Henry M. Yamauchi
Westcoast Energy Inc.
After more than 30 years of discussions and planning, the Vancouver Island (B.C.) pipeline is finally under construction.
The new pipeline will allow natural gas to replace the heavy oil now being used on the island, thereby significantly reducing both air emissions and oil-barge traffic along the southern coast of Canada.
The pipeline is equally owned by Westcoast Energy Inc., Vancouver, and Alberta Energy Co. Ltd., Edmonton.
The selection of pipeline material, the engineering design, and the construction plan of the pipeline will minimize the construction's environmental impact.
Use of materials such as concrete rock shield; a pipeline route that parallels existing roads and powerline corridors; and construction practices such as narrow rights-of-way, water-course crossing techniques using fluming or directional drilling, removal of silted water, and revegetation will all minimize environmental damage.
The pipeline, designed to operate at 14,895 kPa (1,027 psi) over its 590-km (366-mile) length, crosses a variety of terrain conditions in areas ranging from sparse to dense population. The elevation varies from -425 m (-1,394 ft) in Georgia Strait, an arm of the Pacific Ocean, to higher than 1,150 m (3,772 ft) on land.
The subsea portion of the route represents 15% of the length of the project.
This first of two articles on the project provides an overview and discusses the land portions; the conclusion focuses on the marine crossing construction.
The pipeline route (Fig. 1) runs north and west from Vancouver, across Georgia Strait to Victoria on Vancouver Island.
While the pipeline alignment was selected to serve the maximum number of large industrial users and a maximum number of communities, the route was located so that it would parallel or be adjacent to roadways or powerlines where practical to reduce environmental damage and new terrain disturbance.
From the delivery point in Coquitlam, a 610-mm (24-in.) pipe runs approximately 2.0 km north to a compressor station. North of the station, the pipeline follows an existing road for approximately 30 km through the Coquitlam watershed, which supplies part of Greater Vancouver's water.
The pipe through the watershed is 323.9 mm (12 in.) in diameter, which is larger diameter than the rest of the pipe on the project so that future looping in the watershed for additional volumes will not be required.
From the end of the watershed, the 273.1-mm (10-in.) pipeline runs approximately 126 km (78 miles) following, for the most part, existing roads which parallel powerlines. The terrain along this part of the route varies from some 1,150 m above sea level to sea level.
Within this segment, two pulp mills, several communities, and various industrial users will be provided with new natural-gas service.
At the location of the marine crossings, Georgia Strait is divided into two branches by Texada Island, a large island which was used as a land bridge to bypass very difficult terrain along the mainland coast. The crossing of Malaspina Strait (the eastern half of Georgia Strait) to the southern end of Texada Island is via twin 273.1-mm pipelines.
A single pipeline will run north approximately 49 km (30 miles) along Texada Island, mostly following existing roads and powerlines to Kiddie Point.
Four 273.1-mm pipelines will leave Kiddie Point: two lines approximately 11 km long crossing back to the mainland across Georgia Strait to Powell River, and two pipelines approximately 23 km long crossing Georgia Strait westward to Little River on Vancouver Island.
The lines to Powell River will serve one of the largest pulp mills in the world as well as providing other users with natural gas.
At Little River, the pipeline tees with a single 168.1-mm (6-in.) lateral pipeline going north approximately 49 km to Campbell River and a single 273.1 mm pipeline south approximately 210 km (130 miles) to Langford near Victoria.
A series of 168.1-mm laterals branch from the main pipe to serve Port Alberni, Harmac, and Crofton.
The pipeline on Vancouver Island generally follows existing powerline and roadway corridors similar to the mainland and Texada sections. Four pulp mills as well as residential and other industrial users on the island will be served with gas on the island.
Geology, system design
The pipeline route crosses a variety of terrain conditions on the mainland and Vancouver Island.
Terrain on the mainland is typically mountainous with the route following existing river valleys. Bedrock is granitic with surficial deposits of glacial till and river sediments.
Near Squamish, the route crosses an area of geologically recent lava flows and skirts the bases of several high basalt towers.
Sediments on the seafloor of the marine crossing are typically soft silts and clays overlying thick deposits of sand and gravel formed during the last glaciation. Steep rock cliffs occur on parts of Texada Island.
Terrain on Vancouver Island is typically sedimentary and volcanic rock overlain by glacial tills and marine sediments deposited when sea level was higher than present. Karstic (cave forming) limestones also occur on Texada Island, but the route avoids all identified areas of karstic collapse.
The pipeline will be about 590 km long. It is made of approximately 32,000 tons of steel, designed with the latest technology for steel making.
Pipe will be fabricated from a low carbon, low sulfur, alloyed, continuous-cast steel. It will have very high-impact properties to resist fracture initiation and propagation.
The corrosion coating for the marine crossings will be fusion-bonded epoxy (FBE), and for the land portion will be polyethylene extruded over epoxy.
The pipeline is designed to Canadian Standards CSA-Z184 for the land portion and CSA-Z187 for the marine sections.
The latest technology and experience gained through the activities in the North Sea, Gulf of Mexico, Southeast Asia, and West Coast of California will also be used in construction of the marine crossings. The crossing is among the deepest on record.
The marine crossings will be twinned to provide 100% redundancy. The pipe-wall thickness and grade were selected in consideration of internal pressure, external pressure, buckle initiation, buckle propagation, and pipe collapse.
For protection of the pipeline against hydrodynamic forces, the shore approaches will be trenched and the pipe buried.
The burial will extend to water depths of 20-40 m (65-131 ft) at most shore locations and to 90 m (295 ft) depth near Little River.
