand, the buoy can reduce the mooring suspended weight, leading to an increase in the heave platform response. In the case of MSM-BMW, more clump weights increased the mooring line stiffness in the heave direction. In regular and irregular wave conditions, numerical results indicate that the change of natural periods in both MSM-BSW and MSM-BMW case studies leads to the low-frequency resonance having a significant difference from OC4. In contrast, the wave frequency resonance of the three concepts is similar.

Design and Dynamic Stability Analysis of a Submersible Ocean Current Generator Platform Mooring System under Typhoon Irregular Wave

This research proposes a mooring system for an ocean current generator that is working under the impact of typhoon waves. The turbine and the platform are kept stable at a designed water depth to ensure that the generator remains undamaged and continuously generates electricity under excessive water pressure. In this design, the turbine generator is mounted in front of the floating platform by ropes and withstands the force of ocean currents, while the platform is anchored to the deep seabed with lightweight, high-strength PE ropes. In addition, two pontoons are used to connect the generator and the platform with ropes. When the balance is reached, the depth of the generator and the depth of the platform’s dive can be determined by the length of the ropes. In this study, typhoon irregular wave is represented by the Jonswap wave spectrum. The irregular wave is simulated by six regular waves. The equation of motion of the mooring system is derived. The theoretical solution of the dynamic system is presented to determine the dynamic displacements of the platform, pontoon, turbine and the dynamic tensions of the ropes. The dynamic tensions of the ropes increase with the cross-sectional area of pontoon. The natural frequency of the mooring system depends on the parameters, including the mases of elements, the lengths of ropes and the cross-sectional area of pontoons. In the proposed mooring configuration, the dynamic tension of the rope is far less than the breaking strength of the rope; thus, the ocean turbine is stable, and no water that flows through will be disturbed by the floating platform.

Numerical simulation research on hydrodynamic performance of platform with string-type mooring system

Objectives The string-type mooring system is proposed for the permanent berthing demand of special offshore platforms. It is necessary to study the hydrodynamic performance of the platform using this system to verify its adaptability to the permanent berthing demand. Methods Based on the three-dimensional potential flow theory, the numerical model of wharf berthing platform is established by using the boundary element method, and the frequency domain numerical simulation of the mooring platform under different wharf tide conditions is carried out. Two different mooring systems, string mooring and traditional dock mooring, were introduced into the time-domain simulation to simulate the performance of the mooring platform under extreme wave action, and parameterized analysis was conducted to analyze the influence of different tidal levels of the wharf on the motion response of the mooring platform. Results By providing omni-directional restoring force, the String-type mooring system strengthens the rotational motion limitation of the platform, greatly weakens the most violent rolling motion of the mooring platform, transfers the energy of environmental load to translational motion, and further strengthens the restriction of the overall motion of the platform; the motion range of the mooring platform is balanced and stable and controllable; the effect of the system on the movement restriction of the platform is less affected by the change of the tide level. Conclusions Compared with the traditional mooring system, the string mooring system is more suitable for the permanent berthing of special offshore platforms.

Experimental investigation on dynamic responses of a spar-type offshore floating wind turbine and its mooring system behaviour

This study investigated the dynamic behaviour of a spar-type offshore floating wind turbine (OFWT) analysed under various wave loads by conducting an experiment campaign. Both the characteristic of the spar platform itself and the spar with realistic moorings have been investigated. A particular focus of the study was the motions of the mooring lines, which were measured at a series of discrete points along one line using an underwater motion capture system, in parallel with tension measurement at the anchor points. The main objective of this study is to provide experimental measured data to validate numerical software on the research of mooring line motions.A free decay test has been carried out to confirm the spar platform motions natural frequencies and damping characteristics. The platform was then subjected to a range of regular and irregular waves to obtain the motion characteristics of the platform and the mooring lines. For the mooring line motions, which has rarely been discussed in published research, a non-linear snatching phenomenon has been observed at the tank for some wave frequencies. This non-linear behaviour shows that snatching leads to the high instantaneous mooring line tensions and platform accelerations could cause the failure of the mooring lines in the real structure.

Numerical Study on Sectional Loads and Structural Optimization of an Elastic Semi-Submersible Floating Platform

This study investigates the sectional loads on an elastic semi-submersible platform for a 2 MW FOWT (floating offshore wind turbine) used in the Fukushima demonstration project. A water tank test is firstly carried out with an elastic model to study the dynamic responses and sectional loads of the platform in regular and irregular waves. Numerical simulations are then performed using multiple hydrodynamic bodies connected by elastic beams. The dynamic responses of the elastic model are compared to those of a rigid model to clarify the influence of the structural stiffness on the platform motion and mooring tension. The predicted sectional loads on the deck, brace and pontoon by the proposed nonlinear hydrodynamic models show good agreement with the experimental data obtained from the water tank test and a simplified formula is proposed to evaluate the distribution of the moments on the platform. Finally, the structural optimization of the elastic semi-submersible platform is conducted. The sectional moments and fatigue loadings on the pontoons are significantly reduced using the strut between the pontoons since the horizontal wave loads on the side column are dominant and the vertical wave loads acting on the platform are relatively small due to the deep draft.

