Seakeeping Behavior Research Articles

Numerical analysis of coupling effect between sloshing and motion responses of a KCS in uni- and bi-directional waves

The seakeeping behaviour can be severely affected by the coupling between sloshing and ship motions in waves. The sloshing behaviour of liquid-loaded ships in bi-directional waves differs significantly from that in uni-directional waves. Given that ships operate in short-crested or multi-directional waves during their whole service period, it is significant to gain a deeper understanding of the effect of wave directionality on the coupling between sloshing and ship motions. In this study, a KRISO container ship (KCS) model with type-B liquefied natural gas (LNG) cargo tanks is adopted for coupling analysis. The 2D long-crested regular wave is adopted as an uni-directional wave, and three dimensional (3-D) cross wave is adopted as a bi-directional wave. The computational fluid dynamics (CFD) method is applied for the sloshing and seakeeping simulation of the ship model in different wave fields. The differences in wave elevation, sloshing forces, and coupling effects between ship motions and sloshing are studied in these two wave fields by adjusting control parameters such as ship heading angle and filling ratio. The comparative results indicate that sloshing behaviour and coupled motion responses in cross waves should be paid enough attention during ship design and evaluation.

Numerical Seakeeping Analysis for a Floating Helicopter After Ditching in Water

Abstract This paper investigates the seakeeping behavior of helicopters after an emergency landing in water, focusing on a Northern North Sea wave climate and considering a realistic helicopter geometry. Computational fluid dynamics techniques, including the cell-centered finite volume method and boundary element methods, were utilized to analyze motion responses and load distribution. The study ensures numerical result reliability through recommended simulation practices. Results indicate that the inviscid model produces similar outcomes to the viscous model in decay tests with roll, pitch, and heave motions. Natural periods for roll, pitch, and heave motions were obtained. Linearity between incident wave amplitude and pitch/heave response was noted for regular waves, while roll linearity was limited for small angles. In irregular wave conditions, helicopters tended to align perpendicular to waves over time, resulting in increased peak roll angles with higher significant wave heights. Exceedance rates of maximum roll peaks, useful for the assessment of capsizing probability, were quantified for different significant wave heights.

Beyond Jack-Ups: A Moonshot for Future Offshore Wind Turbine Installation Vessels for an Uncertain Market

This paper addresses the growing offshore wind market's demand for larger turbines in deeper waters by highlighting limitations in existing installation solutions and proposing a new concept with a floating monohull, named Moonshot, which will thus be different than traditional jack-up or semi-submersible crane installation vessel options. This paper discusses the design process, which combines Ulstein Rotterdam’s Controlled Innovation and Blended Design to develop the concept. This process is used to explore various market scenarios to determine optimal vessel parameters. Results demonstrate how optimizing for financial performance or seakeeping behavior impacts the design. Moonshot's initial parameters are established, and its performance is compared to existing installation solutions.

The Hull of a Galleon: An Archaic Construction or a Masterpiece of Ship Design

The Galleon was considered a masterpiece of shipbuilding in the sixteenth century, but from a modern point of view, the shape of her hull looks archaic and primitive. However, how accurate is this perception? Is the hull form of Galleon primitive? What were the reasons for its unique design features? This article investigates these questions by directly analysing the hull’s features from the point of view of modern ship’s theory, as well as from a historical perspective. A careful evaluation of specific design criteria of a typical Galleon, together with analysing a 3D model of its hull on modern software, with further insight for the seakeeping behaviour of Galleon with a comparison to more modern full-rigged ship (Barque of 19 CE), showed that these features were not random, but instead had a good rationale behind it, and served specific and carefully decided functions, required at the time.

