Effect Of Sea State Research Articles

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.

Maximizing the surge amplitude of a floater through an adaptable mooring tightening technique

Abstract
 A technique to optimize the response of Wave Energy Converters (WECs) by maximizing the amplitude of motion along the surge direction is presented. This is achieved by utilizing an adaptable mooring tightening technique for a floater moored with tension legs (tendons). To gain a deeper understanding of the effect of sea states and mooring cable lengths on the surge response of the system, a series of numerical simulations were conducted for various wave conditions while varying the length of the mooring cables. WEC-Sim [1] was used to solve the multi-body dynamics of a rectangular cuboid floater by solving the equations of motion using a time-domain formulation. The dynamics of the mooring cables were simulated using the MoorDyn model with a lumped-mass formulation [2]. To validate the accuracy of the numerical methods, a series of experimental tests were performed in a small-scale wave tank. It was observed that tightening the mooring cables by decreasing their length amplifies the surge motion of the floater while the mooring forces in the heave direction rise due to the increased tension in the cables. Stretching the cables further was found to (i) decrease the surge amplitude and (ii) drastically increase the mooring forces, threatening the integrity of the cables. Therefore, there is an optimum value of the length of the cables that maximizes the surge amplitude of the floater while ensuring that the cables will not break. The impact of other mooring cable parameters such as diameter and material properties were also evaluated. More specifically, increasing the stiffness by increasing the diameter or the tensile modulus of elasticity was found to reduce the floater’s surge amplitude. For the geometry used in this study, the optimum length, diameter and properties of the cables are provided for several sea states. The current results lay the foundations for the design of new types of WECs that harness the surge motion of a floater rather than the heave which is the most common approach for floating WECs.
 References
 [1]: Kelley Ruehl, David Ogden, Yi-Hsiang Yu, Adam Keester, Nathan Tom, Dominic Forbush, Jorge Leon, Jeff Grasberger, and Salman Husain. (2022, September), WEC-Sim (Version v5.0.1), DOI: 10.5281/zenodo.7121186.
 [2]: Hall, M., & Goupee, A. (2015). Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data. Ocean Engineering, 104, 590–603. https://doi.org/10.1016/j.oceaneng.2015.05.035

Effect of sea states on the wind velocity fluctuations near the surface under near-neutral conditions

This study examines the effect of sea states on wind velocity fluctuations in the longitudinal direction near the sea surface under near-neutral stratification, characterized by inverse wave age. The analysis is based on wind and wave data collected from a discus buoy measurement off the coast of Taiwan in the Taiwan Strait. It is observed that wind waves and swells modify the overlying turbulence structure differently. In particular, wind-swell coupling results in greater variations and values of the scaled velocity standard deviation and turbulence intensity with increasing wind speed, as well as a more pronounced peak in the scaled wind velocity spectrum compared to pure wind sea conditions. The spectral shapes averaged for pure wind sea and swell conditions are described by a plateau followed by a wave-induced peak in the intermediate-frequency range. These features suggest that near-surface turbulence is driven by eddy impingement on the sea surface, associated with wave-induced fluctuations in the lower part of the eddy surface layer. The study has practical applications in evaluating the wind loads on offshore structures, particularly in the context of offshore wind energy development.

Influence of sea state on density estimates of harbour porpoises (Phocoena phocoena)

A ship-based line transect survey was conducted in the Great Belt, Denmark, from 7-20 April 1994, covering an area of 705 linear kilometres. A total of 497 sightings were collected in sea state 0-3. A comparison of relative abundance stratified by sea state revealed that sea state had a significant effect on the estimated sighting rate, effective search width, density and abundance within sea state 0-3. However, no significant difference was found between sea state 2 and 3. Comparison of abundance estimates of the same area on two different days surveyed in sea state 0, revealed no significant difference. The relative abundance estimate was 1,526 harbour porpoises in sea state 0 within the surveyed area (326.2km2 ) based on the line transect method. This is the highest density of harbour porpoises (4.9 harbour porpoise/km2 ) reported in Europe. There is a strong indication that sea state has a significant effect on abundance estimation of harbour porpoises in ship-based conventional line transect surveys. This is important for future surveys in two ways: (1) the reliability of a comparison of abundance for different surveys strictly depends on the sea state in which the surveys were conducted; and (2) when estimating absolute abundance, effects of sea state should be explicitly addressed. One way is to separately analyse data from each sea state and apply a g(0) estimate for each sea state.

