Abstract
This report includes both an evaluation of procedures for estimating site accessibility and an appraisal of the implications of site accessibility for large-scale deployment of both wave and tidal stream devices and thus represents deliverables 7.4.1 and 7.4.2. Several methods are reviewed for estimating the duration of accessible wave conditions on the basis of the probability of significant wave height exceedance. One of these methods is employed to investigate how the number of occurrences of accessible conditions and the waiting on weather allowance may vary with the annual average significant wave height of a site. Wave climates with average significant wave height in the range 1.5 to 3.5 m are considered. For a threshold wave height of 2 m at these sites, weather windows have average duration of between 2.5 and 1 day and require average waiting on weather of between 2 and 6 days. An estimate is subsequently made of the vessel time required to install a nominal array of 100 MW output. Whilst a statistical approach is straightforward to apply to a single environmental variable, it is not straightforward to apply to a tidal stream site where wave height and both wind-and current-speed are relevant. A time-series method is employed to analyse ten years of environmental variables at a representative tidal stream site. The average waiting time between a 1 day weather window suitable for offshore work is extremely sensitive to current speed and seasonal variations occur due to wind-speed and wave-height. For the site considered it is estimated that between 2 and 5 turbines could be installed per month if installation can be conducted during current speeds up to 1.3 m/s but this falls to consistently less than 2 per month if offshore work is conducted whilst the current speed is less than 1.13 m/s.