Comparative summary of approaches used with the impulse response method

This paper presents a comparison of various methodologies to calculate the seakeeping behaviour of ships using the impulse response method. Developments of several authors incorporate non-linearities regarding the radiation, diffraction and wave forces. In contrast to frequency domain methods, advantages are expected if ship motions become non-linear, for example in waves with high steepness ratios. The comparison is structured into analysis for each force component with and without motions in waves. The main contribution is the quantitative analysis for several ships, forward speeds and methodologies. Multiple frequency domain methods are incorporated in the analysis. The focus of the analysis lies on the evaluation of applicability as well as the limits of the method for all states investigated. The results of the impulse response method show the following: The method is applicable to several forward speeds. In long waves, the choice of the parameters calculated in the frequency domain for the computation of the radiation and diffraction forces has a strong influence on the computed motion amplitudes. In short waves, the computation of the added resistance is a considerable challenge. Especially at high forward speeds, the consideration of the stationary wave system is essential for obtaining accurate results with three-dimensional panel codes.

An experimental study of stabilizing ordinary fishing nets (SOFNets) on a stationary SWATH ship seakeeping behavior under irregular waves

The present research aims to experimentally demonstrate the effectiveness of an innovative method for reducing the ship motions using Stabilizing Ordinary Fishing Nets that we call it SOFNets from now on. To this end, the influence of a range of SOFNets on the seakeeping behavior of a Small-Waterplane-Area Twin Hull (SWATH) ship in irregular waves is investigated physically in a towing tank laboratory. Understanding, controlling, and improving the seakeeping behavior of ships has always been a major concern of ship designers. As a result, various techniques have been utilized to stabilize ships, including the use of anti-rolling tanks, gyrostabilizers, and stabilizer fins, all three of which are bulky, heavy, and costly. These disadvantages have prevented many ships from using conventional rolling reduction methods. In contrast, the present idea, SOFNets, does not suffer from these shortcomings and can be used in any type of ship with any tonnage. The prepared model is tested in the towing tank of the National Iranian Marine Laboratory (NIMALA) using three different net samples in order to compare the behavior of those nets. This research has introduced for the first time the concept of using a SOFNets to reduce motion amplitude in waves. The length of the SWATH model was 2.1 m, and the tests were carried out according to the International Towing Tank Conference (ITTC) guidelines. The research findings for the SWATH ship indicate that the heave and pitch amplitudes in the head sea state were reduced by up to 21%, and the roll amplitude in the beam sea state was reduced by up to 36%.

Seakeeping Behavior of Axe Bow Patrol Boat with the Variation of Waterline Spline Type and Submerged Bow Depth

As a sovereign maritime country, Indonesia has been supported with patrol boats to maintain, protect, and manage the fishery and marine resources. Therefore, enhancing the patrol boat's operability is essentially needed for improving the resources maintenance and protection system. The implementation of an advanced hull forms might be conducted for the improvement of boat performance. The axe-bow hull form is one of the advanced hull forms that might improve the patrol boat operability, especially in high-speed conditions (Fr<0.60). However, in order to determine a reliable axe bow hull form, the body lines and the submerge bow depth should be defined appropriately. Based on the condition, this study focuses on the investigation of seakeeping performance of the developed axe-bow hull forms with variations of waterline spline type and submerged bow depth (the depth of foot). The waterline spline type configurations, which consist of the concave spline, convex spline, and straight spline, have been developed. At the same time, the submerged bow depths configuration have been proposed as 30%, 40%, and 50% of the boat draught. Both parameters should be recognized for the improvement of the axe bow hull seakeeping performance. The strip theory method has been adopted for the calculation of the seakeeping behavior. The results show that the waterline spline type has influenced the heave motion behavior. Otherwise, the larger depth of root might improve the pitch motion performance. It is indicated that suitable combination of spline type and depth of root might generate a reliable seakeeping performance of the axe-bow patrol boat.

