Small Waterplane Area Research Articles

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.

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.

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.

A study on longitudinal motion stability of a variable-structure SWATH USV with and without twin hydrofoils

The form-changeable unmanned surface vehicle (USV) have a high research enthusiasm due to its far-ranging applicability, strong combat capability and low operating cost. This paper constructs a variable-structure Small Waterplane Area Twin Hull (SWATH) USV with twin hydrofoils, and aims at assessing its longitudinal motion stability (LMS) in the catamaran-form state (CFS) by parametric model and CFD method. The parametric representation of the torpedo, double struts and hydrofoils was conducted by uniform B-spline curves, 3-D coordinates conversion and surface generation method. The CFD code was verified, and used to calculate the LMS of the vessel with different strut configurations, form-states and CG position. Based on numerical results, the causes of different instability behaviors were expounded. Further, the LMS of the models with and without hydrofoils was compared at Fr = 0.1–0.8, the adjustment method of fore and aft hydrofoils to sailing attitudes was explained, the rational allocation of attack angles was also offered. The results show that the slender double struts arranged at both ends of torpedo is more conducive to the LMS and resistance, the vessel should sail in the CFS of γs ≤ 10° at Fr > 0.6; the hydrofoils slightly weakens the LMS at low Fr, makes the bow appear slight updip at Fr = 0.4–0.6, but still can sails stably; adjusting attack angles can inhibit the instability, and the most effective allocation of the fore and aft hydrofoil's attack angles is linearly arranged at Fr = 0.7, 0.8, also the adjusting can reduce the upward trim at Fr = 0.9, 1.0, but the hull is lifted higher and arises the bow updip due to the free surface suction force, at this time, the attack angle's graduation should be refined to realize the finer adjustment to the dynamic trim.

Hull form optimization design of SWATH with combination evaluations of resistance and seakeeping performance

With the rapid development of the marine economy, marine operations are gradually diversified. To improve the hydrodynamic performance of SWATH in offshore working conditions, a design optimization method for Small Water-plane Area Twin Hull (SWATH) considering the resistance and seakeeping performance is proposed. The automatic optimization design of SWATH hull form with minimum resistance at different speeds is carried out to obtain a set of Pareto optimal solutions. Then the seakeeping performance of the original and the Pareto optimal solution set are predicted. On this basis, a comprehensive evaluation method based on the composite weight factor of SWATH resistance and seakeeping performance is provided. The results show that the method can obtain a hull form scheme with better hydrodynamic performance than the original hull form. In addition, it can guide the hull form optimization design of SWATH.

On the Nonlinear Response of a Tri-SWACH Vessel in Incident Waves

Abstract In this article, the nonlinear motion response of a Trimaran with SWATH-type center hull vessel (TriSWACH) is investigated and compared with the linear response using a hybrid method combining a regression analysis of the viscous damping generated by the hulls with a nonlinear potential flow simulation of rigid body motions. The TriSWACH is a hybrid trimaran hull form, developed using the SWATH (small waterplane area twin hull) technology, from which a single small waterplane area center hull is retained. It is designed to keep the magnitude of its motion response below its equivalent monohull counterpart while also benefiting from large intact transverse stability provided by its side hulls. Since a significant portion of the total damping generated by the submerged hull girder is viscous in nature, an accurate prediction of the steady-state motion response near natural frequency is highly dependent upon the correct modeling of the boundary layer flow. The total damping coefficients obtained experimentally from motion response transfer functions are used to augment the potential flow solution and improve the numerical prediction method at large angles of inclination, characteristic of operation in rough weather near the peak frequency of the motion response. Introduction SWATH1-type vessels exhibit ship motions less pronounced than a monohull of the same displacement. It has been reported that SWATHs have the potential to significantly reduce the impact of harsh environmental conditions on ship operability (Kennell et al. 1985). Although the interest in SWATH technology in ship design continued over the past few decades, it was recognized that this hull form required modifications in order to accommodate high-speed applications. An example of a TriSWACH concept is shown in Fig. 1, with the main distinguishing features compared to the SWATH-type vessel being the presence of the side hulls and one single main hull.

