Helio Mitio Morishita

SLIDING MODE CONTROL APPLIED TO OFFSHORE DYNAMIC POSITIONING SYSTEMS

Abstract The technique of Dynamic Positioning (DP) consists on keeping the position and heading of a vessel by the use of active thrusters. In Brazil, the offshore oil industry follows the recent tendency of performing several operations aided by DP systems, such as prospection, drilling, offloading and others. Currently, the DPS are based on conventional controllers (PD + Kalman Filter). This paper proposes the application of a controller based on nonlinear sliding mode control technique to the dynamic positioning of a floating vessel, as well as some simulations and experimental validation results.

Laboratory Facilities for Dynamic Positioning System

Abstract Dynamic positioning systems are widely installed in vessels specially those related to the offshore oil industry. Most of these ships require tests in ocean basin to predict their dynamic performance properly since the mathematical models are quite complex and some hydrodynamic phenomena have not been modeled yet. In this paper some aspects of an experimental facility set-up for tests with scale models with dynamic positioning system are described. It includes the topology of the experiment and details of components of the system. Preliminary experimental results are presented and compared with those obtained through digital simulation.

DYNAMIC BEHAVIOUR OF A TURRET FPSO IN SINGLE AND TANDEM CONFIGURATIONS IN REALISTIC SEA ENVIRONMENTS

A comprehensive analysis of the dynamics of ship-type Floating, Production, Storage and Offloading systems anchored through a turret system (Turret-FPSO) is presented. Firstly, the behavior of the FPSO unit alone is examined under the combined action of wind, current and waves. Wave forces are represented by second-order effects such as slow drift forces, as well as wavecurrent interaction corrections to mean drift forces, namely wave drift damping. The influence of the vessel’s draft and of the longitudinal position of the turret is also assessed. The study of the dynamics of the system is performed by calculating static equilibrium solutions and evaluating their stability properties. Relevant ranges of environmental parameters such as wind and current speeds, wave spectra, and respective angles of incidence are investigated. Numerical results are obtained and shown in bifurcation diagrams depicting the evolution of steady-state responses as a function of environmental parameters. The system displays a variety of different regimes of solutions in which both their number and their stability may change as one or more parameters are varied. The stability analysis of the Turret-FPSO system is extended to include the tandem configuration with a shuttle vessel. The main features of this more intricate scenario are summarized in 3D bifurcation diagrams.

Methodology for dynamic analysis of offloading operations

Abstract The relative positioning between the Floating, Production, Storage and Offloading (FPSO) and the shuttle vessel during offloading should be analysed carefully, since the safety of the operation is of primary concern. In order to avoid collision, the shuttle vessel is kept away from the FPSO through the force of a tug-boat or by a dynamic positioning system. In both cases it is convenient to perform a preliminary study of their dynamics in order to obtain a guideline to set the best reference as well as a control approach. This paper presents a methodology for studying the dynamics and control of FPSO and shuttle vessel in tandem configuration systematically, using two software tools. The first tool allows the study of static solutions and the results have shown complex behavior, with a multiplicity of static equilibrium solutions the number and stability properties of which vary according to the combinations of the environmental conditions and vessel parameters. The second tool performs dynamic analysis of the FPSO-shuttle vessel, making it also possible to include a dynamic positioning system. Some results are compared with those obtained experimentally.

Dynamics of a Turret-FPSO and Shuttle Vessel Due to Current

Abstract The dynamic and control of a system consisting of a turret-Floating Production Storage and Offloading and a shuttle ship in tandem formation are investigated. The study comprises the analysis of the influence of the turret’s position along production ship in the static and dynamic behaviour of the system. The static analysis has shown that the shuttle ship is always parallel with the current direction, and the production ship heading angle exhibits a bifurcation as turret position is moved toward the centre of the gravity in the equilibrium configuration. However all solutions of equilibrium are unstable and so some simple control action over shuttle ship is suggested in order to assure a safe offloading operation.

