Active Crane Boom Stabilization by Slew Drive Actuation in Offshore Vessel Operations

Abstract

During offshore monopile installation, the crane boom must be lowered onto its boom-rest while the installation vessel remains afloat. This final landing operation is highly sensitive to wave- and wind-induced disturbances, which can excite oscillatory motion of the crane boom about the slewing axis. In severe cases, the disturbances may excite resonant behaviour, leading to uncontrolled boom oscillations. These uncontrolled oscillations compromise the reliability and efficiency of monopile installation, as well as the operational safety of vessel crew members. Boom landing has historically relied on slow manoeuvring and the skill of the crane operator. However, as offshore cranes continue to grow in scale, maintaining safe and reliable operation through manual control alone becomes increasingly demanding under harsh and unpredictable environmental conditions. This thesis investigates whether such oscillations can instead be actively mitigated using the existing slew drive infrastructure, without requiring additional hardware, structural modification or additional manual interference.

In this thesis, a control-oriented dynamic model of the coupled slew drive, slewing platform, and crane boom is developed using the Euler-Lagrange method, capturing the dominant structural flexibility of the boom while assuming small angular deflections. The model is extended with physically derived disturbance inputs representing first- and second-order wave-induced loading and wind loading, based on representative North Sea sea states. Based on this model, a sliding mode controller is designed, in which the sliding surface is formulated to mitigate both the oscillatory and biased components of the boom motion. The switching control component is derived using a physically motivated bound on the combined offshore disturbances, and the resulting design is validated through a Lyapunov-based stability analysis. The controller is implemented and tuned through a combination of theoretical control authority analysis and systematic simulation-based parameter identification. The resulting design is subsequently validated against a realistic lower-level slew drive motor model in Simulink, confirming that the demanded performance is physically possible within the slew drive motor’s bandwidth, speed and torque limitations.

The proposed sliding mode controller substantially reduces slewing-axis oscillations across all evaluated operating conditions. Under nominal offshore operating conditions, representative of typical sea states encountered during monopile installation, the controller reduces the peak boom tip displacement from 0.65 m in open loop to 0.13 m (80.0%). The RMS displacement is reduced from 0.219 m to 0.045 m (79.5%), while the steady-state bias displacement is reduced by 49.3%. Under harmonic excitation at the boom’s natural frequency, the controller achieves its strongest performance, reducing the peak displacement by 94.9%. In all evaluated scenarios, the slew drive motor tracking ratio remains above the required 95% threshold, confirming that the commanded motor speeds are physically realizable by the existing slew drive.

The controller is benchmarked against an existing pole placement controller developed in collaboration with Huisman Equipment. The comparison shows that the sliding mode controller achieves higher oscillation reduction across all evaluated scenarios and excitation frequencies. Compared with the pole placement controller, the sliding mode controller is especially effective at rejecting bias disturbances. However, the pole placement controller requires less control effort and has a more intuitive tuning process and controller structure. Frequency response and sensitivity analyses for variations in the model parameters show that the sliding mode controller is more robust to bounded disturbances and modelling uncertainties.

These results demonstrate that slewing-axis oscillations during crane boom landing can be substantially and reliably reduced through active slew drive compensation by sliding mode control, offering a viable path toward safer, more reliable, more efficient, and less weather-dependent offshore crane boom landing operations.