Vortex-induced vibration effect of extreme sea states over the structural dynamics of a scaled monopile offshore wind turbine

In order to demonstrate the relevance of considering Vortex-Induced Vibrations (VIV) in the structural design of marine structures, this study proposes an alternative experimental and analytical approach in wet conditions to measure the fluid–structure interaction in the near field and quantify the viscous damping with measured structural and 3D hydrodynamic accelerations. It was demonstrated that VIV caused and incremented 5.00% of the structural damping coefficient, and the extreme wind loading increased 74% of the offshore monopile’s structural damping, demonstrating the relevance of the high non-linear hydrodynamics effects during selecting parameters into the structural design in offshore applications.

An Improved Ship Weather Routing Framework for CII Reduction Accounting for Wind-Assisted Rotors

With the increasingly strict regulations for the energy-saving and emission-reduction technology of ships, minimizing fuel cost and thus reducing the carbon intensity index (CII) is one of the most critical issues in the design and operation of merchant ships. More recently, many wind-assisted devices, such as rotors, wind sails, etc., have been investigated and designed to utilize renewable wind energy. With the equipment of wind-assisted rotors, the optimization of ship routes becomes more important because the effect of this wind-assisted device highly depends on the local wind field along the shipping route. In this paper, an improved ship weather routing framework based on the A* algorithm has been proposed to determine the optimal ship route and ship operations with wind-assisted rotors. The proposed framework effectively utilizes different sources of data, including ship design, weather forecasting and historical sailing information, to produce a better estimation of fuel consumption under the effect of sea states. Several improvements on the classic A* algorithm, including directed searching and three-dimensional extension, are proposed to improve the routing effect and efficiency. Finally, the proposed method was applied to test cases of a VLCC operating from China to the Middle East and the results show that the total fuel consumption could be reduced compared to the minimum distance route.

The effect of sequential storms on the performance of homogeneous berm breakwaters

Reshaping of homogeneous berm breakwaters have been experimentally investigated using 2D physical tests. Berm recession depends on storm duration in addition to breakwater geometry and other storm parameters such as significant wave height and peak period. Furthermore, a berm breakwater may experience some recessions prior to exposing to a design storm for which the breakwater is designed. In this study, 109 tests have been conducted totally to investigate the enhancement of berm recession during design storms and also to study the effect of sequential storms on the ultimate berm recession during the life span of berm breakwaters. The measured cumulative recessions have been compared with direct recessions obtained from a breakwater with no initial reshaping. According to the results, the effect of earlier sea states on final berm recession depends on the design stability index and it can be neglected only if the design storm is too weak ( H 0 T 0 < 40 ) or it is too strong ( H 0 T 0 > 80 ). For weak design storms, earlier reshaping is negligible, while for strong design storm, the final reshaped profile will be governed only by such design storms regardless of earlier sea states. The results show that the predicted berm recession against design storms can be maximum 10% less than the actual recession in the case of missing the effect of earlier sea states. Since this conclusion is valid for both statically and dynamically stable berm breakwaters (the maximum value occurs around H 0 T 0 = 70 ), it is suggested finally that designers consider 10% more recession than the predicted values by existing design formulae to be sure about the endurance of berm width against the cumulative berm recessions during design life of reshaping berm breakwaters. The observations showed also that this width can efficiently dissipate the wave energy and protect underneath layers. • Homogenous reshaping berm breakwaters are studied using 109 physical tests. • A new formula is proposed to calculate berm recession during storms as a function of modified stability index. • Cumulative recession during design life of berm breakwater is investigated and an applicable suggestion is provided. • A modified stability index is proposed for measuring the influence of design storms.

