Severe Sea Conditions Research Articles

Experimental and numerical study on dynamic responses of a TLP-type modular floating structure system

This paper explores a proposed TLP-type modular floating structure system, incorporating both central TLP modules and peripheral floating artificial reef modules. The outermost floating artificial reef module can function as a floating breakwater, and also serving as a habitat for surrounding marine organisms. To assess the system's multi-body coupling dynamics under various wave conditions, both scale model tests (1:80) and time-domain numerical simulations were performed. The simulations accounted for hydrodynamic interactions among the bodies and the mechanical coupling effects. During the scale model test, a survival strategy has been proposed to prevent collision risks between reef modules and adjacent TLP modules, which involving a hinge connector with additional linear spring. A preliminary stiffness recommendation was suggested by a trade-off between the pitch of the reef module and the spring force. Numerical and experimental results have been widely compared and discussed, including the multi-body motion responses and the tension leg loads. A good agreement has been achieved, although responses are slightly overestimated by numerical simulation due to the neglect of the viscous damping effect. In addition, the responses of the proposed system, connected with and without the outermost floating artificial reef module, have been compared and analyzed. The results suggest that the connection of the reef module can significantly reduce the system's surge response and enhance its survivability under severe sea conditions. The validated numerical model can serve as a valuable reference for subsequent system optimization and further engineering applications.

Optimization of Energy Consumption in Ship Propulsion Control under Severe Sea Conditions

Optimization of Energy Consumption in Ship Propulsion Control under Severe Sea Conditions

Experimental and numerical study of wet-deck slamming characteristic for a trimaran in regular head waves

Wet deck may suffer from strong slamming effects when sailing under severe sea conditions, which could damage it locally and compromise the structural integrity of trimarans. The wet-deck slamming characteristic for trimaran sailing in head waves is studied in this work using both experimental and numerical simulation methods. We found that the negative pressure occurred at the front of the wet deck, which is caused by the wet deck exit from the wave crest. Regarding the spatial distribution in longitudinal, the slamming behaviour is more severe at the front part of the wet deck, as the relative vertical impact velocity decreased from the front to the back part of it. Simultaneously, the wet-deck slamming is more severe under the short-wave condition. Regarding the spatial distribution in lateral, we found that the water-entry of the main hull could cause the wet-deck slamming pressure near the outrigger larger than that of the main hull. Moreover, the wet deck has been divided into three regions according to their different lateral distribution characteristics of the wet-deck slamming. These results can provide a better understanding of the wet-deck slamming characteristics when trimaran sailing in waves, which can be considered in future designs for the trimaran.

Theoretical analysis on the behavior of SPM anchor piles embedded in sand under oblique pullout load with emphasis on the padeye depth

Single-point mooring (SPM) system has been used in offshore engineering for its superiority in overcoming severe sea conditions. However, researches focused on the interaction mechanism between the anchor pile and soil in SPM systems remains insufficient, especially regarding the calculation method for oblique pullout bearing capacity that considers padeye depth. Based on the p-y curve method and load transfer method, this paper presents a simple prediction method for the behavior of anchor piles embedded in sand under oblique pullout load. The theoretical predictions are validated through comparing with results from the three-dimensional finite element method and existing centrifuge model tests. The effect of the loading angle and the padeye depth are analyzed and discussed. A negative bending moment peak and axial force discontinuity phenomenon appear at the padeye depth. The optimal padeye depth for anchor piles in this study ranges from approximately 0.53 to 0.80 times the pile length. When the padeye depth is less than 0.33 times of the pile length, the ultimate bearing capacity of the anchor pile increases with increasing load angles. Conversely, when the padeye depth exceeds 0.33 times the pile length, the ultimate bearing capacity decreases with the increasing load angles.

On the Development of a Near-Shore Pivoting Wave Energy Converter

Numerous offshore wave energy converter (WEC) designs have been invented; however, none has achieved full commercialization so far. The primary obstacle impeding WEC commercialization is the elevated levelized cost of energy (LCOE). Consequently, there exists a pressing need to innovate and swiftly diminish the LCOE. A critical challenge faced by WECs is their susceptibility to extreme wave loads during storms. Promising concepts must demonstrate robust design features to ensure resilience in adverse conditions, while maintaining efficiency in harnessing power under normal sea states. It is anticipated that the initial commercial endeavors will concentrate on near-shore WEC technologies due to the cost advantages associated with proximity to the coastline, facilitating more affordable power transmission and maintenance. In response, this manuscript proposes a pioneering near-shore WEC concept designed with a survivability mode that is engineered to mitigate wave loads during severe sea conditions. Moreover, prior investigations have highlighted favorable resonance properties of this novel concept, enhancing wave power extraction during recurrent energetic sea states. This study employs numerical and physical modelling techniques to evaluate wave loads on the proposed WEC. The results indicate a remarkable 65% reduction in wave loads on the moving floater of the WEC during a range of sea states under the implemented survivability mode.

