Green fees There have been a number of voluntary award schemes developed to encourage vessels to be more environmentally-friendly through incentives based on port dues. One possibility is the introduction of green fees or ‘green passports’ in conjunction with port authorities, providing a right of entryand reduced port fees to those vessels meeting environmental requirements. The use of green fees for clean shipping promotion is a good example of a port-driven initiative (though at cost to the port) that can be undertaken in cooperation with shipping companies. The success of the WPCI working group in developing the Environmental Ship Index (ESI) is a good example of collective and proactive action by ports. The WPCI's Environmental Ship Index rates the environmental performance of ships in terms of the emissions of NOx, SO x and CO2 on a scale from 0 to 100 (from highly polluting to emission-free). Ports set their own qualifying benchmarks— Amsterdam, for example, will issue rebates for scores of 20 or above. The WCPI scheme sees potential financial incentives in the form of higher port charges for non-clean ships; discounts for clean ships, and inspection to certify qualifying ships. Green ship promotion may also be a relevant consideration for some UK ports given that several major continental European ports are currently operating such policies (see Table 12). As Table 12 indicates, several environmental indices exist and can be used. A brief review of such indices, their main focus and their advantages/disadvantages, follows (see also Table 13 for a summary).