Oblique Seas Research Articles

Coupled response analysis of dual lifting vessels during collaborative lifting topside module

Offshore assembly and disassembly operations represent a multi-billion-dollar market potential. Collaborative lifting by multiple vessels has emerged as a new operating paradigm for integrated offshore facilities assembly and disassembly. Hence, this paper investigates the hydro-dynamic interaction of dual lifting vessels in collaborative lifting operations. The coupled motions during multi-body operations are simulated using the commercial software SESAM. The feasibility of the numerical model for coupled motions in collaborative lifting is verified by comparing the numerical results of topside motions, vessel motions, and vertical lifting arm loads against experimental measurements. The effects of wave heading and period on the hydrodynamic responses of the topside module, dual lifting vessels, and lifting arms during collaborative operations are studied. Their influence patterns and mechanisms are analysed in detail. The results show that transverse waves and head wave induce significant heave and pitch motions of the topside module and vessels, but the maximum vertical loads on the lifting arms occur in oblique waves. The motion responses of the topside module and vessels increase with longer wave periods under the oblique sea condition, and roll motions are more sensitive to large periods compared to the gradual rise in heave and pitch.

Investigation of wave effects on motion of an unmanned surface vehicle at various scales

Motion characteristics of unmanned surface vehicle are mainly affected by sea wave during its operation. This paper shows the numerical results of the unmanned surface ship at various scales in irregular waves based on the boundary element method. The effects of different sea states on each scaled model in a head sea, bow quartering sea, and beam sea are analyzed to predict the safety region of the ship motion. The significant amplitudes of ship motion are obtained from the ship motion responses in regular waves. Steady wave drift forces in the cases of bow oblique sea are calculated to find the impact of second-order waves on the models. It is proved that the motion of model C is remarkable in the narrow frequency domain from 0.2 to 1.4 rad/s but the ship responses of model A are considerable in the range of frequency of 0.2 to 2.5 rad/s. The significant amplitude of surge and pitch motion reaches the peak values in head sea conditions while the peak values of yaw motion are in the oblique sea of 120 degrees. The left motion is maximum in head sea condition.

Hydroelastic effects on the vertical bending moment of a container ship in head and oblique seas

Large-sized ships like Ultra large container ships (ULCS) experience severe hydroelastic vibrations like springing and whipping. This study numerically investigates the performance of a time domain method to assess the effect of structural vibration in calculating of vertical bending load of an ULCS. The proposed 2-D body nonlinear time-domain numerical method comprises a seakeeping solver based on potential flow theory and a structural solver based on Timoshenko beam theory. The simulations are carried out in the head and oblique waves for various forward speed conditions. The solver captures the effects of high-order springing and whipping in vertical bending. The numerical method identifies the whipping effect based on the slamming associated with the relative motion of the ship. The vertical bending moment is calculated in both regular and irregular waves, and the results are compared with the experimental results. The statistical evaluations of available results for irregular waves are carried out and presented. The numerical results are in good agreement with the experimental ones for most of the analyzed cases.

Numerical study on mean wave drift forces and moment of ships using Rankine panel method in frequency domain

A three-dimensional frequency-domain seakeeping model based on the Rankine panel method is developed to compute the mean wave drift forces and moment acting on ships with forward speeds. Two different linearization models (the double-body and Neumann–Kelvin models) are applied to evaluate the mean wave drift forces and moment using a near-field method. In these models, the Rayleigh’s artificial friction is adopted in the free-surface boundary condition to suppress longer wave components in computational regions apart from the ship. A novel upwind difference scheme is introduced to calculate the first-order and second-order derivatives of the velocity potential on the free surface. Wigley I, Wigley III and S175 containership are adopted in numerical simulations to validate the effectiveness of the developed model. In the present study, the motion response amplitude operators in head, beam and oblique seas obtained using the two models are compared, and their contributions to the mean wave drift forces and moment are analyzed. Results show that in head seas, the pitch motions have significant effects on the surge-directional wave drift force Fx. In beam seas, the heave motions have the most effect on the mean wave drift forces and moment (Fx, Fy and Mz).