Ship motion-sloshing interaction with forward speed in oblique waves

Seakeeping properties of large LNG carriers with partially filled tanks are known to be influenced by sloshing inside their tanks, by wave conditions and wave heading, and by their forward speed. An efficient hybrid numerical method was developed to study wave-induced ship response coupled with sloshing-induced compressible fluid response inside the tanks of a 138,000 m3 LNG carrier at zero forward speed and at the constant forward speeds of 12 and 20 kn. Wave conditions considered were oblique regular waves at heading angles of 0, 30, 60, 90, 120, 150, and 180 deg. The numerical approach combined a nonlinear boundary element Rankine source code to solve the weakly nonlinear seakeeping problem with a Reynolds-averaged Navier-Stokes equations solver to compute the compressible sloshing flow inside the tanks. Coupling effects of the six degrees-of-freedom ship motions in beam waves were validated against comparable model test measurements of ship responses and against free surface elevations in tanks. Results demonstrated that sloshing affected the simulated seakeeping behavior of the subject LNG carrier advancing at forward speeds in oblique waves. As sloshing had the greatest influence on the ship's roll motions, a key aspect was the derivation of roll damping from model decay tests.

INSIGHTS INTO THE FLOW WITHIN THE WELL DOCK OF A MOTHERSHIP DURING FEEDER VESSEL DOCKING MANOEUVRES

An experimental campaign has been undertaken to explore the flow around a feeder vessel as it manoeuvres in and out of the well dock of a mothership. The parent hulls for this study are drawn from the floating harbour transhipper concept created by Sea Transport Corporation. Laser measurement techniques have been employed to analyse the flow field within the well dock while the feeder vessel both enters and departs. For the Master of the feeder vessel to safely perform these manoeuvres, the complex flows resulting from the highly confined nature of the well dock concept need to be understood and potentially mitigated. It is shown that the inclusion of vents in the well dock can significantly influence the flow and that their effectiveness is determined by the size of the vents. This study further progresses the authors’ recent work on the same novel concept where the confined water effect of the well dock and inclusion of vents is quantified for both the seakeeping behaviour and the docking/departure performance. It is concluded that the use of vents is very beneficial when a feeder vessel docks or departs the well dock, however a compromise on the vent size must be reached in order to reduce adverse effects on feeder vessel motions when docked and exposed to a seaway. It is likely that the optimum solution, that covers all operational parameters, only requires the inclusion of relatively small vents.

«Коэффициент мореходности» как обобщенный показатель уровня мореходных качеств судна

Object and purpose of research. This paper discusses the possibility of a ship design process that would consider seakeeping performance to the greatest extent possible. The purpose of this study was to work out a numerical indicator, an index, reflecting all the seakeeping properties relevant for suitability of given ship to its intended operational conditions. Materials and methods. The study was based on the data about various operational parameters of the ship under investigation. These data were further synthesized so as to obtain the most comprehensive picture of ship seakeeping behaviour in different operational conditions. Main results. The study yielded the method and the algorithm for the “seakeeping index” as an average annual probability that seakeeping performance of given ship will be adequate to the conditions of given water area. The method sug-gested in this paper for a generalized comparison of seakeeping properties can handle whatever variety of target parameters and whatever seakeeping standards for any kind of ship intended to operate in given water area, and the result of this comparison is given in form of a single number that can be further used to improve seakeeping parameters of given ship, as well as to estimate possible time of its fully-featured operation in given conditions, including cost efficiency analysis. Conclusion. For more accurate comparison, it is recommended to analyse target parameters as functions of both ship speed and wave heading angle keeping in mind that the assumption introduced, i.e. that these curves as functions of wave heading angle are cosines, is not necessarily true. In other words, it is recommended to rely on more accurate data, experimental or analytical, so as to take into account the effect of apparent frequencies upon these curves.