Operability analysis of SWATH as a service vessel for offshore wind turbine in the southeastern coast of China

The operability of service operation vessels (SOV) is highly concerned with the cost, schedule, and weather window to the operation and maintenance of offshore wind farms. SWATH (Small Waterplane Area Twin Hull) vessel is introduced as a novel application as SOV to be deployed off the coast of southeastern China, an investigation into the operability for walk-to-work (W2W) on-site service using the new type of vessel is conducted in this study. The features of SWATH’s motion behavior are discussed by performing a seakeeping analysis, which is compared to a similar size monohull SOV. W2W operation and various operational limits associated with vessel and gangway dynamic response are modeled by the numerical method established in the frequency domain. Considering different W2W transfer approaches and wave headings, an in-depth analysis regarding the limiting wave height is carried out, the operability of three sites off the southeastern coast of China is predicted. SWATH demonstrates the excellent operability for both transversal and longitudinal transfer approaches as well as the robustness against arbitrary wave heading. The fuel consumption under the high sea-state shows higher usage compared to the monohull by a qualitative estimation using the second-order mean drift forces. With the average operability greater than 95% in the study, SWATH has proven to be an attractive type of vessel against monohull whose operability may drop to 60% to 80% in case of unfavorable heading or transfer approach. Due to lack of damping, the optimization of SWATH geometry and proper damping device may help further improve the operability by suppressing resonant heave and roll.

Numerical Investigation of the Automatic Air Intake Drag Reduction Strut Based on the Venturi Effect

Drag reduction by injecting air is a promising engineering method for improving ship performance. A novel automatic air intake drag reduction strut structure based on the Venturi effect is proposed for the high-speed small water-plane area twin hull vessels in the present study. The drag reduction strut can achieve the function of automatic air intake when the vehicle is moving at high speed, and the air inhaled and the incoming flow form bubbly flows to cover the strut surface, effectively reducing the drag of the strut. Considering the longitudinal symmetry of the strut structure, a two-dimensional single-chip drag reduction strut structure is designed to facilitate analysis and a solution. The volume of fluid model is combined with the k-ω SST turbulence model, and a numerical simulation is carried out to investigate the variation of the air inflow, the air volume fraction in the bubbly flows of the strut and the drag reduction rate of the strut for different sailing speeds. The analysis result shows that when the proposed model reaches a certain speed, the external air is inhaled by the strut intake duct, and the bubbly flows are formed with the incoming flow covering the surface of the strut, thereby reducing the drag coefficient. Meanwhile, it is found that as the sailing speed increases, the drag reduction rate of the strut gradually rises and its maximum value reaches about 30%. For high sailing speeds, the drag reduction rate is affected by wave-making resistance so that it gradually declines.

SUPERIOR SEAWORTHINESS OF A RESONANCE-FREE FAST OCEANGOING SWATH

The speed reduction, additional resistance or slamming caused by the large amplitude ship motions, should be completely restricted for a large fast oceangoing ship because of the strict time-punctuality and the high value of the cargo. A “Resonance-Free SWATH (RFS)”, which has negative restoring moments due to the extremely small water plane area, is introduced to minimize the motion responses. A motion control system using small fins is necessary for the RFS, which has no stability during high speed cruising. Theoretical estimations and experiments to search for the optimum values of PD control gains have been performed. Unsteady characteristics of fin-generated lift such as the time lag and the interaction among the fins and lower hulls have been measured and they are taken into account in the motion equations. Then, experiments using the RFS model with controlling fins have been carried out to validate the theoretical estimation for the motion responses of the RFS in waves. The theoretical and experimental results agree well with each other. The motion responses of the RFS in regular and irregular head waves are compared with those of other hull forms, such as a mono-hull, an ordinary SWATH and a trimaran. The clear advantage of the RFS regarding the seaworthiness has been found. In summary, the heave motion response of the RFS is reduced to 1/60 and the pitch motion becomes1/8, compared with those of the existing mono-hull ship.

