SLAM CHARACTERISTICS OF A HIGH-SPEED WAVE PIERCING CATAMARAN IN IRREGULAR WAVES

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events. The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

Numerical Estimation of Relative Wave Elevation on a Semi-Submersible Vessel

Ambition of this research was to numerically reproduce the relative wave elevations measured on and around a semi-submersible vessel during model tests performed at MARIN (Maritime Research Institute of the Netherlands). Model tests were performed on a semi-submersible model representing, in different setup configurations, a heavy lift vessel and a LPD (Landing Platform Dock). Relative wave elevations were measured at 47 locations on and around the vessel. Tests were made with different regular wave conditions and headings, at anchor, with the model fixed in a soft mooring setup. Data from model tests were used to calibrate and run a time domain potential flow boundary element tool with particular focus on the relative wave elevation at the 47 locations. A comparison between numerical and experimental results is proposed in this paper.

Near-Trapping on a Four-Column Structure and the Reduction of Wave Drift Forces Using Optimized Method

The near-trapping phenomenon, which can lead to high wave elevations and large wave drift forces, is investigated by a floating four-column structure. To solve this wave-structure interaction problem, a numerical model is established by combining the wave interaction theory with a higher-order boundary element method. Based on this numerical model, behaviors of scattered waves at near-trapping conditions are studied; and the superposition principle of free-surface waves is introduced to understand this near-trapping phenomenon. To avoid the near-trapping phenomenon and protect the structure, a way for rotating the structure to change the wave-approach angle is adopted, and improvements of the wave elevations around the structure and the wave drift forces acting on each column are found. Moreover, a genetic-algorithm-based optimization method is adopted in order to minimize the total wave drift force acting on the whole structure at various wavenumbers by controlling the draft of floating bodies, the wave-approach angle and the separation distance between adjacent floating bodies. With the final optimized parameters, the wave drift forces both on each column and on the whole structure can be significantly reduced. The optimized arrangement obtained from a certain wavenumber can work not only at this target wavenumber but also at a range of wavenumbers.

On Wave Diffraction of a Two-Dimensional Moonpool in a Two-Layer Fluid in Finite Water Depth

This paper studies the wave diffraction of a two-dimensional moonpool in a two-layer fluid in finite water depth by using a domain decomposition scheme and an eigenfunction matching method. The formulae of the wave exciting forces, the free surface and internal wave elevations at zero-frequency are derived. Numerical convergence has been assessed by repeating the computations for increasing values of the truncation orders. The present model has been validated by comparing a limiting case with a single-layer fluid case and the comparisons are in general satisfactory. Although the wave exciting forces and free surface wave elevations around resonance frequency are overestimated, the piston mode resonance frequency is well predicted. Two typical configurations with different moonpool widths are selected for computations in both free surface and internal wave modes. It is found that, the wave exciting forces, free surface and internal wave elevations in internal wave mode are much smaller than those in free surface wave mode. In addition, the wave exciting forces in internal wave mode attenuate to zero quickly as incident wave frequency increases. For moonpool with small width, only piston mode resonance can be observed. The piston mode resonance frequencies identified in free surface and internal wave modes are the same. The characteristics of piston mode resonance can also be observed in the horizontal and vertical wave exciting forces. Around the piston mode resonance frequency, the wave exciting forces reach their local maximums. It is revealed that, as moonpool width increases, the piston mode resonance frequency decreases. Meanwhile, it shows that more asymmetric and symmetric sloshing mode resonances appear alternately and occur at higher frequencies than the piston mode resonance. Moreover, the predicted sloshing mode resonance frequencies are compared with those estimated by a simple approximate formula.

