Small Freeboard Research Articles

Numerical study of wave-ice floe interactions and overwash by a meshfree particle method

The interaction between regular waves and ice floes under certain wave conditions involves overwash which is characterized by waves running up over the surface of an ice floe with large deformed free surfaces. An in-house moving particle semi-implicit (IMPS) method is employed to simulate the wave-ice floe interaction, especially for the specific behavior namely overwash due to a small freeboard of sea ice and a large body motion. In this work, a series of regular waves are generated using the Lagrangian particle method based on the Stokes wave theory. Models of the floating sea ice are simplified as circular disks without and with an edge barrier and a circular disk with a central hole, which can be used to study the influences of overwash. Hydrodynamic responses of the ice floe in regular wave conditions with different wave lengths and a constant wave height are analyzed. Compared with the mesh-based methods, the Lagrangian particle-based methods have advantages in treating highly deformed free surfaces during wave overwash processes. A piston-type wave maker and a sponge layer are applied for generating waves and avoiding the wave reflection from wall boundaries, respectively. Validations are presented including the circular ice floe with a diameter of 400 mm and a thickness of 15 mm interacting with regular waves with the wave length λ/D from 1.5 to 3.5. RAOs of the original circular disk are compared with those of the circular disks with an edge barrier and with a central hole. Overwash on the ice floe can be prevented by using an edge barrier. A new type of overwash which waves flow over the upper surface and then into the hole was proposed and simulated. The present results of numerical predictions are in good agreement with the experimental data and the published numerical results using the CFD code Flow-3D.

Forces on a Vertical Dam due to Solitary Impulse Wave Run-Up and Overtopping

Impulse waves feature tsunami-like characteristics and are generated by very rapid mass wasting, including landslides and avalanches, into water bodies. In engineered reservoirs, these waves may run up and overtop the dam, thereby exerting hydrostatic and dynamic forces on its structure. For this work, solitary waves were applied as a proxy for impulse waves and the forces acting on vertical structures during wave run-up and overtopping were investigated with hydraulic laboratory experiments. The solitary waves were generated in a wave channel with a piston-type wave generator and the horizontal pressure forces at a vertical wall as well as a vertical dam-like structure were measured with multiple pressure sensors. The discrete pressure data were used to interpolate pressure distributions and the resulting horizontal forces. Empirical equations were derived, approximating the measured maximum forces and their respective centers of pressure within ±7% for both cases with and without wave overtopping. For small freeboards, already small relative wave amplitudes may exert forces similar to those induced by an earthquake as estimated with a pseudo-static approach.

Numerical modelling of wave transformation over submerged breakwaters using Boussinesq-type models

Predictive skills of two numerical models, which are developed using two widely-recognised Boussinesq-type equations were further investigated in modelling evolution of water waves propagating over submerged breakwaters and the results were compared. These models are often used for practical applications due to their capabilities of simulating complex hydrodynamic characteristics in nearshore region. However, both models are found to be vulnerable to numerical instabilities when simulating wave propagation over submerged breakwaters particularly with small freeboards and steep face slopes. An artificial energy dissipation term was successfully introduced locally into one of the Boussinesq-type models to overcome unrealistic flow patterns that lead to these numerical instabilities near submerged breakwaters and the modified model was verified using a new set of wave flume data.

Application Side Casing on Open Deck RoRo to Improve Ship Stability

RoRo is a vessel that can transport passengers, cargo, container and cars. Open Car Deck is favourite RoRo Vessel in developing countries due to its small GT, small tax and spacious car deck, but it has poor survival of stability. Many accident involve Open Car Deck RoRo which cause fatalities and victim. In order to ensure the safety of the ship, IMO had applied intact stability criteria IS Code 2008 which adapted from Rahola’s Research, but since 2008 IMO improved criteria become probabilistic damage stability SOLAS 2009. The RoRo type Open Car Deck has wide Breadth (B), small Draft (D) and small freeboard. It has difficulties to satisfy the ship’s stability criteria. Side Casings which has been applied in some RoRo have be known reduce freeboard or improve ship’s safety. In this paper investigated the effect side casings to survival of intact dan damage ship’s stability. Calculation has been conducted for four ships without, existing and full side casings. The investigation results shows that defect stability of Open Deck RoRo can be reduce with fitting side casing.