A Multihull Boat’s Fatigue Analysis at Early Design Phase

Fatigue analysis and life cycle prediction of a naval craft or a commercial ship are one of the most critical phases of structural integrity calculations. Many structural failures have occurred due to insufficient structural design, production methods, and inappropriate operational environmental conditions. As a result of the deterioration of the structural integrity, the occurrence of both vital and economic losses becomes inevitable. This paper emphasises the importance of fatigue and life cycle analyses at the early design phase since the fatigue phenomena are generally considered in the late design stages. In this study, fatigue calculations and life predictions were carried out by the Palmgren–Miner method. The wave rosette, also called long-term wave directionality, was used as the wave spectrum approach. Based on wave loads and accelerations, an FE analysis was carried out using the Maestro Marine Altair Partner Alliance (APA) software. The undamaged life prediction of the vessel was calculated based on different sea states and vessel speeds. A specific scenario, which was prepared based on the International Code of Safety for High-Speed Craft, was also analysed to compare the fatigue life of the vessel with regards to safe operating conditions for passengers and crew. In this study, the effect of sea states, environmental conditions, and materials used on the fatigue behaviour of the vessel are discussed and analysed. As a result of this study, the shortest and the longest undamaged life spans based on the loading scenarios are put forth clearly. Analyses and calculations prove that loading scenario 5 is the most effective scenario in terms of the undamaged life span of the boat. On the contrary, loading scenario 1 gives the shortest service life. The main reason behind this phenomenon is the relation between the fatigue endurance limit and load cycles.

Quantitative Evaluation of the Effects of Large-Scale Sea Wave Components on Microwave Backscattering From Ocean Surface

This paper quantitatively examined the effect of these long-wave components with wavelength larger than the Bragg component on radar backscattering using the series-expanded first-order small slope approximation (SSA-1) and the Hwang sea wave spectrum model. Firstly, some roughness with various scales are constructed using different long-wave sea spectrum components and subsequently are cast into SSA-1 to obtain the corresponding normalized backscattering cross-section (NBCS) at different radar frequency, incidence angles, and wind speeds, respectively. Then, the effect of large-scale wave components on radar backscattering is revealed by analyzing the NBCSs with different roughness. Finally, the existing wavelength filtering theory explained the effect of large-scale waves on radar backscattering. It was also verified by another scattering model, i.e., the advanced integral equation model (AIEM) and radar measurements. Besides, we also investigate the effects of sea states and swell waves on the numerical simulation results. The effect of long-scale waves on NBCS clarifies the source of backscattering from the multiscale sea surface, which is helpful for both analytical and numerical scattering models to explore the sea surface scattering properties and better interpret the radar sensing of sea surface. Also, the examination of the effect of sea states and swell waves on radar backscatter is instructive for reducing the inversion errors of sea surface wind speed from radar measurements by means of configuring the appropriate radar parameters (e.g., high frequencies and large incidence angles).

Characterizing Buoy Wind Speed Error in High Winds and Varying Sea State with ASCAT and ERA5

Buoys provide key observations of wind speed over the ocean and are routinely used as a source of validation data for satellite wind products. However, the movement of buoys in high seas and the airflow over waves might cause inaccurate readings, raising concern when buoys are used as a source of wind speed comparison data. The relative accuracy of buoy winds is quantified through a triple collocation (TC) exercise comparing buoy winds to winds from ASCAT and ERA5. Differences between calibrated buoy winds and ASCAT are analyzed through separating the residuals by anemometer height and testing under high wind-wave and swell conditions. First, we converted buoy winds measured near 3, 4, and 5 m to stress-equivalent winds at 10 m (U10S). Buoy U10S from anemometers near 3 m compared notably lower than buoy U10S from anemometers near 4 and 5 m, illustrating the importance of buoy choice in comparisons with remote sensing data. Using TC calibration of buoy U10S to ASCAT in pure wind-wave conditions, we found that there was a small, but statistically significant difference between height adjusted buoy winds from buoys with 4 and 5 m anemometers compared to the same ASCAT wind speed ranges in high seas. However, this result does not follow conventional arguments for wave sheltering of buoy winds, whereby the lower anemometer height winds are distorted more than the higher anemometer height winds in high winds and high seas. We concluded that wave sheltering is not significantly affecting the winds from buoys between 4 and 5 m with high confidence for winds under 18 ms−1. Further differences between buoy U10S and ASCAT winds are observed in high swell conditions, motivating the need to consider the possible effects of sea state on ASCAT winds.

