Dynamic positioning of a turret moored FPSO using slind mode control

This paper presents the application of a sliding mode control technique to the dynamic positioning of a turret moored Floating Production, Storage and Offloading (FPSO) system. The proposed controller is robust with respect to modeling errors and variations in the intensity and direction of environmental forces. Since the system is strongly non-linear, the sliding mode methodology is appropriate because it does not require linearization and uses the complete model for the controller design. Furthermore, the design of the controller term that guarantees robustness is very simple, and can be generalized to any adopted model of dynamic and environmental forces. The main drawback of the sliding controller, related to the high oscillatory control actions, is avoided by the definition of a “boundary layer” near the sliding surface. The controller was tested through numerical simulations of a moored Very Large Crude Carrier (VLCC) under typical environmental conditions at Brazil’s Campos Basin, confirming the stability and performance robustness. INTRODUCTION FPSO system is a modern concept for floating offshore oil exploration units. A tanker is moored in deep water and a production plant is installed on its main deck. Periodically, shuttle tankers are connected to the FPSO in tandem formation bringing the stored oil to shore. Characteristics of tankers such as large deck areas and storage capacity are key factors in oil production at sea. However, due to their large water line area, tankers are exposed to severe environmental loads, which may induce large displacements, eventually causing rupture of the mooring lines and risers. Mooring systems are then designed to minimize such loads, allowing the ship to be aligned with the resultant of the environmental forces. Turret mooring system, for example, is composed by a cylindrical structure (turret) supported by an axial bearing system fixed to the ship and moored to the seabed. Under certain combinations of environmental agents, waves may reach the ship almost perpendicularly, inducing large first order vertical motions. These motions cause dangerous cyclic loads in risers and mooring lines, difficulties in the production process and discomfort for the crew (Leite et al., 1999). When a dynamic positioning system is not available, tugboats are used to relocate the FPSO, avoiding dangerous conditions. Even when the FPSO is equipped with a dynamic positioning system, the controller cannot directly compensate first order motions, due to high frequency components that would require an enormous power to be attenuated. To avoid these motions, it is proposed to head the FPSO in such a way the waves do not induce large first order motions. Furthermore, to minimize fuel consumption, the controller must maintain the FPSO in such a position that surge and sway mean forces are counteracted by the mooring system, and not by the propellers forces. The proposed FPSO control system is composed by two hierarchical layers (Tannuri and Donha, 2000). The first one calculates the optimal set-point position, in order to guarantee minimum first order wave induced motions and minimum fuel consumption. The second level receives the information from the positioning sensors installed onboard and the set point from the superior level and commands the thruster system. The first layer minimizes a cost function relating: roll oscillation amplitude, dynamic traction in risers, static offset of the system and fuel consumption. To exemplify the importance of these operational parameters, a relatively common nonextreme condition in Campos Basin may be considered, composed by swell waves, which come mainly from the south (or southeast), in the presence of current coming from the north (or northeast) direction. In such case, the angle between waves and current is typically between 90 and 180 and, depending on their relative intensities, the FPSO may be subject to beam-sea