A contract for post-lay rock placement for the Nord Stream Project has been awarded to the Dutch joint venture Boskalis-Tideway Offshore. The value of the contract is about 100 million euros. This work is scheduled to start in 2011 and will be completed in 2012.
This contract with Boskalis-Tideway Offshore follows the contract for pre-lay rock placement. Pre-lay rock placement is currently being carried out in certain places along the pipeline route to create a stable base on which to lay the pipeline and to limit resulting freespans to acceptable lengths. In some areas, it will also be necessary to install rock berms after the pipelines have been laid in order to protect them against fatigue.
It is planned that the rock will be sourced from countries on the Baltic Sea coast. The rock material has to meet certain engineering and environmental criteria. The material has to be free from clay, silt, chalk, vegetation, or other deleterious substances. It will behave as any existing inert material on the seabed and will be re-colonised within a few years. In shallow waters algae will soon populate exposed surfaces, and in deeper water, where oxygen and light are scarce, the surface of the rocks will be colonised by micro-organisms.