Mooring System Research Articles

Short-term motion prediction of floating offshore wind turbine based on muti-input LSTM neural network

The motion response of an offshore floating wind turbine (FOWT) platform is closely related to the control operation regarding the safety of a wind turbine. It is affected by various factors such as sea state environments and mooring systems. In practice, how to predict the motion response of the wind turbine platform in the short term has always been a concern of engineering practice. At present, the development of deep learning technology has brought some potential solutions to this problem. In this paper, a Multi-Input Long-Short Term Memory (MI-LSTM) neural network method is proposed to predict the short-term motion response of a floating offshore wind turbine platform. Specifically, the numerical simulation of the 5 MW Braceless platform is carried out under different environmental conditions, and the data of platform motion response, wave elevation, and mooring force are selected as input variables. Then the training and test groups are established after post-processing data. Subsequently, a Single-Input LSTM (SI-LSTM) model and a Multi-Input LSTM (MI-LSTM) model are established to learn the input data. After comparing the overall accuracy of the results, it is found that the additional mooring force and wave elevation positively affects the platform response prediction results. From the aspects of discreteness and overall accuracy, it is verified that the established MI-LSTM model is also applicable, considering the influence of second-order hydrodynamics. Lastly, compared with the prediction results obtained by the multi-input one-dimensional convolutional neural network (MI1D-CNN), the advantages of the two different models are expounded from the perspectives of training time and accuracy, which provides ideas for the optimization of the FOWT motion response prediction model. This study sheds insights on the short-term motion response forecast and platform positioning of a FOWT. Short-term forecasts of a FOWT can be achieved under various sea conditions by combining the global positioning system.

Design optimization of a TetraSpar-type floater and tower for the IEA Wind 15 MW reference wind turbine

We present an optimization study for the conceptual design of wind turbine floaters of the TetraSpar type. The optimization variables include all geometric dimensions of the floater, keel, mooring lines and tower design. A gradient based optimization method is applied to a mass proportional objective cost function. The objective function accounts for the different weight components of the floater, including secondary steel, the wind turbine tower, and the mooring system. A frequency domain response method is utilized, so that each design evaluation also takes into account the dynamic response for 12 wind speeds with associated wave conditions. Nineteen constraints are applied for static and dynamic response, natural frequencies, and fatigue at the bottom of the tower. Two reference designs are presented, namely one with a soft–stiff tower and one with a stiff–stiff tower. Due to the anti-phase coupling of the floater pitch and tower vibration, the soft–stiff tower needs a stronger floater stiffness in pitch. This design thus has a larger water plane area moment than the more compact stiff–stiff floater, which is found to be the least economical. A constraint analysis is next presented based on Lagrange multipliers and a relative cost index. We find that the strongest cost influence is exerted by the 3P tower frequency constraint for the stiff-stiff and soft-stiff designs. Finally, a third design variant with a free optimizable tower frequency is introduced. This design is found to be 11% cheaper than the soft–stiff design and highlights the potential cost savings of tower designs within the 3P region.

Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditions

The concept of a shared mooring system was proposed to reduce mooring and anchoring costs. Shared moorings also add complexity to the floating offshore wind farm system and pose design challenges. To understand the system dynamics, this paper presents a dynamic analysis for a dual-spar floating offshore wind farm with a shared mooring system in extreme environmental conditions. First, a numerical model of the floating offshore wind farm was established in a commercial simulation tool. Then, time-domain simulations were performed for the parked wind farm under extreme wind and wave conditions. A sensitivity study was carried out to investigate the influence of loading directions and shared line mooring properties. To highlight the influence of the shared line, the results were compared to those of a single spar floating wind turbine, and larger platform motions and higher tension loads in single lines are observed for the wind farm with shared moorings. The loading direction affects the platform motions and mooring response of the floating offshore wind farm. Comparing the investigated loading directions to the 0-deg loading direction, the variation of mean mooring tension at the fairlead is up to 84% for single lines and 16% for the shared line. The influence of the shared line properties in the platform motions and the structural responses is limited. These findings improve understanding of the dynamic characteristics of floating offshore wind farms with a shared mooring system.

