ABSTRACT

This paper presents the synthesis of a research on dynamic positioning systems (DPS) for semi submersible platforms, that included both analytic-computational and experimental parts. Firstly it is presented an overview of the mathematical models used for simulation of the vehicle motion in the horizontal plane. The theoretical treatment of the control problem and its implementation Is also focused. Two different schemes were used to get the controller, one of them is fully original in DPS applications. After that, the control system prototype construction is briefly presented. The evaluation of the control system performance may be done by the analysis of computer simulation and experimental trial results. The trials were carried out in the towing tank of Berlin Technical University, using a propelled aluminum model of a semi submersible platform equipped with the control system prototype developed by the authors.

INTRODUCTION

Offshore activities are continuously expanding from shallow coastal waters to the deep regions of the continental shelf. In this regions, normally exposed to worse environmental conditions, new technological problems, such as the impracticability of the floating anchored and fixed conventional systems, are demanding creative solutions. Dynamic positioning systems (DPS) has shown to be one of the best solutions for deep water applications, since tension legs platforms are still not more than an expensive alternative and in deep water conditions anchored structures need too much time for deployment and recovery of anchors and fixed rigs are presently unfeasible. However, DPS still deserves proper attention from the point of view of operational costs as well as safety.

The DPS is basically composed by three subsystems illustred in figure 1: the measurement (sensors) system, which provides information about the vehicle position and heading as well as the environmental conditions; the logical unit (control system), that processes this information, evaluating and supervising the control action; and the actuator system, usually a set of thrusters strategically distributed on the platform hull, frequently kort-nozzle, azimuthally, pitch and speed variable propellers. The thruster system produces the propulsion forces, that will enable the vehicle to keep the desired position (heading), closing the loop and characterizing the system feedback. DPS is designed not to hold the vehicle stationary but within acceptable limits ('watch-circle"), minimizing in this task the energy consumption as well as the wear and tear of the propulsion system. This paper is concerned only with the control system.

Following the research steps, one presents firstly the formulation of the mathematical models used for description of the platform motions at the sea. The development and implementation of the control system are then briefly described, followed by an explanation of the control system prototype construction. After that, test facilities and trial program are presented. Finally, results from simulations and trials are presented and analyzed.

MATHEMATICAL MODELS

Figure 2 shows the elements one must model to perform digital DPS simulations. In the trial conditions the "real" platform model, the environmental disturbance models, the sensors, the propulsion and noise models are replaced by physical models. The control system used for simulations purposes may be used in trial tests with small modifications.

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