Nonlinear URANS model for path-following control problem towards autonomous marine navigation under wave conditions

An accurate prediction of the performance of maritime autonomous surface ships (MASS) to follow a predefined path in waves is of critical importance for ensuring safe autonomous marine navigation. Traditional methods for the study of path-following problems are based on simplified mathematical ship models and are incapable of precisely resolving the complicated interactions between the hull, propeller, rudder, and incident waves under path-following control. In the present study, a CFD-based dynamic model for path-following problems is developed for autonomous marine navigation by means of a fully nonlinear unsteady Reynolds-Averaged Navier-Stokes (RANS) solver combined with the Line-of-Sight (LOS) guidance law. Fortunately, an unsteady RANS solver is capable of incorporating viscous and turbulent effects and the free surface resolution critical to path-following problems, allowing a better prediction of a ship's path-following performance in waves. To obtain practical insight into the performance of the ship executing path-following control for autonomous navigation in waves, a comprehensive analysis of path-following tasks in waves covering the whole range of important wave directions and wave heights was carried out in this study. The numerical results demonstrated that when the ship performed the path-following manoeuvre under the beam and quartering seas, the greatest oscillatory deviation from the predefined path was observed due to the asymmetric pressure distribution on the ship hull caused by the incident waves. It was also confirmed that wave heights strongly affected the trajectories experienced by the ship performing the path-following task in the beam and oblique seas; the deviation from the predefined path was found to increase with the increase in the wave height. As high-performance computational resources become increasingly available, the proposed CFD method will provide an accurate and efficient way to predict the performance of autonomous surface ships performing path-following tasks in waves, providing a valuable contribution to enhancing the safety of autonomous marine navigation.

Estimation of torsional and global loads for a wave-piercing high-speed catamaran at full-scale in irregular bow quartering seas using CFD simulation

Catamarans experience more types of global load than monohulls, these include pitch connecting moment, transverse bending moment, and split force. High-speed catamarans are widely used for passenger transportation but as the speed and size increase the severity of loads rises due to slam induced effects. In this study, full-scale CFD simulations are undertaken to investigate the loads acting on a 98m Incat wave-piercer catamaran (Hull 061) HSV2 Swift. Simulations are performed for conditions of a selected sea trial run in bow quartering seas at 20 knots forward speed undertaken by the Naval Surface Warfare Center, Carderock Division (NSWCCD), enabling comparisons and validation. In order to estimate internal global loads at different sections of the vessel, rigid body dynamics is applied based on the hydrodynamic and inertia forces. Those internal loads include longitudinal bending moment (LBM), pitch connecting moment (PCM), torsional moment (TorM), transverse bending moment (TBM), split force and prying moment. The estimated and measured global loads are also compared with design loads limits provided by DNV GL rules, and peak values of pitch and roll accelerations corresponding to each slam load are determined at the same wave height and wave period as the sea trial. The application of CFD to simulate sea trials runs in oblique seas is shown to provide a reliable estimation of slam induced loads for application in early design stages.

Hydrodynamic analysis of resonant waves within the gap between a vessel and a vertical quay wall

Concerning the side-by-side offloading operations at gravity-based terminals, the wave interaction with a vessel alongside a long vertical wall in both normal and oblique seas was investigated. The imaging principle was applied to convert the original to an equivalent wave-diffraction problem by two symmetrical vessels in open seas exposed to bi-directional incident waves. The boundary integral equation method, in conjunction with a higher-order boundary element method (HOBEM), was used to solve the equivalent problem. Besides the linear free-surface elevation and wave loads, the second-order mean wave drift loads on the vessel was calculated based on either the direct pressure integration or using a semi-cylindrical control surface. Moreover, the solution was also developed for the oblique wave interaction with a two-dimensional barge-wall system using the eigenfunction expansion matching method. Systemic numerical studies were then conducted for the cases of a rectangular barge and a Wigley hull in close proximity to a long wall, respectively. Under oblique incidence, both the odd and even modes of fluid motion within the gap can be induced. In the case of a rectangular barge, the linear wave force can be obviously amplified at the location of the (1, 0) resonant mode, while the linear wave moment at the (2, 0) mode. In addition, the negative mean force along the transverse direction, which tends to push the barge away from the wall, can be apparently enhanced at both the odd and even resonant modes. Besides, in oblique seas, the (1, 0) resonant mode can give rise to the significant amplification of the positive mean yaw moment around the centre of the cross-sectional area of the barge, which tends to pull the stern of the barge (that faces incident waves) closer to the wall, while pushing the bow away from the wall. Such amplification gets less noticeable for higher modes. Numerical results also indicate that the hull geometry imposes a significant impact on the resonant waves in the gap. In the case of a Wigley hull, the amplification of the free-surface response is much less apparent when compared with a rectangular barge.

