Waterplane Area Research Articles

Experimental investigation on seakeeping performance of variable-structure SWATH in heading regular waves

The variable-structure unmanned surface vehicle (VS-USV) have high maneuverability, strong survival, and multitasking characteristics in complex sea environment. In this study, a variable-structure small waterplane area twin hull (VS-SWATH) is proposed, which combines the advantages of straight strut SWATH and inclined strut SWATH. In order to study the seakeeping performance of the VS-SWATH with rotatable strut and hydrofoils at medium to low speeds, a series of seakeeping towing model tests was conducted. A variable-structure device was installed on the test model to change the strut rotation angle, and a detachable hydrofoil adjustment device was designed for the test model enabling the exchange of wingless and winged models. The test equipment includes a seaworthiness instrument system and a data acquisition system to measure values such as pitch, heave, vertical acceleration, and monitor the fluid around the ship. The test results show that the VS-SWATH can avoid resonance by changing the strut rotation angle. Hydrofoils mainly reduce heave and pitch at longer wavelengths, but their improvement in seakeeping performance is relatively limited in short waves.

Nonlinear analysis of hydrodynamics of a shallow-draft floating wind turbine

AbstractThis study investigates numerically the dynamic responses of the T-Omega Wind novel concept of Floating Offshore Wind Turbine. The turbine is light-weight, has a shallow-draft and a relatively high centre of gravity that allows it to glide over harsh marine environments. The turbine responses are studied under regular wave excitation, considering most probable ranges of discrete sea wave heights and periods representative of real ocean conditions. A multibody virtual model is developed, simplified to a rigid 6 DOF system and experimentally validated in the state-of-art Marine Simulator to define the types of dynamical responses for both “Low” and “High” Sea States. The dynamics of coupled heave and pitch DOFs are evaluated with time histories, phase-plane portraits, Poincaré sections and FFT analyses to conclude that period-1 stable solutions exist for all studied cases of “Low Sea States”, whereas period-2, period-3 and period-4 periodic responses are identified for short wave periods of excitation under “High Sea States” conditions. Simulation results show that regions where period-1 responses exist are highly sensitive to wave height and can widen as the wave amplitude reduces. Finally, the turbines’ nonlinearities generated by the floats’ geometry are observed in this dynamical system, which are identified to be related to variation in float waterplane area and particularly observable for “High Sea States”.

Coupling dynamic characteristics of installation vessel - Multi buckets jacket foundation lowering system

Precise control of the splash zone lowering process of the multi-bucket jacket foundation for an offshore wind turbine can save time during the entire construction period, thereby reducing construction and installation costs. This paper focuses on the “installation vessel - multi buckets jacket foundation” coupled dynamic system during the lowering process of the multi-bucket jacket foundation. Taking the three-bucket jacket foundation (TBJF) and the four-bucket jacket foundation (FBJF) as the research objects, four different sling layout types were proposed. The influence of the slings layout types on the motion response of the installation vessel and foundation and the slings' tension in this system were comparatively studied through numerical simulation. Research shows that through the lowering of Type 1 conditions, that is, each chord corresponds to a vertical sling and is directly connected to the crane's lifting point, the motion performance of the foundation and installation vessel can be reduced, ensuring the smooth progress of the lowering process of the structure. Under the same conditions, the FBJF has greater motion response due to the large wave force bearing area and large water plane area. Under different conditions, the DAF value and failure factor of the slings of the two structures during the lowering process all meet the specification requirements.

Fatigue assessment of a new concept for a floating barge with pendulum for offshore wind turbines in MOST

Among floating foundations for offshore wind turbines, the barge is considered one of the most cost-effective platforms, owing to its ease of manufacturing, transport, and installation. However, the barge platform is susceptible to wave-induced stress due to its extensive waterplane area, potentially leading to fatigue in critical structural components such as the platform, tower, nacelle components and moorings. This paper delves into the prospect of mitigating fatigue damage in the barge concept by introducing a pendulum ballast, which can be installed at the offshore site using polyester ropes. The primary focus of this research is the assessment of the ‘barge-pendulum’ concept, aimed at reducing load cycles that could lead to fatigue failure, with a specific emphasis on the tower base as a representative indicator of device damage. The analysis is conducted using MOST, a time-domain simulation code developed in Matlab-Simscape environment. The findings show that the barge-pendulum concept has a poorer performance in terms of fatigue life, due to the increased sensitivity to wave forces.

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.

