Using an offshore accommodation platform combined with fast in-field vessels will be key to maximising efficiencies for large offshore wind farms built far from shore, according to an O&M logistics study carried out by Shoreline for Fred. Olsen Ocean.
Three scenarios with different marine logistics combinations were modelled.
- a single service operation vessel (SOV) with on-board accommodation and access to a helipad
- two walk-to work-vessels (converted platform supply vessels) working alongside an offshore accommodation platform such as Fred. Olsen Windbase
- three large 30m crew transfer vessels (CTVs) working alongside an offshore accommodation platform such as Fred. Olsen Windbase
All three scenarios included the use of a helicopter for troubleshooting and additional CTVs over the summer period for scheduled maintenance. The metocean data was taken from weather observations over 18 years – between February 1994 and January 2012 – so factored in seasonal changes, and included significant wave height and mean wind speed.
The key finding of the simulation, which used a maintenance strategy with specific work order priorities, emergency response time limits and two types of technicians, was that time availability was highest in the third scenario – that of a fixed offshore accommodation platform being used in partnership with larger CTVs.
Fred. Olsen Windbase Project Director David Matthews said that the initial input data for each of the specified base cases was realistic but optimistic, however sensitivity analysis was also included in the report, to consider worst case scenarios as well.
“All three scenarios performed with relatively high time based results, but the best availability – nearly 98% – was with the scenario employing three CTVs combined with an offshore accommodation platform,” Matthews said. “The main difference was that the scenarios relying on the vessels equipped with walk-to-work access system had less availability when the overall workload was increased because of their low in-park speed in dynamic positioning mode and the limited number of access systems deployed.”
“On larger projects the robustness of the solution chosen is tested as the failures rise. We quickly saw which solution started to fail first and quickest. In short the distances between failures on any given day showed that multiple low cost access vessels performed best,” Matthews added. “The modelling coupled with a site specific day rate and scope, allows developers to model a fixed offshore facility against other solutions in offshore wind.”