Challenging, but not impossible

ALL IMAGES COURTESY OF IV-GROEP.

Engineering company Iv-Groep has a track record in developing ofshore constructions for the O & G industry and is also a frontrunner in the ofshore renewables market. The company, for example, designed the world’s frst HVDC (high voltage direct current) substation which was installed at the German BorWin 1 ofshore wind farm in 2010. Together with naval architect Nevesbu, a 50/50 joint venture they have with Damen Shipyards, the company designed one of the frst foating ofshore substation concepts.

Fedor van Veen is COO of Iv-Groep and Managing Director of the Ofshore & Energy division. Iv-Groep is a multi-discipline engineering company, operating in various markets. Ofshore Industry talks with him to learn more about the company and their foating concept. “We act as designer and consulting frm for all kinds of steel structures in the ofshore industry”, Mr Van Veen says. “In the past decades, many ofshore windfarms have been constructed for which we made numerous designs. A substation for an ofshore windfarm has technical similarities with oil & gas platforms that we have designed before, meaning we could use this experience for the renewables market. For ofshore wind, we already designed our frst substation in 2003 and since this frst one, we have built up a nice track record. Next to the Infrastructure division, Ofshore & Energy is Iv-Groep’s most important business unit. The revenues for Ofshore & Energy are to a large extent realised with detailed engineering for large EPC projects for ofshore development and procurement services, including the supply of topside equipment.” >>

Mr Van Veen continues, “For the upcoming years, we expect the market for foating ofshore windfarms to take of. This gives us an opportunity to further combine the knowledge we have built up in ofshore wind with the foating experience of our naval architect Nevesbu. One of the results of this is the study we executed for foating ofshore substations. Nevesbu are experts in the design of foating structures and for the O & G industry, for example, we have jointly executed several projects with them, such as the design of foating platforms and jack-up rigs, as well as FPSO conversions. Together, we took on the challenge of designing a HVAC and HVDC foating substation for the ofshore wind industry. A foating substation has the same functionality as a bottom fxed version, however the movements require various additional technical solutions. Floating solutions focus on, amongst other things, the lifespan of the dynamic power cables and the allowable motions of the transformers, rectifers, and other associated systems.”

“When looking at the lifespan of the cables’’, Mr Van Veen elaborates, “for a bottom fxed platform only the current and associated scour are relevant as cause for movement of the cables, which can lead to wear. For a foating substation, alongside the current, waves and wind also play a role, causing the platform to make horizontal and vertical movements that create accelerations on the equipment, and tension on cables. Therefore, it is important to predict the efect on cables, anchorage, and the platform’s movements. Failure of a transformer at sea should of course be avoided at all times. Operators typically require over 98% availability. This means that a platform can only be out of order for a few days per year. To achieve this for a foating platform, the design margins are very narrow. To guarantee that motions can be kept to a minimum, we

carried out motion analyses to calculate how diferent models behave at sea in strong winds with high waves. Our foating substation concept is designed in such a way that it can be applied in water depths in excess of 150m and with minimal motions in various sea conditions. When the platform is installed at sea, it will be held in position with the help of so-called ‘tendons’ that are vertically anchored to the seabed. Minimum vertical motions can then be realised, and it also reduces the loads on the cables suspended below the platform.”

“Designing a foating substation is a complicated job and the challenge lies in the fact that until now, no project is actual in operation. Also, there is no design code yet available for this kind of structure’’, Mr Van Veen explains. “DNV is currently working on such a code, and we are working with them on a Join Industry Project, which should result in a design framework for future foating substations.” Obviously, the frst ofshore foating substation requires initial investments and the question arises of who will be willing to take the frst step in this. “To be honest”, Mr Van Veen voices, “although we have already performed these studies, we do not yet know when the market will ask for it. Whenever there is a possibility at the location of the windfarm to place a bottom fxed platform, this will remain the proven solution. Floating an ofshore converter station weighing roughly 20,000t with minimal motions is a complex challenge, but our studies have shown that it is possible. Still, we expect that it will take at least fve years before the frst foating substation station will be constructed.” Despite this cautious prediction, the market for foating substation looks promising. Research from the market has identifed projects representing approximately 25GW of foating wind scheduled for commissioning worldwide up to 2035. This fgure indicates the high potential of foating wind and the opportunities it ofers for the innovative foating substation concept.