The NOVA Gas Transmission Ltd. (NGTL) gas pipeline system is expanding northwards as the producers search for and find new gas reserves. This growth has taken the system into the discontinuous permafrost zone, and also into new design problems. One such problem is the structural integrity of a pipeline subjected to the settlement differentials that occur between frozen and unfrozen soils. Adequate integrity design for differential settlement is required by design codes, such as CSA Z662, but the procedures and criteria must be established by the pipeline designers.
This paper presents the methodology of pipeline integrity design for differential settlements used on a number of pipeline projects in Northwest Alberta. Outlined in the paper are the procedures, rationales and models used to: (a) locate discontinuous permafrost; (b) quantify the potential differential settlement; (c) predict pipeline stresses and strains; (d) establish strain limits; and (e) determine the pipe wall thickness to withstand those potential differential settlements. Several design options are available and are briefly discussed. For the projects mentioned, the heavy wall pipe option was identified as a cost effective design for medium to large differential settlements.