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Originally founded in 1875, G&G International is a leading Belgian supplier of complex high-tech welded steel structures including pressure vessels for the oil & gas and chemical industries. Last year’s contract for the supply of 30 monopile transition pieces (TPs) for Germany's Riffgat project marked a breakthrough for the company in the offshore wind market.

At G&G’s Willebroek facilities, “uåX˜äŠÊ˜·³Ç Offshore spoke with chief executive Patrick Van Malis. He explained that initial offshore wind contracts, dating back to 2008, typically involved single-piece and small series of specific components for offshore applications. G&G has now entered a new phase, with production for the Riffgat order in its final stages and a second contract for 72 TPs for the 216MW Belgian Northwind offshore project secured in May.

"On both contracts we cooperate with Idesa of Spain, which manufactured the piles for Riffgat," said Van Melis. "Wind power is one of five business divisions in our group and already responsible for 75% of G&G's total turnover." The company is actively pursuing new solutions, such as optimised jacket foundations. Van Melis adds that these "have already been offered to some clients in the market".

Monopile foundations consist of a pile rammed into the seabed and a TP that loosely slides over it. The circular gap in between is filled with fast-curing concrete called grout. A TP’s main function is to compensate for any slight inclination of the pile during ramming. But TPs also sport a service platform, climbing ladders and J-tubes for the electricity transport cables. Traditional monopile foundations are made of a cylindrical pile and matching cylindrical TP.

Major innovations

The Riffgat monopile foundations contain two structural innovations that are new to the offshore wind industry. The first innovation in the TP consists of a conical lower section that widens towards the bottom, and a top section that narrows conically like a flattened pencil point.

Inside a TP being manufactured, I could clearly see several rows of so-called shear keys, rectangular solid bars, arranged in circles and welded to the inner wall. The top section is fitted with similar circular rows of shear keys. After assembly, and once the TP has been aligned horizontally, the circular gap in between the two components is filled with grout, which provides the necessary axial and radial strength.

The new Riffgat monopile foundation design approach comes in response to a series of pile-slipping incidents caused by imperfect grouted connections between the pile and the TP that did not hold during operation. According to independent experts, offshore wind farms in the UK, Denmark and the Netherlands have witnessed serious TP vertical slippage of up to 40mm since late 2009. The new design promises to fully remedy this problem.

The second major innovation is incorporating the so-called E-module inside the TP rather than mounting it atop. The application of E-modules has become rather common, and for Riffgat it consists of a three-storey platform structure containing the transformer, switchgear and power electronic inverter.

With the Siemens' SWT-3.6-120 turbines applied to the E-module, only the rectifier that converts the generator's AC power into DC power is located inside the nacelle. The grid-side DC-AC inverter that produces grid-compliant 50Hz AC power is located in the E-module. If the rectifier and inverter consist of a single integrated unit, it is called a power electronic converter.

The mounting of an E-module atop a TP and inside the tower foot involves extra offshore operations. Also, the lowering of the tower over the E-module poses additional risks of mechanical and other system damages. The Riffgat solution, on the other hand, requires the TP to be transported in the vertical position and perhaps greater care during onshore and offshore handling and manoeuvring.

During a guided tour of the facilities, Van Melis outlines the challenges linked to entering the offshore wind business: "Pressure vessels are highly specialised and often single-piece products. We had to convince potential customers that we were also capable of series manufacturing offshore foundations in an efficient and cost-effective manner."

Steel expertise

The company processes large quantities of steel sheets, both for internal use and through a steel services division that serves third-party clients. Steel sheets enter one of the big halls as a raw material. Once their properties have been verified, the sheets are cut into the right sizes by either plasma cutting or flame torch, followed by semi-automated preparation for sheet welding.

For rolling circular steel components such as TPs, a single sheet length is used. After the rolling process is completed, the ends are mechanically closed up to a predetermined welding gap, which is then welded internally and externally with submerged arc welding. A large-diameter circle usually requires the joining of several shorter steel sections.

"We have put a lot of effort into welding seam preparation to minimise welding volume and eliminate the need for intermediate grinding when welding thick sheets from two sides," points out Van Melis. "These combined measures reduce welding time and metal deformation due to temperature-related shrinkage, ensuring that the end product is flat and straight." Compared with the alternative process of welding individual pre-rolled sections into full circles, he adds, this process boosts both efficiency and overall product quality.

G&G has invested €5m into specialised equipment, largely designed and built in house, for the transport, handling and ‘cold rolling’ of steel sheets. A sheet-rolling machine can handle sheets of maximum four metres wide and up to 175mm thickness, whereas for most shipyards a maximum of 30–50mm is common. At the plant, several 90mm and 100mm thick sheets were lined up for rolling and assembly into four giant 900-tonne legs for a new jack-up vessel. Each leg will be 86m long and 4.5m in diameter.

 "The Riffgat foundations have been designed for an operational lifetime of at least 23 years," says Van Melis. This requires both strong long-lasting marine coating and the utmost care during component handling inside the factory." Any mechanical surface damage like grooves and notches, he adds, would generate highly concentrated stress at specific points, contributing to damage deriving from premature materials fatigue and ultimately reducing service life.

A large new hall for steel grid blasting and industrial coating was nearing completion when we visited the plant. Near the deep-water quay several bright yellow-coated TPs were lined up, in a vertical position, for loading onto coasters. These transport the pieces to the Dutch seaport of Vlissingen for direct transfer to the huge floating installation vessel Oleg Strashnov, which will take them to the Riffgat offshore building site.