The effects of dynamic ship deflections on friction in monopile seafastening systems

Abstract

The offshore wind market is expanding rapidly due to the growing demand for offshore wind energy. This drives the development of larger, more advanced wind turbines and increases the diameter of monopiles. Transporting these massive monopiles presents challenges, particularly in ensuring secure seafastening. Traditional seafastening systems rely on mechanical support to prevent motion, such as saddles for the transverse direction and lashing wires for the longitudinal direction. With the increasing weight of monopiles, these solutions are becoming increasingly impractical, heavy and costly.

During monopile transportation, waves cause the vessel to move and deflect, with the saddles moving accordingly. The monopile, which has separate stiffness properties, experiences different deflections and displacements. Due to these different deflections and displacements, the vertical forces exerted by the monopile on the individual saddles are not distributed equally. In the least favourable conditions, this can result in loss of contact, also called intermittent contact. Longitudinal vessel accelerations generate forces in the monopile’s longitudinal direction, where friction between the monopile and saddles opposes these forces. However, as friction depends on the contact pressure, any reduction of the vertical force, and so the contact force, affects the friction force.

Seafastening arrangements are designed according to DNV-ST-N001 Marine Operations and Marine Warranty standards. Seafastening requirements are based on seafastening design loads calculated for each shipment and cargo item. DNV-ST-N001 set a minimum seafastening capacity based on cargo weight, not considering friction. The latest update (December 2023) allows cargo transport without seafastening if the friction capacity is at least twice the seafastening design loads. Otherwise, seafastening capacity defaults to a weight-based function, creating a gap between the minimum seafastening capacity and calculated seafastening design loads.

Roll Group’s current seafastening designs do not incorporate friction to prevent longitudinal motion, leading to conservative seafastening systems with high equipment- and installation costs. Therefore, there is growing interest within Roll Group in whether accounting for friction between the monopile and the saddles could reduce the required seafastening arrangement, potentially leading to more efficient and cost-effective designs. This knowledge gap leads to the following research question:

”How does friction between a monopile and its saddles affect the seafastening arrangement of a monopile?”

This research aims to determine the friction resistance between the monopile and the saddles when the vessel is exposed to waves, including cases with intermittent contact of the saddles. Based on this information, the seafastening arrangement can be defined when friction is included in the seafastening calculations.

For this research, a coupled fluid-ship model from another research is revised and used to calculate the forces between the saddles and the monopile. This calculation is based on the vertical displacements of the vessel and monopile for all wave heights that can occur during monopile transport. As a result, the loss-of-contact cases were defined. Since this coupled fluid-ship model only focuses on the vertical displacements of the monopile, a longitudinal monopile model is developed. The longitudinal monopile model focuses on the relative horizontal displacement of the monopile caused by heave and pitch motions. This model is a mass-spring system where the monopile represents the mass, behaving as a rigid body to prevent axial vibrations.

An in-depth analysis of the longitudinal monopile model is done for a case excluding friction and a case including friction. The analysis of the longitudinal displacement of the monopile with lashing wires, but without considering friction, resulted in a seafastening arrangement consisting of sixteen lashing wires to prevent all longitudinal motions of the monopile for all wave heights. When friction is considered, the analysis shows that the total friction capacity remains sufficient for all wave heights to prevent the longitudinal motion of the monopile without applying lashing wires.

In this research case, the total friction capacity of the saddles is at least twice the design loads during the whole wave cycle for all wave heights. Based on this, DNV can agree that meeting only the minimum seafastening capacity requirements is acceptable. The last step of executing a friction-based seafastened monopile transport is getting permission from the marine warranty surveyor and the captain.