Along with the task to construct a 5 km long tunnel of limited diameter, the insertion of the 40” (1069 mm x 19.1 mm) replacement gas pipeline into this tunnel was one of the main project challenges.

As an alternative to the Client’s FEED, which was the concept of roller supports throughout the tunnel for both installation and operation of the pipeline, we submitted an ambitious variant solution within our tender, which proposed the push installation of a submerged pipeline into a water-filled tunnel.

The Client was successfully convinced by this installation method which allowed us to develop our proposals from tender / concept level to the final execution design phase. The corresponding engineering process dealt with a vast number of interrelated technical issues, some more apparent than others.

Beyond the tasks of stringing, welding, testing and moving a 5 km long pipeline weighting 6500 Tons , solutions for cathodic protection, pipeline coating as well as means to control buoyancy and friction of the pipeline inside the tunnel were found. Solutions observed the limitations of qualified materials such as FBE or concrete weight coating.

Several numeric simulations, as well as full scale tests on individual subject areas, were carried out in order to determine all aspects of the push installation method and the operation of the pipeline inside the tunnel. The scale for the project contributed significantly to the complexity of the design, since uncertainties of material properties and production processes had to be accounted for, regarding their effects on pipeline stability.

As well as detailed prediction of the anticipated push forces, other factors such as pipeline compression, deflection and lateral buckling were analysed and trigger points were pre-determined for the purpose of monitoring the execution.