O razvoju oblika trupa katamarana za brod za preradu ribe kao podršci nacionalnom ribarstvu u Indoneziji

Several advantages of multihull, such as catamaran, have been extensively discussed in the previous research. Therefore, this research focuses on developing a catamaran hull form for the fish processing vessel hull. The initial stage is determining the principal dimension and exploring the configurations of catamaran hull forms. The existing high-speed craft catamarans have been adopted to determine the parent model main dimensions using a linear regression equation model. Otherwise, the catamarans single demi-hull geometry was developed by converting and modifying the parent model hull form with enlarging the hull displacement to achieve the deadweight capacity and service speed requirements. The demi-hull spacing configuration with s/L 0.17, s/L 0.20, s/L 0.30, and s/L 0.40 on the resistance characteristics, intact stability, and sea-keeping behaviour were also explored. Furthermore, the comparisons with the previously proposed monohull were presented. Regarding the hull resistance performance, the analysis indicated that the catamaran hull form had better total resistance characteristics than the monohull on the service speed over 23 knots. In the case of intact stability, the analysis results presented that the catamaran hull form has better intact stability characteristics than the monohull. The dynamic stability of the catamarans also gave better dynamic stability at the heeling angle below 41.57°. Otherwise, the catamarans with s/L 0.17 and s/L 0.20 have lower dynamic stability than the monohull at the heeling angle larger than 41.57° and 58.03°, respectively. In the sea-keeping performance, the catamaran hull has shown an excellent rolling motion required for the offshore environment loading/unloading process. The large demi hull spacing of the catamarans hull can reduce the effect of the wave creating load on the roll motion response at the Beam Sea.

HYDRODYNAMIC LOADS AND FLOW STRUCTURE ANALYSIS FOR A SURFACED SUBMARINE USING CAPTIVE MODEL EXPERIMENTS

Submarines are required to operate on the surface for significant periods of time, for example during transit in shallow waters. However, as they are designed primarily for submerged operations, their surfaced sea-keeping behaviour may be poor. Furthermore, the wave making resistance of the vehicle increases in the surfaced state, substantially impacting on its powering requirements. The bow wave encountered when operating in a surfaced state applies a downward heave force and a bow down pitching moment, impacting the vessel’s sea-keeping performance. To provide insight into these effects, a captive model experimental program was undertaken at the Australian Maritime College Towing Tank facility to generate data at model scale for a 1.69m BB2 generic submarine geometry operating on the surface in calm water. The data generated includes the hydrodynamic loads and detailed, time-averaged flow structures on the surface, which will be used to study the behaviour of the vehicle and validate numerical tools.

Optimum design of miniature platforms for marginal fields

Motivated by many recent discoveries of marginal fields in deep water, this paper presents a novel and economical design concept of a minimal floating platform with around 10,000 cubic tons in displacement. The concept characterizes a simple hull geometry and an excellent seakeeping behavior. It incorporates a damping plate at the keel on the basis of a spar-like floater. The design procedure is explained and illustrated. The paper also describes a new design methodology that is capable of efficiently evaluating the seakeeping performance of the platforms with the viscous damping effect included. We integrate this methodology into an Evolutionary Algorithm (EA) to conduct a multi-objective optimization for our novel design. The hull shape is optimized by minimizing the heave motion in waves without sacrificing the cost in construction and installation. Several potential geometric configurations are considered. The optimization results provide a wealth of information that can be used to support practical design decisions.

Numerical study of closed rigid fish cages in waves and comparison with experimental data

Closed fish cages have gained increased interest in marine aquaculture. However, knowledge on the seakeeping behaviour of a floating closed cage and the influence of the contained water inside the cage are still limited. In this paper, a coupled numerical model is developed for the simulation of closed rigid cages in waves. Numerical studies are conducted both in the frequency domain and in the time domain, and compared with scaled physical experiments. Special attention has been drawn to the coupling effects of sloshing on cage response and the resulting mooring line forces. The comparative analyses show that sloshing of the contained water has large influence on the coupled surge and pitch motions of the cage. Sloshing is also found to have significant effect on the mean-drift forces in regular waves. In the tested/simulated irregular waves, the mooring forces are found to be dominated by the slow-drift motions, which indicates that the slowly-varying wave drift forces need to be considered in the design of the mooring system for a floating closed cage.

