"Wimpey Sealab" is a multi-purpose ocean engineering motor vessel operated by Wimpey Laboratories Limited and is equipped with a computer-based dynamic positioning system by GEe Electrical Projects Limited. Originally a cargo ship, the "Wimpey Sealab" was converted to a drill-ship, and completed her commissioning trials in a dynamic positioning mode of operation in November 1974.
A. view of "Wimpey Sealab" at sea is shown in fig. 1; and the main elements of the dynamic positioning system are shown in fig. 2.
The design of the dynamic positioning system depends on the mathematical models (i.e. the set of equations) that define the ship and its environment. A completely theoretical approach to the formulation of these equations is not feasible, and they are determined by means of tests on scaled down physical models of the ship. This paper describes results obtained from the tank and wind tunnel tests, carried out by the National Physical Laboratory, on models of "Wimpey Sealab".
Dynamic positioning for "Wimpey Sealab" is required in water depths between 30 and 300 metres. The ship position must be held within a circle of 7 metres radius (or 3% of water depth, whichever is the greater) in a steady"ind of up to 12.87 m/sec, with waves of significant height 3.54 metres and significant length 91.44 metres, and with a steady sea current of up to 1.54 m/sec. Under the above conditions, but with the wind gusting up to 20.59 m/sec, the ship position must be held within a circle of 11 metres radius. Ship heading is allowed to vary.
Propulsive devices for dynamic positioning may take several forms, including the use of the main engine in conjunction with tunnel thrusters, steerable thrusters, and cycloidal types of thrust unit. Wimpey Laboratories Limited decided to use steerable thrusters, and the hull of the "Wimpey Sealab" was modified to accommodate four retractable, rotatable and controllable pitch thrusters. These thrusters are each rated at 746 kW; two are located side by side at the stern of the ship, and two are located, slightly offset from one another, at the bow.
The S.1. system of units is used throughout this paper. Conversion factors to imperial units are given in Appendix 1.
The objectives of the tests can be considered in three main parts as follows:-
Part 1: the estimation of the steady forces and moments acting on the ship due to the environment of current, wind and waves (see sections 3, 4 and 5 of the paper respectively); and the estimation of wave induced ship motions (see section 5).
Part 2: the estimation of forces and moments applied to the ship by the action of the four thrusters; and, in particular, the interaction effects that occur between the thruster jets and the hull, in still water and in the presence of current flow (see section 6).
Part 3: the formulation of the equations of motion of the ship, which define its motions due to the forces and moments applied by the thrusters and by the environment (see section 7).