Comparative study of realistic ship motion simulation for optimal ship routing of a bulk carrier in rough seas

Accurate estimation of ship motions is inevitable for safe, economical, and environmentally friendly ship routing. Onboard measurements have been conducted for a bulk carrier in 2010–2016, including eight rough seas in the Northern and Southern Hemispheres. Rough sea conditions were reproduced using a wave model, WaveWATCH III, with wind inputs from meteorological organizations. Ship performances were evaluated using traditional seakeeping theories, with the main focus being on transverse motion. The 3D frequency-domain Rankine Panel Method (RPM), New Strip Method (NSM), and Enhanced Unified Theory (EUT) were compared. First, roll decay tests and simplified Ikeda's methods of roll damping were applied in each method. Second, the results of the model tests in oblique seas in regular waves were additionally considered for the accurate estimation of ship motions in actual seas. These were validated with measured ship motions in eight rough sea voyages. It was found that errors are smaller in the RPM than those in the EUT and NSM for measured roll and pitch motions in actual seas. The favorable results obtained using the proposed methods may provide a new alternative for optimal ship routing to predict ship roll and pitch motions in rough seas with reliable accuracy and computational efficiency.

Influence of an active T-foil on motions and passenger comfort of a wave-piercing catamaran based on sea trials in oblique seas

In this paper the influence of a Ride-Control System (RCS) on the Response Amplitude Operator (RAO) of a full-scale high-speed catamaran was investigated using sea trials data. A T-foil and stern tabs were installed on a Wave-Piercing Catamaran (Incat Tasmania Hull 061) to improve ship motions and passenger comfort. More than 40 total effective hours of sea trials were conducted by the US Navy in 2004, encountering sea states 4–5 in the Atlantic Ocean near the United Kingdom. The reduction in Motion Sickness Incidence (MSI) was estimated in order to examine the effectiveness of the RCS in improving passenger comfort. By comparing the case of active RCS (T-foil plus stern tabs) with the case of active stern tabs only, it was found that the T-foil plays a vital role in the passenger comfort enhancement. Based on ISO recommended MSI calculation of a 2-h seaway, the percentage reduction in MSI was estimated, and hence the effectiveness of the T-foil deployment, along with the influence of speeds, headings, location on board and encountered wave height were analysed. A notable improvement in passenger comfort was observed in the real world bow quartering sea by deploying the RCS. An MSI reduction of 21% in high speeds (30–35 knots, [Formula: see text] 0.6) was observed, which was almost twice the MSI reduction (11%) in low speeds (15–20 knots, [Formula: see text] 0.3). However, in terms of MSI percentage reduction, the ability of T-foil in vessel motion control in oblique seas was found to be limited compared to the results in head seas.

Seabed influence on wave scattering by a porous flexible barrier in a two-layer oblique sea

Seabed influence on wave scattering by a porous flexible barrier in a two-layer oblique sea

Bottom topography influence on wave transformation by a porous-flexible membrane in a two-layer oblique sea

In a two-dimensional context, the interaction between an oblique gravity wave and a partial porous-flexible membrane barrier in a two-layer fluid with an undulating bottom has been studied using linear wave theory. Two cases (a) wave scattering and (b) wave trapping by porous-flexible membrane barrier in presence of an impermeable sea-wall are discussed. The impact of surface wave, interfacial wave, and the induced energy among these by bottom undulation has been taken into consideration. An approximation technique Mild slope equation (MSE) and Fourier method along with least square approximation have been adopted as a solution technique. The solutions obtained by former methods are matched at interfaces under suitable physical conditions which reduces the problem to algebraic system of equations. This study finds that energy dissipation of surface and interfacial wave acquires an inverse pattern for a given particular gap length between barrier and free-surface. Concave upward bottom is seen to be responsible for less dissipation of surface and interfacial waves. An opposite behavior of surface and interfacial waves on the membrane deformation has been observed. The study carried out in this article holds good potential in deepening our understanding of scattering and trapping of waves by a partial porous-flexible barrier.

