An investigation into fatigue damage of monopiles during transport

Abstract

Monopiles are the predominant support structure used in offshore wind farms. Since the early 2000s, the exponential growth in turbine power output has necessitated corresponding increases in monopile diameter, length, and weight. Designing these structures for offshore environments involves accounting for dynamic loading cycles that induce significant stress ranges and pose risks of fatigue damage.

While fatigue damage is routinely assessed for operational and installation phases, the transport phase remains underexplored despite its potential contribution to cumulative fatigue. This thesis investigates fatigue damage incurred during monopile transport by comparing three approaches: conservative estimates based on DNV codes, vessel motion simulations, and recorded acceleration data from a transoceanic voyage. A sensor placed at the seafastening cradle captured six degrees of freedom (DOF) accelerations, which were filtered and processed to estimate stress ranges and fatigue cycles using finite element modelling (FEM). Results show that recorded accelerations were 70–80% lower than those predicted by vessel motion calculations, and that phase differences between DOFs significantly influence stress response. A secondary case study during installation further supports the conclusion that DNV-based fatigue estimates are overly conservative. These findings suggest that incorporating recorded data and phase-aware modelling can improve fatigue predictions and reduce overdesign in monopile transport systems.