Operation, maintenance and service (OMS) are the combined functions which, during the lifetime of the wind farm, support the ongoing operation of the wind turbines, balance of plant and associated transmission assets. OMS activities formally start at the wind farm construction works completion date.
The focus of these activities during the operational phase is to ensure safe operations, to maintain the physical integrity of the wind farm assets and to optimise electricity generation.
About £75 million per annum for a 1GW wind farm. This includes insurance, environmental studies, compensation payments and other internal asset owner costs (not itemised in sections below).
The wind farm owner will oversee and fulfil overall site operations activities.
In terms of wind turbine planned maintenance and service in response to faults, wind turbines are typically under warranty for the first three to ten years of operations and the wind turbine suppliers offer a service level agreement during this period to provide turbine maintenance and service.
After this initial warranty period, the wind farm owner may maintain and service the wind farm using an in-house team, contract to a specialist company or develop an intermediate arrangement where turbine technicians transfer to the wind farm owner at the end of the warranty period.
The focus of OMS is to maximise the financial return from the owners’ investment. Owners aim to optimise the balance between operational expenditure and turbine yield. By scheduling downtime during the low wind speed summer months, owners can secure high availability during the winter months when wind speeds and energy outputs are typically higher. Contractual arrangements, which award energy production, are increasingly common.
Turbine availability is the percentage of time the wind turbine is ready to produce power if the wind speed is within the operational range of the turbine. Modern onshore turbines have a technical availability of around 98%. The performance of offshore wind turbines has improved with optimised design, and offshore turbines often have availabilities in a similar range to onshore. The planning of logistics and access is vital to securing higher availabilities. Where there are access restrictions then availability may be in the range 95-98%.
Operational support is provided to the wind farm 24 hours a day seven days a week, 365 days a year, including responding to unexpected events and turbine faults, weather monitoring and live turbine monitoring. Outside normal operating hours this support is provided from remote control rooms which monitor wind farm SCADA data.
Maintenance and service includes scheduled and unscheduled activities and requires the regular transfer of personnel and equipment to the wind turbines and offshore substation. Safe access to the turbines is a critical area for further focused innovation.
Repairs and replacement of major turbine components, including the blades, are carried out by the turbine supplier or, less commonly, by specialist 3rd party providers.
In the UK, transmission assets (substations and export cables) are transferred to an OFTO within 18 months of wind farm commissioning. The OFTO may contract some maintenance and service functions to the wind farm owner because it has onsite personnel and has a strong interest in minimising transmission downtime. In other European territories, typically a transmission operator is responsible for building the offshore transmission.
Operations relate to management of the asset such as health and safety, control and operation of the asset including wind turbines and balance of plant, remote site monitoring, environmental monitoring, electricity sales, administration, marine operations supervision, operation of vessels and quayside infrastructure, and back office tasks.
About £25 million per annum for a 1GW wind farm. This includes training, onshore and offshore logistics support and management, overheads, health and safety inspections and insurance.
The owner of the wind farm typically creates a special-purpose vehicle to operate the project. This may have several shareholders, one of which is likely to take a lead role.
Operations tasks for offshore wind farms are typically provided by the majority wind farm owner.
Some aspects of wind farm operations are contracted to companies such as Deutsche Windtechnik, James Fisher Marine Services and Worley.
An onshore control room provides access via SCADA and other systems to detailed real-time and historical data for the wind turbines, substation, met station, offshore crew and vessels. Systems ensure that the operations duty manager knows where all personnel and vessels are located. This control room is often responsible for the monitoring of multiple sites.
Wind farms are monitored remotely on an ongoing basis using SCADA and condition monitoring systems and periodically by way of active inspections, including of subsea infrastructure.
A senior authorised person (SAP) is available at all times with coordination responsibility for the switching operations of all high voltage equipment.
Review of SCADA data and prognostic condition monitoring can help to time preventative maintenance before failure occurs. The industry is steadily adopting more advanced data driven approaches to maximising asset value, including the increased use of performance analytics, performance benchmarking and integrated digital systems.
In addition to hardware-related activity, environmental monitoring to understand the effect of the wind farm on the local environment and wildlife is also carried out.
