Castoro Sei: Tensioners
During the pipelaying process, the pipe is held by a number of heavy duty tensioners that retain it in position against installation stresses. Each tensioner consists of an upper and a lower track, which look similar to tank tracks. The track loops hold the pipeline in place by applying pressure. On the Castoro Sei, there are three tensioners that control the pipeline's movement by maintaining a set tension on the pipe string as it is installed onto the seabed. They each can exert 110 tonnes of operational pressure.
Source for the image and text, is owned by Nord Stream AG