Abstract
This report identifies substantial differences between Environmental Impact Assessment (EIA) predictions and as‑built offshore wind farm (OWF) developments, focusing on parameters influencing cumulative disturbance from pile driving. Analysis of 12 OWFs shows that foundation installation commenced on average 6.5 years after EIA submission, by which time technological developments often meant the original design assumptions at EIA were outdated. On average, only 49% of the number of wind turbine generators (WTGs) and 12% of the number of piles assessed at EIA were ultimately installed, and piling typically occurred later and for shorter durations than predicted. The combination of broad Project Design Envelopes and precautionary worst‑case assessment requirements therefore leads to cumulative effects assessments (CEAs) that frequently overestimate disturbance potential. To improve the accuracy and realism of future CEAs, it is recommended that EIAs explicitly report the number of expected piling days (calendar days of activity) for all offshore infrastructure, specify the year(s) in which piling is anticipated, and describe the temporal assumptions used to distribute piling days within the construction period. These refinements would enable more representative cumulative assessments derived from EIA values and improve the robustness of population‑level modelling for marine mammals.