Virtually constrained generalized relative motion modeling and a control parameter optimizer for automatic carrier landing

PurposeThis paper aims to propose a novel control scheme and offer a control parameter optimizer to achieve better automatic carrier landing. Carrier landing is a challenging work because of the severe sea conditions, high demand for accuracy and non-linearity and maneuvering coupling of the aircraft. Consequently, the automatic carrier landing system raises the need for a control scheme that combines high robustness, rapidity and accuracy. In addition, to exploit the capability of the proposed control scheme and alleviate the difficulty of manual parameter tuning, a control parameter optimizer is constructed.Design/methodology/approachA novel reference model is constructed by considering the desired state and the actual state as constrained generalized relative motion, which works as a virtual terminal spring-damper system. An improved particle swarm optimization algorithm with dynamic boundary adjustment and Pareto set analysis is introduced to optimize the control parameters.FindingsThe control parameter optimizer makes it efficient and effective to obtain well-tuned control parameters. Furthermore, the proposed control scheme with the optimized parameters can achieve safe carrier landings under various severe sea conditions.Originality/valueThe proposed control scheme shows stronger robustness, accuracy and rapidity than sliding-mode control and Proportion-integration-differentiation (PID). Also, the small number and efficiency of control parameters make this paper realize the first simultaneous optimization of all control parameters in the field of flight control.

Numerical analysis of wet-deck slamming characteristics for trimaran section with different main-hull profiles

Wet-deck slamming affects the structural integrity of trimarans sailing under severe sea conditions. In this study, a Cartesian-grid computational fluid dynamics code is adopted to simulate the water-entry process of a trimaran cross-section. The numerical simulation results of the wet-deck slamming pressure and evolution of the free surface agree well with the experimental results. The spatial distribution of the wet-deck slamming pressure and the process by which the slamming pressure increases to its maximum are analyzed via simulation. Simultaneously, the wet-deck slamming characteristics are compared with three different main-hull profiles. We found that the occurred of the flow separation is related to the bilge curvature of the trimaran, which makes the slamming pressure near the main hull smaller than the other positions on wet deck. Moreover, the dimensionless peak slamming pressure on wet deck is less affected by the main-hull profile, while the time required for slamming pressure rise to its maximum is longest for the main-hull profile with smallest bilge curvature. These results can provide a better understanding of the relationship between the main-hull profile and the wet-deck slamming characteristics, which may be considered in future design for the trimaran section.

A computational framework for the co-optimization of platform hydrodynamic and passive structural control of floating offshore wind turbines

Floating offshore wind turbines extract rich and stable wind energy in the deep sea. However, they face severe sea conditions and high maintenance costs, and a promising method is needed to generate a high-performing, reliable, and robust design. Traditional sequential methods overlook the coupling between sub-systems and may miss the global optimum. The control co-design method, in contrast, focuses on the integrated optimization of the whole system. However, implementing co-simulation and co-optimization for co-design is challenging. In this study, a control co-design framework is developed and validated to couple the platform hydrodynamics with multi-physical wind turbines for integrated simulations and optimization. Based on this framework, the floating platform and tuned mass damper are co-optimized by establishing an efficient Kriging-enhanced Genetic algorithm. A case study is conducted using a 10 MW floating offshore wind turbine for demonstration. The platform shape and tuned mass dampers are parameterized by five and three design variables, respectively. To minimize platform mass, two formulations are proposed, that is, multi- and single-objective, for scenarios with and without consideration of structural loads. These formulas are solved under pre-defined turbulent wind and irregular wave conditions. Compared to the baseline model, the optimal design significantly reduces platform mass, enhancing both power generation and platform motion stability. After involving structural load in optimization, the optimal platform mass, energy production, and platform motion stability are degraded. These findings demonstrate the effectiveness of the co-design framework in optimizing platform geometry and passive structural control.

