Session: 06-03-01 Fluid-Structure, Multi-body and Wave-body Interaction I

Paper Number: 123620

123620 - The Influence of Wave Diffraction on the Motion of a Crew Transfer Vessel Behind a Monopile 

The safety of offshore technicians during maintenance operations at offshore wind turbines (OWTs) is of high importance today and will gain even more importance with the rising number of wind farms around the globe, especially in the North Sea. For wind farms close to the shore, technician transport is performed using Crew Transfer Vessels (CTVs). During these operations, the transition from vessel to OWT is the most critical aspect regarding technician safety. With the trend of increasing OWT sizes to Offshore Megastructures, the diameter of monopile foundations increases steadily. This increase in diameter can result in a shift in wave regime: From a hydrodynamic transparent monopile to a hydrodynamic compact monopile. For the former, no wave field changes are expected, while wave diffraction effects should occur for the latter. These effects need to be considered during the technician transit (i.e., when CTV pushes against the monopile) as well as during the approach of the CTV.

In this experimental study, the incidence of wave diffraction behind monopile foundations and their influence on the motion response of a CTV are investigated. The model tests were conducted in the wave flume Schneiderberg (German: Wellenkanal Schneiderberg, WKS) in a scale of 1/40. The waterdepth was fixed to 40 m (all following quantities are given in prototype scale). Four sets of tests were conducted: (i) Wave-only tests without vessel or monopile to measure the undisturbed wave, (ii) Vessel-only tests without monopile to measure the reference motion response of the vessel, (iii) monopile-only tests without the vessel to measure the wave field behind the monopile, which will subsequently cause the motion response of the vessel behind the monopile. This is investigated in test set (iv): monopile-vessel tests to measure the motion response of the vessel behind the monopile.

A horizontal mooring system comprising four lines with low stiffness was used to ensure an undisturbed vessel response at wave frequency, preventing excessive drift motions. An infrared motion tracking system was used to measure the motion response of the vessel. Ultrasonic wave gauges measured the wave height at different locations in front and behind the monopile.

Monopiles with different diameters D of 6.4 m, 8 m and 10 m were tested, while the distance between vessel bow and monopile had three fixed values of 10 m, 18 m and 40 m as well. Regular waves with wave periods T between 5 s and 10 s were generated, while the wave height H was set to 1 m, 2 m, and 3 m.

The results show that the expected diffraction effects occur behind the monopiles, especially for short period waves and large diameter monopiles, leading to a large ratio of D/L > 0.2. These diffraction effects have a significant influence on the vessel’s motion response. Furthermore, hydrodynamic interactions (e.g., vessel’s radiated wave interacts with monopile’s diffracted wave) are observed.

Presenting Author: Jannik Meyer Leibniz University Hannover, Ludwig-Franzius-Institute

Presenting Author Biography: Jannik Meyer obtained his BSc in Civil and Environmental Engineering from University of Hannover in 2016 and his MSc in Hydraulic, Environmental and Coastal Engineering from University of Hannover in 2018. Afterwards, he started his PhD at the Ludwig-Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering at the University of Hannover. His research work focuses on floating structures, with applications in the fields of wave energy and offshore logistics (e.g., seakeeping of crew transfer vessels or crane vessels).

Authors:

Jannik Meyer Leibniz University Hannover, Ludwig-Franzius-Institute
Thilo Grotebrune Leibniz University Hannover, Ludwig-Franzius-Institute
Mareile Wynants Leibniz University Hannover, Ludwig-Franzius-Institute
Arndt Hildebrandt Leibniz University Hannover, Ludwig-Franzius-Institute
Torsten Schlurmann Leibniz University Hannover, Ludwig-Franzius-Institute