This part of ISO 21072 specifies a methodology for establishing quantitative performance data for oil skimmers for recovery of oil with high viscosity, so the end user can objectively judge, compare and evaluate the design and performance of different skimmers. The methodology applies to testing in a basin and requires control of oil properties and oil slick characteristics. The method is applicable to all types of skimmer provided that the equipment dimensions are within the physical limitations of the test basin. The test procedure provides full-scale test results for the unit tested, under controlled conditions, and for one or more classes of highly viscous oil. Attention is drawn to the care required when applying the test results to predict a realistic skimmer performance under field conditions. For dedicated/inbuilt systems the test procedures outlined in this part of ISO 21072 are only applicable to the skimming device as such, not the entire skimming system.