The growing usage of ocean facilities and land constraints have necessitated transfers of equipment and cargos to and from support vessels, complex lifts and other stevedoring operations in open sea conditions. Recently, there has been increasing research interests in three-dimensional control and automation of offshore cranes. Suspended cargos in a ship-mounted crane system are caused to swing due to the vibratory motion of the ship induced by ocean waves, which can lead to collision between cargos and deck. Therefore, it is crucial for offshore crane systems to satisfy rigorous requirements in terms of safety and efficiency. This paper presents the modelling and control development for three-dimensional offshore boom cranes. A second-order sliding mode control law is proposed for trajectory tracking and anti-sway control, making use of its capability of actuator chattering alleviation while achieving high tracking performance and preserving strong robustness. The asymptotic stability of the closed-loop system is guaranteed in the Lyapunov sense. Simulation results indicate that the proposed controller can significantly reduce the effect of disturbances coming from gusty waves and other dynamic loadings.
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