Mass Transfer at the Ocean–Atmosphere Interface: The Role of Wave Breaking, Droplets, and Bubbles

Breaking waves modulate the transfer of energy, momentum, and mass between the ocean and atmosphere, controlling processes critical to the climate system, from gas exchange of carbon dioxide and oxygen to the generation of sea spray aerosols that can be transported in the atmosphere and serve as cloud condensation nuclei. The smallest components, i.e., drops and bubbles generated by breaking waves, play an outsize role. This fascinating problem is characterized by a wide range of length scales, from wind forcing the wave field at scales of [Formula: see text](1 km–0.1 m) to the dynamics of wave breaking at [Formula: see text](10–0.1 m); air bubble entrainment, dynamics, and dissolution in the water column at [Formula: see text](1 m–10 μm); and bubbles bursting at [Formula: see text](10 mm–1 μm), generating sea spray droplets at [Formula: see text](0.5 mm–0.5 μm) that are ejected into atmospheric turbulent boundary layers. I discuss recent progress to bridge these length scales, identifying the controlling processes and proposing a path toward mechanistic parameterizations of air–sea mass exchange that naturally accounts for sea state effects.

Numerical Simulation of the Motion of a Large Scale Unmanned Surface Vessel in High Sea State Waves

The motion stability of the Unmanned Surface Vessel (USV) is threatened by the action of waves under a rough sea state. In the present paper, the motion of a large-scale USV is numerically simulated under high sea state of level 5 and 7. The overset grid method and Reynolds Averaged Navier–Stokes (RANS) approach are employed to solve Navier–Stokes (N-S) equations. For the case of wave incident angle 0° and 30°, the heave, pitch and roll motion response of a large scale USV are investigated by using the six Degrees of Freedom (6-DOF) numerical model. The effects of different sea states, as well as different wave directions, on the motion of USV are compared. The comparative results indicate that the response of this USV in waves is the periodic free-motion according to the corresponding amplitude, which does not exceed the stable range, and there are no overturning and other situations that may affect the safety, in the case of level 5 and 7 sea states. The corresponding pressure at the bottom of this USV meets the range of material strength, and no structural damage or injury to the hull occurs, although the pressure varies at different wave periods. For the case of different wave directions, the analysis of the boundary layer thickness shows that the wave direction is of great importance to the boundary layer thickness distribution, both in the level 5 and level 7 sea states.

A practical direct URANS CFD approach for the speed loss and propulsion performance evaluation in short-crested irregular head waves

This paper employed a direct URANS CFD based approach to investigate the speed loss and waterjet performance for a waterjet-propelled trimaran in short-crested head waves. Firstly, a reasonable quantity of linear superposed regular waves was validated to simulate the realistic ocean wave adopted on the ITTC directional wave spectrum. Then the wave parameters were applied to the URANS CFD simulations, to avoid the wave reflection due to structures and to narrow the simulation domain, the Euler Overlay Method (EOM) based wave forcing zone was defined in the outer domain, the time history of the wave elevation and the wave patterns matched well with the theoretical results. Moreover, by employing the PID-controller and Body-Force method, the 1/30 scale waterjet-propelled trimaran free-running numerical model was validated through the trimaran model tests. Eventually, the free-running simulations in short-crested head waves of the trimaran were performed considering the effects of sea state, traveling speed, and wind, respectively. The added resistance in short-crested waves was mainly caused by the pitch and heave variation, and lead to the speed loss and waterjet performance deterioration. Notable, the wind-induced speed loss could account for nearly one half of the total while the wind speed exceeds the vessel speed. The proposed approach of direct evaluating the traveling and propulsion performance in short-crested waves shows reasonable results and practical value.

