Irregular Waves Research Articles

Predictive Control of a Heaving Compensation System Based on Machine Learning Prediction Algorithm

Floating structures have become a major part of offshore structure communities as offshore engineering moves from shallow waters to deeper ones. Floating installation ships or platforms are widely used in these engineering operations. Unexpected wave-induced motions affect floating structures, especially in harsh sea conditions. Horizontal motions on the sea surface can be offset by a dynamic positioning system, and heave motions can be controlled by a heave compensation system. Active heave compensation (AHC) systems are applied to control vertical heave motions and improve safety and efficiency. Predictive control based on machine learning prediction algorithms further improves the performance of active heave compensation control systems. This study proposes a predictive control strategy for an active heave compensation system with a machine learning prediction algorithm to minimise the heave motion of crane payload. A predictive active compensation model is presented to verify the proposed predictive control strategy, and proportion–integration–differentiation control with predictive control is adopted. The reliability of back propagation neural network (BPNN) and long short-term memory recurrent neural network (LSTM RNN) prediction algorithms is proven. The influence of the predictive error on compensation performance is analysed by comparing predictive feedforward cases with actual-data feedforward cases. Predictive feedforward control with regular and irregular wave conditions is discussed, and the possible strategies are examined. After implementing the proposed predictive control strategy based on a machine learning algorithm in an active heave compensation system, the heave motion of the payload is reduced considerably. This investigation is expected to contribute to the motion control strategy of floating structures.

Numerical investigation of hydrodynamic responses and statistical analysis of imposed forces for various geometries of the crown structure of caisson breakwater

One of the techniques used to protect the shores and to create an undisturbed basin for vessels to build a rigid wall against waves; which is called the caisson breakwater. In this paper, interactions between irregular waves and a caisson breakwater are examined using the FLOW-3D software. The results of the free surface, pressure on the caisson, and overtopping from the numerical simulation are validated and calibrated against the existing experimental data. The rate of error in the numerical model (average error) when predicting the overtopping and the maximum pressure on the caisson wall is equal to 12% and 3%, respectively. Additionally, the following five various geometries of the crown stricture, values of the overtopping, the force on the crown structure, the velocity of waves at the moment of the overtopping, and the wave reflection coefficient are determined. It is demonstrated that the highest and lowest wave reflection coefficients are associated with geometries C and E of the crown structure, respectively. The maximum overtopping and pressure magnitude are related to geometry D and geometry C respectively. Next, the best probability distribution function that fits the imposing force on the crown structure is established. Based on the results provided, the Weibull distribution showes the most accurate fit for the data.

Validation of short-term prediction of added resistance in head seas considering wave steepness nonlinearity with probability density function method

The added resistance in waves, which is an important component in the estimation of actual sea performance of ships, has been evaluated as the average value by short-term prediction methods. In this paper, a novel probabilistic method to account for non-linearities of added resistance with respect to wave height in short-term sea conditions is proposed using the joint occurrence probability distribution of wave height and period and non-linear added resistance response in the frequency region. It is a method of averaging the combination of crest and trough that ships encounter in irregular waves, which is considered as one wave of the regular waves, and the resulting added resistance is combined stochastically. The non-linearity with respect to wave height is empirically expressed as a “correction coefficient” obtained from the comparison of CFD data and potential-based linear calculation results in regular waves. The results of the proposed method have a reasonable agreement in the acceptable range of accuracy with results of RANSE based CFD numerical simulations in long crested irregular waves and the experimental model tests. As a result, the proposed method is applicable for obtaining added resistance in irregular head waves having the non-linearity with respect to wave height with a scope of further improvement in the method.

Time-domain simulation of sum-frequency hydrodynamic performance of a TLP-type floating wind turbine in multidirectional seas