Critical Analysis of Control and Filtering Algorithms Used in Real Dynamic Positioning Systems

Dynamic positioning systems (DPS) comprise the utilization of active propulsion to maintain the position and heading of a vessel. Sensors are used to measure the actual position of the floating body, and a control algorithm is responsible for the calculation of forces to be applied to each propeller, in order to counteract all environmental forces, including wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous power to be attenuated, possibly causing damage to the propeller system. A filtering algorithm is then used to separate high frequency components from the low frequency ones, which are indeed controlled. Usual commercial systems apply Kalman filtering technique to perform such task, which includes a full model of the system. Furthermore, an adaptive on-line estimation algorithm is also used to evaluate the wave peak frequency, since the model in Kalman Filter depends on such parameter. The controller itself is based on a simple proportional-derivative (PD) actions. This paper presents all the mathematical formulation of the Kalman Filter, adaptive algorithm and the controller used in commercial DPS and performs a critical analysis of those models. Some illustrative results of a dynamic positioned shuttle vessel are presented, considering the incidence of waves, current and winds.© 2005 ASME

Dynamics and control of a two-body floating system under realistic environmental loads

In this paper a systematic study of the dynamics of turret FPSO in single and tandem configuration is carried out for further application in the formulation of control strategy of the shuttle vessel. As a preliminary step equilibria are calculated along with their stability properties for a range of values of the main parameters defining each configuration: wind speed, significant height and mean period of waves and relative angles of incidence. The influence of the position of the turret is also assessed. Static solutions are obtained numerically due to the complexity of the mathematical models and then their stability properties are investigated through time-domain simulations. The system displays a variety of different regimes of solutions in which both their number and their stability may change as one or more parameters are varied. The results are summarized in a series of bifurcation diagrams in which the evolution of steady-state heading response of the ships is displayed as a function of parameters characterizing the environmental state. Finally the application of the bifurcation diagram for a control strategy for the shuttle vessel is suggested.

DEVELOPMENT AND APPLICATION OF A SHIP MANOEUVRING DIGITAL SIMULATOR FOR RESTRICTED WATERS

Abstract This paper reports on two case studies making use of a digital simulator to investigate the manoeuvring motions of ships in canals with shallow and restricted waters. The first case study corresponds to a manoeuvring analysis conducted for the Port of Rio Grande (RS – Brazil), whose aim was to assess the potential impact upon manoeuvres of the presence of a large offshore platform (the Petrobras P-53) which is to remain docked for several months at the Port to complete its construction. The second study made use of the simulator to evaluate the manoeuvring conditions along the approach route and manoeuvring basin of the Port of Ponta do Felix (PR – Brazil). The simulator includes a complete mathematical model of the ship dynamics in the horizontal plane when subjected to wind and current forces. It also comprises detailed models for the action of thrusters and propellers, both fixed and azimuth, employed to control manoeuvres and dynamically position ships, as well as rudders and tugboats. The models used by the simulator allow for the effects of shallow and restricted waters, including the increase in resistance and lateral forces, increase in additional mass and the appearance of lateral and vertical suction (squatting). The simulator is implemented via an interactive interface through which the user is able to apply control actions (rudder angle, main engine, thrusters and tugboats) in real time during manoeuvres, thereby reproducing to some extent the action of a pilot.

EVOLUTION OF CRUDE OIL OFFSHORE LOADING IN BRAZILIAN WATERS

Abstract This paper gives an overview on the oil exploitation in Brazilian waters, the operational aspects, and the experience to implement DP shuttle vessel operations. Besides that, the Shuttle Tankers (ST) basic requirements for Campos Basin conditions and the acceptance tests required by PETROBRAS, at the present time, are also presented. In addition, the authors make comparisons between converted and new building shuttle tankers, describing the pros and cons of each one. Naturally, the most cost effective alternative for the Campos Basin scenario is discussed.

Observer-Lagrangian-based controller applied to two dynamically controlled vessels

Abstract The application of the Observer-Lagrangian-based controller for a typical offshore system comprised of two dynamically controlled vessels connected by a hawser in tandem configuration is investigated. The control law is obtained by introducing some virtual constraints in the Lagrangian control model. The virtual constraints are defined in terms of the relative position and heading between the vessels. The state variables (position, heading and velocities) and the external forces of the system required by the controller are estimated through a nonlinear observer based on the passivity approach. The closed-loop dynamics is proved to be exponentially stable in the sense of the Lyapunov stability theory. Some results of dynamical simulations have indicated a very good performance of the controller.