Dynamic positioning system is usually employed to keep offshore vessels at a desired position for safe operations. Positioning ability is crucially affected by the selection of thrusters and their configuration for a dynamically positioned (DP) vessel. The positioning abilities of the vessels with different thruster configurations are investigated by time domain simulations. A novel synthesized positioning ability criterion has been proposed to quantify the positioning ability of a DP vessel. A semi-submersible was adopted to perform the comparison. Based on the aforementioned synthesized criterion, the positioning ability of the vessel is quantified. The results may provide useful guidance to the design of thrust system of the vessel. For the thruster configuration design, the thruster should be deployed as far as possible to the center of the marine structure. Furthermore, it would be better to deploy the thrusters dispersedly.
For oil and gas offshore Exploration and Production (E&P) operations in deeper and deeper waters, floating platforms such as Floating Production Storage and Offloading (FPSO) and semi-submersible are widely used. Dynamic positioning system (DPS) is usually employed to keep these offshore vessels at a desired position for safe operations (Xu et al., 2015e). A dynamically positioned (DP) vessel is by the International Maritime Organization (IMO) and the certifying class societies (DNV, ABS, LR, etc.) defined as a vessel that automatically maintains its position and heading (fixed position or pre-determined track) exclusively by means of active thrusters. DPS has been widely used in offshore engineering over the last five decades. Brink and Chung (1981) has carried out an extensive computer simulation and performance analysis and assessment for the dynamic positioning control of a 300,000 tons ship. Description of DPSs, including their early history can be found in (Morgan, 1978; Fay, 1990).
In the design of a thrust system, there are many factors are subject to changing (i.e. the number of the thrusters, the thruster types, the maximum thrust of the thruster and the thruster configuration, etc.). Many efforts can be found in the literature focus on the number of the thrusters and their configuration. Mahfouz and El-Tahan (2006) has proposed a developed software program as a marine tool in the selection of thrusters, in their configuration, and during preliminary investigation of the positioning capability of a newly designed vessel DPS. However, the comparison is based on the rough observation of the polar plots, which may cause confusion when these polar plots overlap with each other. A synthesized positioning capability criterion which can quantify the synthesized positioning capability has been adopted to compare the polar plots (Xu et al., 2015a, d).
