OWF in operation require regular maintenance to minimize downtime and maximize generation of electricity. These activities include:
›Management of the asset: remote monitoring, environmental monitoring, el. sales, administration etc.
›Preventive maintenance: routine inspections, change of lubrication oils and preventive repair of parts known to wear down over time
›Corrective maintenance: repair or replacement of failed or damaged components
O&M strategy differs from one operator (or Original Equipment Manufacturer, OEM) to the next but always tries to find optimal intersection of:
›Access to the asset: transit time and time period in which a turbine can be reached by particular means
›Onshore support: availability of parts and services taking part in maintenance or repair
While the development of O&M infrastructure represents a small portion of the initial offshore wind capital investment, over the long-term (typical lifetime of 25 years), O&M make up a larger portion of the overall cost of energy. Operating expenses can comprise up to 30-40% of the LCOE . Hence, early planning of
O&M strategies and identification of suitable O&M infrastructure can make a significant difference to a project’s economic viability.
Although O&M ports must satisfy technical requirements, discussions with developers are mostly commercial. Another factor is strategic commitment of the port to support these operations as it lasts throughout the lifecycle .
O&M ports can be entirely different from the installation ports, as their main requirement is a close proximity to the farm and as infrastructure requirements are less demanding compared to installation.
Based on European experience, a building at the port of at least 300 m² is needed for storage of spare parts and a small workshop. Spare parts and consumables that need to be stored for O&M activity could include components such as bolts, cables, tools and lubricants, necessary for both scheduled and unscheduled maintenance of the wind farm and substation(s). The workshop should facilitate planned and unplanned maintenance and repair activity of minor components.
A staff office is usually established at the port and should include facilities for incidental office work. There should also be showers, changing rooms as well as facilities for drying of work clothes.
2.3.1 Vessel portfolio
Two principal models to address this optimization challenge for regular inspection and maintenance activities are to use either Crew Transfer Vessels (CTV) or Service Operation Vessels (SOV). Helicopters are possibility as well but not discussed in this study because they are used in relatively few cases.
CTVs are smaller vessels limited to return trips within a single day. There are many examples in European experience where short distance to the shore favored the CTV approach to O&M.
Assuming 1.5 to 2 hours transport time to OWF and speed between 15 and 25 knots, this limits the distance between the base and the OWF to 90km or 50nm for use of a CTV vessel.
These boats are usually aluminum catamaran designs, with overall lengths ranging from 14 to 26m. Those at the larger end are governed by the logic that such vessel offers better comfort (reduced motion sickness) and can operate in wider range of weather conditions (significant wave height below 2.5m).
Development of the larger vessels is driven in part by increased distance of wind turbine sites from the shore.
In all cases, work boats are limited to a 12-passenger capacity to maintain the classification of non-convention vessels according to SOLAS (vessel not engaged on international voyages). Vessels are fitted with a fender-lined push-on bow that facilitates transfer of personnel to the turbine landing. With some
producers, a gripping mechanism at the bow allows safer transfer and possibility to operate in larger wave conditions. Work boats do not have overnight stay possibility for passengers (but do for crew). Two examples of CTV vessels are given in Figure 2-16.
Vessel type: Crew Transfer Vessel Name: Damen Fast Crew
LOA: 26.3 m
Beam: 10.3 m Draft: 2.4 m
Comment: 12 personnel and 100 m² deck area
Name: Ribcraft CRC Voyager
LOA: 15.0 m
Beam: 3.6 m Draft: 0.7 m
Comment: 12 personnel and 1500 kg payload
Figure 2-16: Examples of Crew Transfer Vessels (CTV)
With distance of OWF to land rising, the use of SOV's is also increasing. SOVs are larger vessels, as illustrated in Figure 2-17, that also include
accommodation, workshops and spare part storage. They can spend weeks at sea and usually return to port only to restock, refuel and exchange crew. A unique feature of these vessels is "walk to work" where gyro-stabilized
gangways give safe access to turbines even in high wave conditions, up to 3m.
Name: Esvagt Faraday
LOA: 83.7 m
Beam: 17.6 m Draft: 6.5 m
Comment: 40 personnel and 450m² deck area
Figure 2-17: Example of service operation vessel (SOV)
The decision on whether to use CTV or SOV must not depend on distance alone but also on the overall O&M strategy of each operator. It is not uncommon that both types of vessels are used for same windfarm, such as at the English OWF Hornsea 2.
If a major component replacement is needed, such for example, a blade, hub or generator, a jack-up vessel with a crane must be engaged and requirements will be similar to those used in installation.
2.3.2 Distance to site
Based on the data set given in Appendix A, COWI has analyzed the distances between major OWF and their O&M port. The results are shown in Figure 2-18, below.
Figure 2-18: Distances between OWFs and installation port facilities
The analysis shows that OWF that use CTV vessels are generally at the distance to O&M base between 20 and 80km. Those that use STV vessels group between