Previous studies (Ren et al., 2022) by the authors have confirmed that the second-order diffracted wave loads are very sensitive to the wave directional spreading of the so-called storm-swell mixed seas where the storm-driven local seas combined with long-period swells propagating from different angles. In this paper, particular attention is paid to estimate the sum-frequency hydrodynamic forces and motion responses of a TLP-type floating wind turbine (FWT) in such complex sea states. A time-domain potential flow model is developed for the second-order interaction between the multidirectional irregular waves and moored structures. The boundary value problems for the scattered potential are solved by a higher-order boundary element method (HOBEM). The fourth-order Adams–Bashforth–Moulton method is used to update the wave surface and the Newmark method combined with the Newton–Raphson iteration scheme is adopted to solve the body motion equations.For validation, a freely floating hemisphere in unidirectional bichromatic waves is investigated and good agreement is found by comparing with Kim and Yue's frequency-domain results. Numerical tests are conducted on the TLP-type FWT with different combinations of incident angles and spreading factors of the storm-swell mixed seas. The effects of the wave headings and directional spreading on the sum-frequency hydrodynamic response are discussed. It is noted that when the swells approach the storm seas from large relative angles, the sum-frequency vertical response of the FWT can be up to seven times greater than that in the collinear mixed seas. In addition, the present model can extend the rotor-floater-tether coupled dynamic simulator of FWT to more realistic seas such as storm-swell mixed seas.

Analysis of hydrofoil craft in regular and irregular waves

Analysis of hydrofoil craft in regular and irregular waves

Development of model-based and model-free reactive control scheme: considering copper loss and movable-floater-displacement constraint for a wave energy converter

The utilization of wave energy is expected since ocean wave energy has a high potential. The improvement of the feasibility of wave energy converters requires control that maximizes the electric output energy, including the copper loss under a displacement constraint. Several model-based and model-free reactive controls have been developed. Although model-based reactive control attains high performance, it struggles to deal with modeling errors and forecasting wave excitation forces. On the other hand, the model-free reactive control can adapt to dynamic modeling, including modeling errors; however, it requires a vast amount of learning data and considerable time and effort to consider the displacement constraint. Model-based and model-free reactive controls each have advantages and disadvantages. Combined model-based and model-free reactive controls are desirable to freely switch between the model-based and model-free reactive controls based on various ocean situations. In this study, two equivalent model-based and model-free reactive controls that can consider the copper loss and displacement constraints without forecasting the wave excitation forces were proposed. The model-free reactive control was compared with the model-based reactive control and a conventional control using numerical simulations in irregular waves. The results of the simulation show that the proposed model-based reactive control achieves superior performance compared to that of the conventional control. The proposed model-free reactive control achieved comparable performance to that of the proposed model-based reactive control under various wave conditions. Moreover, the proposed model-free reactive control decreased the required training trials. The development of the two equivalent control schemes will lead to the proposal of combined model-based and model-free reactive controls in the future.

Speed-and-pressure combined control for the CPHPTO of an inverse pendulum wave energy converter

Constant pressure hydraulic power take-off (CPHPTO) utilizes a high-pressure accumulator to weaken its oil pressure fluctuations. However, CPHPTO operating point still changes violently under the circumstance of the changeable irregular waves as well as the limited volume of the high-pressure accumulator. This causes CPHPTO to work away from the designed operating point and even to run in the inefficient and unstable region. This paper proposes a speed-and-pressure combined control (SPCC) strategy, which keeps system speed/pressure around the designed value via the vector control of a PMSG or the displacement tuning of a hydraulic motor, for the efficient and stable operation of a CPHPTO. A SPCC strategy for the CPHPTO embedded in a novel inverse pendulum wave energy converter (NIPWEC) is designed in detail. Furthermore, the overall wave-to-wire dynamic model is simulated in irregular waves to research the effect of SPCC. Results show that both the SPCC with an oil pressure open-loop regulator (OPOLR) and the SPCC with an oil pressure closed-loop regulator (OPCLR) can effectively stabilize the system speed/pressure. Besides, SPCC, especially the SPCC with OPCLR, can effectively improve both the efficiency and stability of CPHPTO. Moreover, system-speed control is more significant than system-pressure control in terms of improving CPHPTO efficiency.

Splash Zone Dynamic Analysis of A Suspended Semi-Submerged Subsea Module for Irregular Waves

Splash Zone Dynamic Analysis of A Suspended Semi-Submerged Subsea Module for Irregular Waves

Dynamic performance of a point absorber WEC considering nonlinear hardening spring under irregular waves

Wave energy is a kind of clean renewable energy that is huge and still need further research. Point absorber wave energy converter (PA-WEC) is widely utilized for offshore power devices. The PA-WEC utilizes the heave motion to drive linear electric generator or a hydraulic motor to generator using power take off (PTO) system. And the PTO system usually is assumed to be a linear spring and a linear damper. Because of the larger motion, the PTO system, which exhibited non-linear behavior, is considered as linear spring with the nonlinear hardening spring and a linear damper. Through the time domain method, the dynamic model of the PA-WEC is developed under irregular waves. The state space model replaced the convolution term in the frequency domain equation. The dynamic response of the PA-WEC is researched considering different environmental parameters by Runge-Kutta method. Then the power and power ratio performance of the system is obtained. Compare with the linear WEC, the results show that the nonlinear WEC can increase the power captured in irregular waves.

