Eduardo A. Tannuri

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.

The Lagrange equations for systems with mass varying explicitly with position: some applications to offshore engineering

The usual Lagrange equations of motion cannot be directly applied to systems with mass varying explicitly with position. In this particular context, a naive application, without any special consideration on non-conservative generalized forces, leads to equations of motions which lack (or exceed) terms of the form 1/2(¶m/¶q.2), where q is a generalized coordinate. This paper intends to discuss the issue a little further, by treating some applications in offshore engineering under the analytic mechanics point of view.

Wind Shielding Effects on DP System of a Shuttle Tanker

The effects of hydrodynamic and aerodynamic interactions on a typical shuttle tanker (ST), when offloading a FPSO moored in Spread Mooring System (SMS) configuration are being studied in an extensive research project conducted at the University of Sao Paulo, in collaboration with Petrobras. The numerical models will be incorporated in the simulators in order to evaluate the impact of such effects on the dynamics of the ST and on its DP System power requirement. Present paper focuses the wind shielding effect, since current and wave wake effects were already treated in previous works (Illuminatti et al., 2009; Queiroz Filho et al., 2009). A detailed CFD model of the FPSO and ST were used to evaluate the horizontal forces and yaw moment induced in the ST by the wind when inside the shadow zone defined by the FPSO. The CFD model was calibrated using wind tunnel measurements of a similar system. Typical tandem configurations were considered for the connection and oil-transfer stages of the operation. The power requirements for each DP thruster were then calculated, considering a thrust allocation algorithm. The comparisons with the stand-alone ST configuration indicated that the wind shielding effect is important concerning DP power. Besides, it is proposed a simplified model for wind forces and moment in the ST, considering the wind velocity field in the wake of the FPSO (CFD calculation). The procedure is based on report [7] where the wind force is evaluated through de summation of forces and moment in the stern, middle and bow parts of the ST. Such procedure has the advantage of requiring only one CFD calculation (for obtaining the velocity field downstream FPSO).© 2010 ASME

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.

Higher Order Sliding Mode Control Applied to Dynamic Positioning Systems

Abstract This paper presents the application of Higher Order Sliding Mode Control (HOSM) theory to the problem of Dynamic Positioning (DP) of offshore vessels. Robustness aspects of Sliding Mode Control are suitable for DP design, since the vessels are subjected to several unmeasured environmental disturbances and there are also modeling uncertainties inherent to such a complex system. However, first-order sliding mode control presents chattering (high frequency oscillation) in the control action. Such effect is eliminated by changing the original formulation of the controller, replacing the sign function by a smooth transition function (such as sigmoid or saturation). Such replacement introduces a steady state error, that is then eliminated by the introduction of an integral action. The application of HOSM control naturally eliminates the chattering, since the order of the system is artificially increased and the control action is in fact the time-integral (low-pass filter) of a high-frequency oscillatory signal. So, there is no necessity of changing the original formulation of sliding mode control to obtain a smooth control action, while still keeping the good robustness and model-error insensibility properties of sliding mode control. Problems associated with the necessity of second order derivative of the position are discussed, and an exact-robust differentiator is then applied. A numerical simulator is used to assess the performance and robustness of the HOSM control applied to a typical DP shuttle tanker. Preliminary experimental analysis is also exposed.

Adaptive control strategy for the dynamic positioning of a shuttle tanker during offloading operations

In deep water oil production, Dynamic positioning systems (DPS) strategy has shown to be an effective alternative to tugboats, in order to control the position of the shuttle tanker during offloading operations from a FPSO (floating production, storage, and offloading system). DPS reduces time, cost, and risks. Commercial DPS systems are usually based on control algorithms which associate Kalman filtering techniques with proportional-derivative (PD) or optimal linear quadratic (LQ) controllers. Since those algorithms are, in general, based on constant gain controllers, performance degradation may be encountered in some situations, as those related to mass variation during the loading operation of the shuttle tanker. The positioning performance of the shuttle changes significantly, as the displacement of the vessel increases by a factor of three. The control parameters are adjusted for one specific draught, making the controller performance to vary. In order to avoid such variability, a human-based periodic adjustment procedure might be cogitated. Instead and much safer, the present work addresses the problem of designing an invariant-performance control algorithm through the use of a robust model-reference adaptive scheme, cascaded with a Kalman filter Such a strategy has the advantage of preserving the simple structure of the usual PD and LQ controllers, the adaptive algorithm itself being responsible for the on-line correction of the controller gains, thus insuring a steady performance during the whole operation. As the standard formulation of adaptive controllers does not guarantee robustness regarding modeling errors, an extra term was included in the controller to cope with strong environmental disturbances that could affect the overall performance. The controller was developed and tested in a complete mathematical simulator, considering a shuttle tanker operating in Brazilian waters subjected to waves, wind and current. The proposed strategy is shown to be rather practical and effective, compared with the performance of constant gain controllers.