UNSTEADY CHARACTERISTICS OF LIFT GENERATED BY SMALL UNDERWATER CONTROL FIN

Speed reduction or slamming must be restricted for a high-speed oceangoing vessel because of the requirement for punctuality and the high value of the cargo. Speed reduction and slamming are caused by large amplitude motions in waves. A promising ship form for such vessels is so-called “Resonance-Free SWATH (RFS)”, which has negative pitch and roll restoring moments due to the extraordinary small water plane area. As a consequence, the resonance peak is removed from the motion response. The attitude of the RFS with negative restoring moments is adjusted by four pairs of control fins attached to the fore and aft ends of the lower hulls. In previous studies, the steady value of the lift-curve slope is usually used in the motion equation of the frequency domain. However, when working in waves, the controlling fins are not working in a steady state and the lift coefficient is no longer a constant. In addition, there exists a phase lag between the change in the attack angle and the fin-generated lift. In the present study, theoretical predictions using a frequency-domain 3D-Rankine Panel Method, as well as experimental measurement, have been made to analyze the phenomena of the lift generation including the phase lag and the interference between fins, the lower hulls and the struts. The theoretical results agree well with the experimental results in spite of the potential theory being without viscosity. Next, the unsteady characteristics of fin-generated lift are expressed as the function of the encountering wave frequency. Then the effects of the fore fins, the lower hulls and struts on the lift curve-slope of the aft fins are discussed.

Design parameter sensitivity analysis for SWATH with minimum resistance at design and service speeds

Parametric hull form is efficient in geometric modification, and can help to obtain new hull forms with excellent hydrodynamic performance. Achieving parametric hull modeling depends on the reasonable definition of design parameters. Currently, in parametric hull modeling and optimization, the lack of an effective parameter selection method usually results in blindness and inefficiency in the hull design. This paper provides an automatic calculation framework for SWATH (Small Water-plane Area Twin Hull) resistance at different speeds. A fully parametric SWATH model is established to realize automatic geometric transformation. SOBOL (Sobol’ quasi-random sequence) algorithm is used for space sampling. The sensitivity analysis of all the parameters of SWATH for resistance is carried out. The correlation study between design variables and resistance is performed and the correlation coefficients of all design parameters are given and discussed. Among the parameters of SWATH, 4 design parameters, those control the longitudinal deformation of the bow of the underwater hull, the transverse deformation of the stern, the position of the parallel middle body of the strut, and the inclination of the strut, respectively, are more sensitive to the resistance. The suggestions for the parametric design of SWATH are given based on the sensitivity analysis and optimization results.

Главное о судах с малой площадью ватерлинии

Object and purpose of research. The object of study is small waterplane area twin hull (SWATH) ships whose main advantage is their excellent seaworthiness. The purpose of this work is to demonstrate the broad capabilities of such vessels. Materials and methods. The knowledge from foreign literature along with the author’s data are employed. Comparison methods focus on the main specific features of the technology for different fields of application. Main results. Brief review of the SWATH background is given highlighting the main trends of development. Common features and differences of each SWATH type is identified. The revealed merits and disadvantages, design details point to a broad range of possible applications. Conclusion. SWATH architecture may provide a considerable improvement in seaworthiness of small and medium displacement vessels. SWATH is an economically viable option for all types of marine vessels with a relatively large deck area, so-called capacity carriers, like passenger, research, ro-ro, naval ships, etc. For more efficient application it is recommended that alternative SWATH projects should be developed at an early design of vessels for the above missions.

SHIPS WITH SMALL WATER-PLANE AREA: THE MAIN INFORMATION

Ships with small water-plane area (below “SWA ships”) have very special shape of hulls. It is the reason of characteristic specificity of such ships. SWA ships usually are multi-hull ones. And as all multi-hulls, SWA ships differ from the comparable mono-hulls by bigger deck area relative the displacement and by simple ensuring of stability. There are some architectural types of SWA ships. Their differences are shown by the article. Relative area of deck and initial transverse stability of SWA ships are compared. Statistical data on performance: twin-hull SWA versus catamarans. Common specificity of and . The main external loads. Special method of designing. Main disadvantages. Conclusions, recommendation.

EXPERIMENTAL AND NUMERICAL INVESTIGATION ON MANEUVERING PERFORMANCE OF SMALL WATERPLANE AREA TWIN HULL

Free running model tests and a system-based method are employed to evaluate maneuvering performance for a Small Waterplane Area Twin Hull (SWATH) ship in this paper. A 3 degrees of freedom Maneuvering Modeling Group (MMG) model is implemented to numerically simulate the maneuvering motions in calm water. Virtual captive model tests are performed by using a Reynolds-averaged Navier-Stokes (RANS) method to acquire hydrodynamic derivatives, after a convergence study to check the numerical accuracy. The turning and zigzag maneuvers are simulated by solving the maneuvering motion model and the predicted results agree well with the experimental data. Moreover, free running model tests are carried out for three lateral separations and the influence of the lateral separations on maneuvering performance is investigated. The research results of this paper will be helpful for the maneuvering prediction of the small waterplane area twin hull ship.