CFD SIMULATION OF MOTION RESPONSES OF A TRIMARAN IN REGULAR HEAD WAVES

CFD has proved to be an effective method in solving unsteady Reynolds–Averaged Navier-Stokes (RANS) equations for analysing ships in free surface viscous flow. The research reported in this paper is intended to develop a better understanding of the parameters influencing high-speed trimaran motions responses. Variations of gridding system and time step have been investigated and reliability analysis was performed in solving the RANS equations. Different turbulence models were investigated, and the SST Menter K Omega turbulence model proved a more accurate model than Realizable K-epsilon model. In order to validate the CFD method, the results of the motions response of a high- speed trimaran are compared against a set of experimental and numerical results from a 1.6 m trimaran model tested in various head seas conditions. The results suggest that CFD offers a reliable method for predicting pitch and heave motions of trimarans in regular head waves when compared to traditional low speed strip theory methods. Unlike strip theory, the effect of breaking waves, hull shape above waterline and green seas are considered in CFD application. A wave resonance phenomenon was observed and wave deformation as a result of wave-current-wind interaction in CFD was identified as the main source of discrepancy. The results from this work form the basis for future analysis of trimaran motions in oblique seas for developing a better understanding of the parameters influencing the seakeeping response, as well as passenger comfort.

Time-domain TEBEM method for mean drift force and moment of ships with forward speed under the oblique seas

A numerical method for solving 3D unsteady potential flow problem of ship advancing in waves is put forward. The flow field is divided into an inner and an outer domain by introducing an artificial matching surface. The inner domain is surrounded by ship wetted surface and matching surface as well as part of the free surface. The free surface condition for the inner domain is formulated by perturbation about the double-body flow or uniform incoming flow assumption. The outer domain is surrounded by matching surface and the rest free surface as well as infinite far-field radiation boundary. The free surface condition for the outer domain is formulated by perturbation about uniform incoming flow. The simple Green function and transient free surface Green function are used to form the boundary integral equation (BIE) for the inner and outer domains, respectively. Taylor Expansion Boundary Element Method (TEBEM) is utilized to solve the double-body flow and inner domain and outer domain unsteady flow BIE. Matching conditions for the inner domain flow and outer domain flow are enforced by the continuity of velocity potential and normal velocity on the matching surface. Direct pressure integration on ship wetted surface is used to obtain the first-order and second-order wave forces (moments). The numerical predictions on the displacement, added resistance, sway mean drift force and yaw mean drift moment of the modified KVLCC2 ship at different forward speeds are investigated by the proposed TEBEM method. It is also compared with the other numerical results. The physical tank experiment results are also developed to validate the accuracy of numerical tank results. Compared with the experiment solutions, a good agreement can be obtained by TEBEM method.

A simplified method for nonlinear hydroelastic responses of a truss-type VLFS in oblique seas based on orthotropic plate model

ABSTRACT The load response characteristics of VLFS in oblique waves have very important impact on the structure safety assessment of VLFS. However, due to the complexity of the truss-type VLFS structure, the only research achievements focus on the box-typed VLFS. Thus, a simplified time-domain nonlinear hydroelastic method for a truss-type VLFS is developed. In the structure part, according to the structural characteristics of the truss-type VLFS, the VLFS is simplified as a two-dimensional equal stiffness orthotropic plate for finite element modal analysis. In the hydrodynamic part, the radiation force and diffraction force are obtained by the mean wetted surface and linear interpolation to improve the calculation efficiency, while the nonlinear hydrostatic restoring force and nonlinear Froude-Krylov force are obtained on the instantaneous wetted surface to guarantee good calculation accuracy. Moreover, the Morison force of small-scale components is taken into consideration. Finally, by analysing the results, some useful conclusions are drawn.

Experimental and numerical investigation of wave-induced hydrodynamic interactions of a sub-floating hydrocarbon storage tank system in shallow waters

A subsystem of a large compliant floating hydrocarbon storage facility (FHSF) is established as a simplified model to validate its hydrodynamic performance. The subsystem is composed of two floating hydrocarbon storage tanks (floating tanks) placed in a large moonpool of a rigid barge frame. The barge frame is expected to serve as a breakwater as well as a protection to the floating tanks. The wave-induced responses of the subsystem with the existence of moonpool/gaps are investigated through experimental and numerical studies. The experimental studies followed a Froude scaling ratio 1:45 were conducted in an ocean basin under various wave conditions corresponding to nearshore environments. Both free surface elevations in the moonpool/gaps and the hydrodynamic responses of the three floating bodies were obtained. Besides, a numerical model based on linear potential flow theory is developed, and a dipole lid is introduced to incorporate the viscous effect on the resonant free surface motions. Comparable experimental and numerical results show that the free surface elevations in the moonpool/gaps can be reasonably predicted. Next, four different configurations of the subsystem were investigated, and different hydrodynamic coupling behaviors between the fluid resonant motions, the motions of the barge frame, and the floating tanks are revealed. It is demonstrated that the motions of the floating bodies and the resonant fluid motions in the narrow gaps are the smallest in the oblique sea. The present study provides a useful reference for the design and hydrodynamic analysis of floating multi-body systems for nearshore applications.

