Operability Study Monopile Upending Smart Tool

Abstract

The installation of large diameter monopiles for offshore wind turbines presents new challenges as turbine sizes and water depths increase. Traditional deck-based upending methods are constrained by vessel capacity and workability. Seaway7’s Monopile Upending Smart Tool (MUST) enables suspended upending using crane based rigging and grommet connections, offering a more flexible alternative. While time-domain simulations have been recommended in prior studies, a fully coupled, continuous implementation has not yet been applied to this operation.

This thesis investigates how hydrodynamic effects influence the upending phase of a monopile suspended from a crane vessel. The increasing added mass due to larger submergence of the monopile generally increases the natural period of the pendulum oscillation. Partial submergence of the monopile causes intermittent slack in the upending lines, resulting in snap loads that are considered unacceptable for safe operation. For head seas conditions, the 60 degree upending angle was identified as the critical upending angle and governing configuration for the operation, due to the likelihood of snap loads in the upending line. However, the results also show that proposed mitigation measures, including damping strategies and monopile design, can significantly improve operability at this upending angle and thereby improve the overall feasibility of the operation.

A coupled time domain model was developed in OrcaFlex, including the vessel, crane, rigging, and a 125 m monopile. Hydrodynamic forces, including wave excitation, added mass, and damping, were applied based on site-specific sea states from a U.S. offshore wind location.

A central contribution is the development of a continuous upending simulation, capturing the full 0° through 90° upending motion in one simulation. This allows for dynamic updating of hydrodynamic properties and monopile orientation throughout the operation. Compared to quasi static analysis, the continuous approach avoids oscillation build up and tends to produce less conservative operability estimates.

The developed model offers a practical foundation for design optimisation and for assessing whether a continuous or quasi static simulation is more appropriate for a given analysis.