Sensitivity-Based Robust NMPC for Close-Proximity Offshore Wind Turbine Inspection with a Tilted Multirotor

TL;DR

This paper introduces a sensitivity-based robust NMPC method for tilted multirotors to ensure safe offshore wind turbine inspections despite uncertainties and disturbances.

Contribution

It develops an online constraint tightening approach using parametric sensitivities to enhance safety in complex, uncertain environments without changing the NMPC structure.

Findings

Eliminates clearance violations compared to nominal NMPC.

Handles wind gusts with stage-dependent additive margins.

Moderate increase in computational solve time.

Abstract

Close-proximity offshore wind turbine inspection requires strict clearance control around large cylindrical structures under wind and model mismatch. Nominal Nonlinear Model Predictive Control (NMPC) may violate safety constraints when mass, inertia, thrust effectiveness, drag, or wind conditions differ from nominal assumptions. We propose a sensitivity-based robust NMPC for a tilted multirotor that robustifies the tower-clearance constraint via online constraint tightening. First-order parametric state sensitivities provide a structured-uncertainty margin, while bounded gusts are handled by a stage-dependent additive margin. The formulation augments the nominal NMPC with sensitivity propagation and margin evaluation only, leaving the receding-horizon optimization structure unchanged. Monte-Carlo evaluation over 500 uncertainty realizations on a boundary-critical helical inspection trajectory shows that the proposed controller eliminates the clearance violations observed under nominal NMPC at the cost of a moderate increase in solve time.