Robust Dynamic Positioning of offshore vessels using mixed-μ synthesis modeling, design, and practice

This paper describes a procedure to design robust controllers for Dynamic Positioning (DP) of ships and offshore rigs subjected to the influence of sea waves, currents, and wind loads using H∞ and mixed-μ techniques. The proposed method will increase operational weather window and robustness of the DP vessel and associated DP system. To this effect, practical assumptions are exploited in order to obtain a linear design model with parametric uncertainties describing the dynamics of the vessel. Appropriate frequency weighting functions are selected to capture the required performance specifications at the controller design phase. The proposed model and weighting functions are then used to design robust controllers. The problem of wave filtering is also addressed during the process of modelling and controller design. The key contribution of the paper is threefold: (i) it affords system designers a new method to efficiently obtain linearized design models that fit naturally in the framework of H∞ control theory, (ii) it describes, in a systematic manner, the different steps involved in the controller design process for DP systems operating under different sea conditions, and (iii) it contains the details of simulations and results of experimental model tests in a towing tank equipped with a hydraulic wave maker.