Currently being tested

Monday, 8 June, 2015 - 12:30
Luxury out at sea or compensation for a strenuous job. C-Bed floating hotels are on par with high-quality hotels on land. (Photo: C-Bed)
Luxury out at sea or compensation for a strenuous job. C-Bed floating hotels are on par with high-quality hotels on land. (Photo: C-Bed)

Maintenance costs rank among the greatest financial risks for an offshore wind farm. Of course, the sector does not yet have long-term experience in this area. Various concepts are now ­being tested in the field.

Servicing an offshore wind farm is costly. The German Wind Energy Association (BWE) estimates the share of servicing to amount to as much as a quarter of the ­total investment costs. However, the good news is that out of all the components and services, maintenance is one of the easiest to optimize – a ­coherent concept alone solves many problems. In the first place, distance to shore plays a key role. The relationship can be expressed as a general principle: the farther from shore, the costlier the maintenance – although this has not yet been borne out in ­practice. However, ­various concepts make maintenance of ­offshore ­turbines a calculable endeavour.

Land-based maintenance ­operations

Onshore-based concepts are suitable for nearshore wind farms within about 30 km. For nearshore wind farms – as implemented in the early Scandinavian wind farms in the Baltic Sea – servicing operations always start on land. With this type of transfer, technicians embark in the supply ports and are taken to the offshore wind farm by a crew transfer vessel (CTV). The vessels carry smaller wear parts and spare parts along with them. Their transportation volume comprises up to a dozen maintenance technicians including equipment and material. This ship transfer may also be done with larger vessels, which can carry larger parts and are faster. However, the most ­crucial advantage is that such ships are not as restricted in terms of weather because they can travel in higher swells than smaller CTVs when transferring technicians. While smaller vessels can only cope with a significant wave height of up to 1.5 m, the limit for the bigger ones is about 2 m.  

A maximum distance to shore of 30 km is not set in stone. A maintenance concept relies on many factors, Dong Energy’s Iris Franco Fratini emphasises. She makes the astonishing statement that Dong services all its ­German wind farms from an on-land base. “We always start on shore,” she says. This comes as a surprise because the most distant Dong wind farm is 50 km from shore, which is almost double the 30 km that is generally the limit in the industry. Franco Fratini does not see why this should be a problem: “Our personnel set off from the port of Norddeich in East Frisia to our offshore farms using a specialist vessel or a helicopter. The vessel was especially designed to serve our wind farms. It is a catamaran that is more stable than a vessel with a conventional hull.”

For Dong, an onshore-based maintenance concept ­includes a clearly defined personnel policy, Franco ­Fratini explains, “We only employ people from the ­region and those who move here because of that.” The reasoning for this personnel policy is that with weather conditions that can only be forecast to a limited extent, Dong’s ­technicians can wait for better weather at home – and can quickly go from being on stand-by to working at their site.

However, Dong’s onshore-based maintenance ­concept is not a dogma; in the case of the German ­offshore wind farms, practical issues are certainly taken into account, according to Franco Fratini. There will be different concepts for really long distances, such as 80 to 100  m off the coast.”

Living on board

An alternative to land-based maintenance concepts is floating hotels and accommodation platforms as seen in the offshore oil and gas industry. They are...

You can read the complete article written by Jörn Iken
in OFFSHORE WIND INDUSTRY 2/2015.

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