The Effect of Sea State on the Polarization of Reflected Beidou B1 Signals

Background: Oceanographic buoys generally employ satellites for positioning and data transmission. However, sea surface conditions can affect these signals. The Signal to Noise Ratio (SNR) of small observation buoys can be improved by exploiting polarization diversity. Methods: This paper discusses the effect of sea surface conditions on the polarization and reflection loss of Beidou B1 reflected signals for the purposes of exploiting polarization diversity. The Rayleigh roughness criterion is used to assess the roughness of the sea surface. The Fresnel reflection coefficients are derived to analyze the polarization and reflection loss of the Beidou B1 reflected signals with different sea surface states. Results: The results obtained show that for the Beidou B1 signals, the sea surface is considered rough for most sea surface states and incident angles. For smooth sea surfaces, the Beidou B1 reflected signals are mainly Left Hand Circularly Polarized (LHCP) waves, but Right Hand Circularly Polarized (RHCP) waves dominate when the incident angles are larger than the Brewster angle. The reflected loss is between -2 dB to -3.4 dB. In rough sea surfaces and the signals propagation is dominated by diffuse reflection. The reflection loss decreases with the incident angle and there is a fluctuation when the incident angle is around 49 degrees. The specular reflection signal has a significant amplitude when the angle of incidence is large. RHCP waves are the main component of the reflected signals, and the reflection loss is relatively small which can be employed for polarization diversity or marine remote sensing. Conclusion: polarization diversity is only useful with good sea conditions, and the corresponding gain decreases with the deterioration of the sea surface conditions.

The MoonWEC, a new technology for wave energy conversion in the Mediterranean Sea

This work presents the MoonWEC, a new device for wave energy conversion. This new concept encompasses several working principles as the heaving point absorber, the oscillating water column and the overtopping. The development of the new WEC has been carried out through numerical modelling. Three main parts of the device have been simulated: a hollow floating structure, a central moonpool and a CALM mooring system. All the parts of the model have been coupled to obtained the general behaviour of the device under the effect of irregular sea states. The device has been designed according to the Mediterranean Sea wave climate. Its dimensioning aims to maximize the power absorption by tuning both,device and moonpool, natural periods with the identified prevailing sea states. Six mooring system configurations have been tested in order to obtain a stable, safe and yet effective behaviour. Finally, performance indicators have been drawn to assess power production in two selected locations in the Mediterranean Sea.

Revisiting dependences of the drag coefficient at the sea surface on wind speed and sea state

Accurate representation of wind stress is important for modelling and predictions of marine environmental conditions. The effect of sea state on wind stress is usually parameterized by relating the equivalent sea surface roughness to the wave age or wave steepness. In this study, a new parameterization is presented for the dependences of sea surface roughness on the wind speed and sea state based on observations. Analyses of observations demonstrate that the sea surface roughness is correlated with the wave age stronger than the wave steepness. As a result, the wave-dependent sea surface roughness is parameterized in terms of the wave age in the new parameterization. The new parameterization features that the sea surface roughness has different dependences on the wave age under wind-sea-dominated, mixed and swell-dominated sea states. Younger waves have larger values of the drag coefficient under wind-sea-dominated and mixed sea states but older waves have larger values under swell-dominated sea states. The drag coefficient predicted by the new parameterization is enhanced at low winds and levels off at high winds. The applicability of the new parameterization is investigated using a nested-grid wave model for the northwest Atlantic based on WAVEWATCH III. The wave model results during a winter storm in March 2014 are compared with the available measurements from buoys and satellite altimeters over the study region. Our results demonstrate that the new parameterization reduces deficiencies of three existing parameterizations of underestimating significant wave heights (SWHs) at low winds (or old waves) and overestimating SWHs at moderate and high winds (or young waves). Overall, the new parameterization performs the best in predicting SWHs during the winter storm in comparison with the existing schemes.

A coupled modeling study of mechanical and thermodynamical air-ocean interface processes under sea storm conditions

