This study developed the “ECUME” risk-based approach to identify and prioritize critical impact pathways to be considered in cumulative impact assessment of offshore windfarms, and for future research. The prioritization framework has been tested on two offshore windfarms projects located in the French part of the English Channel off the coast of Normandy, those of Fécamp and Courseulles-sur-Mer. The approach is based on a complete inventory of impact pathways, prioritizing those for which an impact assessment will be carried out. The aim was to avoid a “quantification bias” and elaborate a systemic vision. The novelty of the study is to apply a combination of expert judgement, consensus building, and a scoring system, to prioritize the pairs of pressures and receptors of the marine environment to work on. The scoring system is based on the ecological importance of receptors, the degree of knowledge on the effect of a pressure on a receptor and the sensitivity of each receptor to pressures. Priorities for research were also determined during the same process. Bringing together a large set of specialized marine environment scientists, the initial challenge was to build a common vocabulary, and a shared understanding of the risk-based prioritization approach. This required significant time and effort but secured foundations for further work. This study confirms the increasingly shared view that adopting a risk-based approach considering adverse effects on receptors is an efficient way to assess cumulative impacts, to focus on critical impact pathways, and manage the scientific complexity and the significant uncertainties.