Real-time model for wave attenuation using active plate breakwater based on deep reinforcement learning

This paper presents a numerical study of the interaction between irregular waves and actively controlled plate breakwater based on deep reinforcement learning (DRL), in which wave attenuation performance is mainly discussed. A numerical wave flume (NWF) is built and an in-house solver is developed to integrate CFD and DRL. Validations on wave generation and interaction between incident waves and the plate are carried out. An NWF-DRL coupled model is proposed to simulate the actively controlled plate moving to attenuate waves, where the motion of the plate is controlled by DRL. It is shown that under the regular wave and trichromatic wave group, both the trained agents can obviously reduce the wave height compared to that under fixed plate. Moreover, the further trained agents can reduce the transmitted wave heights to a significantly low level in the three more complex and unknown irregular wave cases. Meanwhile, the model does a great job of preventing the development of focused wave height in an irregular wave train. This research first examines the wave attenuation of a DRL-controlled breakwater in irregular wave environments, which may offer fresh perspectives on wave attenuation and even the design of intelligent marine equipment.

Deformation of Spartina patens and Spartina alterniflora stems under irregular wave action

ABSTRACT Physical modeling studies have endeavored to quantify the influence of plant biophysical parameters and hydrodynamics on wave-vegetation interactions and coastal protection. The accuracy with which such studies have characterized stem motion is limited by the predominant use of plant surrogates, and the few saltmarsh species considered in live vegetation studies. To address this, prototype-scale experiments were conducted in the outdoor Large Wave Canal of the Institut National de la Recherche Scientifique, Québec, in collaboration with the University of Ottawa and the National Research Council Canada, allowing novel characterization of live vegetation deformation under wave action. Two saltmarsh species were investigated (Spartina alterniflora, Spartina patens) under various irregular wave conditions (0.03 m< H s <0.28 m, T s 2.5, 10 s). Stem deformation was characterized using submerged cameras and bending angle tracking, coupled with wave height and velocity measurements. Significant differences in stem flexibility were observed between species, with S. alterniflora exhibiting more rigid stems (EI alterniflora =0.051 Nm2) than S. patens (EI patens =0.0015 Nm2; t-test; p<0.05). The two species consequexhibited different bending angles under similar hydrodynamic conditions, expected to influence their relative coastal protection capacity. These findings provide critical insight into the design of marsh construction or restoration for coastal protection.

Dynamic responses of serially connected truss pontoon-MOB – A numerical investigation

Very large floating structures (VLFS) are typically made up of several modules of offshore platforms, such as semi-submersibles, connected by mechanical connectors. The proposed Truss Pontoon Mobile Offshore Base (TP-MOB) is a new type of offshore platform based on the truss pontoon semi-submersible design. The design of the connectors is crucial for the functionality of the structure. In this study, the flexible connectors are modeled with a 3D source distribution method and the wave forces are estimated with 3D linear potential theory. The open-source finite element-based numerical model, HYDRAN-XR, is used to analyze the structural responses and connector forces in both regular and irregular wave conditions. The results indicate significant differences in responses between single and multi-module cases, especially in extreme waves. Loads on the connector change greatly for three- and five-module cases, with maximum values at specific wave angles.