Towards an Operational Stereo System for Directional Wave Measurements From Moving Platforms

Stereo video imaging of water surface has become an effective instrumentation to gather wind waves 3-D data from small to medium range spatial scales. Indeed, recent applications of stereo techniques provided new insights of space-time distributions of sea wave elevations, small scale wave statistics, and directional wave spectra. Like most photogrammetric applications, an accurate calibration of the optical acquisition machinery is required to provide a low-noise, precise and reliable surface reconstruction adequate to extract meaningful wavy surfaces. However, for practical open field applications, there is a strong interest to provide a calibration procedure apt to be performed in an uncomfortable environment in which it may be unfeasible to take apart or even physically access the device. Here, we propose a stereo pipeline suitable for 3-D wave measurements from fixed and moving platforms. In particular, within the Wave Acquisition Stereo System (WASS) framework, we contemplated a self-calibration technique for a robust estimation of the stereo extrinsic parameters, a fast dense stereo correspondence algorithm, and a two-step correction of the cameras motion. As for other applications, wave information collected by WASS includes synthetic wave parameters (e.g., significant wave height, wave periods, and directions), wavenumber and frequency-direction spectra, spatial distribution of wave elevations, heights, and lengths. The new pipeline features has been firstly assessed by installing WASS on top of the “Acqua Alta” oceanographic tower in the northern Adriatic Sea (Italy) and comparing WASS measurements against those acquired at the tower with reference instrumentation. Afterwards, the stereo system has been mounted on board the R/V “Urania” during a cruise on April 2013 in the southern Adriatic Sea. During the cruise, to correct for ship’s motion, WASS has been synchronized to the motion unit used for the vessel’s navigation. For validation, sea wave state gathered by WASS has been compared to theoretical models and results from the numerical wave model SWAN. Results presented show that the accuracy of 3-D waves provided by the new algorithms is comparable to that of other WASS applications, although significantly reducing the installation effort and the computational time by more than one order of magnitude. Additionally, encompassing for ship’s motion makes stereo system a perspective instrumentation for operational wave measuring from research and opportunity vessels. The manuscript is completed by a discussion on the limitations and troubleshooting related to the proposed pipeline.

Slam Characteristics of a High-Speed Wave Piercing Catamaran in Irregular Waves

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events. The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

Improving Offshore Platforms Wave Measurements

Offshore platforms are equipped with wave instrumentation at deck extremities to measure incoming wave elevations. When those instruments are close to large structural members they record the diffracted wave as well as the incident wave. This paper studies the effect of the diffracted wave on the measured wave height. First and second order diffracted wave elevations are computed for a model Tension Leg Platform (TLP) that was tested in the Offshore Basin of the Maritime Research Institute Netherlands (MARIN) offshore basin as part of the CresT Joint Industry Project (JIP). Their respective contributions to the wave spectrum are compared at locations near the structure. These calculations are useful for identifying the best locations for wave probes. The diffraction solution is used in forward calculations to compute the wave height and wave crest at locations under the deck from the undisturbed wave. These calculations can be used to set the air gap under the deck. Conversely, this paper introduces an inverse method to retrieve the undisturbed wave height and crest from the measured data by inverting the diffracted wave coefficients. The calculations are verified using measurements of undisturbed and diffracted waves under the TLP model. This work was sponsored by the Climatology and Simulation of Eddies (CASE) JIP.

Higher Order Wave-Current Elevations in Deep Water

Wave-current interaction refers to the interaction between surface gravity waves and ocean current flow. This interaction implies an exchange of energy, i.e. both the waves and the current are affected. The present paper describes the calculation of wave elevations in higher order unidirectional, irregular waves with a uniform current in deep water. Results for regular waves are compared with those obtained for Stokes second and third order waves with uniform current according to the method described by Fenton [1]. The results for higher order wave elevations in irregular waves have been compared with waves and current generated in a model test basin and reasonable agreement has been found.

Estimates of Wave Elevations Around Structures

When designing offshore platforms the still water air gap has to be large enough to avoid major wave-in-deck impact. Since wave elevation in harsh weather is highly non-linear, corrections to the calculated first order solution are necessary. The present method is a pragmatic approach to estimate the higher order contributions, utilizing the first order response amplitude operator and higher order wave elevations. For infinite water depth it is shown that regular wave theory is a good approximation for calculating second order wave elevation in irregular seas. So the higher order waves are calculated with regular wave theory, and the QTF and higher order terms are approximated by the first order RAO. Comparison with model test results have been performed for a GBS in moderate water depth and a semi-submersible is relatively deep water. The agreements with model tests are satisfactory.