Abstract

Offshore Arctic development presents unique challenges. Typical oil and gas pipeline designs used in other regions of the world can bring with them unforeseen consequences in the arctic. Such is the case of pipeline thermal insulation. Thermal insulation, in the form of an external foam coating or a pipe-in-pipe (PIP) configuration limits heat transfer from warm operating pipelines to the colder environment and improves flow assurance performance. However, the insulation also slows the heat absorption by the pipeline as it enters the water during winter on-ice construction, which is a field-proven installation method used to install three subsea pipelines on the North Slope of Alaska.

Historically, the industry assumed that un-insulated pipelines installed from floating ice into seawater filled trenches during winter will warm up from the subzero arctic air temperatures to the warmer seawater temperature in a matter of minutes. This assumption has been validated by thermo-dynamic calculations and OLGA verification simulations. These analyses also show that one or more days may be required to warm an insulated pipeline to the warmer seawater temperature during the same installation conditions. As a result, the pipeline steel may either need to be heated prior to lowering into the flooded trench or a lower as-installed pipeline temperature may need to be assumed in pipeline design calculations.

An arctic pipeline that is installed and backfilled with a colder installation temperature than expected in design calculations will develop a larger differential temperature between its as-installed temperature and its maximum design operating temperature. Larger axial compressive forces will develop in a pipeline system with a larger differential temperature. These forces, combined with vertical imperfections in the pipeline trench, may lead to upheaval buckling and potential exposure of the pipeline to ice contact and other failure modes. Thus, properly accounting for the potential as- installed pipeline temperature is an important design task.

This paper summarizes the analytical work for determining warm-up times and ultimate as-installed pipeline temperatures for arctic subsea pipelines installed in winter, as well as the application of pipeline heating solutions to the thermal challenges of constructing and installing an insulated arctic subsea pipeline in winter.

Introduction

Three offshore Beaufort Sea developments on Alaska's North Slope have had their associated subsea pipelines installed in the winter from floating and bottom fast ice. These three projects are BP Northstar (2000) [Ref. 6 & 7], Pioneer Oooguruk (2007) [Ref. 5], and Eni Nikaitchuq (2009) [Ref. 4]. Northstar had a 10-inch single phase oil pipeline and a 10-inch single phase gas pipeline, both un-insulated, installed as a bundle (Figure 1). Oooguruk and Nikaitchuq each had a three phase Pipe-In-Pipe (PIP) pipeline, a foam insulated water injection pipeline, a single phase gas / spare pipeline, and an arctic heating fuel pipeline (Figure 2).

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