Fast Reconstruction Model of the Ship Hull NURBS Surface with Uniform Continuity for Calculating the Hydrostatic Elements

The fast reconstruction of the ship hull nonuniform rational B-spline (NURBS) surface with uniform continuity is essential for calculating hydrostatic elements such as waterplane area and molded volume in real time. Thus, this study proposes a fast reconstruction model with uniform continuity to solve the problem of uniform continuity and splicing in the separate model of hull bow and stern surfaces. The proposed model includes the NURBS curve generation (UCG) algorithm with uniform continuity and the hybrid NURBS surface generation (HSG) algorithm. The UCG algorithm initially fits the feature points using the global interpolation algorithm and then precisely constructs straight-line segments in the curve using the improved flattening algorithm. In comparison, the HSG algorithm adaptively selects the surface knot vectors according to the parameters of the section curves. In this study, the profile of discontinuous compartments is uniformly expressed, effectively avoiding various articulation problems in separation modeling. The results of comparative experiments show that the NURBS surface generated using the HSG algorithm can accurately express the characteristics of various parts of the hull with uniform continuity, and the calculation speed of the proposed model can be increased by up to 8.314% compared with the existing best-performing algorithms. Thus, the proposed model is effective and can improve computational efficiency to a certain extent. The NURBS surfaces generated by the proposed model can be further applied to calculating the hydrostatic elements of hulls and compartments.

Study on the Effect of Hull Attitude on the Resistance Reduction of SWATH with Airflow Injection

With the development of the green ship concept in design and construction, how to reduce the resistance to reduce fuel consumption has become a focus of ship research. As an important drag reduction method, the air lubrication method has been applied to various ship types, but it is still a new method in the study of SWATH (small waterplane area twin hull) drag reduction. In this paper, the air lubrication method is applied to a SWATH model with an overall length of 2.5 m to numerically study the influence of the hull attitude on the air coverage and resistance reduction. The grid is verified by the grid independence and the experiment results. Then, the resistance of the SWATH model under different trim angles and drafts is calculated, and the air coverage on the surface is observed. The drag reduction rates of different areas, including the strut, underwater body, and fins, are analyzed, too. The results show that the slight trim by the head is more conducive to the resistance reduction of the SWATH model, and the resistance reduction rate can reach 39.11%. The draft mainly affects the resistance reduction of the strut, and the difference is more than 10%.

Numerical study of a novel small waterplane area USV advancing in calm water and in waves using the higher-order Rankine source method

The wave loads and motion responses of an Unmanned Surface Vehicle (USV) in water will directly influence the powering requirement, energy supply design and function of the installed sensors. In the present article, to investigate the wave loads and motions response characteristics of a USV advancing on the free surface, a Higher-Order Rankine Source (HORS) method is proposed. During the discretization of the boundary elements, bi-quadratic B-splines are applied to distribute the velocity potentials on the body surface and free surface. Based on the proposed HORS method, steady state simulation and time-domain simulation approaches are both used for predicting the wave loads and motion responses of a novel Small Waterplane Area (SWA) USV in calm water and regular waves, respectively. The predicted wave loads in calm water and the predicted heave and pitch in heading wave are compared with corresponding model test data. The good agreement found indicates the validity of the proposed HORS method. Finally, the heave and pitch motion responses and the wave components of the USV including diffraction and radiation forces and Froude−Krylov forces are investigated when the SWA USV is running against heading wave under different conditions of wavelength, wave steepness and advancing velocity.

Numerical analysis of the offshore wind turbine pre-mating process using a low-height lifting system for a nonconventional installation vessel

The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept.

Experimental study on the drop test on wet deck slamming for a SWATH segment model

Wet deck slamming is a serious issue for high-speed catamarans if the relative vertical motions are higher. In this study, a drop test on wet deck slamming is conducted for a small waterplane area twin hull segment model, and the vertical water-entry model test is performed under different drop heights and speeds. The slamming pressure under the wet deck and the acceleration are measured. The flow field is obtained through particle image velocimetry measurement. A high-speed camera observation during wet deck slamming shows that air is pressed into water, and a water–gas mixture is formed due to the air compressibility effect. The slamming pressure shows an oscillating property with multiple frequency components, which is due to the air cushion effect caused by the volume variation of the water–gas mixture and the acoustic wave oscillation with the air pocket. The effects of the drop height variation and the constant-drop speed on the slamming load are analyzed. The results show that the pressure peak appears during the demihull's bottom slamming and wet deck slamming. The acceleration time record produces two peaks, one for the demihull's bottom slamming and another for the wet deck slamming. In conclusion, the air cushion effect can reduce the wet deck slamming pressure.

