Monopiles Driven at Danemark’s Anholt Offshore Wind Farm

DONG Energy, which has erected half of the world’s major offshore wind farms, is currently building Denmark’s large scale offshore wind farm, which includes an area of 88 km2 between Grenaa and the island of Anholt. The Anholt windfarm will become Denmark’s largest offshore wind farm with an overall capacity of 400MW and will cover 4% of Denmark’s total power consumption.

THE PROJECT

Dong Energy wanted to test Vibrodriver’s technology as an alternative solution to impact hammers, for driving monopile foundations in offshore wind farm projects while reducing noise emissions under water.

The job required driving monopiles of conical shape with a bottom diameter exceeding 5 m and weighing 275 t. The complexity of this job with harsh offshore weather conditions and heavy piles, required a customised solution that PTC had the capability to provide in a very short delivery time.

The pile driving equipment proposed by PTC was a Trio of 2OOHD Vibrodrivers perfectly synchronized to get an eccentric moment of 600 m.kg. These Vibrodrivers were powered by 3 PTC power packs 1200C model, supplying a total power of 2418 kW. A PTC Vibmaster® monitoring system was used to display and registered important piling data, such as depth, vibration amplitude and power.

FIRST PHASE: ONSHORE TEST, SYNCHRONISATION IS THE KEY

The synchronisation of the 3 Vibrodrivers as well as the 3 power packs was a complex task. PTC studies in this matter, resulted in a patented synchronization technology based in PTC’s long time know how in mastering vibration and producing high-end power packs.

In order to adjust the synchronisation of the PTC equipment before taken it to the barge, the job site begun with a onshore test. In this initial phase taken place in the Aalborg harbour, the Trio of PTC 200HD drove into the ground a shorter test pile,

he Trio 3x2OOHD driving monopiles in the Anholt offshore wind farm, Danemark which was then extracted with the same equipment. PTC’s technological approach was confirmed and the equipment was then taken to the barge.

SECOND PHASE: OFFSHORE PILE DRIVING

The next phase of the project was offshore in the waters of Kattegat between the North and the Baltic seas, and just a few kilometers away from the Anholt island.

The PTC pile driving equipment was installed in the SVANEN barge owned by the company Ballast-Nedam. PTC service technicians qualified for offshore operations were on board as well, and assured the operations and the equipment installation, commissioning and support until the end of the jobsite. The driving of the 275 ton piles started as soon as the weather conditions were favourable.

The piles were driven to the stipulated depth in less than 30 min in a soil consisting of dense sand and compact silty sand. In addition of an excellent driving speed of the pile, PTC Vibrodrivers proved a significant environmental advantage, as these vibrators can drive the pile respecting the most recent regulations in noise emissions under water.

Dong Energy performed several sound measurements and confirmed its satisfaction in the PTC driving solution for offshore wind farms.