Wind farms can be broadly categorised as having:
In practice, wind farm operators adopt a flexible approach, particularly during peaks of activity. In both cases, helicopters may be used in addition to CTVs and SOVs. Careful planning of routine and unscheduled activities with due consideration of weather conditions and availability of spares and specialist vessels is critical.
For groups of smaller wind farms located in the same geographical area, it can be cost effective to service them all from a single offshore base.
Training ensures that OMS personnel are qualified to fulfil the roles needed by the wind farm while ensuring their own safety and those of colleagues.
About £500,000 per annum for a 1GW wind farm.
Suppliers include AIS, ARCH, B&FC, CWind, Heightec, Maersk Training, MRS Training and Rescue, National Wind Farm Training Centres, Offshore Marine Academy, ProntoPort and RelyOn Nutec.
The Global Wind Organisation (GWO) training standards are now widely adopted in the offshore wind industry. The GWO is a non-profit body founded by leading wind turbine suppliers and/or operators.
Training is related to both technical aspects and to health and safety skills and awareness.
A number of certificates are required by all personnel likely to be present on the wind farm site, including:
The technical training required is dependent on the requirements of the client, but as a minimum will cover specific technician training for the relevant turbine model.
Other key training qualification requirements includes operational safety rules for high voltage switching and wind turbine operations
Training courses
Training examinations
Certification
Onshore logistics involves support and resources to the wind farm operations, including quayside infrastructure, warehousing, logistics and operational planning.
About £450,000 per annum for a 1GW wind farm.
The wind farm operator will establish OMS port facilities during the installation process, as many support vessels active during the operational phase will operate from local ports. The wind farm owner will typically occupy quayside facilities, operating on a long-term lease with the owner of the port infrastructure.
Typically, wind farm owners will look to use the nearest port that meets its specifications to minimise transfer times and reduce the risk of time being lost due to bad weather. Nevertheless, owners will typically competitively tender the contract for the provision of port services. For wind farms further from shore, the use of offshore accommodation and other facilities (possibly shared with other wind farms) becomes more attractive.
Port location is critical – far from shore port requirements will differ from a wind farm that is operated using CTVs and workboats only.
Consideration is given to the scope for future expansion to support additional project phases.
Port facilities are required to be flexible to accommodate variable demand with maintenance and service campaigns and site activities. Ideally, the warehousing and logistics buildings are close to the quayside to minimise the time loading support vessels.
24/7 access from a chosen port in all states of tide will increase flexibility to perform maintenance and service operations without delay to enable weather windows to be exploited – this may require port agreements to include requirements for dredging to maintain adequate water depths.
Safe means of transfer onto vessels is needed – this often requires the installation of pontoons to ensure a level access route in all tidal conditions.
A 1GW wind farm may employ up to 100 people onsite, of which about half will be turbine technicians. The availability of skilled and experienced technicians is a crucial factor in the successful operation of an offshore wind farm for wind farm owners and operators. OMS facilities need 24/7 access, 365 days a year.
As well as the port facility, operators will use remote land-based support, such as specific engineering advice and support, performance monitoring and 24/7 control room monitoring.
Each support vessel will need a berth of up to 30m. A 1GW wind farm may require the operation of between four and seven vessels, depending on the distance from the wind farm to shore and the maintenance and service strategies chosen, although up to 10 berths may be specified in order to provide capacity for peak periods. Uninterrupted access requires the availability of a non-drying harbour with minimal tidal restrictions.
An onshore base consists of:
Facilities management
Offshore logistics involves management and coordination of all marine based activities and operations.
About £1.6 million per annum for a 1GW wind farm.
The wind farm owner will setup and manage a marine operations centre at the main OMS port. Third party suppliers of marine coordination services and software include James Fisher Marine Services, SeaRoc, Vissim, and Windandwater.dk.
Marine coordination involves the 24/7 monitoring of the locations of all vessels and personnel within the vicinity of the project, including the supply and interpretation of specialist tools such as marine coordination software.
Cameras are often located on selected offshore structures to enable CCTV feeds to review conditions and monitor offshore activities.
Operators a need to make judgements about the priority of activities based on the scheduled maintenance and unscheduled service workload and weather forecast. The industry is increasingly adopting software simulation tools to maximise operational efficiency in relation to scheduling tasks and deploying resources, taking account of weather conditions, sea state, vessel capability and operational priorities.