Biofouling on mooring systems in the Talaud and Halmahera Seas, Indonesia

Many deep-sea scientific discoveries have been driven by sampling from mooring systems. We observed biofouling assemblages on five mooring systems in the Talaud Sea and the Halmahera Sea. Biofoulings on all the mooring components extending from the sea surface to the depth of 1800–2000 m were documented. We found mollusks, barnacles, annelids, algae, and sponges assemblages on buoys, instruments, and cables of the mooring systems. Barnacle Heteralepas sp. was the most dominant biofouling attached to the float instruments of all mooring systems. At a depth of 200 m, we found mollusks, barnacles, and sponges; while algae were founded at a depth of 750 m, 1000 m, 1200 m, and 1800 m. In comparison, sponges were detected at a depth of 200 m, 250 m, 500 m, 750 m, 1000 m, 1200 m, and 1800 m. Nevertheless, at a depth of 2000 m, we did not find any biofouling attached to the mooring systems.

Selection of mooring system configuration of a FSO using multi-criteria methods

In order to maintain and restrain the movement of a temporary storage tanker during the loading and offloading process, it is necessary to design a mooring system that is safe and reliable for its operation needs. The main problem that must be solved in this research is choosing the appropriate mooring system configuration for the temporary storage tanker equipped with a catenary mooring configuration that can be used for oil loading operations from the storage tanker to a transport tanker using the ship to ship method. This research focusses in the selection of mooring system configurations based on the strength of the anchor chain. There are nine alternatives configurations and predetermined criteria that were carried out in the study. Each alternative configuration will provide a different value of tension in each mooring line and ship movement. The analysis of the mooring tension is carried out using dynamic method with time domain approach. There are many criteria to be considered in the selection of the alternatives. The analytical hierarchical process (AHP) method is used in the selection of the best configuration based on multi criteria conditions. As case study of the study is a temporary storage tanker at Papa Field in Pertamina (Persero) ONWJ operation area as temporary storage taker is an Aframax tanker size.

Design and Analysis of a Mooring System for an Offshore Platform in the Concept Design Phase

Design and Analysis of a Mooring System for an Offshore Platform in the Concept Design Phase

Rigid-flexible coupling multi-body dynamics modeling of a semi-submersible floating offshore wind turbine

A 14-degree-of-freedom (14-DOF) flexible multibody dynamics model is developed and verified for a semi-submersible floating offshore wind turbine (FOWT). The model considers the coupled dynamics of the platform, tower, nacelle, blades, and mooring subjected to external wind and wave loads. The platform is simplified as a rigid body connected to the seabed by the mooring system. The tower and blade are modeled as flexible cantilever beams. The energy method is used to derive the governing equations of motion, where the kinetic energy, potential energy, and work done by external wind-wave forces are all deduced in a global coordinate system. The 5-MW baseline semi-submersible FOWT is used to verify the derived model against the results simulated from FAST developed by the US National Renewable Energy Laboratory (NREL) at two scenarios: free decay state and different wind-wave load cases. Results show that the established FOWT model can well reflect the vibration characteristics of FAST model. The application of the model to the control of platform pitch with a tuned mass damper is studied. The simplified model could provide a low-order method for the structural dynamics analysis and advanced vibration control design for the multi-body components of the semi-submersible FOWT in the future.

Numerical developments on the E-Motions wave energy converter: Hull design, power take-off tuning and mooring system configuration

This paper addresses a recently conducted numerical study on the E-Motions wave energy converter, aimed at improving its energy performance and efficiency. The Potential Flow Theory-based ANSYS® Aqwa™ software was used, being a robust industry-standard code that has been successfully applied to the E-Motions case study, in the past. Data from a preceding physical modelling campaign was applied towards calibration procedures for three platform variants – half-cylinder, half-sphere and trapezoidal prism, with and without the power take-off and subjected to regular waves. A non-dimensional root mean square error minimization approach was employed. Afterwards, the focus shifted towards defining and comparing the performance of sixteen E-Motions sub-variants, which included a new hull shape: enclosed frustum. Two additional stages were carried out: one oriented towards assessing ten power take-off mass-damping combinations and another to the study of four mooring system configurations. Results point towards a very significant hydrodynamic response, particularly within the resonance range and for the half-cylinder and half-sphere sub-variants, and maximum average power outputs of nearly 40 kW. For the best half-cylinder, half-sphere, trapezoidal prism and enclosed frustum sub-variants, the annual energy production estimates, per device, were 141, 110, 106 and 91 MWh/yr, respectively.

