Tall ships in tight spots

HTC-ship model during manoeuvring test. (Photo: Marin)

Increases in both ship sizes and traffic density in harbours necessitate better manoeuvrability of modern ocean giants. PhD researcher Serge Toxopeus improved manoeuvring simulations considerably, thus paving the way for less unwieldy designs.

Simulations of a ship performing special manoeuvres like steering alternatively full port and starboard for about a minute, or steering full rudder and making a 360-degree turn, can go painfully wrong. In examples quoted by PhD researcher, Serge Toxopeus, the simulation underestimated deviations from the zigzag course by almost 30 percent, while overestimating the diameter of the narrowest turn by 46 percent.

Toxopeus, who after graduation from TU Delft in 1996 went to work at the Maritime Research Institute Marin in Wageningen, set out to prove that the simulation could be improved if different input data were used.

Traditionally, the input data describing the forces and moments on the ship’s hull are based on measurements made of a scale model in a large tank. It’s perhaps a bit confusing that what seems to be a calculation – a simulation – is in fact based on empirical data, but so it is. Nonetheless, tank tests provide a set of parameters that are used in simulations to calculate the forces on the ship’s hull and, hence, its trajectory through the water and manoeuvrability.

Alternatively, one can calculate the forces on the hull under a certain course using computational fluid dynamics (CFD). Progress made in software and computing power now render this a feasible approach, although it still typically requires a couple of days to evaluate a hull design. Toxopeus has used both a physical and numerical model of a large container ship, called Hamburg Test Case (or HTC), to evaluate this numerical approach.

He found that simulations based on the CFD data corresponded better with the tank tests than those based on empirical parameters. This means that a ship’s manoeuvring behaviour can be forecast much more reliably while still under design, thanks to CFD.

Does that mean that tank tests will become obsolete for testing ship designs? Absolutely not, says Professor Rene Huijsmans (Mechanical, Maritime and Materials Engineering), who was Toxopeus’ PhD supervisor. “CFD was used by Toxopeus in addition to tank tests, to determine a couple of parameters, which then could be used in a simulation. But to replace the entire tank test by CFD calculations is of another magnitude entirely, and way beyond today’s computational power.”