Experimental study of wet deck slamming for a SWATH in regular waves

Small Waterplane Area Twin Hull (SWATH) ships are subjected to severe sea conditions during ocean voyages, which may cause damage to the wet deck structures due to slamming. The flow process of wet deck slamming in catamarans is complex and involves wave breaking and splashing caused by wave impact on the wet deck. The slamming load under the wet deck of vessels is investigated in this paper by experiment. The test measures the slamming pressure of the wet deck in regular waves, and the flow field during slamming is observed using cameras. The experimental data are analyzed to determine the characteristics of the slamming pressure. The influence of wave height and wave length parameters on the magnitude and distribution of the slamming pressure are examined.

Numerical investigations of dam-break flow impacting on an elastic beam using various FSI coupling algorithms

The occurrence of green water and slamming in severe sea conditions can often be simplified as a dam-break problem. The flow characteristics of dam-break are similar with those on a ship’s deck and can result in structural damage. In this study, the fluid-structure interaction (FSI) of dam-break flow impacting on an elastic beam is investigated based on CFD-FEM method. The FSI coupling is solved in a partitioned manner. The fluid field is solved by OpenFOAM and the structure is calculated by self-developed code using FEM. The two-way coupling algorithm has been implemented to update the mesh of the fluid field. An investigation on mesh independence and the comparison of results with other methods have been conducted to validate the simulation results. After that, three types of FSI coupling algorithms have been utilized to examine the impact on both computational accuracy and efficiency. It is studied that the computational accuracy of all three FSI coupling algorithms is in good agreement. The FSI coupling algorithm utilizing weak iteration exhibits excellent performance in terms of both computational accuracy and efficiency. For the strong coupling algorithm, it is crucial to make a rational selection of the outer and inner iteration numbers in PIMPLE loops.

Assessing the coastal hazard of Medicane Ianos through ensemble modelling

Abstract. On 18 September 2020, Medicane Ianos hit the western coast of Greece, resulting in flooding and severe damage at several coastal locations. In this work, we aim at evaluating its impact on sea conditions and the associated uncertainty through the use of an ensemble of numerical simulations. We applied a coupled wave–current model to an unstructured mesh, representing the whole Mediterranean Sea, with a grid resolution increasing in the Ionian Sea along the cyclone path and the landfall area. To investigate the uncertainty in modelling sea levels and waves for such an intense event, we performed an ensemble of ocean simulations using several coarse (10 km) and high-resolution (2 km) meteorological forcings from different mesoscale models. The performance of the ocean and wave models was evaluated against observations retrieved from fixed monitoring stations and satellites. All model runs emphasized the occurrence of severe sea conditions along the cyclone path and at the coast. Due to the rugged and complex coastline, extreme sea levels are localized at specific coastal sites. However, numerical results show a large spread of the simulated sea conditions for both the sea level and waves, highlighting the large uncertainty in simulating this kind of extreme event. The multi-model and multi-physics approach allows us to assess how the uncertainty propagates from meteorological to ocean variables and the subsequent coastal impact. The ensemble mean and standard deviation were combined to prove the hazard scenarios of the potential impact of such an extreme event to be used in a flood risk management plan.

Dynamic Response Analysis of Offshore Flaoting Hose Strings Under Wave Load

A floating hose is key item for offshore oil transportation. In this paper, a mathematical model is derived for the overall dynamic performance of a single point-moored floating hose string under wave and current load. Airy wave theory was selected to calculate the wave force on the floating hose string. The dynamics of buoy, floating hose string and FSO were modelled using elastic foundation beam theory and discretised by using a fixed grid finite difference method. Discontinuities in the bending stiffness due to the flange connections are also considered. To verify the accuracy of the mathematical model, the finite element model of the buoy, floating hose string and FSO as a system was established by use of Orcaflex software tool. Numerical results for the displacement and bending moment at each section of the floating hose under severe sea conditions were obtained.

Carrier Aircraft Flight Controller Design by Synthesizing Preview and Nonlinear Control Laws

This paper proposes an innovative automatic carrier landing control law for carrier-based aircraft considering complex ship motion and wind environment. Specifically, a strategy is proposed to synthesize preview control with an adaptive nonlinear control scheme. Firstly, incremental nonlinear backstepping control law is adopted in the attitude control loop to enhance the anti-disturbance capability of the aircraft. Secondly, to enhance the glide slope tracking performance under severe sea conditions, the carrier motion is predicted, and the forecasted motion is adopted in an optimal preview control guidance law to compensate influences induced by carrier motion. However, synthesizing the inner-loop and outer-loop control is not that straightforward since the preview control is naturally an optimal control law which requires a state-space model. Therefore, low-order equivalent fitting of the attitude-to-altitude high-order system model needs to be performed; furthermore, a state observer needs to be designed for the low-order equivalent system to supply required states to the landing controller. Finally, to validate the proposed methodology, an unmanned tailless aircraft model is used to perform the automatic landing tasks under variant sea conditions. Results show that the automatic carrier landing system can lead to satisfactory landing precision and success rate even under severe sea conditions.