Multi-objective optimization for an autonomous unmoored offshore wind energy system substructure

Autonomous unmoored floating offshore wind energy systems are an unconventional but promising technological solution to access the greatest wind resource in deep waters far offshore. This study proposes a trimaran substructure for such an autonomous unmoored system producing green hydrogen, named the Wind Trawler, which serves as both a power generation system and an energy transport vessel. Using a multi-objective optimization approach founded on the non-dominated sorting genetic algorithm (NSGA-II), the principal geometric parameters of the trimaran substructure hulls (primary hull and two symmetrically-spaced equivalent outriggers) are optimized with respect to minimization of system steel mass and minimization of the average unit power consumption per unit generation. Three discrete cases of maximum allowable overturning angle in heel (5°, 10°, and 15°) are considered. The results show that outriggers provide the majority of stability while the primary hull is most significant in influencing power performance and steel mass. Constraints of system stability and minimum outrigger draught induce a tradeoff between the outrigger waterplane area and the outrigger spacing from the primary hull. Steel mass minimization correlates with small hull spacing, shallow hull draughts, and greater outrigger proportion of the total mass. Large hull spacing, deep hull draughts, and reduced outrigger size maximizes the proportion of total mass in the streamlined primary hull, and therefore minimizes power consumption relative to generation. The effect of increasing maximum allowable overturning angle is that outrigger draught is much deeper at equal spacing from the primary hull. This induces a much greater outrigger mass and less efficient system overall, which drives up power consumption relative to generation. Ultimately, the optimum Wind Trawler substructures yield steel mass results that promise to compete with that of conventional systems, especially considering the elimination of mooring and electrical transmission subsystems. Metrics of power consumption relative to generation are unique to this type of unmoored system and thus need to be translated to conventional units of net energy production. This can be performed by considering the varying expected duration and trajectories of each operational mode.

Parametric Generation of Small Ship Hulls with CAD Software

CAD software is a daily tool in ship design offices and shipyards, and every software uses NURBS or B-splines curves and surfaces as common foundations. The CAD tools of today are not static software products and most of them now include parametric design modules, which enable users to change the shape of an object based on its key geometric feature parameters with the use of sliders or equivalent controls. Although B-spline techniques are commonly applied to the representation of the ship hull curves and surfaces, the parametric deformation of the hull surfaces based on geometric parameters is less used. This paper presents a methodology to define the parametric definition of a ship hull with the use of a standard and non-specialized CAD software that is of common use in the ship design offices and universities: Rhinoceros. The presented parametric design methodology will use specific ship hull parameters or feature parameters with a clear geometric meaning, such as displacement, waterplane area, LCB, and LCF, together with the properties of the B-spline curves and the power of Grasshopper, the parametric design tool inside Rhinoceros, to create parametric ship hulls.

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.

Hydrodynamic analysis of one-way energy capture by an oscillating water column wave energy device

This work evaluates the hydrodynamic performance of an oscillating water column wave energy converter, with a focus on comparing conventional two-way energy capture to one-way energy capture where only the up- or down-stroke is used drive the turbine. Small-scale model test experiments are performed, and numerical calculations are made using weakly-nonlinear potential flow theory. The air turbine is represented experimentally by an orifice plate with a flow area equal to about 1% of the internal-chamber water-plane area. One-way energy capture by the experimental model is realized by incorporating a passive, low-inertia, non-return valve which vents the air inside the chamber on one half-cycle of the internal water-column oscillation. In the numerical calculations, there is little difference between the two venting configurations, due to the simplified weakly non-linear model. However, the experimental results show that up-stroke venting generally yields a higher power absorption than down-stroke venting and the two-way energy capture generally yields a higher power absorption compared to the one-way energy capture. The calculations agree well with the experiments for two-way absorption, but substantially over-predict the absorbed power in the one-way configuration. This is mainly attributed to the imperfect venting system in the physical model, but further tests and/or CFD calculations are needed to confirm this conclusion.

Simplified estimation formula for frequency response function of roll motion of ship in waves

This study proposes a simplified formula for estimating the frequency response function of ship roll motion expressed by the main hull-form parameters of a ship (length L, breadth B, draft d, block coefficient Cb, waterplane area coefficient Cw, height of center of gravity KG‾, and metacenter height GM‾). By taking a theoretical approach, starting from the equation for roll motion and approximating each fluid force component, the authors have developed a formula that is physically clear and simple, yet engineering accurate and extendible. For nonlinear effects, this study assumes the case where decay coefficients or Bertin's N-coefficient are given, and it considers the nonlinearities in the quadratic damping force by equivalently linearizing the force. For equivalent linearization, a convergence calculation is usually required, but in this study, it is replaced by an explicit formula. To validate the proposed equation, numerical calculations using the linear 3D-Green's function method program were performed for 154 actual merchant ships with a wide parameter range, and it was confirmed that the equation is applicable regardless of ship type and size.

