Screw anchors economically control pipeline buoyancy in muskeg.

To control buoyancy of large-diameter pipelines that cross such wet areas as muskeg swamps in Alberta Province, Canada, NOVA Gas Transmission Ltd., Calgary, for more than 6 years investigated alternatives to traditional concrete weights.

The company's research indicated that helical screw anchors offer cost-effective buoyancy control for naturalgas pipelines 20-in. OD and larger.

NOVA's system encompasses 19,000 km (11,800 miles) of pipelines ranging from 2 to 48 in. OD. In the last 3 years, the company has constructed more than 300 km (180 miles) of 42 and 48 in. pipelines.

This expansion has occurred mainly along the eastern slopes of the Rocky Mountains and in Alberta's northern forests: areas characterized by intermittent muskeg swamps. Muskegs are organic soils with high water tables, low shear strengths, and low densities the winter. The frozen muskeg provides access and support for pipe, concrete weights, and heavy equipment.

Trucks which are used to haul the concrete weights produce some of the highest earth loads encountered during construction. These loads may cause the frozen muskeg to fail, resulting in construction delays and increased costs.

NOVA's buoyancy-control research initially focused on refinements of the swamp-weight spacing requirements, by investigating backfill properties to reduce conservatism in the design.

Advancements in pipeline-buoyancy control with geotextile swamp weights(1) and plate weights(2) on small diameter pipelines have now been complemented by use of helical screw anchors on large diameter pipelines.

While the use of screw anchors is not new to the pipeline industry, they have seen limited application in high production situations on large diameter pipelines in Canada.

NOVA's experience demonstrated practical and cost benefits of screw anchors to resist uplift forces. And among other benefits, substitution of screw anchors for concrete weights significantly reduced the number of heavy trucks that must be moved across the muskeg.

Buoyancy control; anchors

Pipelines will "float" to the surface when the weight of the pipe and contents, plus the resistance provided by the backfill, is less than the buoyant force on the pipeline.

The following equations describe the forces acting on the pipeline shown in Fig. 1: [B.sub.p] - [M.sub.p] (g) = (2) [F.sub.S], for Fig. 1a;[B.sub.p], - [M.sub.p] (g) = [M.sub.c] (g) - [B.sub.c], for Fig. lb.

The net buoyant force on the pipeline (BP - MP [g]) must be counteracted by either the force developed by a screw anchor assembly (2FS) or the force exerted by a submerged concrete weight ([M.sub.c] [g] - [B.sub.c]).

A screw anchor is a circular helical plate welded to the end of a steel shaft (Fig. 2). Rotation of the shaft advances the …