Maintenance And Operation Activities

Published Date: 02 Nov 2017

It is well recognised that harvesting energy from wave and tidal stream has the potential to make a significant contribution of up to 20% of the U.K.’s energy mix. As the industry moves forward from testing commercial scale prototypes to the first to arrays there are many challenges facing this new industry. In order to make this industry cost-effective and therefore viable in the future the following areas of innovation have the most potential to reduce the cost of energy:

Lowering the costs of manufacture and improving performance of the components within the device

Installation, maintenance and operation activities

Improving the next generation of device through design as a result of "learning by doing"

Improving the weather window/sea state operability period for deployment

Decreasing the current speed limitations vessels and vehicles can operate in

This paper will discuss the challenges facing developers in the installation and maintenance of devices in the harsh marine environment.

The European Marine Energy Centre in Orkney, Scotland is at the forefront of this industry. With its two main grid connected sites, it provides 14 berths for developers to install and test their devices in the real environment. Established in 2003, EMEC is celebrating its 10th anniversary this year. EMEC remains the world’s first and only grid connected, accredited test facility for marine renewable devices.

The geographic location, the frequency and extent of the natural power which makes these sites so suitable for the testing of renewable energy devices poses some unique challenges to developers when considering the deployment, maintenance and recovery of the renewable energy devices. Of the 14 berths on the two main sites all are currently under contract and 11 are in use by developers with two more developers planning to deploy in 2013.

In addition to the 2 sites detailed above, EMEC have in 2011 commissioned another 2 facilities within Orkney waters. These sites provide facilities to enable developers to test devices at smaller scales in more benign conditions. Their locations are:

Shapinsay Sound

East side of Scapa Flow

The United Kingdom has very favourable resources of renewable energy which includes the already explored offshore and onshore wind and the huge potential of wave and tidal energy off its shores. The marine renewable energy potential is estimated at a technically exploitable potential of some 15 TWh. Recent studies have suggested that this figure could be significantly higher but due to the current technical maturity of the industry little of this potential has yet been realised. Globally, there are other areas with significant potential for the harvesting of this energy and this paper will cover one such area later, the Bay of Fundy in Nova Scotia, Canada which itself has huge tidal stream resource; about 160 billion tonnes of water every tide pass through the Bay - more than all the freshwater rivers and streams in the world combined.

The offshore marine environment is quite rightly considered to be one of the most challenging working environments on the planet. The biggest issue associated with exploiting marine renewable energy is the need to operate in this uniquely hostile environment. The huge potential that renewable energy offers is both an opportunity and a challenge because the areas where the greatest energy is available are the areas where it’s most difficult to engineer a means of harvesting that energy resource. Wave and tidal stream energy devices are generally complex machines which incorporate various structures and mechanical and electrical sub-systems; all interlinked by control systems. When such innovative technologies are deployed in the aggressive marine environment they can experience a wide range of challenges resulting from technical malfunctions through to difficulties associated with the physical deployment and retrieval of the device itself, with which experience is currently limited. The resolution, or mitigation, of these challenges is critical to enable cost effective, reliable and optimised level of power generation to be achieved whilst also minimising the risk of injury, damage or failure which may have significant repercussions for the future of this industry.

Although the potential for marine energy conversion clearly exists, the technology is presently still in a pre-commercial phase and only a handful of devices have so far been tested at full scale in open waters. With current prototype device installation methods requiring flat calm, benign conditions before installation can proceed weather has a significant impact on installation. A major reason for this is that current installation methodologies rely on equipment and installation techniques from the oil and gas industry, who typically have tried to avoid or work around high energy areas with high tidal flows and waves. When possible, vessels from the oil & gas sector based in Aberdeen have been used but these add the costs of transit to and from the site into the operational costs and it means that equipment hire is dependent on demand from the oil and gas market, which can fluctuate dramatically. Because of the vessel charter rates and the amount of money available in the oil and gas industry, this fledgling industry cannot compete for the resources. The rates for vessels do lower during the winter months when the oil & gas sector tend to plan projects for the forthcoming season but the weather in the winter is a significant factor.

Wind and wave conditions are inherently seasonal but can limit access to installations at any time throughout the year. Consequently, operational planning issues and the provision of suitable plant and vessels are much more significant in the marine environment than they are on land. Operational planning requires a wide range of environmental information including real time wave and tidal data to ensure optimisation of operation and maintenance activities. The provision of hind-cast and forecast met-ocean data to allow accurate planning and then to define suitable weather windows is fundamental in optimising the installation period.

For wave devices, the wave height on site may exceed the safe operating limit of standard installation vessels for a significant amount of the time. This has the potential to introduce costly weather delays to the installation and commissioning phases. Winter isn’t a good time to plan work. Weather windows tend to be fairly small with the probability of there being a low sea state for installation activities of about 20% ie one day in every five. Even in the summer months the low sea states can only be available for approximately 80% of the month giving a requirement to build in a weather contingency of at least 20% into operational planning. The author has experience of riding out a force 9-10 easterly gale, breathing a sigh of relief and preparing to start operations only to have a westerly swell from a different Atlantic weather system come in to the site and put crane operations out of safe limits. If you’re not careful you can get a little bit of a persecution complex!

