In one concept for a Mobile Offshore Base (MOB), the several modules are kept in alignment and properly positioned with respect to one another by the use of many steerable thruster units. The research reported here develops an algorithm using linear programming to rationally allocate commanded thrust and thruster azimuth to multiple thrusters (more than two) in order to best achieve the desired force system on the MOB module. The linear programming approach allows explicit consideration of many of the realworld characteristics of the thrusters, including maximum thrust capability, maximum slew rate of the thrusters, etc. The algorithm was programmed and the results of the optimum allocation for two different scenarios are presented.
Offshore platforms often have a system of thrusters for dynamic positioning (see Figure 1). For instance, in the case of offshore oil platforms, the thrusters are used to keep the platform over the drill hole. This is. accomplished by commanding the thrust units to develop a timevarying force system that opposes the time-varying environmental loads. This force system is composed of a horizontal force in a given direction and a yaw moment. If the thrusters can be oriented to any azimuth, then any such instantaneous force system can be created with only two thrusters, as long as the maximum thrust capability of each thruster is not exceeded and the thrust direction of each is chosen properly. With more than two thrusters, the system is redundant and there exists an infinite number of different combinations of thrust and thrust direction by the individual thrusters to achieve any given instantaneous force system. In some of the MOB concepts as many as eight thrusters are anticipated to be used for each module because the thrust capability of current single thrusters is insufficient only two thrusters are used.