Page 463 - Practical Design Ships and Floating Structures

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1 INTRODUCTION
The design and construction of dredgers is the core business of MC. An important contingent of these
dredgers is of the trailing suction hopper type. The hull form of such vessels is characterized by a high
block coefficient, a low UB ratio and a high B!T ratio. Because of these characteristics and the high
power density of the propeller, hopper dredgers have typically twin-screw propulsion. Nozzles are
nearly always applied in view of the need of high thrust during dredging at low speeds.
As an exceptional case, IHC received recently an order for a single-screw Hopper Dredger. This vessel
happened to be the first single-screw dredger for the yard since 28 years, all intermediate hopper
newbuildings being twin-screw ships. The performance of that former single-screw case - in the sequel
referred to as “the old hull” - had been disappointing. The combination of the full aft body, the heavily
loaded propellers and the small propeller-hull clearance led to unacceptably high vibration levels,
which had to be remedied by expensive retro-fit solutions.
Such incidents, which put a burden on the relation with the client and certainly cause damage to the
reputation of the yard, should preferably be avoided. However, the client’s choice of a single-screw
Hopper Dredger because of its relatively low price is of come legitimate. This brings out the need for
an early detection of possible problems with a new design. In this respect, model tests play an
important role. However, time constraints ask for checks even at a pre-model testing stage. In this
paper we show that computational fluid dynamics can assist to avoid major problems.

2 THECASE
Early 2000, MC got an order from a small French dredging company to build a low- budget single-
screw hopper dredger in an extremely short delivery time of less than 11 months. The block coefficient
of 0.85, the thrust loading coefficient of 2.7, the propeller power density of 420 kW/m2 and small
propeller-hull clearances all pointed to a serious risk of vibration problems. It was clear therefore that
the design of the aft body should be thoroughly investigated, but lack of money (low budget) and time
(very short building period) seemed to frustrate that. At least model tests were under the given
circumstances no viable option. The application of computational fluid dynamics, in particular the
PARNASSOS code, was proposed by MARIN as an alternative to check the lines of the aft body and
to investigate possible shortcomings.

3 DESIGN CONSIDERATIONS
Hopper dredgers have undergone drastic changes during the last decades as a result of changing
demands of the owners. Not only the loading capacity has increased, also the operation range and the
maximum speed. Besides, more stringent requirements on the comfort for the crew are imposed
nowadays. Hence, the design has become critical, particularly with regard to vibration hindrance,
basically caused by poor inflow to the propeller.
From the beginning it was clear that the hull form of the old vessel was not a proper starting point for
the new design. The aft body of that hull may be classified as V-shaped, which for high-block ships
naturally lead to steep buttocks. Ships based on such a design concept often suffer from a bad wake
field at the propeller location, notably in the top sector of the propeller disk.