Study on the Effect of Air Injection Location on the Drag Reduction in SWATH with Air Lubrication

In this paper, the air lubrication method is applied to high-speed SWATH (small waterplane area twin hull) to reduce the resistance. The influence of air injection locations on drag reduction is investigated. The computed results are compared with experimental results, and the grid independence test is performed. Then, the numerical method is used to simulate the resistance of SWATH in calm water with airflow. The hull is divided into different areas, and the influence of the air injection location on the drag reduction in different areas of the hull is studied through different air injection locations and airflow rates. It could be seen that the air injection location on the underwater body is more conducive to the drag reduction in SWATH than the air injection location on the strut. Besides, the air injection location near the bow of the underwater body could cause a better drag reduction effect of SWATH. There are obvious differences in the drag reduction effect of different areas of the underwater body.

Experimental and numerical investigation on anti-rolling performance of a DP semi-submersible platform based on thruster group biasing

For semi-submersible platforms equipped with dynamic positioning (DP) system, which has a small water-plane area and low metacentric height, roll motion will be affected by the lateral control force of the thrusters, causing a larger roll amplitude. In view of this problem, present paper proposes a thrust allocation method based on thruster group biasing. The thruster axial forces in group counteract each other while the normal force directions are opposite to form an anti-rolling moment, so that the roll damping can be enhanced without affecting the positioning performance in the horizontal plane. The roll stabilization mechanism is studied experimentally and numerically. A numerical wave tank based on computational fluid dynamics (CFD) method is established, verified and validated against the experiments. Both experimental and numerical results show that the thruster group biasing strategy can increase roll damping and reduce the roll motion of the platform by approximately 16% under the given sea condition.

Comparative study of the mating process for a spar-type floating wind turbine using two alternative installation vessels

The offshore wind industry faces many challenges as the water depth of offshore sites increases. For installation of the spar floating offshore wind turbines (OWTs), cost-effective installation methods are needed to facilitate installations at moderate or deep-water depths. In this paper, a novel installation concept initially designed for installing pre-assembled wind turbines is further studied. Two alternative installation vessels (i.e., a catamaran and a small water-plane area twin hull vessel (SWATH)) equipped with a dynamic positioning (DP) system are considered for installing 10-MW OWTs. Hydrodynamic responses of the installation systems are obtained by the fully coupled nonlinear time domain method. Based on the numerical analysis in SIMO-Riflex, a comparative study of the two vessel concepts is conducted to study selected response variables of two mating points. Compared with the catamaran installation vessel, the SWATH substantially reduces dynamic motions during the critical mating phase. The findings of this paper provide references for industrial stakeholders selecting alternative installation vessels and serve as a basis for further optimizing the concept.

Dynamic response of a SWATH vessel for installing pre-assembled floating wind turbines

New and efficient installation concepts which can reduce the cost of developing an offshore wind farm are of particular interest. This paper explores a promising concept using the small water-plane area twin-hull vessel (SWATH) to install pre-assembled wind turbines (OWT) onto floating spar foundations. A focus is placed on the hydrodynamic performance of the SWATH and the response analysis of the coupled SWATH-spar system. Firstly, the numerically calculated difference-frequency wave force effect and damping forces of the original SWATH were verified with experimental data. Secondly, the original SWATH was modified to satisfy the criteria of weight-carrying capacity and hydrostatic stability. Thirdly, a multibody numerical model for the SWATH-spar system was developed, in which the hydrodynamic and mechanical couplings between the SWATH and a spar were considered. The SWATH is equipped with a dynamic positioning system to counteract the slow-drift wave force effects. The nonlinear time-domain simulations were carried out for the mating stage when a wind turbine is lifted above the spar foundation. Based on the analysis of statistics of the relative displacement and velocity of the tower bottom and the spar top, the installation concept with SWATH is found to be of decent performance. Finally, recommendations are provided for future research on this concept, which contributes to developing next-generation installation concepts for bottom-fixed and floating wind farms.

Comparative Analysis of Slamming Phenomenon Prediction between U and V Hulls using Strip Theory Method

Choosing the right hull shape is important in designing a ship, for example, a U-section or V-section of the hull. The hull shape will affect various aspects, such as design, resistance, seakeeping, structure and production. The ship hull must be properly designed so that it can operate according to the ship’s mission. From the seakeeping aspect of the ship's motion at sea, the difference in the hull shape will result in different motions and dynamic effects such as the slamming phenomenon. Based on the difference in the hull shape cases, this study analyzed the difference in the probability of slamming between the U and V hulls. Both hulls were made based on Formdata and almost all parameters were made the same. Parameters that cannot be forced to be the same are WSA (wetted surface area), (coefficient of waterplane area), and (distance of keel to buoyancy), where those parameters determine the difference in the results. The calculation of RAO (operator amplitude response) was obtained using the strip theory method which assisted by Maxsurf Motion software. The results became the input for the calculation of the slamming probability. The study results show that the U hull has a higher probability of slamming occurrence than that of the V hull, with the difference in values ranging from 20% to 35%. Therefore, the U hull will get more frequent slamming loads, so it has the potential to have a higher structural failure (fatigue) than that of the V hull.