A Critical Analysis and Validation of the Accuracy of Wave Overtopping Prediction Formulae for OWECs

The development of wave energy devices is growing in recent years. One type of device is the overtopping wave energy converter (OWEC), for which the knowledge of the wave overtopping rates is a basic and crucial aspect in their design. In particular, the most interesting range to study is for OWECs with steep slopes to vertical walls, and with very small freeboards and zero freeboards where the overtopping rate is maximized, and which can be generalized as steep low-crested structures. Recently, wave overtopping prediction formulae have been published for this type of structures, although their accuracy has not been fully assessed, as the overtopping data available in this range is scarce. We performed a critical analysis of the overtopping prediction formulae for steep low-crested structures and the validation of the accuracy of these formulae, based on new overtopping data for steep low-crested structures obtained at Ghent University. This paper summarizes the existing knowledge about average wave overtopping, describes the physical model tests performed, analyses the results and compares them to existing prediction formulae. The new dataset extends the wave overtopping data towards vertical walls and zero freeboard structures. In general, the new dataset validated the more recent overtopping formulae focused on steep slopes with small freeboards, although the formulae are underpredicting the average overtopping rates for very small and zero relative crest freeboards.

Computational fluid dynamics analysis on the added resistance of submarine due to Deck wetness at surface condition

This article describes the evaluation of the wave profile of submarine at surface condition and deck flooding which occurred by the wave making pattern at the bow. Movement of ships and submarines on the free surface of calm water creates the surface wave. Because of the difference in the bow shape and freeboard height, the wave making system in ships and submarines is different. Rounded or elliptical bow shape of submarines generates a high bow wave which causes deck and bow wetness. This is because of the fact that in submarines, this situation arises a small freeboard. In submarines, Deck wetness (because of deck flooding) is a very important subject that has some remarkable consequences, such as increase in resistance and added weigh. The focus of this article is on the added frictional resistance in the deck wetness condition. The bow wave profile, deck wetness and added resistance are studied in several Froude numbers by computational fluid dynamics method. This analysis is performed for a bare hull model at two different drafts by Flow Vision (V.2.3) software based on computational fluid dynamics method and solving the Reynolds-averaged Navier–Stokes equations.

SPH simulation of ship behaviour in severe water-shipping situations

Fishing vessels, having relatively small freeboard, are prone to encounter a water-shipping event in severe sea states. The water impact and accumulated water effects could make fishing vessels be unstable and capsize in the worst-case scenario. Therefore securing the safety of fishing vessels under water-shipping situations is an important subject, but it is not easy to predict the water behaviour in water-shipping situations, where over-topping, water impact, largely deformed free-surface and strong coupling with ship motions appear compositely. In this paper, SPH simulations using GPU are applied to the prediction of 6DoF ship motions in severe water-shipping condition. Then the prediction accuracy of the SPH method is systematically investigated through comparisons with dedicated captive and free motion tests.

EXPERIMENTAL STUDY OF OVERTOPPING PERFORMANCE FOR THE CASES OF VERY STEEP SLOPES AND VERTICAL WALLS WITH VERY SMALL FREEBOARDS

This article describes the investigation of average wave overtopping performance for sloping coastal structures characterised by very steep slopes (typically cot α < 0.5) and very small freeboards. Based on experimental tests, performed in the large wave flume of Ghent University, the influence of the relevant hydraulic and geometrical parameters on overtopping is examined. Test set-up and test parameters are presented and discussed in detail in this paper. Furthermore, the established dataset, called UG13 dataset, is compared to prediction formulae from existing literature, including the limiting cases of zero freeboard and vertical wall.

Numerical simulation of wave-induced nonlinear motions of a two-dimensional floating body by the moving particle semi-implicit method

The moving particle semi-implicit (MPS) method was applied to compute nonlinear motions of a floating body influenced by the water on deck. To compute the motions of a rigid body, the fluid pressure at the position of each particle on the body surface was directly integrated in space and the equations of translational and rotational motions were integrated in time to determine the correct position of the rigid-body surface at each time step of the time-domain calculation. The performance of this method was validated through a comparison with measured results in an experiment that was newly conducted using a model of a box-shaped floating body with a small freeboard. Although the overall agreement was good, some discrepancies were observed for a shorter wave period, especially for the drift motion in sway. The effect of numerical resolution on the results was checked by changing the number of particles. With a higher number of particles, no obvious improvement was seen in the global body motions, but the resolution of the local free-surface profile, including the water on deck, was improved.