Bigger wind farms further offshore and with more complex operational systems will increase the logistical challenge.
Robust communication equipment and infrastructure is a key element of offshore logistics in order to ensure live communication between all personnel.
O.1.3.1 Crew transfer vessels
O.1.3.2 Service operation vessels
O.1.3.3 Turbine access systems
O.1.3.4 Helicopters
I.8.3 Weather forecasting and metocean data
Marine planning software
Communications equipment including radio and asset tracking
Safety planning and systems
CTVs provide access for technicians and contractors to the wind turbines from the onshore OMS base to turbine locations and substation. CTVs are the preferred access solution for projects closer to shore.
The charter day rate for a CTV is about £2,500, depending on specification, availability and contract period.
Vessel operators: Acta Marine Wind Services, MPI Workboats, Northern Offshore Services, North Sea Logistics, Turbine Transfers and Windcat Workboats.
Manufacturers: Alicat, Fjellstrand, Fred. Olsen WindCarrier, Manor Renewables, South Boats and Umoe.
CTVs transport personnel to the wind farm on a daily basis and do not have overnight facilities.
Key requirements are robust vessels that can operate in adverse weather conditions. Wind farm operators typically use aluminium catamarans up to 30m long with capacity for 12 to 16 technicians.
CTVs are typically Class I passenger ships, as classified by the Maritime and Coastguard Agency, which enable them to work further than 60nm from a safe haven. These vessels can be built to carry up to 24 passengers. Vessel speeds can be up to 30kn and are designed to transfer maintenance and service team members in comfort and safety to the wind farm ready to start work.
There is an oversupply of small CTVs (less than 20m), with operators typically opting for larger vessels with longer ranges and better sea keeping.
There is interest in SWATH (small waterplane area, twin hull) and SWASH (small waterplane area, single hull) type vessels to increase technician comfort and lower weather downtime.
CTVs may have fixed or controlled pitch propellers but operators may prefer the increased manoeuvrability of water jets. Vessels with a smaller draught (less than 2m) may be used where harbours are more challenging to operate from due to water depths.
CTVs have a load capacity up to 30t for turbine components and consumables, as equipment. Fuel is not typically included in the charter cost and there is an important emphasis on fuel efficiency of vessels.
SOVs provide an offshore OMS base, with staff working from the vessel for periods of two to four weeks at sea. SOVs are the preferred way to maintain and service wind farms located far from shore.
Charter costs around £25,000 per day depending on size and fit out (excluding fuel).
Vessel operators: Acta Marine, Bernard Schulte, Bibby Marine, Esvagt, Louis Dreyfus Travocean, Østensjø Rederi and Vroon.
Manufacturers: Astilleros Gondanm Cemre, Damen, Royal IHC and Ulstein.
SOVs offer accommodation, mess and welfare facilities for wind farm technician staff, as well as workshop and spares storage. SOVs will stay at the wind farm for up to four weeks at a time, at which point they will return to home port to restock and change crews.
Access to the wind turbines is achieved either by smaller crew transfer vessel, daughter craft, by helicopter, or directly from the SOV using a turbine access system.
SOVs have operational speeds of up to 15kn. They are equipped with dynamic positions systems. Vessel manoeuvrability is a key requirement to reduce positioning time and therefore costs. For this reason, there is little use of surplus platform support vessels (PSVs) from the oil and gas industry. PSVs have a more important role in supporting installation and commissioning.
SOVs can typically accommodate between a crew between 50 and 100, of which up to 50 may be wind farm workers.
Accommodation berths
Mess, welfare and leisure facilities
Spares and tooling storage
Workshop facilities
Walk to work system
Turbine access systems provide access to the turbine from a CTV or SOV. Systems are designed to permit access to the turbines in as wide a range of sea-states as possible, in the interests of maximising possible maintenance and service time and turbine availability.
Costs typically included in vessel costs.
Suppliers include Ampelmann, Fjellstrand, Houlder, Osbit, Uptime and Windcat.
Many SOV turbine access systems are based on motion compensated gangways that react in real-time to changes in the sea surface, providing a stable platform to allow personnel to walk from the vessel onto the turbine. Motion compensating gangways have been trialled on CTVs.
Such systems are designed within operational limits, and will not permit access in the most severe sea-states.