Numerical Investigation into the Performance of an OWC Device under Regular and Irregular Waves

A numerical investigation into the hydrodynamic efficiency of an oscillating water column (OWC) device for the production of energy from sea waves under the conditions of regular and irregular waves is proposed. The numerical simulations were carried out using a two-dimensional version of a recently published three-dimensional free-surface nonhydrostatic numerical model, which is based on a conservative form of the contravariant Navier–Stokes equations written for a moving co-ordinate system. The governing equations are spatially discretized by a finite volume shock-capturing scheme based on high-order wave-targeted essentially nonoscillatory reconstructions and an exact Riemann solver. Time discretization was performed by a predictor-corrector method that took into account the nonhydrostatic pressure component. The proposed numerical model allowed us to highlight the significant differences between the hydrodynamic efficiency obtained under irregular waves and those obtained under regular monochromatic waves and provides more realistic evaluations of the OWC device performances. The results of the above comparison showed a reduction in the hydrodynamic efficiency of the OWC from 0.78 to about 0.54 when passing from regular waves to the corresponding irregular ones. The model was applied to assess the potential energy production obtainable by a set of OWCs at the Cetraro harbor (southern Italy). The numerical results show that, by adopting the optimal dimensions of the OWC, the estimated mean annual energy production obtainable at the Cetraro harbor is equal to 1540.52 MWh, which corresponds to the energy production of about 10 wind turbines with a nominal power of 60 KW.

Experimental study on dynamic pressure under the crest of unidirectional nonlinear waves

The dynamic pressure under wave crests is experimentally studied in three different scenarios, and the influence of nonlinearity as well as the dispersive effect is revealed by comparing with the corresponding linear theoretical predictions. For regular waves, the nonlinear effect plays different roles in modifying the upper and lower sections of the dynamic pressure under the wave crests with the boundary determined by the strength of nonlinearity. Regarding irregular waves, the dynamic pressure under wave crests is mainly affected by the surrounding individual waves within the same wave group, and in most cases its variation is dominated by the dispersive effect, exhibiting a greater magnitude than its counterpart in regular waves. As to focused waves, the same large crests can be realized in the time domain with different methods, but the discrepancy of dynamic pressure is very noticeable due to the fundamentally different nonlinearities involved in the focusing process. Therefore, to accurately evaluate the dynamic pressure under wave crests in the ocean, the dominant nonlinearity at the current evolutionary stage, as well as the dispersive effect, must be considered.

A hybrid VMD-LSTM/GRU model to predict non-stationary and irregular waves on the east coast of China

Accurate wave forecasting is essential for the safety of port and offshore structure operations and ship navigation. Computational fluid dynamics (CFD) and traditional time series models are ineffective in dealing with non-linearities and non-smoothness. However, long short-term memory (LSTM) and gate recurrent units (GRU) have strong non-linear handling capabilities but are deficient in non-stationary situations. Variational mode decomposition (VMD) can effectively separate the non-linearity and non-smoothness in data. In this report, a VMD-LSTM/GRU model is proposed by combining the advantages of the LSTM model, the GRU model, and the VMD technique. Based on the one-tenth maximum wave height at three locations on the east coast of China, the error of the VMD-LSTM/GRU model is shown to be lower than that of the LSTM/GRU model, and significantly lower than that of the single LSTM and single GRU models. By analyzing different forecast durations, it was found that the correlation of the VMD-LSTM/GRU model improved from 10.75% to 20.99% compared to the corresponding LSTM/GRU model. The RMSE and relative errors of the VMD-LSTM/GRU model were reduced by 96.77% and 95.52%, respectively, for the most difficult forecast result 10 h ahead. Thus, this model has proved to be superior in predicting non-linear and non-stationary waves.

Experimental and numerical investigation of the dynamic responses of longline aquacultural structures under waves

Mariculture is an important activity in the exploitation of marine resources, of which longline raft culture accounts for a large part of the production. The flexibility due to the structural characteristics and the front-to-back shielding effect between suspended culture units have great influence on the dynamic response of the structure under waves. There are relatively few systematic studies on the impacts of this flexibility and shielding effect. Laboratory experiments are carried out in our work to investigate the dynamic response of different forms of aquacultural structures under the action of regular and irregular waves, in which the structural flexibility is reasonably considered and modeled. Then based on the experimental results, a numerical model developed in our previous work is optimized to take the shielding effect into account. Through the improved numerical model, the influence of structural parameters and wave parameters on the mooring line tension is analyzed. The results indicate that the shielding effect reduces with the increase of wave frequency. The maximum mooring line tension generally decreases with the decrease of culture density, and the decreasing trend is gradually weakened. The increase of anchoring angle results in a smaller maximum mooring line tension in most cases due to the reducing anchoring system constrains. The maximum value of the mooring line tension under the action of regular and irregular waves generally appears around the relative structure length B/L=1.15∼1.35. The conclusions of the experiment and analysis can be used as a reference for the design of similar longline aquacultural structures to avoid structural failure.

