The environmental aspect can be critical in the decision-making process at the onset of offshore E&P projects, especially in sensitive areas. Thus, we developed a methodological framework and a web tool to support the selection of technological and locational alternatives in subsea system installation and decommissioning activities. This framework relies on the assessment of environmental pressures and biological sensitivities. Our model's main assumption is that an activity can cause a number of environmental pressures, which can have different effects on each environmental component (habitats or biological groups). In the end, a potential environmental impact of a technology can be the result of its pressures on the components at a location. The work relied on experts' judgement and it was based on some previously established criteria for the classification of these environmental pressures and components' sensitivities. Scores were set and ranked in these assessments and a formula calculated the potential environmental impact of each activity/technology. A web tool called SCENARIO was developed to incorporate these results in a user-friendly interface, where one can compare scenarios made up with different technological options and different habitats or biological groups. This tool points out the potential environmental impact of each available choice. It also details how each component relates to that impact. Finally, one can acknowledge the environmental performance of its own scenario choices within the range of the best and worst possible alternatives. The tool was tested and proved to be technically sound and capable of contributing effectively to support the decision making process in the environmental aspects of offshore E&P subsea projects. It is also flexible to incorporate any pressure review, new activities and technological updates whenever necessary.