Nonlinear dynamic characteristics analysis of the surge motion of a tender-assisted drilling operation system

ABSTRACT In this study, a numerical analysis is carried out to investigate the coupled behaviour of a TLP (tension leg platform) -TAD (tender-assisted drilling) system under a series of regular waves in surge direction. The numerical model is established in the time domain considering the nonlinear characteristics of the mooring systems and the hydrodynamic interference. After the case validations, the effects of the wave periods and amplitudes on the nonlinear dynamic characteristics of the surge motions are investigated in detail. Then, the observed nonlinear dynamic characteristics are interpreted in detail through the existing theory of phase dynamics. The surge motions of the coupled system can show multiperiod nonlinear characteristics under regular waves in certain wave periods. Three forms of phase dynamic characteristics: phase locking, phase trapping, and phase drifting and slipping are observed in the surge motions of the tender-assisted drilling operation system.

Analysis of the Mooring Effects of Future Ultra-Large Container Vessels (ULCV) on Port Infrastructures

The size of container vessels is continuously growing, always exceeding expectations. Port authorities and terminals need to constantly adapt and face challenges related to maritime infrastructure, equipment, and operations, as these are the principal areas affected by the future Ultra Large Container Vessels (ULCVs). Maneuvring areas are at their limits, and mooring equipment is at an increased risk of being overloaded. This study aims to analyze the limitations that present mooring systems may face when ULCVs are subjected to wind and passing-ship forces exerted by a future ULCV and wind forces through Dynamic Mooring Analysis (DMA). A hypothetical and massive future ULCV with a capacity of 40,000 TEU is compared to the Emma Maersk, which is a present vessel that regularly calls at container terminals. The Emma Maersk, with its current mooring arrangement, experiences higher motion than future ULCVs, which experience higher forces but are also moored with more and stronger lines. This translates into considerably higher loads in the mooring system, potentially compromising safe mooring conditions at the terminal. Mitigating measures are proposed in the study to face these limitations. In addition, the study explores the potential of new and innovative mooring technologies, such as high-strength synthetic ropes and smart mooring systems, to address the challenges posed by ULCVs. A container terminal at the Port of Rotterdam, Europe’s largest sea port, has been analyzed as a case study. The terminal is located next to a busy fairway that leads to other container terminals, justifying the need to analyze both wind and passing-ship effects on moored ships.

Mathematical modelling of a floating Clam-type wave energy converter

In this paper, wave power extraction from a floating Clam-type wave energy converter is investigated. The device is mainly composed of a Clam, which is formed from two pieces of floating flaps hinged at a submerged body. The Clam is closed by a flexible impermeable bag with the two hinged floating flaps kept apart by a Power Take-Off system. As waves propagate through the device, the Clam motion of the device is excited, which can be used to drive the Power Take-Off system to capture wave power. To evaluate the response and also the wave power absorption of the device, a mathematical model is developed based on the linear potential flow theory, in which a generalised mode method is adopted to model the Clam action. Theoretical expressions of the maximum wave power absorption and the corresponding optimised Power Take-Off system and mooring parameters are derived. Good agreement between the present numerical results of the device response and the physical observations is obtained. The validated model is then applied to do a series of case studies. It is revealed that the optimised Power Take-Off stiffness and mooring stiffness are independent of the Power Take-Off damping. The maximum wave power absorption can be achieved when the device is fixed in heave mode or free-floating without any constraints from the mooring system.

On improving the wave-to-wire efficiency of a two-body hinge-barge wave energy converter

The multi-body hinge-barge wave energy converter has the advantages of high capture width ratio and wide bandwidth of wave-excited response, which is the current hotspot of studies on wave energy converters. However, to correctly and throughfully analyse its overall efficiency, hydrodynamics, mechanical dynamics and electrical dynamics, a fully coupled wave-to-wire approach by numerical analysis and physical validation is required, which is not concerned by current studies. The work conducted in this study improves the overall wave-to-wire efficiency by optimising the mooring system, the structure and geometry of the primary capture system, and the external electric load. Results show that an optimised prototype achieves a wave-to-wire efficiency up to 25% in flume experiments, where a significant improvement of 53% is contributed by employing a damping plate to the right barge. Numerical results indicate that a wave-to-wire efficiency of up to 53% is obtained by further optimisation of the damping plate.