Control systems
Hydraulics
Steel infrastructure
Helicopters are used to provide access for technicians and contractors to the wind turbines and offshore substation.
Around £1.5 million per year, though this is very dependent on the level of expected use (such as flying hours and helicopter type) defined by the operational strategy.
Operators: Babcock, Bond Aviation Group, Heli Service International and Northern Helicopter.
Manufacturers: Airbus, Leonardo and Sikorsky.
Helicopters allow access in otherwise inaccessible sea state conditions. Their high speeds and low carrying capacities fits well with the dispersed nature of offshore wind projects and the high frequency of low effort interventions that make up a large proportion of offshore visits.
The high costs mean that helicopters are not used as primary means of technician transport. They can be cost-effective for projects at the limit of the effective range of CTVs for which the fixed cost of SOVs is unattractive.
Arrangements to use local airports need to be developed or a dedicated helicopter base set up at the operations port. This usually requires additional planning consent. It is important to locate the helicopter close to the operations base to reduce inefficiencies in journey time.
Helicopters rarely land on the offshore installations, with technicians being winched down to the turbine. Helicopters are limited by weight restrictions and typically carry two to six technicians depending on the type of helicopter. The type of spare parts and tools that can be carried is limited by weight and size.
Helicopters are normally contracted on a long-term basis, with either exclusive or shared access to the aircraft.
Specialist offshore pilot training
Health and safety inspections are a crucial activity to ensure the ongoing safe operation of wind farm infrastructure and systems, and to fulfil statutory obligations to inspect safety critical systems on a regular basis.
About £400,000 per annum for a 1GW wind farm.
Suppliers include Bureau Veritas, DNV-GL, SGS and TÜV SÜD.
Inspections of safety-critical devices and equipment including:
Safety critical items are subject to a statutory inspection regime, where there are legal requirements including recommended inspection frequencies and method of inspection. Inspections are carried out by qualified personnel, either as part of the primary turbine maintenance works or by a team of independent inspectors. Inspection frequency will be six-monthly or annual, depending on the equipment. Drills of health and safety procedures are routine.
Most owners will train their own technicians for these roles as they are frequent but require minimal time. Where there is a requirement for periodic statutory inspections and certification, such as for fall arrest systems, independent certifiers will provide these services.
Owners will seek to perform inspections prior to other planned work being carried out in the summer months to minimise the likelihood of weather delays and ensure equipment remains certified for use.
Health and safety equipment provides personnel with access to vital equipment to reduce the risk of injury, and to provide equipment to assist in emergency situations.
Aspli, Trauma Resus, Viking Life Saving Equipment and WFE Safety.
A comprehensive set of health, safety and personal protection equipment is carried in the project vessels or stored in each turbine. Running stock will be maintained at the onshore OMS logistics facilities.
Turbines have basic emergency equipment to permit overnight occupation in the turbine in the event of personnel being stranded due to access restrictions.
Typical health, safety and personal protection equipment includes:
Rescue equipment including descenders, spinal boards and stretchers, hub rescue equipment.
Inventory tracking
Maintenance and service activities ensure the ongoing operational integrity of the wind turbines and associated balance of plant, including planned maintenance and unplanned service in response to faults, either proactive or reactive.
About £50 million per annum for a 1GW wind farm.
Maintenance and service activities are provided by a combination of the owner’s in-house resources, wind turbine suppliers and third party service providers. These are further defined under the sub-headings below.
There is considerable focus in the industry on optimising maintenance and service activities to reduce OPEX whilst also achieving the targeted levels of availability and reliability.
This optimisation is best achieved by taking a lifetime view of the project economics, focussing on the levelised cost of energy. Operational management teams will consider the whole operational system in order to achieve this.
Effective turbine maintenance and service ensures the long-term productivity of the turbines.
About £33 million per annum for a 1GW wind farm.
The wind turbine supplier, during the defect notification period (DNP) and for the duration of any agreed contract beyond the DNP.
The wind farm owner may seek to bring service capability in-house or to engage an independent service provider (ISP). This typically requires seeking agreement with the manufacturer for the supply of spares, software systems and specialist expertise.
ISPs include Deutsche Windtechnik, James Fisher Marine Services and Worley.
