Assisted Dynamic Positioning of a Moored FPSO: Robustness Aspects Regarding Current Forces Modeling

This work treats the problem of assisted dynamic positioning (DP) of moored FPSOs, focusing on robustness issues regarding the modeling of hydrodynamic current forces and moment. Modern DP systems usually employ Extended Kalman Filters associated to LQG or Adaptive controllers to estimate current forces and velocity, performing a direct compensation of these environmental loads. Such a procedure guarantees better performance of the controller under a wide range of environmental conditions, which is known as a “large environmental window”. However, it demands a large number of control parameters, usually requiring a time consuming tuning process. More recently, Sliding Mode Controllers (SMC) with an alternative compensation technique [1] have also been investigated and proved to be an appropriate choice. Robustness properties of SMC allow the use of a simple hydrodynamic current force modeling and rough measurements of current velocity and direction to perform a real time estimation of current loads. Robust performance and stability are guaranteed even in the presence of current measuring and modeling errors. This procedure preserves the advantages of current compensation and avoids the parameter tuning process. In the present paper the so-called ‘Short Wing-Cross Flow Current Model’ [2] is used. This current force model has the advantage of being semi-explicit, in the sense that it depends on only three experimental coefficients, obtained from simple rotating-arm experiments in a towing tank. Nevertheless, it is a common practice in Ocean Engineering to use the well-known ‘Hydrodynamic Derivative Models’, for low maneuvering speeds as, e.g., the ‘Takashina model’. Those models require a large number of towing tank tests, and are usually believed to represent current loads with very small errors. The present work shows simulations of the SMC technique (with the simpler Short Wing Current Model), controlling a tanker that is otherwise modeled with a ‘Hydrodynamic Derivative Model’. The results confirm that, for control purposes, the ‘Short Wing-Cross Flow Current Model’ current model can be used, since performance and stability requirements are preserved.