The interaction mechanisms between the lower part of the atmosphere and the ocean surface comprise of a complicated issue with many parameters to be studied. Heat, moisture and momentum exchanges in the air-sea system exert a significant role on both lower atmosphere and sea state conditions.Working towards the understanding of how wave characteristics link the atmosphere with the ocean, an atmospheric model is fully coupled with a wave spectral model. The RAMS/ICLAMS atmospheric model is integrated with the WAM wave spectral model under the OASIS-MCT coupler. The combined modeling system utilizes schemes for ocean surface roughness and sea spray droplet thermodynamics.A series of sensitivity tests related to midlatitude cyclones is performed to examine the associated effects of sea state and sea spray impact on wave conditions and lower atmosphere. Under increased wind conditions, sea drag is enhanced as illustrated in the case of the two-way coupling simulations. In addition, increased values of roughness length were traced in near-shore areas in the case of expressions that employ wave parameters such as the wave age. The magnitude and the role of energy yield from sea water droplets depend on the prevailing weather conditions. Sea spray fluxes are amplified in areas with intensified surface winds that also results in an increase of the total heat fluxes. This induces a moistening of the near surface air, an effect in stability profile and an increase of wind speed. The most noticeable impact on marine layer structure emerged close to the first hundred meters. The study revealed an enrichment of the description of the air-sea system through the transition of information between the different models.

Analysis of the Dynamic System of Wave Glider with a Towed Body

Mobile observation platforms are widely used in oceanographic and marine resource exploration and other applications. Wave Glider is a mobile platform that can transform wave energy into locomotion power and overcome the bottleneck of low energy supply. Wave Glider has recently been applied to tow underwater sensors fulfilling observation tasks. In this paper, the dynamic system of Wave Glider with a towed body is studied by applying multibody mechanics, and the relevant motion conditions of the system are investigated. Dynamic models of Wave Glider with a towed body and tether are first developed individually and then integrated into a whole model. The numerical method is used to obtain the dynamic responses and assess performance of the towed body pulled by the submerged glider of Wave Glider. The effects of sea state, mass of the towed body, and length of the towed cable are investigated on the basis of simulation results. This work can be used for the design and analysis of Wave Glider-towed body systems.

Predicting Radiometric Effects of a Rough Sea Surface, Whitecaps, Foam, and Spray Using SURFER 2D

Whitecap (WC) formation due to waves breaking on a wind-roughened sea surface facilitates the exchange of mass, momentum, and energy across the air-sea interface. While approximate analytical electromagnetic (EM) models paired with surface gravity wave spectra have been used to predict surface roughness emissivity enhancement, these methods do not reveal details of the response to foam, breaking wave, and WC geometry. They also ignore possible coupling between the roughness and WC emissivity effects. We report the application of a full-wave finite-difference time-domain (FDTD) EM model to investigate the separate and combined emissivity effects of specific surface roughness profiles, associated WC fields, and overlying spray. The model solves Maxwell's equations directly for an arbitrary free space and dielectric configuration. It is applied to multiple dielectric layers representing foam and spray overlying flat and rough sea surfaces. The foam layer profiles are adapted from Anguelova's L-band radiative transfer model and the rough surface is a statistical realization of the Kudryavtsev gravity wave spectrum. The model is also used to investigate the secondary effect of sea state on emissivity for a given mean square slope, which is the primary factor governing rough surface emissivity enhancement. The accuracy and precision of the FDTD model emissivity estimates and the detectability of WCs using L-band radiometry are assessed under various wind conditions, including those of tropical storm and category 1 hurricane strength. The prospects for performing Monte Carlo simulations for stronger winds and deterministic simulations of breakers with WCs of various void fractions, shapes, and scales are also considered.

Numerical analysis of offshore integrated meteorological mast for wind farms during wet towing transportation

An offshore integrated meteorological mast (OIMM) featured by integrating the offshore transportation of the foundation and steel mast into one operation is proposed. The most critical technique of the OIMM is a self-floating towing operation with a reasonable subdivision inside the floating tank. To find out the towing dynamic behaviors of the OIMM with different drafts, mooring positions, towing velocities and towrope lengths in various sea conditions, the hydrodynamic analyses were carried out by using MOSES. The OIMM model in prototype scale was established in MOSES to simulate the static stability and dynamic responses induced by coupled wave and wind conditions. A small tugboat and a single towrope are employed for this towing system. The results show that the OIMM has sufficient intact stability and acceptable damage stability for its inner subdivision configuration. The towrope length exerts the most impact on fluctuation response than draft and mooring positions. Draft depths and mooring positions impact more on pitch motion than on heave motion. The normalized maximum values of motion responses are equally influenced by draft and mooring position. Coupled effects of sea states and towing speed on dynamic features are also illustrated.