To anyone driving past the old lock in Zeebrugge, the image of the tugboats moored there is a familiar sight. In the past, on-board generators were used to provide ships with the necessary electricity. This is no longer necessary because the tugboats are now connected to shore power. This is called "cold ironing", shore-to-ship power (SSP) or alternative maritime power (AMP). The term cold ironing refers to the cooling down of the iron-coal engines that were used to fuel ships many years ago. So, once a ship is at berth, the plug goes in and the engines can be turned off. This is also part of Zeebrugge's "Clean Port" sustainability strategy.