Numerical simulation of the mooring capacity comparison of the subsea suspended manifold with two mooring schemes

The next-generation underwater production system (NUPS) is based on the suspension cluster manifold (SCM) as a new conceptual scheme. SCM mooring stability is essential for establishing NUPS. Therefore, comparing the SCM mooring stability in different mooring systems is vital for evaluating system adaptability. This paper detailed two mooring schemes designed for the SCM, including the steel catenary riser (SCR) mooring system and the new steep wave (NSWR) mooring system. OrcaFlex software was used to establish the mooring system model, analyzing the static motion response of the SCM under the current and fluid density. Furthermore, the mooring system adaptability in the cluster wellhead layout was also evaluated and compared. The results showed that the maximum offset of the SCM with the SCR mooring system was within 2 m under the current, while the deflection of the SCM with the NSWR mooring system was within 1.5° in extreme fluid densities. Furthermore, the SCM with the SCR mooring system displayed superior station-keeping capability in the current, while the NSWR mooring system exhibited better stability when transporting extreme fluid densities and was more adaptable in cluster wellhead layouts.

Adaptive RBF -SMC control for multi-point mooring system

A stable adaptive control scheme for multi-point mooring system (MPMS) with uncertain dynamics is proposed in this paper. The control scheme is designed by a hybrid controller based on RBF (Radial Basis Function) NN (Neural Network) and SMC (Sliding Mode Control), which learns the MPMS dynamic changes, and the compensation of external disturbances is realized through adaptive RBFNN control. Meanwhile the RBF-SMC control parameters are adapted by the Lyapunov method to minimize squares dynamic positioning (DP) error. The convergence of the hybrid controller is proved theoretically, and the proposed mooring control scheme is applied to the “Kantan3” mooring simulation system. Finally, the simulation results are compared with the traditional PID controller and standard RBF controller to demonstrate the effective mooring positioning performance of the control scheme for the MPMS.

Dynamic Stability of the Coupled Pontoon-Ocean Turbine-Floater Platform-Rope System under Harmonic Wave Excitation and Steady Ocean Current

This research proposes a mooring design which keeps the turbine ocean current, static, balanced, and fixed at a predetermined depth under water, to ensure that the ocean current generator can effectively use current to generate electricity, and that the water pressure remains adequate value before critical pressure damage occurs. In this design, the turbine generator, which withstands the force of ocean currents, is mounted in front of a floating platform by ropes, and the platform is anchored to the deep seabed with light-weight high-strength PE ropes. In addition, a pontoon is connected to the ocean current generator with a rope. The balance is reached by the ocean current generator weight, floating pontoon, and the tension of the ropes which are connected between the generator and floating platform. Therefore, both horizontal and vertical forces become static and the depth can be determined by the length of the rope. Because the floating platform and pontoons on the water surface are significantly affected by waves, the two devices subjected to the wave exciting forces are further affected by the movement of the platform, pontoons, turbines, and the tensions of the ropes. Among them, the exciting forces depend on the operating volume of the two devices. Moreover, there is a phase difference between the floating platform and the pontoon under the action of the waves. In this study, the linear elastic model is used to simulate the motion equation of the overall mooring system. A theoretical solution of the static and dynamic stability analysis of the mooring system is proposed. The dynamic behaviors of the turbine, the floating platform, the pontoon, and the tension of the rope under the effects of waves and ocean currents are investigated. The study found the relationship of the phase difference and the direction difference of waves and ocean currents, the wavelength, and the length of the rope between the carrier and the turbine. It was found that the phase difference has a great influence on the dynamic behaviors of the system. The length of the rope can be adjusted to avoid resonance and reduce the rope tension. In addition, a buffer spring can be used to reduce the dynamic tension of the rope significantly to ensure the safety and life of the rope.

STANDARDISING THE DESIGN AND PRODUCTION OF MOORING WINCHES THROUGH MORE COHESIVE REGULATIONS: A NECESSARY STEP

Stricter guidelines, hand in hand with continuous technological advances, mean that vessels are safer and more efficient. At the same time, these changes make ships more complex and expensive. However, this is not the case with mooring equipment. Regulations vary in content, but have the same objective. They fail to improve the ships’ safety and prices are negatively affected. A vessel’s mooring system has to take into account various sets of regulations, including those of the country under whose flag it is sailing; the classification societies; ship owner associations and the terminal ports. This paper will analyse the contents of all these regulations in an attempt to find what they have in common. Its next purpose is to propose guidelines with unified contents. The proposal is to harmonise regulations that affect the mooring winch’s operation, design and construction. With standardised regulations, it would be possible to carry out a single design process and one calculation for components. These steps will be apt for every kind of project. By unifying this process, the final price of manufacturing equipment will be reduced.