Dynamic Positioning Systems: Comparison Between Wave Filtering Algorithms and Their Influence on Performance

Wave filtering is an essential function of a Dynamic Positioning System, being responsible for the separation between high-frequency wave induced motions and low-frequency ones, which must be controlled. Low attenuation of the first components may cause oscillatory control action, high fuel consumption and can damage propeller systems. On the other hand, depending on filtering design, high levels of attenuation may be associated with non-admissible delay-times, which may cause instability in the closed-loop system. Traditionally, low-pass or notch-type filters have been used since the first DP systems, due to the simplicity, acceptable performance and possibility of being implemented using analog circuits. Alternatively, observers based on Kalman Filtering Theory have also been used, based on simplified ship models, separating low and high frequency motions. Two wave filters, representing each of those categories, were implemented in a complete DP simulator. All DP components and algorithms are considered and modeled in the simulator, including propellers, thrust allocation, wind-feedforward and different control strategies. Environmental loads are evaluated using fully validated models, including wind, current and wave actions. A pipe-laying barge under typical Campos Basin environmental conditions has been considered as example. Several aspects of the filtering algorithms were analyzed and compared, involving the influence of each filter in the overall DP performance, relationship of design parameters with physical system, ease of commissioning and tuning. The trade off between low frequency tracking (which is associated with the delay time) and wave response suppression is analyzed and discussed for both categories of filters.Copyright

Adaptive techniques applied to offshore dynamic positioning systems

Dynamic positioning systems (DPS) comprise the deployment of active propulsion to maintain the position and heading of a vessel. Several sensors are used to measure the actual position of the floating body, while a control algorithm is responsible for the calculation of forces to be delivered by each propeller, in order to counteract all environmental forces, such as wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous amount of power to be attenuated, possibly causing damage to the propeller system. That is the reason why a filtering algorithm is to be put in place to separate high-frequency components from the low-frequency ones, which are, then, fed into the control loop. Usual commercial systems apply Kalman filtering technique to perform such task, due to the smaller phase-lag introduced in the control loop compared to conventional low-pass filters. The Kalman filter draws on a model of the system to be controlled, which, in turn, depends on an unknown parameter, related to the wave frequency. Adaptive filtering is called upon with a view to perform an on-line estimation of such parameter. Most control algorithms, however, rely on fixed gains, thus making it possible for a noticeable performance degradation to take place in some situations, as those associated to mass variation during a loading operation. This paper presents the application of model-reference adaptive control (MRAC) technique to DPSs, cascaded with the commonly used adaptive Kalman filter. The model of a dynamically-positioned shuttle tanker exposed to waves and current is employed to highlight the advantages of the adaptive controller compared to commonplace fixed-gain controllers.

USP Active Absorption Wave Basin: From Conception to Commissioning

This paper presents the new active absorption wave basin constructed at the University of Sao Paulo (USP), in the Numerical Offshore Tank (TPN) Laboratory. The square (14m × 14m) tank is able to generate and absorb waves from 0.5Hz to 2.0Hz, by means of 148 active flap-type wavemakers. An independent mechanical system drives each flap by means of a 1HP servo-motor and a ball-screw based transmission system. A customized ultrasonic wave probe is installed in each flap, and is responsible for the measurement of wave elevation in the flap. These sensors do not require constant calibration, differently from the capacitive or resistive sensors normally used in similar tanks. A complex automation architecture was implemented, with 3 Programmable Logic Computers (PLC), and a low-level software is responsible for all the interlocks and maintenance functions of the tank. Furthermore, all the control algorithms for the generation and absorption are implemented using higher level software (MATLAB®/Simulink block diagrams). These algorithms calculate the motions of the wavemakers both to generate and absorb the required wave field by taking into account the layout of the flaps and the limits of wave generation. The experimental transfer function that relates the flap motion to the generated wave is used for the calculation of the motion of each flap. Absorption tests were conducted with a prototype wave generator in a 2D wave flume with regular waves. Two different algorithms were tested. The first one is the frequency domain method based on Maeda et al. (2004), in which the commanded variable is the motor velocity. Furthermore, the time domain algorithm proposed by Schaffer (1996) was also tested. It is based on a digital filter and uses the position of the motor as the commanded variable. Both algorithms have hydrodynamic feedback based on the measurement of surface elevation at each flap. The first algorithm needs an extensive test procedure to calibrate its control parameters while the second one, after optimizing the digital filter, is ready to use. Both algorithms presented similar results with reflection coefficient smaller than 10.7% for regular waves in the frequency range of 0.5 to 2.0 Hz. The paper also presents the first results obtained in the tank.Copyright