Offshore renewable energy facilities have become increasingly important in the last decade and training for the construction and maintenance of these facilities has developed in parallel. In this second part of a two-part story, SCT’s Mario Pierobon reports on how differing offshore wind farm maintenance strategies may influence training design.

Maintenance Logistics

A very critical task in offshore wind energy is maintenance logistics and supply chain management and these need to be considered as part of wind farm maintenance.

A failure to deliver proper maintenance services may adversely affect the wind farm availability and thereby reducing power output as well as profitability, according to Shafiee[i]. “‘Maintenance logistics’ is known as an important competitive factor in the offshore wind energy industry having a significant impact on profitability of wind projects,” he says.

Maintenance logistics must be designed and developed considering the life cycle of the project, as the service lifetime is relatively long (20-40 years), according to Shafiee. He highlights a framework based on a three-echelon architecture: strategic/tactical/operational model. The model is related to three echelons of decision-making: Strategic (long-term), tactical (medium-term) and operational (short-term).

The strategic echelon deals with decisions that have long-lasting effect (i.e., over whole life cycle) on the operation and maintenance of the offshore wind farms. The tactical echelon typically includes decisions that are updated anytime during the overhaul interval. The operational echelon refers to day-to-day decisions within offshore wind farms, according to Shafiee.

Major Components Replacement

From a maintenance point of view, when it comes to the replacement of major components on floating wind farms, there are four different possible strategies that can be considered, according to World Forum Offshore Wind (WFO)[ii].

The first strategy is floating-to-floating, and this scenario relates to offshore overhaul, i.e., the replacement is going to be entirely carried out onsite, according to WFO. “A heavy lift vessel working at the wind farm site is equipped with cranage that can transfer components to and from the FOWT (floating offshore wind turbine) directly,” says WFO. “This requires an important and prompt evolution of the maritime industry to speed up with new design in order to provide vessels and solutions capable of carrying out the replacement operations in a floating-to-floating setup. These need to be efficient, quick and operating directly by the floating wind unit.”

The second strategy is tow-to-port, and this scenario relates to onshore (dry-dock or inshore) overhaul, meaning that the replacement is going to be executed onshore (offsite), after having carried out a reversed-installation process and towed the unit to the harbour, affirms WFO. “The FOWT is towed by readily available and relatively inexpensive vessels and then repaired with cranage in onshore-type weather,” says WFO. “This procedure might be complex, lengthy and costly, both in terms of operations as well as downtime, and also requires MCR capabilities at the O&M sites (which is not always the case). Nevertheless, as long as the floating-to-floating solution cannot yet rely on a newly developed maritime/vessel industry, this is currently the main option considered in the floating wind business cases.”

The third strategy is tow-to-shore, a hybrid solution combining the reverse-installation procedure of the tow-to-port strategy although the unit is towed closer to shore where a fixed jack-up vessel is installed (i.e. vessel with fixed support structures), according to WFO. “This strategy can be typically considered in case of suitable project conditions (geography, floater design) and/or synergies with other bottom-fixed assets. This strategy may be more costly due to the rental of the jack-up vessel in comparison to an onshore crane,” says WFO.

The fourth strategy has to do with the use of self-hoisting equipment, and it has the objective of having the replacement of major components done by new self-hosting or climbing cranes using support barge-type (or similar) at the site or involving expensive tow-to-port operations, explains WFO. “The adoption of different types of crane technology/vessel combinations are now being discussed as the technological development progresses and new companies approach the market. This opens up discussion around lifting capacity, operations time at sea, responsibilities among Original Equipment Manufacturers (OEMs), etc.”

Onsite Repairs

Onsite repair is related to the replacement of a major component at the project site with the help of specialised vessels and cranage. It is used when floating-to-floating and self-hoisting equipment strategies are pursued, according to WFO.

In relation to the floating-to-floating approach, as for bottom-fixed offshore wind farms, onsite solutions can be used for minor correctives, explains WFO. “However, as the floating wind array ages, the needs for maintenance increase. For this reason, tow-to-port may not be a feasible strategy when dealing with major correctives (e.g., blade exchanges) or multiple, large FOWTs,” says WFO. “Rather, supporting vessels that can perform maintenance onsite are considered more suitable though not yet fully developed and commercialised.”

In case of use of self-hoisting equipment, cranes are already used for onshore projects and multiple members of the forum are investigating their applications for offshore wind, explains WFO. “However, the main concern with self-hoisting cranes for floating offshore wind is whether they are able to lift major components at the height of the turbine and distance from the tower base (which is increased due to the floater),” says WFO.

There are two layers of complexity to the self-hoisting equipment. The first is the lift of the major component from the self-hoisting crane assuming that the crane is already embedded in the turbine; the second level of complexity occurs if the crane is not embedded, therefore necessitating an additional operation to equip the turbine with the crane. In both cases, the technology is currently not proven for dynamic lifts and remains an interesting development, says WFO.

Outsourcing Repair Services

According to Shafiee, there is a need for significant investment to keep the wind farm maintenance in-house as it is very expensive and complex to build new infrastructures, train the maintenance engineers, and equip specialised facilities. “For this reason, in recent years there has been a growing trend towards outsourcing the maintenance activities to external service providers with the goal of getting higher quality at faster, safer and lower costs,” he affirms.

In this article we have covered the specific maintenance and logistics considerations that should be accounted for in training for the maintenance of offshore wind farms. Together with the design considerations that were covered in the first part of this story we have given an overview of considerations that lead this fast-growing and ever more relevant domain of energy production.

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