Adaptive neural network projection analytical fault-tolerant control of underwater salvage robot with event trigger.

To solve the problem of control failure caused by system failure of deep-water salvage equipment under severe sea conditions, an event-triggered fault-tolerant control method (PEFC) based on proportional logarithmic projection analysis is proposed innovatively. First, taking the claw-type underwater salvage robot as the research object, amore universal thruster fault model was established to describe the fault state of equipment failure, interruption, stuck, and poor contact. Second, the controller was designed by the proportional logarithmic projection analytical method. The system input signal was amplified and projected as a virtual input, which replaces the original input to isolate and learn the fault factor online by the analytical algorithm. The terminal sliding mode observer was used to compensate for the external disturbance of the system, and the adaptive neural network was used to fit the dynamic uncertainty of the system. The system input was introduced into the event-triggered mechanism to reduce the output regulation frequency of the fault thruster. Finally, the simulation results showed that the method adopted in this study reduced the power output by 28.95% and the update frequency of power output by 75% compared with the traditional adaptive overdrive fault-tolerant control (AOFC) method and realized accurate pose tracking under external disturbance and system dynamic uncertain disturbance. It has been proven that the algorithm used in this research can still reasonably allocate power to reduce the load of a fault thruster and complete the tracking task under fault conditions.

Impact of Propeller Emergence on Hull, Propeller, Engine, and Fuel Consumption Performance in Regular Head Waves

Abstract In this study, the impact of propeller emergence on the performance of a ship (speed), propeller (thrust, torque, and RPM), a diesel engine (torque and RPM) and fuel consumption are analysed under severe sea conditions. The goal is to describe the variation in the system variables and fuel consumption rather than analysing the motion of the ship or the phenomenon of propeller ventilation in itself. A mathematical model of the hull, propeller, and engine interactions is developed in which the propeller emergence is included. The system parameters are set using model experiments, empirical formulae, and available data for the engine. The dynamic response of the system is examined in regular head waves under submerged and emerged conditions of the propeller. The pulsatility and the extent of variation of 20 selected variables for the coupled system of hull, propeller, and engine are elaborated using quantitative and qualitative terms and absolute and relative scales. The simulation begins with a ship moving on a straight path, in calm water, with a constant speed for the ship, propeller and engine under steady conditions. The ship then encounters regular head waves with a known time series of the total resistance of the ship in waves. Large motions of the ship create propeller emergence, which in turn reduces the propeller thrust and torque. This study shows that for a specific ship, the mean ship speed, shaft angular velocity, and engine power were slightly reduced in submerged conditions with respect to calm water. We compared the mean values of the variables to those in the emerged condition, and found that the shaft angular velocity was almost the same, the ship speed was considerably reduced, and the engine power significantly dropped with respect to calm water. The ratios of the amplitude of fluctuation to the mean (Amp/Mean) for the ship speed and angular velocity of the shaft under both conditions were considerable, while the Amp/Mean for the power delivered by the engine was extremely high. The outcomes of the study show the degree of influence of propeller emergence on these variables. We identify the extent of each change and categorise the variables into three main groups based on the results.

Experimental study of slamming pressure for a trimaran section with different drop heights and heel angles

Trimarans may suffer from large motion and strong slamming accompanied by repeated water entry and water exit motions when sailing under severe sea conditions, and the huge instantaneous slamming load is prone to causing local structural damage. In this study, a free-drop water-entry model test of a trimaran section with different drop heights and heel angles was conducted. The motion and slamming pressure under different test conditions were compared, and the evolution of the free surface and flow field was observed using PIV (Particle Image Velocimetry) technology. We found that the peak value of the pressure was proportional to the square of the impact velocity, and that the peak pressure increased with the increasing heel angle on the lower side of the trimaran. Moreover, the flow separation phenomenon was more obvious at higher drop heights and larger heel angles. Finally, a Gaussian distribution function fitting model is proposed to fit the time-history curve of the slamming pressure within the slamming duration time on cross deck. The non-dimensional pressure coefficient and slamming action time under different conditions are summarised, and the fitting form which considers the impact velocity and impact angle in our results shows good agreement with the experimental data.