The initial service agreement typically covers the period of the turbine defect warranty, which is usually five years. During this period, turbine technicians are typically employed by the wind turbine supplier. The service agreement may specify that on expiry technicians’ contracts are transferred to the wind farm owner. This ensures continuity of staffing and removes technicians’ disincentive to relocate to the wind farm site.
Activity is divided into preventive maintenance (scheduled) and corrective service (unscheduled) works. The bulk of preventive works will typically be carried out during periods of low wind speeds (usually the summer months) to minimise the impact on production. This is not always achievable in practice.
Corrective service is performed in response to unscheduled outages and is often viewed as more critical, due to accruement of downtime until the fault is resolved. The primary skills required are mechanical or electrical engineering, with further turbine-maintenance training often provided by the relevant turbine provider.
Typical maintenance includes inspection, checking of bolted joints, and replacement of worn parts (with design life less than the design life of the project).
Unscheduled interventions are in response to events or failures. These may be proactive, before failure occurs, for example responding to inspections of from condition monitoring or reactive (after failure that affects generation has occurred).
O.2.1.1 Blade inspection and repair
O.2.1.2 Main component refurbishment, replacement and repair
Electrical transmission system maintenance
Blade inspection and repair consists of the inspection of the condition of blades and replacing or repairing blades in a timely and cost effective manner.
Service suppliers include Bladefence, Cyberhawk, Deutche Windtechnik, DNV-GL, FORCE Technology, GEV, Global Wind Service, James Fisher Marine Services, Mistras, Natural Power (Fred. Olsen) and Worley.
Inspection technology suppliers include ABJ, Cornis, Scoptico, SkySpecs and TSR Wind.
Blade maintenance and service is an area of specific focus in the offshore wind industry. Issues such as leading edge erosion have been the source of availability issues in the industry and proactive blade inspection and preventative repair is now widely pursued in response.
Blade inspections are performed by drones equipped with high-resolution cameras, by rope-access technicians or by high-resolution camera equipment located on the transition piece or vessel.
Where minor repairs are required, this is sometimes possible using rope access teams often using a blade platform suspended from the hub. Where a blade cannot be repaired in-situ a jack-up vessel is typically required in order to deliver the swap-out, although smaller vessels than those used during turbine installation can be used. Exchange is carried out in one visit, followed by off-site or deck-based repair. Retrofit programmes are carefully planned to ensure effective vessel utilisation taking into account repair turnaround times.
Blade inspection work typically requires the turbines to be stationary, therefore there is a focus on performing inspection work during the less windy periods of the year to minimise lost energy production.
Specialist expertise is required to undertake damage diagnostics and repair activities.
Automation of blade inspection and damage diagnostics is an active area of innovation, as is the ability run the diagnostics on an operating turbine (that is, without the need for the rotor to be stationary).
Unmanned aerial vehicles (UAVs) provide low cost and safer external inspections of turbines.
Manufacturers: Aerial Vision, ASV Global, DJI and SkyFront.
Operators: Cyvberhawk, Esvagt, Force Technology, Perceptual Robotics and SkySpecs.
Most UAVs for wind turbine inspection are multi-rotor copter drones.
Drones are typically provided by specialist operators and are rented with qualified pilots.
Drones can perform an inspection in a fraction of the time required for a traditional rope-access inspection
The drone can be equipped with a digital camera, a thermographic camera or a combination, depending on the scope of the inspection task. A digital camera provides proof of the visual failures and damages to the tower, nacelle, rotor blades and bolt jointing.
Thermographic inspection is a non-contact and non-destructive inspection method that makes it possible to examine a large area of the blade for structural defects and weaknesses in the blade. With infrared thermography, the drone monitors variations in the surface temperature of the blades.
A number of specialist suppliers supply the industry with integrated drone inspection, image diagnostics and data archiving services.
Flight planning
Data storage and archiving
Main component refurbishment, replacement and repair consists of the replacement of large components such as gearboxes, blades, transformers and generators in a timely and cost effective manner.
Suppliers include James Fisher Marine Services, Seajacks, Fred. Olsen WindCarrier and Ziton.
Design methodologies for offshore turbines to facilitate easier large component repair and replacement with less external intervention.
On-board turbine service cranes can in some cases lift substantial loads. Some components on turbines however need a jack-up barge to enable replacement, although smaller vessels than those used during turbine installation can be used. Exchange is carried out in one visit, followed by off-site refurbishment. Retrofit programmes are carefully planned to ensure effective vessel utilisation taking into account repair turnaround times.