CFD Simulation of Motion Response of a Trimaran in Regular Head Waves

CFD has proved to be an effective method in solving unsteady Reynolds–Averaged Navier-Stokes (RANS) equations for analysing ships in free surface viscous flow. The research reported in this paper is intended to develop a better understanding of the parameters influencing high-speed trimaran motions responses. Variations of gridding system and time step have been investigated and reliability analysis was performed in solving the RANS equations. Different turbulence models were investigated, and the SST Menter K Omega turbulence model proved a more accurate model than Realizable K-epsilon model. In order to validate the CFD method, the results of the motions response of a highspeed trimaran are compared against a set of experimental and numerical results from a 1.6 m trimaran model tested in various head seas conditions. The results suggest that CFD offers a reliable method for predicting pitch and heave motions of trimarans in regular head waves when compared to traditional low speed strip theory methods. Unlike strip theory, the effect of breaking waves, hull shape above waterline and green seas are considered in CFD application. A wave resonance phenomenon was observed and wave deformation as a result of wave-current-wind interaction in CFD was identified as the main source of discrepancy. The results from this work form the basis for future analysis of trimaran motions in oblique seas for developing a better understanding of the parameters influencing the seakeeping response, as well as passenger comfort.

Simulation research on mooring stability of oil tankers in Changxing Island Port Area considering open environmental conditions

The contribution of this paper is to study the mooring stability of oil tankers by OPTIMOOR software considering the open environmental conditions in Changxing Island Port Area. Firstly, influence factors on mooring stability is analyzed, which are considered as the inputs in the numerical models. Then, the experimental cases under the combinations of waves, water level, tidal current, and wind are developed. Next, the standard of mooring stability used in this paper is given after comprehensively considering the standards used in Norwegian, German, Japan and British. Finally, by feeding the environmental conditions in Changxing Island Port Area, all the numerical experimental tests are conducted and six motion components of moored ships are obtained. The results show that (1) The water level has little effect on six motion components. (2) Compared with oblique sea and following sea, beam sea has the greatest impact on the mooring stability of oil tankers.

Numerical Study on Prediction of Flare Slamming Load on Container Ship under Head Sea and Oblique Sea Conditions

Numerical Study on Prediction of Flare Slamming Load on Container Ship under Head Sea and Oblique Sea Conditions

Experimental and numerical studies on added resistance of ship in oblique sea conditions

In this paper, the added resistance of a large tanker is estimated experimentally and numerically in oblique sea. Experiments on ship motion response and added resistance in oblique sea are performed in the SSPA seakeeping basin. The experiments are conducted using the self-propulsion test for seven wave directions between 180° and 0°. In the self-propulsion test, the added resistance is estimated from the difference between the thrust of the propeller in calm water and waves. In the case of the head sea, the results are compared with those obtained from the captive test at the towing test of Seoul National University. As numerical method, two methods are selected: the strip method and the 3D Rankine panel method. The maximum value of the added resistance is observed between the incident wave directions of 180° and 150°. From 120°, the added resistance tends to decrease and the peak of the added resistance shifted to the short waves. Through the two numerical analysis methods, the tendency of added resistance and the cause of the change of the added resistance in the oblique sea are investigated.

Wave Overtopping over Coastal Structures with Oblique Wind and Swell Waves

Most guidelines on wave overtopping over coastal structures are based on conditions with waves from one direction only. Here, wave basin tests with oblique wave attack are presented where waves from one direction are combined with waves from another direction. This is especially important for locations where wind waves approach a coastal structure under a specific direction while swell waves approach the coastal structure under another direction. The tested structure was a dike with a smooth and impermeable 1:4 slope. The test programme consisted of four types of wave loading: (1) Wind waves only: “sea” (approaching the structure with an angle of 45°), (2) Wind waves and swell waves from the same direction (45°), (3) Wind waves and swell waves, simultaneously from two different directions (45° and −45°, thus perpendicular to each other), and (4) Wind waves, simultaneously from two different directions (45° and −45°, thus perpendicular to each other). Existing guidelines on wave overtopping have been extended to predict wave overtopping discharges under the mentioned types of wave loading (oblique sea and swell conditions).