Engineering Possibility Studies of a Novel Cylinder-Type FOWT Using Torus Structure with Annular Flow

This paper proposes and researches a novel cylinder-type FOWT using a neutrally buoyant double-layer torus structure with annular flow; its oscillatory motion in severe sea conditions is controlled by a spinning top device designed as a neutrally buoyant double-layer torus structure with annular flow water in a torus structure with a small internal radius, and welded to the periphery of the cylinder-type FOWT underwater buoyancy-providing part. The rotational axis retention effect and the gyroscopic effect are considered appropriate approaches to suppress the oscillating motion of FOWT. To obtain a better hydrodynamic response, the scale of the torus structure, such as its radius, the radius of the internal annular flow water, and the angular velocity of the annular flow water are taken as the design parameters, and a large number of comparative calculations based on the fluid–solid coupling theory of potential flow are carried out to determine the appropriate design parameters. Eventually, on the basis of the obtained suitable design parameters, the proposed conceptual design approach is demonstrated to be feasible in view of the energy consumption.

Experimental analysis of wave-induced vertical bending moment in steep regular waves

Recent studies have benchmarked the prediction of the wave vertical bending moment (VBM) of ships in waves and found significant scatter among the numerical codes. A long series of tests with segmented hulls have been reported in the literature over the past 70 years to assess the vertical bending moment. However, alongside the improvement in numerical tools and the modeling of ships’ internal loads, the scientific community still requires accurate experimental data measured in steep waves (ratio of wave height H to wavelength λ, H/λ= 0.1) where the ship behavior and loads are modified by non-linearities. Thus, in order to validate simulation codes, and to establish criteria and requirements that make ships safer to sail in severe sea conditions, some experiments were carried out in the 50 m × 30 m × 5 m hydrodynamic and ocean engineering tank of the Ecole Centrale Nantes. A 1/65th scaled model of a 6750-TEU containership was used. The ship was moored and several combinations of wavelength and wave height were tested. The present experiments were performed on a segmented hull with a 6DOF load sensor located close to amidship, where some measurements were available from previous studies. The design, which did not use strain gauges, was done specifically to increase the first structural mode frequency and the stiffness of the model in order to reduce and make it possible to filter out hydro-elastic effects. The model motion was measured through a Qualisys IR tracking system and accelerometers were located on the fore and aft of the beam. Also, each of the 9 segments is equipped with a 3DOF dynamometer to measure the hydrodynamic loads on the hull. A mathematical description of the relations between the hydrodynamic forces, the load measurements and the internal forces is given in the paper. This makes it possible to retrieve the hydrodynamic loads on the segments and then to compute the shear force and bending moment discretized at each intersegment all over the ship length. Particular care was paid to the reproducibility and repeatability of the tests. The experimental setup and the measured data are presented in the paper. Hydrodynamic loads are reconstructed and the obtained VBM is compared with the 6DOF sensor measurement. The non-linear effects of increasing the wave steepness on motions and bending moment are discussed.

Effects of mooring line failure on the dynamic responses of a semisubmersible floating offshore wind turbine including gearbox dynamics analysis

Floating offshore wind turbines (FOWTs) are located in complex ocean environments; therefore, they typically exhibit more significant dynamic responses than land-based wind turbines. Examples from the marine and offshore industries have shown that mooring line systems may fail under mild to severe sea conditions during their life cycle, possibly changing the global responses of the FOWTs and affecting their internal drivetrain dynamics. Therefore, this paper considered an OC4 DeepCwind semisubmersible FOWT as a representative model to investigate the influence of one broken mooring line on the steady-state and transient responses of the system, as well as on the internal drivetrain responses when the turbine operated normally. A fully coupled wind turbine model was constructed to analyze the global response, and then a decoupled method was applied to analyze the internal drivetrain responses. Furthermore, a set of load cases were considered to perform the simulations. The results showed that accidental fracture of the upwind line significantly affected platform motions, turbine structural loads, and tensions in the remaining lines. However, the effects of upwind line failure on the electrical power output and the internal gearbox dynamics were relatively small due to the almost unaffected nacelle yaw errors and the actions of pitch-torque control.

Analysis of biaxial proportional low-cycle fatigue crack propagation for hull inclined-crack plate based on accumulative plasticity

Fracture failures of ship plates subjected to in-plane biaxial low-cycle fatigue loading are generally the coupling result of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of hull structure that accounts for the accumulative plasticity effect can be more suitable for the actual evaluation of the overall fracture performance of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for ∆CTOD is presented for hull inclined-crack plate. A test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behavior and the effects of biaxiality and stress ratios were investigated. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.