Nord Stream Project - Route Selection And Optimised Intervention Work Design To Meet Design Criteria

Design of the two 48-inch Nord Stream Pipelines involved the crossing, along the Gulf of Finland and the Baltic Sea, of severe and very long uneven seabed areas in combination with very low seabed soil strength. Since the routing phase, attention has been paid in avoiding the sharpest outcrops and selecting the smoothest passages in the transversal rocky ridges. Gravel supports, designed for free-span mitigation against static and fatigue stress, have been optimized in order to avoid very large gravel volumes, which in many cases would have been not feasible, and in the meantime reduce the expected high settlements due to low strength of the foundation soil together with the severe load conditions (support height and pipeline loads). The design optimization involved in particular the post-lay gravel dumping which was scheduled through three different campaigns to be performed, namely 1) immediately after laying, 2) before the planned flooding and 3) after pipeline de-watering. The 1^st post-lay dumping campaign was introduced in Nord Stream Project to optimise the gravel works required to mitigate the pipe fatigue stress in temporary condition and/or the static stress in the occurrence of an accidental flooding condition, avoiding a bigger prelay support for the same purpose. A new approach to assess the effect of a storm on short term free-span fatigue has been adopted. The 2^nd post-lay dumping campaign was foreseen in order to guarantee the pipe structural integrity against pressure test. The 3^rd post-lay dumping campaign was instead included to dump all gravel supports needed for operating condition only, which will not face the high load from pressure test with consequent gravel saving (approach already successfully adopted during Langeled Project in the North Sea). The very low soil bearing capacity pushed towards an unconventional solution also for the cable crossings.