Where two pipelines have to cross over each other there is a need to provide some protection to the pipeline crossing. There are several ways which this can be achieved. Protection is mostly provided at crossings by laying small pieces of rock to form a bridge over the bottom pipeline which in turn is covered by rock. This method is undertaken using a rock placement vessel, as seen in the picture to the right.

These specialist vessels are capable of laying the rock in exactly the correct location, direct from the surface. Stone mattresses may also be used in conjunction with rock placement, as seen in the image below.

Source : http://fishsafe.eu/en/offshore-structures/pipelines.aspx