Long-term extreme response analysis for semi-submersible platform mooring systems

The accurate assessment of long-term extreme responses of floating-structure mooring system designs is important because of small failure probabilities caused by long-term and complex ocean conditions. The most accurate assessment would involve considering all conceivable sea states in which each sea state is regarded as a stochastic process and performing nonlinear time-domain numerical simulations of mooring systems to estimate the extreme response from a long-term analysis. This procedure would be computationally intensive because of the numerous short-term sea states involved. Here, a more feasible approach to evaluate the long-term extreme response is presented through immediate integration combined with Monte Carlo simulations. A parameter fitting procedure of the short-term extreme response distribution under irregular wave conditions is employed to solve the long-term response integration. Case studies were conducted on a semi-submersible platform using environmental data measurements of the Gulf of Mexico and a joint distribution model of the environmental parameters was considered. This approach was observed to be effective and the results were compared with those of traditional methodologies (univariate extreme value design and environmental contour methods). The differences were reflected using a reliability analysis of mooring lines, which indicated that the design standards must be stricter when using long-term analysis.

Platform Stabilization of Floating Offshore Wind Turbines by Artificial Muscle Based Active Mooring Line Force Control

The floating offshore wind turbine (FOWT) presents underactuation challenge for controls in terms of platform stability, power regulation, and increased structural loads, which demands for simple, low-cost, low-power, and high-bandwidth actuation concepts. In this article, an active mooring line force control (AMLFC) strategy is proposed based on a novel thermally actuated fishing line artificial muscle (FLAM) actuator. The proposed FLAM actuator consists of multiple bundles of twisted nylon fishing lines, which is added to the junction between the mooring lines and platform bars of FOWT with tensioned-leg platform (TLP). A simulation model of the FLAM actuator is developed in Simulink, along with an interface to the mooring line model of TLP-FOWT in NREL's FAST. The dynamic model of the FLAM actuator is obtained with ANSYS simulation, and a control-oriented model is obtained for the FOWT platform motion. A linear quadratic regulator is implemented for the FLAM-based AMLFC. Simulations are performed on the 5-MW WindPACT model for one Region-2 and one Region-3 scenarios. Simulation results show that, with mild power consumption, the proposed strategy can significantly reduce the platform roll motion and the tower-base side–side bending loads without little impact on the rotor speed and power output.

Influence of bedding-in on the tensile performance of HMPE fiber ropes

Synthetic fiber ropes are widely used in the maritime industry, with applications from sailboat rigging to offshore platform mooring lines, thanks to their numerous qualities compared to steel cables: light weight, good specific mechanical properties and excellent marine resistance. A key feature of these ropes is their sensitivity to loading history, as this affects subsequent performance. In the present work the influence of the maximum bedding-in load level and the loading path were studied at the different rope component scales (filament, yarn and braided rope), in order to develop an optimal bedding-in procedure for braided ropes. The load levels were: 10%, 30% and 50% of the tensile break load of the sample. From the results obtained, the influence of braid construction and material have been studied, and an efficient bedding-in process has been defined. It was shown that the main parameter is the maximum load level, the loading path is not important. The load required to bed in HMPE ropes is closely related to changes in the braid angle under load.

Mooring positioning performance of jack-up platform

ABSTRACT This study focussed on the mooring positioning of a jack-up platform. A numerical analysis was performed using ANSYS to examine the performance of a mooring system in time and frequency domains. The hydrodynamic response of the platform was analysed, and response amplitude operators were obtained in the frequency domain. A coupling analysis of platform motion was performed in the time domain, which examined the effects of environmental loading directions on platform displacements and the influence of mooring line parameters on mooring positioning performance. The primary concern was effects of the incident angle of environmental load on six components of platform displacement and mooring line tension. Moreover, a multi-objective optimisation technique was employed to control the length and tension of the mooring line in towing and positioning, which can determine the optimal length of the mooring line and then assist in achieving accurate platform positioning and steady mooring.

Numerical simulation of vertex induced motion for Spar platform with helical strakes

Objectives Long-lasting Vortex Induced Motion(VIM)may cause the Spar platform mooring system fatigue and even the structure damage. Therefore, the adverse effects of VIM must be fully taken into account. Methods In this paper, the CFD solver naoe-FOAM-SJTU and dynamic grid method developed by ourselves are used to make numerical simulation of the characteristics of the vortex induced flow field around the Spar platform. The time Delayed Detached-Eddy Simulation(DDES) turbulence model based on shear-transport-stress equation is used to simulate the three-dimensional fine wake vortex structure of the Spar platform with helical strakes. The longitudinal motion, transverse motion and yawing of the Spar platform with helical strakes at different reduced velocities are studied. The platform mooring system is stimulated by using the linear spring system. The research also takes into account the effects that the platform coupling effect at different degrees has on VIM. Then the numerical simulation results are compared with the model test results, to analyze the time history, spectral characteristics and locking of longitudinal and transverse VIM of Spar platform with helical strakes and to reveal the intrinsic mechanism of VIM. Results The results show that the helical strakes can effectively reduce the response amplitude of Spar platform's VIM; the CFD solver naoe-FOAM-SJTU has good accuracy and reliability for the calculation and simulation of the offshore platforms' VIM. Conclusions The numerical simulation and analysis of the VIM of Spar platform with helical strakes is of great significance to the practical design of Spar platform.

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.