In the future, certain maintenance tasks may also be carried out remotely (see
DOWES, Sect. 4.2.4).
Until now, O&M visits are carried out when the significant wave heights
(SWH) are less than or equal to 1.5 m. According to Stavenuiter (2009), each
support vessel has a certain maximum allowable significant wave height for several
operations. Therefore, the availability of a vessel is correlated with the occurrence
of certain significant wave heights. The cumulative occurrence of significant wave
heights less than or equal to 1.5 m is 68%. Waves up to 2.0 m increases the
cumulative occurrence to 83% (Stavenuiter 2009). Significant wave heights measured close to two Dutch offshore wind farm locations (OWEZ and PAWP) are
almost identical, despite a distance of 40 NM between each other (Rijkswaterstaat
2009).
The number of days per month that offshore locations are accessible or not due
to weather downtime varies over the year, with more no access days during the
windy and stormy winter months (Stavenuiter 2009). The maximum number (22) of
no access days of two offshore locations, very close to the Dutch offshore wind
farms OWEZ and PAWP, in 2009 was in November. In spring and summer months,
weather downtime fluctuated between 6 and 11 days between April and August
2009, and 16 in September 2009 (Stavenuiter 2009).
The possibility of larger wave heights will require new systems for safe O&M
personnel transfer. If transfers are to be restricted to wave heights of 1.5 m, this will
limit offshore work to about 200 days a year (Noordzeewind 2010; Miedema 2012).
Noordzeewind (2010) estimated a total of approximately 218 possible access days
in 2009, and the remaining time of the year was considered non-productive time
(‘weather downtime’) in 2009. According to a study in the German North Sea,
available working days were estimated to be even lower, in a range of 30–
100 days/year (Michler-Cieluch et al. 2009a). Increasing the workable significant
wave height from 1.5 to 2 m, could increase the accessibility of wind farms by 15%
(Stavenuiter 2009). An increase of the safe working wave height to 3 m and above
could increase the number of days available for transfers up to 310 days per year.
Hence, increasing overall accessibility can lead to cost reduction of wind energy
production. To achieve this, new ships with motion stabilizers are required to
guaranty safe transfers of personnel and material. Current solutions are offshore
access systems such as ‘Ampelmann’, a hydraulic ship-based transfer platform that
compensates the vessel’s motions, thereby enabling safe offshore access and
operations. But even if these new systems for operating in far-offshore conditions
are developed, a constant shuttling of workboats to and from the coast is impractical
and costly. Therefore, developers and offshore service providers are looking for
new methods, one of which is the ‘mother ship’ approach. A single large vessel
would then service one or more offshore wind farms staying in the neighborhood of
these farms for long periods of time and deploying multiple smaller craft for daily
servicing.
100
C. Röckmann et al.
DOWES, Sect. 4.2.4).
Until now, O&M visits are carried out when the significant wave heights
(SWH) are less than or equal to 1.5 m. According to Stavenuiter (2009), each
support vessel has a certain maximum allowable significant wave height for several
operations. Therefore, the availability of a vessel is correlated with the occurrence
of certain significant wave heights. The cumulative occurrence of significant wave
heights less than or equal to 1.5 m is 68%. Waves up to 2.0 m increases the
cumulative occurrence to 83% (Stavenuiter 2009). Significant wave heights measured close to two Dutch offshore wind farm locations (OWEZ and PAWP) are
almost identical, despite a distance of 40 NM between each other (Rijkswaterstaat
2009).
The number of days per month that offshore locations are accessible or not due
to weather downtime varies over the year, with more no access days during the
windy and stormy winter months (Stavenuiter 2009). The maximum number (22) of
no access days of two offshore locations, very close to the Dutch offshore wind
farms OWEZ and PAWP, in 2009 was in November. In spring and summer months,
weather downtime fluctuated between 6 and 11 days between April and August
2009, and 16 in September 2009 (Stavenuiter 2009).
The possibility of larger wave heights will require new systems for safe O&M
personnel transfer. If transfers are to be restricted to wave heights of 1.5 m, this will
limit offshore work to about 200 days a year (Noordzeewind 2010; Miedema 2012).
Noordzeewind (2010) estimated a total of approximately 218 possible access days
in 2009, and the remaining time of the year was considered non-productive time
(‘weather downtime’) in 2009. According to a study in the German North Sea,
available working days were estimated to be even lower, in a range of 30–
100 days/year (Michler-Cieluch et al. 2009a). Increasing the workable significant
wave height from 1.5 to 2 m, could increase the accessibility of wind farms by 15%
(Stavenuiter 2009). An increase of the safe working wave height to 3 m and above
could increase the number of days available for transfers up to 310 days per year.
Hence, increasing overall accessibility can lead to cost reduction of wind energy
production. To achieve this, new ships with motion stabilizers are required to
guaranty safe transfers of personnel and material. Current solutions are offshore
access systems such as ‘Ampelmann’, a hydraulic ship-based transfer platform that
compensates the vessel’s motions, thereby enabling safe offshore access and
operations. But even if these new systems for operating in far-offshore conditions
are developed, a constant shuttling of workboats to and from the coast is impractical
and costly. Therefore, developers and offshore service providers are looking for
new methods, one of which is the ‘mother ship’ approach. A single large vessel
would then service one or more offshore wind farms staying in the neighborhood of
these farms for long periods of time and deploying multiple smaller craft for daily
servicing.
100
C. Röckmann et al.