Control Of Secondary Regulated Offshore Crane In The Splash Zone

In the offshore lift operations,the water entry phase during which the payload transits from air to water is referred to as ‘lowering through the splash zone'.In the splash zone,the payload is subject to potentially large and rapidly changing hydrodynamic forces under the influence of waves,which may cause damage to the load and can lead to large variations in wire tension or a wire break.To ensure the safety of lift operation in the splash zone,a hydraulic crane driven by a secondary controlled unit is investigated,and the impedance control is introduced to design the controller for the involved crane.The main contributions are summarized as follows:Firstly,the main component of the crane system is introduced.The detailed models of the crane system and the offshore operating conditions are established.Based on these models,analysis of the water entry phase of the payload is conducted,from which the design requirement of the controller is deduced.Secondly,indirect adaptive robust control method is proposed to control the angle of the secondary control unit.Adaptive backstepping method is utilized to design strong robust controller to ensure stability in the presence of external disturbances and parametric uncertainty,which guarantee the separation of controller and identifier.And the ‘explosion of terms' inherent in the backstepping design procedure is avoided by the command filters.Meanwhile,a parameter model is derived from the original models by using low pass filter,and a robust identification law is proposed to estimate unknown parameters under the relaxed excitation conditions.The stability of the close loop systems is guaranteed by Lyapunov stability theory.Simulation results show that the indirect adaptive robust control method can ensure smaller tracking error and accurate parameter estimation.Finally,impedance controller is proposed to assure the safety of lift operations in water entry phase.A cascade control structure is constructed.In the outer loop,an extended state observer is used to estimate the unmeasured payload states.Then,an impedance controller is designed to realize trajectory tracking and impedance regulation of the payload.The desired angle of the secondary control unit is generated from the outer loop,and the aforementioned indirect adaptive robust control method is designed in the inner loop to maximize the achievable tracking performance of the secondary control unit.The proposed impedance controller can reduce the variation in the hydrodynamic forces at the beginning of the water entry phase and ensure accurate trajectory tracking after the payload is completely submerged.Comparative simulations between the impedance control method and the parallel force/position control method are conducted.Simulation results verify the effectiveness of the proposed control method.