Wind and waves affect all offshore operations but working on the tidal test site is also compounded by the tides. By their very nature the sites are areas with high tidal currents. The tidal currents experienced on site can limit the period of safe working for subsea operations to less than one hour in each tidal cycle around the high or low water slack period. This is also compounded by being during the neap tide periods which give a developer two four to five day operational windows per month. Factor in the risk of adverse weather and you soon see that planning for tidal operations is a high risk task. Operations requiring longer duration interventions, where holding position is required through the full flow of the tide, such as cable jointing or connections will require the use of a significant mooring spread to hold the vessel in place or a Dynamic Positioning (DP) capable vessel to be used.

The physical size and weight of the devices and associated support structures being developed is another technical challenge to the industry. Structures designed to withstand the environmental forces prevalent in offshore locations require large areas of dockside for fabrication and storage. They also demand equally large pieces of specialist plant to transport and install them. A gravity base tidal device weighs in the order of one thousand tonnes for a 1MW device. Because of this, installation and testing of the devices is expensive with costs probably similar the cost associated with the design and manufacture the device. Furthermore, the number of external factors that can affect installation means that this process is very difficult to control, potentially leading to spiralling costs should significant problems or delays be encountered. The reason for this is that marine renewable devices, by their very nature, are placed in highly energetic environments. These are the very places where even experienced mariners normally strive to avoid. Because of this the vessels, tools and techniques developed for offshore oil and gas exploration and exploitation are not designed to operate effectively in such an environment particularly when considering the tidal environment. In order for technologies to be transferred they must first be adapted to the environment and specific application. This will only happen as experience grows and the industry becomes commercially viable attracting the necessary investment.

A further challenge is presented by the variation in device designs currently available. There are not yet broadly defined methods of installation that are consistent across several different device types. Deployments at EMEC’s tidal test site have a number of different mooring solutions from gravity based structures, pin piled tripods to monopole and twin piled structures and indeed floating turbines. All these different methods of mounting require differing installation procedures. Installation of tidal devices may require specialist dynamic positioning (DP) vessels to overcome the tidal currents during installation and hold position between tides. Installation of wave devices may seem to be more straightforward, for example if they can be towed to site and moored using relatively standard methods and readily available vessels but in order to get to that position the required moorings, cable (or pipeline) and connection have to be pre-installed. This still required a significant amount of vessel time and effort.

Generally speaking, the mooring systems and foundation structures for devices may need to be installed prior to or at the same time as the device. In some cases, it may be cost effective to complete installation prior to deploying the device as it allows disruptive, high impact or weather dependent activities such as piling to be carried out with no risk to the device. Installation methods are largely dependent on device design. Gravity structures require minimal site preparation or post installation activities, and the focus is on accurate positioning and detailed foundation design. In comparison, methods involving piling will require several stages to deployment and the use of specialist installation tooling. In some cases mooring systems may be quickly deployed by anchor handling vessels whereas others will require further subsea operations to fully secure them.

Dependent on the device type there are requirements for:

Specialist vessels able to carry out complex installation procedures;

Supporting vessels locally sourced to provide construction and monitoring support;

Supply and operation of specialist tooling and ROVs during installation activities;

Diving operations, if required, need to be very carefully planned.

One major factor when using any type of surface vessel is station keeping. A DP vessel is specifically designed to hold a position, typically within a 5 metre radius, for an extended period of time; although this is limited by the current, wind speed and the vessel’s aspect in relation to these forces. This is achieved by utilising a series of thrusters mounted at various positions on the hull of the vessel. Most DP vessel systems are designed and proved for operations in tidal streams of up to 5 knots (2.5ms-1). EMEC’s test site has tidal currents of up to 4.5ms-1 and the Bay of Fundy experiences over 5ms-1. Future deployments in the Pentland Firth between Orkney and mainland Scotland will experience even higher currents. Current DP vessels are operating at their upper limits of capability. Experience at EMEC has shown that automated systems can be slow to respond to changes required to keep a vessel in position leading to manual intervention. The requirements of current systems have not been specified to operate in these areas and this is an example of where improvements can be made specifically for marine renewables.

For a moored barge typically large gravity anchors are used to hold position often on scoured rock bottoms in the flow. This has several drawbacks as the anchors have to be installed on a semi-permanent basis, remaining on site throughout the period of installation and access operations. Also because this is a relatively new application a significant factor of safety is typically applied to any gravity anchor or foundation. This has the potential for the anchors to be over engineered, driving up costs. There are also other costs to be considered as the moorings themselves pose a significant navigational hazard and will need to be marked or guarded.