Dynamic Response of 15 MW Floating Wind Turbine With Non-Redundant and Redundant Mooring Systems Under Extreme and Accidental Conditions

AbstractThis work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

Assessment of nylon versus polyester ropes for mooring of floating wind turbines

With the development of floating wind turbines comes the need for mooring systems suitable for shallow water depths. Compliance becomes challenging for mooring systems composed of chain and wire rope. Polyester ropes can improve compliance, but the even lower stiffness of nylon may further reduce mooring line loads. However, fatigue lifetime of nylon ropes has been limiting. This paper presents results of stiffness and fatigue tests on long lay polyester and nylon ropes and evaluates the performance of two mooring systems for a floating wind turbine. The fibre ropes’ complex stiffness behaviour is modelled by the Syrope model, with mooring analyses based on fully coupled time-domain simulations. Nylon ropes reduce the dynamic loads in the mooring system, particularly wave frequency loads. This improves the fatigue lifetime of all components, allowing a reduction of the chain cross-section area by ∼40% compared to the polyester-based mooring system. The long lay nylon ropes show excellent fatigue properties. With the reduced loading, the lifetime is longer for the nylon ropes than the polyester ropes. Ultimate loads are also reduced when using nylon, allowing reduction of the breaking load by ∼30%. This gives a thinner and lighter rope than the equivalent polyester rope.

Hydrodynamic investigation on the submerged tunnel suspended from a fixed platform using SPH method

In this paper, the coupled dynamic motion characteristics of a tunnel element suspended from a platform have been studied at a 1:50 scaled model by using the weakly compressible Smoothed Particle Hydrodynamic (WCSPH) method. The corresponding numerical model was validated with the experimental published data of the motion characteristics of the tunnel suspended from a fixed platform, and the case of a floating structure with its mooring lines. Multiple linear wave conditions and different sinking depths of the submerged were involved in the numerical model for tunnel dynamic response and cable behavior investigation. Effects of the mooring system, the wave parameters, the tunnel sinking depth and its mooring configurations were studied. The numerical results were arranged to inform about the dynamic characteristics and mooring behavior for different case studies. The results indicate that the tunnel motions decrease with the increment of the tunnel sinking depth and decrease with the decreasing mooring angle. The natural periods of the tunnel-platform system play key roles in the wave condition impacts on the tunnel motions. It was found that the additional mooring system anchoring the tunnel to seabed played a specific role in reducing the tunnel motions in roll and sway, the motion amplitude of the tunnel in heave is mainly controlled by the suspension cables, particularly when subjected to lower sinking depth and longer period waves.

Design of Wave Energy Capture Structure and Mooring System with Additional Mass

There is huge amounts of wave energy in ocean, and how to convert its unstable energy into stable and sustainable output energy is a major problem that needs to be solved urgently. The power production performance of a wave energy capture device is closely related to its anti-wave performance, so the design of the mooring system is very important for it, In this paper, a truss-type three-sided wave energy capture device is used, and the mooring system with additional mass is used for research. The study found that the mooring system with additional mass blocks could significantly restrain the platform’s surge displacement under wind and wave loads.

Computational and experimental studies of wave–structure interaction: Wave attenuation by a floating breakwater

Interactions between surface gravity waves and a floating rigid body are complex, as waves may reflect from, break on, and be transmitted behind the body. Studies of these phenomena are critically important in improving the safety and functional efficiency of offshore structures. Here, the wave attenuation performance and motions of a type of floating breakwater (FB) are studied through numerical and experimental approaches. A numerical wave tank (NWT) is developed based on the software OpenFOAM and properties of wave channel from a laboratory. In the NWT, the air–water interface is captured by the volume of fluid method. The motions of FB are tracked by the six degrees of freedom model. A mooring system model is developed to simulate the constraints of the FB. Large eddy simulation turbulence modeling is implemented for the wave breaking processes. A model FB with a scale of 1:20 is tested in both the experimental and numerical wave channel. Wave heights at the back/front of the FB and the constraint forces of the mooring wires are measured. The numerical models are validated by comparing the results with experimental measurements. The variations of transmission/reflection coefficients, energy dissipation rate, and maximum mooring force are calculated. Changes of the response amplitude operators with the ratio of FB width to wavelength ( B / L w) and wave steepness are analyzed. The wave transmission coefficient will drop below 0.8 if the value of B / L w is larger than 0.3, but will go over 0.95 if B / L w is less than 0.1. Wave steepness has a large influence on FB motions and the mooring system. The effect of Stokes drift is observed by the shift of position of the FB.