Because the lower mainland and Vancouver Island are susceptible to earthquakes (Canadian Building Code Zones 3-6), special practices in terms of route location, additional wall thickness, anchor support, and other considerations have been incorporated into the design.
However, no special design requirements for the pipeline itself are needed. Built to the existing code, the pipeline can withstand ground shaking associated with the predicted 100-year seismic event.
Reducing environmental disturbance has been an important part of the design. Various design practices have been used:
- The pipeline route was selected to maximize the use of existing corridors for roads and powerlines (Fig. 2).
- The right-of-way clearing and grading width will be limited to 8-13 m, sufficient only for laying pipe (Fig. 3).
- The amount of grading and slope reduction will also be limited at specifications on steep slopes.
Rather than these slopes being graded to an angle less than the angle of repose of the backfill, grading will be done to produce a smooth but steep slope, equipment being on cables.
The pipe, concrete coated to protect the corrosion coating, will be welded on top of the slope and "stove-piped" downwards.
The trench and the pipe will be covered with polyurethane foam. The surface of the foam will be coated with polyethylene for protection against ultraviolet rays.
- The route crosses several small watersheds as well as the Coquitlam watershed which supplies water to Greater Vancouver.
The Vancouver watersheds have a long-standing policy of restricted access in order to provide protection for the water supply.
Special construction practices, such as hauling away clayey material, will be used in the Coquitlam watershed to reduce potential siltation and turbidity in the water reservoir.
The construction crew will have medical checks, periodic seminars on minimizing impact in the watershed, and will follow special provisions for personal hygiene and cleaning of equipment.
Special procedures to avoid hydrocarbon spills will also be used.
Approximately 20% (100 km) of the onshore pipeline will be placed in shot-rock ditch.
As a substitute for granular backfill material, which must be excavated and hauled, a special concrete coating will be used to protect the corrosion coating and the pipe.
The rockshield concrete coating is a reinforced modified cementitious mix with a compressive strength of 20 MPa.
The design of the coating permits the pipe to be cold bent to standard radius requirements of 1.5/diameter (Fig. 4). The coating will not spall during bending, nor will it damage the corrosion coating.
The use of concrete-coated pipe in the rocky terrain has two other advantages:
- A decreased ditch depth because padding is not required below the pipe.
- A decrease in the right-of-way width required because granular material need not be hauled.
The remainder of the route is primarily in glacial till which may contain small boulders and cobbles.
As a substitute for pipe padding of granular material, a heavy corrosion coating of approximately 2 mm of extruded polyethylene over epoxy will be employed.
Rivers and streams
The pipeline alignment crosses approximately 250 rivers and streams. These water courses will be crossed using various types of aerial structures, directional drilling, or by conventional open-cut methods using fluming and/or pumping water control methods.
Criteria for choosing between these methods depend upon the stream configuration, stream flow conditions, potential for turbid water, structural constraints, and the river channel geology.
- Spans. The span technique uses a larger support pipe to span sharply incised streams up to approximately 40 m wide.
The pipeline is insulated and inserted through the larger diameter pipe which is supported by concrete piers located above water level.
- Aerial. With an aerial method, the pipeline is suspended on cables or trusses across wide and deep canyons, with supporting piers located above water level.
Because of prevailing topography, some of the aerial crossings will be inclined at up to 15 from the horizontal.
- Directional drilling. Construction by directional drilling will be used on a few rivers which are environmentally sensitive, have restricted working space or construction periods, and have favorable geological conditions.
Directional drilling is a two-stage process. The first stage consists of drilling a small-diameter pilot hole along a designed directional profile. The actual path of the pilot hole is monitored during drilling by periodic readings of the inclination and the azimuth of the hole just behind the bit.
The path of the hole is corrected as required during drilling in order to hit the target exit location.
The second stage involves back-reaming the pilot hole to a diameter which will accommodate the pipeline. The reaming is done while the reamer is being pulled under the river back toward the drill rig with the pipe attached to the end of the drillstring on swivels.
Additional strings of pipe are welded onto the pipe being pulled back into the hole as required.
- Conventional open cut. In the majority of the cases where the open-cut method of stream crossing will be used, the stream flow will be diverted around the excavation.
Diversion techniques may include fluming and dam and pump.
The fluming technique involves damming the stream flow with earth or water-filled bags upstream of the crossing location and channeling the ponded water through one or more pipes or culverts (the flume).
Excavation, followed by pipe laying, then progresses across the stream, passing beneath the flume. Where water quality is the major concern, such as in the Coquitlam watershed, any water seeping into the trench will be pumped out to a suitable absorption area.
The dam-and-pump method is an alternative to fluming to divert water around the in-stream excavation. The method involves damming the stream above the crossing point and pumping the water around the construction site.
The dam is typically constructed of earth or water-filled bags. The ponded water is pumped across the excavation site. This technique is particularly favorable for low flow and sharply incised streams.
Immediately upon completion of a stream crossing, the stream bed will be restored as close as possible to its original condition.
Backfill of the excavation will be placed in reverse order of excavation. Fine-grained material which may cause turbidity or siltation will be covered by coarser material.
Stream banks that have been cut down to permit access for the pipe will be graded and terraced to prevent channeling of run-off water, and erosion. In some cases, gravel riprap will be placed to provide additional erosion protection.
Following construction in an other than rock area, the disturbed part of the pipeline right-of-way will be selectively revegetated to reduce erosion and to restore the area.
In some areas as in the Coquitlam watershed, trees will be transplanted at the entry areas to replace those required to be removed to create necessary working areas.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.