Future developments must consider the environmental resource that the vessel is to operate in.

The development of streamlined installation and retrieval techniques that are less sensitive to environmental conditions and less reliant on the use of specialist vessels (and equipment).

Diving operations are to be minimised, however, it is recognised that these will be required on occasion. Developers’ attention is drawn to the extremely limited slack water periods on the tidal test site and to the depth of water at both main wave and tidal test sites berths. Extreme caution must be exercised in planning and conducting diving operations.

For ROVs, attention is drawn again to the operational windows available for the use of such vessels on the tidal test site. Problems have been experienced on site due to attempts to work longer than the planned operating window. ROVs seem to be able to operate in up to 2 knots of tide. Above 2 knots then there are issues with operation and recovery of the ROV.

Operation and maintenance – the problem of access

Once an offshore renewable energy installation has been built and commissioned, the focus turns to how it can be supported in terms of operation and maintenance. There is an ongoing need for up to date met-ocean data so that energy outputs can be estimated to obtain the most favourable prices on spot markets.

In terms of maintenance, there is a fundamental difference between operating a plant in the marine environment and operating a plant on land. Generally, land based infrastructure in Europe is readily accessible for most of the year without huge constraints: In the sea this is not the case.

"...the reliability of any mechanical plant installed on an offshore renewable energy installation must be very high if it is going to be commercially viable."

This has precipitated the design and development of a brand new class of vessel that fills a gap in the market between unsuitable inshore craft and unacceptably expensive offshore oil and gas support vessels.

Because it is difficult to intervene easily at sea, the reliability of any mechanical plant installed on an offshore renewable energy installation must be very high if it is going to be commercially viable.

If a wind turbine on land breaks down in the middle of winter, it is still possible to affect a repair. If a wind turbine 50 miles offshore were to breakdown in the middle of winter, repair is much more difficult and might conceivably not be possible for perhaps months, due to wind strength and sea state.

For OREIs to be commercially viable a high degree of reliability must be engineered into all of the components of the installation.

End of life-time – then what?

One final factor that must also be considered, but has yet to be implemented due to the relative immaturity of the marine renewables sector is that of decommissioning.

The challenge currently facing the offshore oil and gas will be repeated. Potentially in 50 or 60 years time there will be thousands and thousands of turbines that have all reached the end of their working lives and will need to be either removed or fully refurbished.

"There is no time to re-learn lessons of the past: We must get it right and get it right first time."

However, with this knowledge and access to the lessons learned by the offshore oil and gas industry, offshore renewable energy installations can be deigned to ensure ease of decommissioning and removal. The challenges associated with removing these structures in the latter part of the 21st century can be considered and addressed almost before they are built.

There is certainly a steep learning curve for developers and operators of offshore renewable energy installations to go through. It is not simply a case of taking plant, machinery and working practices that are designed to operate on land and transposing them into the sea.

The oil and gas industry learned the hard way that it doesn’t always work and the offshore renewable energy industry need not follow the same path. Developers have the opportunity to work with companies like BMT that have experience in all the skill-sets required to provide through-life consultancy and support for offshore offshore renewable energy projects.

In order to support the design and construction of these devices we must make the best possible use of the wealth of knowledge and experience available in the marine engineering, science and technology sector.

There is no time to re-learn lessons of the past: We must get it right and get it right first time.

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Installation of electrical systems

Typically, installation of both onshore and offshore substations and cabling is required. The onshore

activities are conventional and so not covered in more detail in this report.

Offshore electrical installation activities may include:

● Directional drilling at the cable landing point. This requires expert geotechnical knowledge

and specialist equipment;

● Draw-through and installation of several kilometres of subsea cabling to avoid geohazards;

● Cable protection and securing using rock dumping and, where appropriate, pinning and

active positioning around seabed features using ROVs. Installation methods and challenges

are quite different for tidal energy projects compared to wave. Particular care must be taken

at tidal sites to avoid cable vibration (strum) that can severely reduce life expectancy;

● Installation and connection of the offshore substation, which may be on a piled foundation

or subsea; and

● Installation and connection of array cabling between devices and the offshore substation.

These are installed prior to the devices in most cases.

Cable laying makes use of specialist vessels but also requires the support of a range of other vessels

to ensure navigational and operational safety

Stuart Baird is the Operations Director at the European Marine Energy Centre. He took up this role in 2009 following completion of a Masters degree in renewable energy development at Herriot Watt’s International Centre for Island Technology based in Stromness Orkney. Stuart started his career as an engineer in the Royal Navy has spent 20 years serving in various roles. Since leaving the Royal Navy Stuart has been involved in in projects in various different industries from defence to communications to construction and the legal sector. Since joining EMEC in 2009 he has been involved in expansion of the main test sites including laying of over 11km of new cables providing three new berths and the design and implementation of new scale test sites providing facilities for smaller scale devices in more benign environments. Stuart has responsibility for the management of all activities on site to ensure all operations are carried out safely.