Friction Behaviour of Synthetic Slings in Offshore Monopile Lifting

Abstract

The installation of offshore wind farms increasingly relies on synthetic slings for monopile lifting, as these provide operational advantages over traditional steel rigging. However, the safety of horizontal monopile lifts depends critically on the frictional behaviour at the monopile-sling interface, which is often simplified or conservatively assumed in practice. Overestimating friction can lead to unsafe designs, while underestimation reduces operational efficiency. This thesis investigates the friction behaviour of high-modulus polyethylene (HMPE) slings in contact with coated and uncoated monopile surfaces, aiming to improve the reliability of lift stability assessments.

A systematic experimental campaign was carried out using a custom test setup to quantify static friction under varying conditions, including fibre set, fabric structure, consolidation time, rate of loading, lubrication, temperature, and contact pressure. The results confirm static friction decreases and appears to approach a limiting value as the normal force increases. Fibre set was found to consistently reduce static friction, representing the conservative case for safety assessment, while lubrication and temperature showed interface-dependent effects. Distinct frictional responses were identified for coated versus uncoated steel, underscoring the need to treat these cases separately.

From the experimental tests, load-dependent relations were derived based on the measured data. For each interface, static coefficients of friction were obtained from the COF-contact pressure relation and empirical constants a and n were determined from the friction force-normal force relation (F=aNⁿ). These experimentally derived parameters were subsequently implemented as input for the monopile lifting stability assessment.

Three analytical approaches were compared: a baseline approach (A), based on current industry guidance, assumes uniform friction and load distribution, which simplifies the calculation but can overestimate lift stability. Approach B, which includes capstan effects, provides a more realistic representation by accounting for friction along the sling contact arc. This results in a reduction of the overall normal load and consequently a more conservative stability prediction. Approach C, includes capstan effects with non-linear friction, further refining the behaviour by making the normal force friction dependent along the contact arc. This produces the lowest transverse friction resistance for the bottom interface and a similar resistance for the top interface compared to approach B, offering the most conservative and reliable stability prediction for design applications.This work contributes to closing the gap between friction theory, experimental data, and practical offshore lifting assessments. It provides new insights into textile friction under realistic offshore conditions and demonstrates how including load-dependent friction can enhance the safety assessment of monopile lifting operations.