Large component repair vessels support the change-out of large nacelle and rotor components that need a stable hook height at hub height.
Large component repair vessels are typically self-propelled jack-ups that can install, or have previously installed turbines.
Most large component repair vessels are installation vessels that are no longer able to install current turbine models or that are sub-optimal for the purpose. Given the large number of such vessels, day rates are competitive and owners typically seek to negotiate call-off contracts for vessel or charter them for several months for intensive maintenance or service campaigns.
Older installation vessels are not necessarily well suited for maintenance and service without modification. They may not be able to work at sufficient water depths for some projects and they may be over specified in terms of crane capacity and deck space. A site specific assessment must be carried out prior to commencement to ensure the vessel can safely work with the site soil conditions, weather limitations and water depths.
Marine planning
Vessel maintenance and service
Balance of plant maintenance and service is focused on ensuring the operational integrity and reliability of all wind farm assets other than the wind turbines, including the substation(s), foundations, array cables, export cables, scour protection and corrosion protection systems.
About £18 million per annum for a 1GW wind farm.
CWind (Global Offshore), Fred. Olsen WindCarrier, James Fisher Marine Services and Worley.
The balance of plant forms an integral part of the wind farm system. Proactive balance of plant maintenance is a key aspect of a reliability based preventative maintenance regime.
Regular inspections of all balance of plant elements are required to ensure emerging issues are highlighted and remedial service work is planned in order to avoid loss of generation.
Foundation inspection and repair identifies and addresses corrosion and structural problems above and below the water line.
Suppliers include CWind (Global Offshore), Deutsche Windtechnik, Fugro, Global Wind Service, Mistras, Offtech Wind and Strainstall (James Fisher).
Maintenance consists of visual inspections, non-destructive testing (NDT) and sea bed survey work with remedial service work completed when required.
Inspections focus on structural integrity, lifting, safety equipment, corrosion protection and scour protection.
Inspection work is managed by the wind farm owner, although is often subcontracted to a specialist third party provider.
Routine surveys are likely to be undertaken in the first two years but thereafter on a five or ten year cycle. Surveying the status of the protection installed to prevent sediment erosion, where the turbine foundation meets the sea bed (scour), can be carried out by side-scan sonar from a survey vessel or by using a ROV.
Regular inspections are required on secondary steelwork such as ladders, gates, grills and platforms. On some sites, cleaning is needed to remove sea bird guano, which can be a serious health and safety hazard.
Surface inspections and surveys include monopile internal inspections of the grouted or bolted connections and splash zone inspections. Activity needing subsea operations may include infrequent structural and J-tube cathodic protection inspections and weld inspections and can generally be carried out using ROVs.
Diving is required only in exceptional circumstances and efforts are being made to maximise the use of safer, remote techniques.
ROVs are used to inspect wind farm underwater structures.
Manufacturers: ECA Hytec, Saab Seaeye and Seatronics.
Operators: Fugro, James Fisher Marine Services and ROVCO.
Inspection class ROVs are used to inspect the foundation below the water line and the cable route, particularly in areas at risk of scour or other sea bed movements, and at other high risk locations, such as crossings with other cables.
Inspection ROVs typically have a speed of 3-5kn, weigh 8-12kg and have dimensions 1m x 0.7m x 0.5m.
They are equipped with propulsion systems, lighting and a range of imaging equipment.
ROVs are launched form a DP2 vessel equipped with an A frame or moon pool.
The communication between the operator and the vehicle is controlled by an umbilical or tether cable, transmit electrical power, optical signals and mechanical payloads. It strengthened, usually with steel wire, to support the mechanical loads of the ROV underwater.
The development and use of unmanned subsea inspection vessels is an area of innovation.
Propulsion system
Control system
Remote camera
Lighting system
Power supply
Manipulator arm
Provide low cost means of surveying below water, focussing on balance of plant assets such as cables and foundations.
Manufactures: ECA Hytec, General Dynamics, Teledyne Marine Gavia and Woods Hole Oceanographic Institution.
Operators: Fugro, Modus and UTEC.
Vehicle maintenance and service
Identify faults and replace whole or sections of cable.
