Marc Vantorre

Experimental investigation of the validity of linear theory to assess the behaviour of a heaving point absorber at the Belgian Continental Shelf

The results of an experimental investigation on a heaving point absorber are presented. The physical tests are used to validate numerical simulations of the behaviour of the point absorber based on linear theory in the frequency and time domain. Floater response and power absorption are evaluated in regular and irregular waves representing a mild wave climate. A good correspondence is found between the physical and numerical test results. In irregular waves the difference between numerical and experimental power absorption is generally smaller than 20%. In regular waves the correspondence is good as well, except in the resonance zone; i.e. when the natural frequency of the buoy is tuned towards the resonance frequency of the incident wave. In this case, non-linear effects such as viscosity and a non-linear hydrostatic restoring force become important due to the high velocities and motion amplitudes of the point absorber. However, because of these large amplitudes, pure resonant cases are often not preferred in practical applications. In general it is concluded that the numerical results are in good accordance with the experimental results and can be used to predict the point absorber behaviour in mild energetic waves in non-resonance conditions.Copyright

Ship Motions in Shallow Water As the Base for a Probabilistic Approach Policy

A calculation tool has been developed for determining tidal windows for deep-drafted ships approaching and leaving the Belgian harbors according to probabilistic criteria. The calculations are based on a database containing response functions for the vertical motions in waves and squat data for a selection of representative ships. The database contains both results of model tests carried out in the Towing tank for maneuvers in shallow water – co-operation Flanders Hydraulics Research & Ghent University in Antwerp (Belgium), as well as calculated values. During the experiments, draft, trim, under keel clearance (7 to 20% of draft) and speed have been varied. The tests were performed in regular waves with lengths which are small compared to ship length, and in wave spectra that are typical for the Belgian coastal area. For given input data (ship characteristics, speed, tide, directional wave spectra, bottom, trajectory, current, departure time), the tool calculates the probability of bottom touch during the transit, so that a tidal window can be determined. Other restrictions, such as penetration into fluid mud layers and current, are taken into account as well.Copyright

A methodology for evaluating the controllability of a ship navigating in a restricted channel

A methodology is presented for evaluating the controllability of a ship navigating in a restricted channel by means of a hydrodynamic force analysis. This method is applied to assess the controllability of a container vessel in straight channel reaches and in bends in two practical cases. By comparing different initial conditions and bottom configurations the influence of different ship characteristics (main dimensions, draft, rudder and propeller characteristics), operational parameters (such as speed, propeller commands, and bank clearance), environmental parameters (such as current and tidal level), and channel characteristics (water depth, bank slope, bend radius) on this controllability can be evaluated. For estimating the components of the force analysis, use is made of results of captive model tests in shallow and restricted waters.

Investigation of vertical slamming on point absorbers

This paper focuses on the impact of vertical slamming on floating point absorber systems and the associated pressures that might be expected when these phenomena occur. In a first part it will be shown how the occurrence probability of slamming can be reduced by implementing a slamming restriction, i.e. by controlling the motion of the point absorber. The impact of these slamming restrictions on power absorption will be discussed. Secondly an investigation is made of the pressures that occur when the buoys are subject to vertical bottom slamming. Analytical results are presented, which give a pressure prediction of an impacting body with conical and hemispherical shape, using Wagner theory. Laboratory experiments have been carried out at Ghent University. Impact pressures were measured during drop tests with both hemispherical and conical buoy shapes. These pressures were measured by ICP pressure sensors with a range up to 345 kPa with small membrane and very high resonance frequency (> 250 kHz). Analytical and physical results are compared and conclusions are drawn.Copyright

Validation of ship manoeuvring in shallow water through free-running tests

The shallow water effect on ship manoeuvring cannot be neglected. Most sea-going ships become more course stable when they sail from deep to (very) shallow water. International collaborations such as SIMMAN intend to grade up the knowledge on ship manoeuvring prediction through model tests and system based and numerical methods. Free-running model tests executed with the very large crude carrier KVLCC2 at two laboratories have been compared with the results of simulated turning circles and zigzag manoeuvres from two different mathematical models. It was concluded that the type of mathematical model has an important influence on the simulated behaviour. Moreover, further research is necessary as simulations result into a more course stable behaviour compared to free-running tests at model scale.

Prediction of squat for underkeel clearance

This chapter presents a summary of ship squat and its effect on vessel underkeel clearance. An overview of squat research and its importance in safe and efficient design of entrance channels is presented. Representative PIANC empirical formulas for predicting squat in canals and in restricted and open channels are discussed and illustrated with examples. Most of these formulas are based on hard bottoms and single ships. Ongoing research on passing and overtaking ships in confined channels, and offset distances and drift angles is presented. The effect of fluid bottoms or mud is described. Numerical modeling of squat is an area of future research and some comparisons are presented and discussed.

The Towing Tank for Manoeuvres in Shallow Water

cm Centimetre FHR Flanders Hydraulic Research GSP Groningen Seaports kHz Kilohertz kn Knots (nautical speed) KSN Keep Sediments Navigable N.A.P. Normaal Amsterdams Peil (reference height) m Meter Pa.s Pascal second UKC Under Keel Clearance W&P Wiertsema & Partners UKC210kHz UKC with respect to 210kHz based on survey of 2 of May 2015 UKC33kHz UKC with respect to 33kHz based on survey of 2 of May 2015

Requirements for standard harmonic captive manoeuvring tests

Abstract Within the scope of the determination of a standard PMM testing procedure, a review of existing guidelines for the execution of harmonic captive manoeuvring tests with ship models is given, with an emphasis on the selection of the oscillation frequency. These guidelines are discussed, making use of the results of systematic PMM-test series, mainly carried out at low speeds in shallow water conditions. In this way, the validity of rules of thumb in situations that are considerably different from the design conditions can be assessed.

Running sinkage and trim of the DTC container carrier in harmonic sway and yaw motion : open model test data for validation purposes

After successful conferences on bank effects, ship – ship interaction and ship behaviour in locks, the Fourth International Conference on Ship Manoeuvring in Shallow and Confined Water (MASHCON) has a non-exclusive focus on ship – bottom interaction. With increasing ship sizes in vertical and horizontal dimensions, a clear understanding of the interaction between a ship and the bottom of the waterway will help to improve the operations and increase the safety of manoeuvring ships. To open a joined research effort on the validation and verification of the different research methods, the Knowledge Centre Manoeuvring in Shallow and Confined Water has selected model test data which were obtained while executing tests with the DTC container carrier in the framework of the European SHOPERA project. The benchmark data are harmonic yaw and harmonic sway tests with the bare hull of the DTC at full draft and 20% under keel clearance at rest.

Requirements for Standard Harmonic Captive Manoeuvring Tests

Abstract In the scope of the determination of a standard PMM testing procedure, a review of existing guidelines for the execution of harmonic captive manoeuvring tests with ship models is given, with emphasis on the selection of the oscillation frequency. These guidelines are discussed making use of the results of systematic PMM-test series, mainly carried out at low speed in shallow water conditions. In this way, the validity of rules of thumb in situations which are considerably different from design conditions can be assessed.