Motion characteristics of vertically loaded anchor during drag embedment in layered clay

As a widely used taut-wire mooring system for deepwater platforms, the Vertically Loaded Anchor (VLA) has better performance in bearing capacity, angle adaptability, and deepwater installation than other systems. However, the installation process of the VLA and its motion characteristics are significantly impacted by multi-layered seabed soil. In this paper, the coupled Eulerian‒Lagrangian (CEL) large deformation finite element analysis method has been applied to analyse the continuous penetration of a VLA in nonuniform clay with an interbedded stiff layer. A detailed parametric study has been carried out to explore the trajectory, drag angle, movement direction and drag force of the VLA in layered clay with different embedded depths, thicknesses and undrained shear strength of the stiff layer. The CEL numerical analysis results have been validated by comparison with the analytical solutions from the inverse catenary equation. Excellent agreement has been obtained between the results from the CEL analyses and the analytical solutions. The stiff layer leads to concave and convex shapes on the trend lines of the movement direction angle and drag forces, respectively. The embedded depth of the stiff layer determines where the concave and convex shapes appear on the trend lines, while the thickness affects the sizes of the openings of the shapes. The most decisive parameter, an abrupt variation in the undrained shear strength, causes predominant rotation at the interface of layered clay. It diminishes the final embedment depth and ultimate stable drag force, meaning that the bearing capacity of the VLA severely declined in layered clay.

Comparative Analysis of Seakeeping of Offshore Photovoltaic Floating Unit

Background: In recent years, offshore photovoltaic (PV) power systems have become an emerging hot issue. For offshore PV, the marine environmental loads are more complex, especially wave loads will put higher demands on the mooring system, and a failure of the mooring system will cause great harm. In order to ensure the safe operation of the system, the floating unit should be designed for high seakeeping. Objective: In this paper, six common configurations of floating units are selected, and the six configurations are divided into two experimental groups, one of which includes floating units with different shapes of holes, and mainly discusses the effects of different shapes of holes on the seakeeping of floating units, and the other experimental group includes common configurations of catamarans and trimarans in ships, and mainly discusses the effects of different bottom structures on the seakeeping of floating units. And after determining the better configuration, further shape optimization is performed. Methods: The problem described in this paper cannot ignore the change of the solid deformation on the flow field boundary, when both interaction processes between the two phases need to be calculated. Therefore, this paper discusses a bi-directional fluid-solid coupling process. For the six float units, a bi-directional fluid-structure coupling calculation analysis was carried out with high seakeeping as the research objective, and the effects of different float unit configurations on the seakeeping performance of floats were discussed. Results: Ultimately, the trimaran configuration is considered to have better seakeeping performance than the remaining configurations among the six configurations. On this basis, the bottom structure of the trimaran configuration was further optimized, and the cross-sectional shape of the bottom structure with better seakeeping performance was obtained. Conclusion: The wave resistance of the trimaran configuration was calculated to be superior compared with other configurations. The cross-sectional shape of the bottom structure of the trimaran was further analyzed. It is concluded that the trimaran structure with a bottom structure cross-section whose bottom edge is at an angle of 0° with the horizontal plane has the best wave resistance. It provides some guidance for the design of similar marine structures with high seakeeping.

Offshore Fish Farms: A Review of Standards and Guidelines for Design and Analysis

While moving fish farms to offshore sites can be a more sustainable way to expand farmed fish production, the fish pens have to contend with a harsher environment. Thus, it is necessary to draw on offshore engineering competences for designing and analysing the offshore fish farming infrastructure. This paper reviews existing design and analysis guidance from maritime classification and national/international authorities that can be applicable for offshore fish farms. Based on the existing design guidelines, a review of design criteria for offshore fish farms under the following subtopics is provided: design life, design environmental loads, combining environmental loads, and miscellaneous load conditions. This review on the global performance analysis procedures and methods is presented based on practices used for neighbouring industries, such as offshore oil and gas and wind energy production, under the following subtopics: hydrostatic analysis, hydrodynamic analysis, and mooring system analysis with introducing theoretical background and modelling techniques. This paper also highlights limitations and cautions when using these design and analysis methods. Providing this comprehensive information, as well as commentary on their applications, will help engineers and designers to develop offshore fish farming infrastructure with confidence.