Scale model tests with the semi-submersible GVA 4000 under position and heading control in irregular waves and winds are presented and discussed. The propeller rates and force directions of four azimuthing thruster units were controlled by a dynamic positioning system implemented in a micro-computer Motorola 68000 at the tests. The data from the tests are useful for the development of a mathematical simulation model for further analysis of the rig with different dynamic positioning systems as discussed m the chapter
Much theoretical and experimental work has been devoted to investigations of the behaviour of floating offshore structures in different wind, wave and current conditions. The forces then acting on the structures are usually divided into two parts, one high-frequency wave induced part and one slowly oscillating part. The aim of a mooring or Dynamic Positioning (DP) system is to compensate for the slowly oscillating forces. This chapter describes model tests of a semisubmersible rig, which was positioned on-line by a DP-system.
This work is a continuation of simulation studies of DP-systems at SSPA, see refs (I), (2), (3). In these simulation studies the high-frequency wave induced motions and the measurement disturbances were filtered away using a Kalman Filter The estimates of surge, sway and heading were then used in a regulator designed with linear-quadratic control theory.
The mean optimal thruster settings for a certain given weather condition with respect to total power were initially calculated with the SSPA program THRUST 2–4, and the DP-system thereafter operated around these optimal settings.
The results discussed in this chapter are the first steps m a research program for studies of DP-systems in connection with model tests. The selected rig is GVA 4000, which during the tests was positioned using a simplified DP-system under influence of Irregular waves and gusting winds.
During several years SSPA has developed simulation models based on seakeeping tests using soft mooring in combination with thruster/thruster and thruster/hull interaction tests See refs (1)–(4). Scale model tests under DP-control, as described in this chapter, can be regarded as an alternative approach.
The characteristics of GVA 4000 are summarized in Section 2. The tests were carried out in the SSPA Maritime Dynamics Laboratory (MDL) with a model in the scale 1:25. The model was equipped with four thruster units, one in each column, with a diameter of 2.8 m (full-scale)
The DP-system was implemented In a Motorola 68000. The positions of two infra-red spots on the platform were measured and used as input to the DP-system From these measurements surge, sway and yaw of the rig were calculated.
The DP-system m this case consists of PID-controllers, and the high-frequency wave induced motions and measurement noise were filtered away by a low-pass filter The filtered signals were then used m the controllers and the thruster settings were calculated for the four thruster units See Section 4.
