The SAR-DRIFT project aims at developing a forecast system for localisation of drifting objects, as well as decision aid tools for Maritime Search and Rescue. Its basic principle is to predict the position of drifting objects, when combining environmental measurements and forecast through sophisticated stochastic modelling. Among the different tasks of the project, one is to assess the hydrodynamic behaviour of various objects drifting at sea when submitted to the joint action of wind, waves and currents. Thousands of cargo containers are reported to fall overboard each year. A significant part of these containers will stay afloat from several hours to several weeks and represent a major hazard to navigation. In order to evaluate the drift coefficients of such cargo containers, trials were conducted at sea during which a 20 feet container was let adrift for 24 hours while measuring environmental parameters. Experimental set-up and operational process are described in this report. Main results on evaluation of the behaviour of the drifting container are also presented.