The River Tamar was a hive of activity as tugs, barges and other vessels responded to a simulated oil spill from a fuel depot.
The test was part of a Ministry of Defence-led exercise to rehearse its emergency plans in the event of a spill from the Thanckes oil depot, at Torpoint, opposite HM Naval Base, Devonport.
Agencies were faced with the scenario of a spill being driven up river by wind and tide into the River Lynher.
Inflatable booms were deployed to prevent it spreading. The surface oil was then "dispersed" within the boomed area by the movement of tugs and removed by a suction machine and soaking floats.
Commander Carl Necker, who as Queen's Harbour Master headed the exercise, said: "This was a good opportunity to exercise our contingency plans for fuel spills with all relevant agencies.
"This worked successfully and we were able to stand up the command and control centre within Devonport Naval Base from which to direct the exercise. The second aim was to deploy effectively the containment and clean-up measures.
"Having achieved this we will now provide the feedback to outside independent consultants who will provide us with a report which we will analyse and we can see how to integrate improvements into our future contingency plans."