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Just as offshore wind installation starts to accelerate, there is a worry that this potential bottleneck could drive up the costs of offshore wind operations and maintenance (O&M).

ROV services company SeaView Systems describes the vehicles as "unoccupied, highly manoeuvrable, underwater robots operated by a person on the surface". And, according to the International Marine Contractors Association (IMCA), an international body representing offshore, marine and underwater engineering companies, ROV operations for marine tasks are expanding dramatically. In the June 2014 issue of its in-house Making Waves magazine, IMCA pointed to estimates that at least 2,000 additional pilot technicians will be required by 2017-2019 to keep up with demand.

However, the outlook may not be alarming as some have feared. "Mechanisms are being put in place, and training providers and contractors are working hard to train new pilot technicians," says an IMCA spokesperson. "IMCA's ROV committee has also worked hard to try to facilitate that ongoing work."

Qualified

"We currently have all the ROV staff required in-house, but we are always looking for more suitably qualified personnel," says Graham Evans, business development coordinator of UK company Red7Marine Offshore, which provides subsea engineering, diving, diving support vessels, ROVs and seabed excavation equipment and services to the offshore oil, gas and renewables markets. "There is a problem with manpower across the oil, gas and marine sectors," he agrees. "There are more projects than there is manpower to support them," says Evans.

But what about the availability of ROVs, and just how vital are they to the offshore wind O&M sector? There are around 1,100 work-class ROVs, owned by 25 companies around the world, of which more than 500 are generally in use in the offshore oil and gas sector at any one time, according to the 2013 annual market survey on work-class ROV operations, which is carried out by energy business advisers Douglas-Westwood.

This indicates that there is spare capacity for use in the offshore wind sector.

ROV availability

"The key question is whether the ROVs can be made available in the right place at the right time," says John Westwood, group chairman of Douglas-Westwood. "After all, both the oil and gas sector and the offshore wind industry focus on fair weather periods for getting construction and O&M work done," he says.

In the oil and gas sectors there are around 300 floating drilling rigs, which are all equipped with one or more ROVs for use as a tool system and for observation. They provide what Westwood describes as a "very powerful insurance policy for getting things right in the drilling process". But with around 7,500 offshore wind turbines due to be installed over the next ten years, ROV use in the sector can be expected to grow, he adds.

The three biggest global ROV operators are: Texas-headquartered Oceaneering, which in 2013 owned around 316 ROVs used almost entirely for oil and gas drilling support; UK-based Subsea7, which has about 175 ROVs, mainly for use in oil and gas offshore construction; and Dutch firm Fugro, which has 158 ROVs, mainly specialising in surveying and inspection, especially for cable routes and marine geotechnical surveys, the Douglas-Westwood research revealed.

For the offshore wind O&M sector, observation-class ROVs are widely used for activities that include visual inspection of structures, scour monitoring, cable touchdown-point monitoring and measuring cathodic potential, says IMCA. Indeed, any work involving linear structures such as cables and cable routes moving away from a vessel, and where observation is required, would be an indicator for use of an ROV, says Westwood.

Work-class ROVs are used for activities such as debris clearance, mattress installation and retrofitting anodes. Many of the cable burial jetting tools are actually ROVs, says IMCA.

Health and safety

Evans of Red7Marine agrees that ROVs tend to be used to carry out site surveys and debris and unexploded ordnance clearance, as well as to provide support during foundation installation and other jobs involved in the installation of offshore wind turbines. He also stresses the health and safety benefits of using ROVs. They would be deployed in the first instance when it is believed that it would not be safe to send down divers because of potentially unsafe debris or other possible health and safety risks, he notes.

ROVs are not always right for the job, however. Red7Marine is currently working on the London Array project, which, like many offshore wind farms, is built on sandbanks. Being in shallow waters, London Array has involved diver support vessels as well as rigid inflatable boats (RIB) and Argocat amphibian vehicles for support, but has not required ROVs.

However, says Evans: "Our DP2 diver support vessels, Red7Reel and Red7Tonjer, both have ROV capabilities on board. These may be used for an initial inspection before divers are deployed," he says.

Bob Allwood, CEO of the Society for Underwater Technology, explains some of the drawbacks of ROV deployment. Offshore wind-turbine maintenance often involves visual inspection, for instance of possible scour on the seabed where sediment may be washed away by water currents. The inspection can be done by small "eyeball" ROVs. But in shallow depths, in the so-called splash zone where the water swirls and visibility is poor, the work is difficult for a diver, but even harder for an ROV. Therefore this work is usually carried out by divers, says Allwood.

A scour problem can be rectified by laying sand bags, another job more easily done by divers than by ROVs, he notes. A work-type ROV equipped with a seven-degree freedom of movement manipulating arm for carrying out the work, and a three-degree arm for grab and stabilising the ROV itself, could in theory be used. However, the sophistication of movement required and the conditions underwater mean the job of unloading sand bags from a box lowered from a vessel and placing them correctly around the foundation pile is carried out more easily and quickly by divers, says Allwood.

"Typically, a diving solution will be quicker to install each mattress," says IMCA in agreement. "However, if there are only a few mattresses to install, then the ROV solution may be lower cost. There may also be higher risks in having divers exposed to some of these operations. For example, a diver died in a German project last year installing concrete mattresses."

Deeper waters

Yet a large work-class type ROV, the size of a small room and weighing up to two tonnes, is by no means a low-cost solution because of the vessel costs involved, notes Allwood.

Potentially, ROVs may have to be used during installation and for O&M work in deeper waters, exceeding 50 metres, where floating offshore turbines could be anchored to the sea bed. Divers could be used, but the cost will be higher due to the decompression time required when they return to the surface from such depths.

In broad terms, IMCA highlights that divers are extremely good at dealing with unforeseen issues on location. They can use standard, readily available tools and can get in to smaller spaces than work-class ROVs. In many ways, they are therefore well suited to unplanned O&M-type activities.

By contrast, ROVs work most efficiently on planned activities. It is possible to invest in specialist tooling and their work can then be made very efficient. This should be ideal for offshore wind, where there is a higher degree of standardisation and repetition than in the oil and gas sectors, suggests IMCA.

According to Evans it will never be possible to completely replace divers with ROVs, even in shallower waters, not least because the ROV pilots are not on the spot, but are using visual display units from the ROV's on-board cameras in a mini-control room on the deploying vessel, added Evans.

Next steps

We may see different solutions in the near future, argues IMCA. Projects are being developed in the offshore oil and gas industry in water depths of up to 3,000 metres, where every installation activity and intervention is deliberately planned to be carried out by ROVs. In offshore wind, developers and their project designers may yet have limited understanding of ROV capabilities, meaning ROV intervention is not as simple as it could be.

Current developments in autonomous underwater vehicles (AUV) - robot vehicles that are programmed to operate underwater without input from an operator - could also see increase use in future wind projects. We may soon see the possibility of running an AUV housed on an underwater substation to perform routine underwater inspections rather than bringing in a specialised support vessel from port, IMCA suggests.

Clearly the types of work done by divers and ROVs will remain in flux for as long as the offshore wind and oil and gas sectors continue to explore better and more efficient modes of operation, and share the knowledge between them.