Offshore works: Boskalis, Briggs Marine, CWind (Global Offshore), Offshore Marine Management and Pharos Offshore.
Electrical works: Boskalis, Power Cable Services.
The frequency of inspections depends on sea bed mobility and results of the initial surveys. Surface surveys can be used to detect substantial cable exposure, but ROV surveys will be required for more accurate burial depth data. Insufficient burial or cable exposure is typically resolved by remedial measures including protective mattressing and rock dumping, normally using a dynamically positioned fall pipe vessel, or occasionally side-dumping vessels.
Cable damage may come from the mechanical loads of wave and tidal action if the cable is exposed, from anchors or fishing gear, or as a result of handling during transport or installation that exceeds the cable’s specification. Although cables typically come with a two-year warranty, none of the main causes of damage is covered by the warranty
The owner is responsible for monitoring and surveying the cable and repairing it when required. The survey work and remedial work is likely to be subcontracted to a specialist provider. For array cables and export cables before the transfer of transmission assets to the OFTO (UK only, up to 18 months after works completion date), the wind farm owner is responsible. For the export cable the transmission operator or OFTO (UK only) is responsible, although the wind farm owner has a strong interest in ensuring that export cable faults are rapidly fixed to reduce and reduction in transmission capacity.
Some offshore wind farms have redundant export cables so a fault on one cable will not necessarily lead to loss of wind farm output.
Cable repair will normally require a full cable laying spread consisting of a cable laying barge with cable plough or jetting equipment, with a quadrant to ensure that the minimum bend radius is not exceeded.
On deck, the cable is cut and a new section inserted with cable joints linking the new and old sections. Unlike in subsea telecoms, where cables are largely standardised, subsea power cables may differ substantially. In the past, bespoke joints have been used but there is high interest by transmission operators in developing universal joints.
For array cables, shorter cable lengths and challenges in joining shorter cables mean that replacement of the cable may be more cost effective than repair. If so, the cable will be cut at the bases of the foundations, the internal section of cable removed, and a new cable laid using the same process as installation.
Maintenance and service record management
Mitigates the risk of undermining sea bed movements on subsea structures.
Inspection: CodaOctopus, DHI.
Management: HR Wallingford, Norfolk Marine and Subsea Protection Systems.
The presence of scour (erosion of the sea bed surface) around marine structures including offshore wind farm foundations is common.
Larger diameter structures are particularly prone to scour because of the deflection of water movement around the structure. Monopile foundations are at a higher risk of scour. Jackets may still suffer from scour but design features can mitigate the risk.
Scour is generally managed through rock (or grout, sand or gravel) dumping around the base of the foundation. Mats are generally laced on top and these stabilise the infill material and prevent secondary scour. Frond mats, tyre-filled sacks and tyre-based mats have also been used.
Concrete mattresses may also be used, potentially with protective mats, where cables have become exposed.
Sea bed inspection
Ensures there is no interruption to transmission from electrical failures or structural problems with the offshore platform.
High voltage electrical contractors such as ABB, Alstom, GE, Schneider Group and Siemens Power Transmission and Distribution.
Offshore contractors such as Deutsche Windtechnik, Petrofac.
Maintenance and service of the offshore substation primarily consists of non-intrusive inspections of topside switchgear and transformers, sampling of transformer oil, foundation and topside structural inspection and resulting infrequent service interventions.
The owner carries out paint repairs and secondary steelwork repairs (for example to railings, gratings, gates, stairs and ladders).
Serious repair operations, such as replacing transformers, require heavy lift vessels.
Rapid turnover parts and consumables are stored in a large warehouse at the onshore base.
Back-up diesel generators require periodic maintenance and refuelling.
Access to the substation may be by vessel or helicopter but since few failures require urgent attention, the weather downtime of vessels may not be as important a consideration as it is for turbines. During planned power outages to support detailed inspection and service operations, careful planning is required to ensure weather windows are used to avoid excessive wind farm downtime if work cannot be completed and assets re-energised.
Onshore substation maintenance comprises non-intrusive inspections of switchgear, transformers and any reactive power compensation equipment. Infrequent service in response may be required.
Unlike many of the systems of an offshore wind farm, the onshore substation is almost entirely non-offshore wind specific – consisting of standard high-voltage electrical equipment.
Inspection
Maintenance and service record management
