IN-DEPTH: Siemens floats novel idea

Hoisting the 9,300-tonne DolWin Alpha converter station onto its offshore foundation was a high-price affair.

Day rates for the vessel brought in for the job — Heerema’s gargantuan Thialf, a titan from the offshore oil industry capable of lifting 12,000 tonnes — can run as high as €1m ($1.35m) a day.

But it was a cost that had to be borne by transmission system operator TenneT to catch up on its behind-schedule programme to build Germany’s North Sea offshore grid network — budgeted at €1bn per GW.

Tens of converter stations like the 800MW DolWin Alpha — which will soon channel power from a cluster of three German offshore projects known as DolWin1 — are on the slate for the vast wind-power zones off the UK and Germany, yet there are very few heavy-lift vessels up to the task.

The desire to drive the cost of energy under the magic €100/MWh mark has the European offshore wind industry hunting for alternatives to these oversized vessels.

The offshore oil industry has been a source of inspiration. A first self-installing substation platform — in which the unit is towed out to site and its built-in legs are jacked down to the seabed — was installed this year by Keppel Verolme at Germany’s Global Tech 1; and the Fukushima Forward project off Japan recently floated out a substation on a semisubmersible hull.

Now Siemens is exploring another installation concept from the oil sector: a float-over barge that derives from designs used for a decade in the generally calm waters of Southeast Asia.

“We have very tough challenges ahead to build the [UK’s] Round 3 wind farms and in other regions off Europe — the hostile environment, how far offshore they are,” says Steve Jones, director of engineering and technology at Siemens UK Transmission & Distribution. “The float-over barge is an idea we are pursuing to lengthen the weather windows [periods during which construction offshore is safe], to de-risk the operation [of substation installation] and lower costs.

“Winds, waves and tidal speeds all need to be carefully considered — while deciding how to manoeuvre topsides that [for an HVDC platform] could weigh 10,000-12,000 tonnes. We think the float-over barge might be one answer.”

The float-over technique is appealingly low-tech and low-cost. A topsides is strapped to a giant seagoing barge that is towed offshore and slotted between the trussed steel legs of an installed jacket foundation. The barge then drops anchor, and by shifting ballast transfers the load onto the jacket legs, at which point the barge is pulled out and towed back to port.

But carrying out this operation in bucking North Sea waves has never been done before and remains high-risk.

In April, Siemens agreed a deal with Seaway Heavy Lifting to float-over the 14,000-tonne topsides for its SylWin Alpha transmission hub, using one of Dockwise’s vessels from the oil industry.

And now the German giant has just finished first tests of a lead-weighted 1:40 scale model of a float-over barge, designed with British industrial marine transport outfit ALE, at a wave tank at Newcastle University, northeast England. 

In simulated North Sea waves equivalent to 15 metres high, the model performed “admirably” during mating of jacket and topsides, according to motion analyses.

“The trickiest part is the lift-off — just as the weight of the topsides comes off the barge. That is where the potential for impact and damage to jacket, topside and even barge is greatest,” states Peter Bowes, facilities manager at the university’s School of Marine Science and Technology, who oversaw the model tests.

“We have collected a huge amount of information from these tests that will be fed back to Siemens and ALE... to cross-check the [computer] simulation against the reality, to see if this concept could indeed open the window of operation wider in harsh environments such as the North Sea.

“Our analyses will allow us to determine if this technique makes sense. And then we might need another round of tests before we know if it can be commercialised.”

The market scope in the European offshore wind sector for float-over barges is “somewhat muddy”, says Siemens’ head of grid-access solutions, Steve Aughton, but for the UK’s Round 3 alone “well in excess of a dozen” HVDC platforms could be needed.

“There are merits and disadvantages to each [of the various installation methods being piloted],” he notes. “We’re not ruling anything out but we like what we have seen in these tests.”

HVDC converter stations are as big as office blocks — DolWin Alpha is ten storeys tall, 62 metres long and 42 metres wide; the concept being developed by Siemens would measure 110 metres long, 47 metres wide and six storeys high.

“With an offshore converter platform you are looking at supporting a very large plant area on a jacket that has to be fashioned so that a barge can pull up into it,” notes Siemens offshore platform consultant Bill Fahy.

Siemens is in discussions with two shipbuilders to construct float-over barges for future European offshore transmission platforms.

There are also suitable barges in ports around the world that could be refitted to handle the HVDC stations, but questions remain over “whether there are enough of them, all-in”, says Fahy, given that many are tied up on long-term charters in Southeast Asia.

Jones adds: “The industry is at a stage where there is a concerted effort being made to get costs down and this is an offshore concept that should get costs down.

“The offshore oil business can teach us a great deal, but ultimately, offshore wind power is a whole new proposition.”