CFD Investigation into Seakeeping Performance of a Training Ship

The numerous ship accidents at sea have usually resulted in tremendous loss and casualties. To prevent such disastrous accidents, a comprehensive investigation into reliable prediction of seakeeping performance of a ship is necessarily required. This paper presents computational fluid dynamics (CFD) analysis on seakeeping performance of a training ship (full scale model) quantified through a Response of Amplitude Operators (RAO) for heave and pitch motions. The effects of wavelengths, wave directions and ship forward velocities have been accordingly taken into account. In general, the results revealed that the shorter wavelengths (l/L ? 1.0) have insignificant effect to the heave and pitch motions performance of the training ship, which means that the ship has good seakeeping behavior. However, the further increase of wavelength was proportional with the increase of RAO for her heave and pitch motions; whilst it may lead to degrade her seakeeping quality. In addition, the vertical motions behavior in the following-seas dealt with higher RAO as compared with case of the head-seas condition. Similarly, the subsequent increase of the ship forward velocity was prone to relatively increase of the RAO for her heave and pitch motions especially at l/L ? 2.0. It was merely concluded that this seakeeping prediction using CFD approach provides useful outcomes in the preliminary design stage for safety assessment of the training ship navigation during sailing.

Performance of Rescue Boat Operation when Operated in Waves

Search and rescue operations, especially in ship accidents, require speed and reliability from the rescue vessels in facing the challenges of extreme weather and sea waves. A rescue boat hydrodynamic performance study is very important to do to determine the ship’s ability to withstand dangerous conditions while operating. Therefore, a rescue boat design is needed that is capable of high acceleration and can penetrate various extreme sea condition. Three rescue boat hull models have been developed to obtain the hull model with the best seakeeping performance. A numerical arrangement based on computational fluid dynamics (CFD) techniques is used to predict seakeeping behaviour from three rescue boat hull model in wave. Three-dimensional diffraction panel method uses as a CFD solver method to predict the seakeeping performance of rescue boat high-speed operation in wave. From the study results, it was found that the first rescue boat hull model had a good seakeeping performance compared to other hull models.

CFD prediction of ship seakeeping behavior in bi-directional cross wave compared with in uni-directional regular wave

The seakeeping behavior of a vessel in multi-directional wave differs significantly from that in uni-directional wave. Given that a vessel operates in multi-directional or short-crested waves during its whole service period, it is important to have a better understanding of the influence of wave directionality on ship seakeeping performance. In this study, CFD method is used to simulate ship motions in both uni- and bi-directional regular waves. A S175 hull model is adopted for seakeeping analysis. Both uni- and bi-directional waves are generated by velocity boundary method. Overset grid technique is adopted to simulate ship motions in waves. Ship motions in different uni-directional regular wave conditions are predicted and validated by experimental data at first. Then ship motions in monochromatic bi-directional wave conditions with different control parameters such as wave length, ship speed and ship heading angle are predicted and analyzed. The comparative results indicate that ship motion response in bi-directional waves is generally larger than that in uni-directional waves. Moreover, severe bow slamming and green water under bi-directional waves are also observed and investigated. This study indicates that ship seakeeping behavior in bi-directional wave should be paid enough attention during ship design and evaluation.

CFD prediction of stern flap effect on Catamaran seakeeping behavior in long crest head wave

The paper presents the numerical studies of seakeeping behavior for a high-speed catamaran with stern flap advancing in long crest head wave. Both model-scale and full-scale simulations have been carried out in sea state 6 using unsteady Reynolds-averaged Navier-Stokes (URANS) solver. Uncertainty analyses were performed to verify the accuracy of the numerical solver. Model tests were conducted for the catamaran including resistance tests in calm water and seakeeping tests in regular waves to validate simulation results. The comparisons between simulation results and experimental results show good agreement. Seakeeping behaviors of models with/without stern flaps were predicted for the comparison and evaluation. Model-scale simulation results indicate that using stern flap could reduce catamaran's total resistance and the effects on heave and pitch motions are obvious in sea state 6. The installation of stern flap shows significant effect on the reduction of vertical acceleration's amplitudes, especially for reducing the occurrence of slamming impact due to the effective control of model's trim angle. Full-scale simulations have also been conducted and compared with results at model scale. Results show that effects of stern flap on heave and pitch motions are the same for both model-scale and full-scale catamaran. Residuary resistance coefficients on full-scale catamarans with/without stern flap are almost the same with model-scale simulations.