Mooring Sense Project: A Risk-Based Integrity Management Strategy for Mooring System of Floating Offshore Wind

While Floating Offshore Wind (FOW) represents a significant opportunity to foster wind energy development and to contribute to remarkable CO2 emissions reductions, its associated operational costs are still substantially above grid parity, and significant innovation is needed. MooringSense is a research and innovation project which explores digitisation technologies to enable the implementation of risk-based integrity management strategies for mooring systems in the FOW sector with the aim to optimise Operations and Maintenance (O&M) activities, reduce costs, and increase energy production. As part of this project, a risk-based assessment methodology specific for the mooring system of Floating Offshore Wind Turbines (FOWT) has been developed; this allows the development of a risk-based Mooring Integrity Management Strategy that can result in more cost-effective inspection planning. The methodology shall utilise the information made available by numerical tools, sensors, and algorithms developed in the project to update the risk level of the mooring system and set the required plan to mitigate the risk. Leveraging the additional information from monitoring technologies and predictive capabilities to determine the mooring system condition and remaining lifetime, the strategy provides the criteria for optimal decision making with regards to selection of O&M activities. The risk-based strategy developed allows for optimal planning of inspection and maintenance activities based on dynamic risk level that is periodically updated through the interface with the Digital Twin (DT). The validation of the strategy will demonstrate potential cost saving and economic advantages, however, it is expected that the overall MooringSense approach can reduce FOW farm operational costs by 10-15% and increase operational efficiency by means of an Annual Energy Production increase by 2-3%. The MooringSense project comprises of the development and validation of innovative solutions coming from multiple disciplines such as numerical modelling, simulation, Global Navigation Satellite System (GNSS), Structural Health Monitoring (SHM), and control systems which will provide valuable input to the risk-based mooring integrity management strategy.

Proposal and Performance Study of a Novel Mooring System with Six Mooring Lines for Spar-Type Offshore Wind Turbines

Spar-type floating offshore wind turbines commonly vibrate excessively when under the coupling impact of wind and wave. The wind turbine vibration can be controlled by developing its mooring system. Thus, this study proposes a novel mooring system for the spar-type floating offshore wind turbine. The proposed mooring system has six mooring lines, which are divided into three groups, with two mooring lines in the same group being connected to the same fairlead. Subsequently, the effects of the included angle between the two mooring lines on the mooring-system’s performance are investigated. Then, these six mooring lines are connected to six independent fairleads for comparison. FAST is utilized to calculate wind turbine dynamic response. Wind turbine surge, pitch, and yaw movements are presented and analyzed in time and frequency domains to quantitatively evaluate the performances of the proposed mooring systems. Compared with the mooring system with six fairleads, the mooring system with three fairleads performed better. When the included angle was 40°, surge, pitch, and yaw movement amplitudes of the wind turbine reduced by 39.51%, 6.8%, and 12.34%, respectively, when under regular waves; they reduced by 56.08%, 25.00%, and 47.5%, respectively, when under irregular waves. Thus, the mooring system with three fairleads and 40° included angle is recommended.

Proposal of a Novel Mooring System Using Three-Bifurcated Mooring Lines for Spar-Type Off-Shore Wind Turbines

Floating wind turbine vibration controlling becomes more and more important with the increase in wind turbine size. Thus, a novel three-bifurcated mooring system is proposed for Spar-type floating wind turbines. Compared with the original mooring system using three mooring lines, three-bifurcated sub-mooring-lines are added into the novel mooring system. Specifically, each three-bifurcated sub-mooring-line is first connected to a Spar-type platform using three fairleads, then it is connected to the anchor using the main mooring line. Six fairleads are involved in the proposed mooring system, theoretically resulting in larger overturning and torsional stiffness. For further improvement, a clump mass is attached onto the main mooring lines of the proposed mooring system. The wind turbine surge, pitch, and yaw movements under regular and irregular waves are calculated to quantitatively examine the mooring system performances. A recommended configuration for the proposed mooring system is presented: the three-bifurcated sub-mooring-line and main mooring line lengths should be (0.0166, 0.0111, 0.0166) and 0.9723 times the total mooring line length in the traditional mooring system. The proposed mooring system can at most reduce the wind turbine surge movement 37.15% and 54.5% when under regular and irregular waves, respectively, and can at most reduce the yaw movement 30.1% and 40% when under regular and irregular waves, respectively.

Development of a Multi-Objective Optimization Tool for Screening Designs of Taut Synthetic Mooring Systems to Minimize Mooring Component Cost and Footprint

As the offshore wind industry develops, more lease sites in the intermediate water depth (50–85 m) are being released to developers. In these water depths floating wind turbines with chain catenary systems and fixed-bottom turbines with jacketed structures become cost prohibitive. As such, industry and researchers have shifted focus to floating turbines with taut or semi-taut synthetic rope mooring systems. In addition to reducing the cost of the mooring systems, synthetic systems can also reduce the footprint compared to a chain catenary system which frees areas around the turbine for other maritime uses such as commercial fishing. Both the mooring systems component cost and footprint are pertinent design criteria that lend themselves naturally to a multi-objective optimization routine. In this paper a new approach for efficiently screening the design space for plausible mooring systems that balance component cost and footprint using a multi-objective genetic algorithm is presented. This method uses a tiered-constraint method to avoid performing computationally expensive time domain simulations of mooring system designs that are infeasible. Performance metrics for assessing the constraints of candidate designs are performed using open-source software such as Mooring Analysis Program (MAP++), OpenFAST and MoorDyn. A case study is presented providing a Pareto-optimal design front for a taut synthetic mooring system of a 6-MW floating offshore wind turbine.