Joint estimation of vessel position and mooring stiffness during offshore crane operations

Offshore heavy lift vessels are designed for transporting, installing and removing offshore facilities. Previous studies have shown that the time-varying forces acting on the main crane may result in an unstable Dynamic Positioning (DP) control system, which adversely affects the safety of the heavy lifting operation. Solutions based on force feedforward control thus has been proposed, i.e., the low-frequency horizontal component of the crane force is forwarded to the DP controller or observer. Accurate estimation of the horizontal component of the crane force is essential for the feedforward solution, but it is extremely difficult to obtain it from a measurement due to its time-varying nature and the non-linear system dynamics stemming from the ship-environment interaction. Therefore, in this work, we focus on the estimation of the low-frequency crane force in the horizontal direction during loading and offloading stages of a heavy lift operation in various environmental conditions. To achieve this goal, a novel time-varying mooring stiffness term is introduced, which is included in the nonlinear passive observer of the vessel and used in a joint parameter-state estimator to estimate the horizontal crane force and vessel states. Afterward, simulations are performed to test the estimator in various environmental and loading conditions. The results indicate that an accurate estimation of the low-frequency horizontal component of the crane force can be achieved by measuring the position of the ship and the tension in the wires even in the presence of large parameter uncertainty.