A data driven method for multi-step prediction of ship roll motion in high sea states

Ship roll motion in high sea states has large amplitudes and nonlinear dynamics, and its prediction is significant for operability, safety, and survivability. This paper presents a novel data-driven methodology to provide a multi-step prediction of ship roll motions in high sea states. A hybrid neural network is proposed that combines long short-term memory (LSTM) and convolutional neural network (CNN) in parallel. The motivation is to extract the nonlinear dynamic characteristics and the hydrodynamic memory information through the advantage of CNN and LSTM, respectively. For the feature selection, the time histories of motion states and wave heights are selected to involve sufficient information. Taken a scaled KCS as the study object, the ship motions in sea state 7 irregular long-crested waves are simulated and used for the validation. The results show that at least one period of roll motion can be accurately predicted. Compared with the single LSTM and CNN methods, the proposed method has better performance in predicting the amplitude of roll angles. Besides, the comparison results also demonstrate that selecting motion states and wave heights as feature space improves the prediction accuracy, verifying the effectiveness of the proposed method.

Effect of bottom unlatch and top breakage mooring lines on the dynamic behavior of a semisubmersible platform system

Mooring disconnections occur frequently in a floating production system (FPS), probably resulting in the instability of structures or even offshore accidents. Therefore, this paper investigates the influence of mooring disconnections on the response of a SEMI system under regular waves and irregular waves. Two failure modes were performed and compared according to the disconnected positions of mooring lines: the top breakage and bottom unlatch. A numerical model of a fully coupled hull/mooring/riser system was established through a self-developed program to obtain the detailed behavior of each component. The results indicated that the platform motion became a 4-period nonlinear response containing a super-harmonic motion when three moorings unlatched. The disconnected moorings produced a large deformation due to the sliding of anchor points. The riser tension exhibited robustness under mooring failure because of its low tension level and the robustness of platform heave. Compared with bottom unlatch conditions, the platform under top breakage conditions showed a larger-amplitude fluctuation and a slightly farther offset particularly three moorings all disconnected. The total mooring force under top breakage conditions would lose more fairlead tension at the failure time, thus inducing a lower mooring force for the platform.

Conceptual design and analysis for a novel parallel configuration-type wave energy converter

Oscillating body wave energy converter (OBWEC) is an essential way to exploit wave energy. Existing OBWECs with a single degree of freedom (DOF) suffer from a low energy conversion ratio. Although this power generation limit of OBWECs can be alleviated by increasing DOF, for the multi-DOF OBWEC, specific structure design guidelines, accurate energy conversion modeling and economic cost-related performance analysis at the conceptual design stage are still missing. To address this gap, this study proposes a novel parallel configuration WEC (PCWEC) with the idea of combining the advantages of multi-DOF OBWEC and parallel structure. The PCWEC energy conversion model is established under irregular wave states. Furthermore, we propose an energy cost-efficiency indicator and perform the performance comparison analysis and simulation experiments. Compared with typical OBWECs, results validate that the proposed PCWEC has the following advantages: (i) a significant power generation increase under the same device scale, (ii) greater optimal output power by power take-off control, and (iii) higher energy cost-efficiency under changing wave states. Such a novel PCWEC design concept improves the current OBWEC's power generation performance and application prospect and offers certain guidance for the future commercial development of OBWECs.

Multi-directional irregular wave forces acting on a tandem pile group

In this study, the interactions between multi-directional irregular waves and a tandem pile group are experimentally investigated. A systematic statistical analysis of the force coefficients of the pile group and the wave force on a tandem pile group under multi-directional waves is first carried out. The results show that the wave force of the pile group is significantly affected by the wave directionality. Further, Reynolds-averaged Navier–Stokes equations are used to establish a 3D numerical wave tank by considering the interactions between the waves and the pile groups, and this is verified by using experimental data. The characteristics of the wave force on the first and the last pile in the tandem pile group were numerically examined. The flow field around the front first pile was affected by its rear pile such that the pressure at its rear increased. However, without the rear pile, the pressure difference between the upward and rearward zone of the last pile significantly increased compared with that of the first pile, and this caused the wave force of the last pile to be larger than that of the first pile.