Insight into the Flow Within the Well Dock of a Mothership During Feeder Vessel Docking Manoeuvres

An experimental campaign has been undertaken to explore the flow around a feeder vessel as it manoeuvres in and out of the well dock of a mothership. The parent hulls for this study are drawn from the floating harbour transhipper concept created by Sea Transport Corporation. Laser measurement techniques have been employed to analyse the flow field within the well dock while the feeder vessel both enters and departs. For the Master of the feeder vessel to safely perform these manoeuvres, the complex flows resulting from the highly confined nature of the well dock concept need to be understood and potentially mitigated. It is shown that the inclusion of vents in the well dock can significantly influence the flow and that their effectiveness is determined by the size of the vents. This study further progresses the authors’ recent work on the same novel concept where the confined water effect of the well dock and inclusion of vents is quantified for both the seakeeping behaviour and the docking/departure performance. It is concluded that the use of vents is very beneficial when a feeder vessel docks or departs the well dock, however a compromise on the vent size must be reached in order to reduce adverse effects on feeder vessel motions when docked and exposed to a seaway. It is likely that the optimum solution, that covers all operational parameters, only requires the inclusion of relatively small vents.

Linear wave-induced dynamic structural stress analysis of a 2D semi-flexible closed fish cage

Closed fish cages in the sea are proposed as a new concept in marine aquaculture, replacing the conventional net cages in order to meet ecological challenges related to fish lice and escapes. A closed fish cage can be compared to a floating tank structure with an internal free surface. Several types of closed cages have been suggested, and they are categorised according to structural properties as flexible membrane structures (fabric), semi-flexible structures (glass fibre) and rigid structures (steel or concrete). To be able to develop safe and reliable structures, more knowledge is required on the seakeeping behaviour of closed cages in waves and the structural response to the wave loads. This paper builds on a theory presented in Strand and Faltinsen (2019) on the linear wave loads on a 2D closed flexible fish cage. A modelling error has been found in Strand and Faltinsen (2019), however, all the main conclusions are in hold. The error has been corrected in the model in the present paper. The present paper extends the model to include bending in the structural model to be able to handle semi-flexible structures where bending stiffness is significant. In this paper, the linear theory of a 2D semi-flexible closed fish cage in waves is developed and analysed to investigate the structural response of the semi-flexible closed cage in waves. We have compared a quasi-static analysis with a fully coupled hydroelastic analysis to investigate if it is a valid and conservative assumption to assume that the stresses in the structure can be assumed quasi-static. If a hydroelastic analysis is necessary or not, is dependent on the stiffness of the structure. We have investigated what happens with the stress in the curved beam part of the closed fish cage for increasing and decreasing stiffness relative to a reference composite structure. One stiffer and two softer cases have been analysed. One major concern for the structural stresses in a closed cage is the effect of sloshing. Sloshing is internal wave motion inside the cage and have multiple resonance periods. The results indicate that to use the quasi-static assumption in structural stress calculation is conservative within the given frequency range for all examined stiffnesses and frequencies, except the frequencies very close to the second sloshing frequency. Close to the second sloshing frequency for all the examined stiffnesses, a localised peak can be observed in the coupled hydroelastic results. The second sloshing frequency is a frequency connected to a symmetric sloshing mode. Rigid body motion is not affected at the symmetric sloshing frequency for an assumed rigid structure, and are therefore also not visible in the stress results from the quasi-static analysis. The structural stress in irregular sea was calculated. These results show no indication of increased stress close to the second sloshing frequency. However, this is not a surprising result since the stress peak is very localised in frequency, and the accumulated effect on the stress standard deviation is therefore small.