Trans RINA, Vol 157, Part C1, Intl J Marine Design, Jan –Dec 2015


Weather is the main constraint: a maintenance operation cannot be launched outside a specified weather window. One of the main factors determining the high costs of es.


energy production is the accessibility of the turbine Even for UK Round 1, close to the shore with relatively mild environmental conditions, turbines were accessible sel


on average for only 69% of the time [9]. Thus vess motion characteristics and ability to access turbines in a range of sea sttates play a significant role in reducing the electricity production costs (Figure 3).


For the UCL MSc study, a Significant Wave Height (SWH) of 1.5 m was assumed to be the limit for safe direct transfer from vessel to a turbine (‘step over’ onto the turbine ladder). However, in the most onerous region of the Southern North Sea (Dogger Bank and Hornsea), a transfer vessel (with or without a transfer system on bo


oard) operating in sea states up to a s ignificant wave height of 2.5m may still deliver 90% accessibility. A system operating in significant wave heights of more than 2.5m would not


be feasible as performance


improvements would be minimal with significant cost increase [10].


3.3 MARINE SUPPORT TO WINDFARMS


With currently operating wind ffarms, the trend is to use small boats, such as catamarans or SWATH vessels. However, as the distance from shore increases other approaches should be considere


ed. An offshore substation


is of interest for large wind farms situated at more than 75 km from the shore [11]. Tablle 2 summarises the three main marine support strategies under consideration.


Using a large ship as an O&M base provides more flexibility as the vessel can offer crew accommodations and can be equipped with a cra


ane. Alternatively, a small


ship can be used as an O&M base to support small offshore windfarms.


The different vessel types available for maintenance operations


for offshore wiind summarized in Table 3. farms have been


Figure 3: Influence of vessel access threshold on operational costs [9] Strategy


Table 2 Marine Support Strategies overview [8] Relative


Cost


Work boats: Onshore-based marine access, with specialised work boats of some description, based at a coastal port


Helicopter access, either as support to work bo


oats or as the primary access Fixed or floating offshore base


accommodation platforms, ‘mo etc)


(offshore therships’


Low High High


Operability in Range of Weather Conditions


Liimited: Maximum wave height of 1.5m


High: Insensitive to wave heights, though some visibility restrictions


High: Assuming helicopters used with fixed base, and direct access sy


ystem with floating base


Transit Speed


ds


Slow: ~20 knots


Quick: ~135 knots


Criterion is less relevant as close to site


Distance From Port toWind Farm


 75km  75km  75km


C-138


© 2015: The Royal Instittu


ution of Naval Architects


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