Incorporating Muskeg Soil Shear Strength Into Buoyancy Control Design

Abstract

For the past four years, NOVA Gas Transmission Ltd. (NGTL) has included the backfill shear strength in buoyancy control design for natural gas pipelines built and operated in muskeg soil laden terrain. Incorporation of muskeg soil shear strength has resulted in major cost reductions. In typical muskeg soil laden terrain, the weighting requirement for NPS 4 and smaller pipelines has been eliminated, while the need for weighting of larger pipelines has been significantly reduced. Traditionally, muskeg soil shear strength has not been considered due to an absence of adequate theoretical and performance data. NGTL has constructed a series of laboratory and field studies in an attempt to quantify the contribution of “muskeg soil shear strength.” The results of this research program, when combined with a rudimentary field investigation, are part of NGTL’s current buoyancy control design methodology for pipelines traversing muskeg soil laden terrain. This paper presents a practical methodology used by NGTL for buoyancy control. Design examples are provided to calculate two design components; the maximum tolerable muskeg depth and optimum weight spacing. The paper concludes with a discussion of construction and operational considerations which influence buoyancy control design.