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Offshore Wind Journal

Ability to submerge will give installation ship an edge over jack-ups

Thu 21 Feb 2019 by David Foxwell

Ability to submerge will give installation ship an edge over jack-ups
An artist’s impression of OHT’s semi-submersible installation vessel installing monopile foundations

A shipowner used to the operation of semi-submersible vessels and a naval architect experienced in the design of heavy lift ships have come together to build a revolutionary new design that they believe will outperform existing offshore wind installation vessels.

To-date, jack-up vessels have predominated in the offshore wind installation vessel market, but a new ship being built for Offshore Heavy Transport (OHT) could see that change. When weather conditions dictate, the ship in question can submerge, and will be able to continue working, installing next generation, ultra-large jacket foundations and monopiles in conditions that would defeat a conventional design.

As Ulstein Design & Solutions project manager and lead naval architect Dolf Manschot for OHT’s Alfa Lift project told the 2019 OWJ conference in London in February, jack-ups have proved to be suitable for foundation and turbine installation and are workable in high sea states but are relatively slow because they need to jack up and down at each location at which they install a turbine or foundation.  

“Jacking up and down offshore and in port is time consuming,” said Mr Manschot, “and the jacking process is sensitive to seabed conditions. Jack-ups also have a limited payload capacity. Compared with jack-ups, floating installation vessels are primarily suitable for foundation installation, but they are sensitive to higher sea states.”

But compared with a jack-up and a floating vessel, an installation vessel that can submerge like a semi-submersible heavy lift ship of the type that OHT is used to operating has a number of important advantages: being able to submerge reduces motions, increasing workability in installation mode. Submerging also provides the vessel with a secondary role as a heavy transport vessel capable of transporting heavy modules, jack-ups and other cargoes. 

“The key to the concept is the fact that a submersible vessel has better motion characteristics when submerged than a typical monohull vessel” Mr Manschot said. “What is more, when the vessel is submerged the cargo doesn’t have to pass through the splash zone during lifting operations, which has a positive influence on the crane dynamics. ”

He explained that the dynamically-positioned, 48,000 dwt vessel has a 3,000-tonne lifting capacity main crane from Liebherr that matches the future requirements of the offshore wind industry, allowing the OHT Alfa Lift vessel to transport and install up to ten 1,500-tonne ultra-large jacket foundations or 11  2,000-tonne XXL monopiles, plus transition pieces for next-generation wind turbines. 

It is designed to install components such as the these, without submerging, in significant wave heights of up to 2.0 m (Hs 2.0 m). If conditions become more challenging, the vessel can submerge and continue to work in Hs 2.5 m. 

“Having submerged, the Alfa Lift vessel remains in that state until all foundation installation work is finished, remaining submerged and moving in DP mode to the next foundation,” Mr Manschot explained. “The vessel only comes back to the surface if the weather picks up significantly, or if operations have been completed and a new batch of foundations needs to be loaded in port. You gain a lot of time whilst also getting a very stable platform,” he said. 

The new vessel has a length overall of 216.3 m and beam of 56 m. It has a submerged draft of 25.6 m and a service speed of 13 knots. The DNV GL-classed dynamic positioning class 2 vessel will have a deck strength of 30 tonnes/m2 and accommodation for 100 people. The first vessel is being constructed by China Merchant Heavy Industry in China and will be available for construction and installation activities from early 2021. 

 

 

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