Henry M. Yamauchi
Westcoast Energy Inc.
Vancouver, B.C.
Willem J. Timmermans
Intec Engineering Inc.
Houston
Installation of a gas pipeline system connecting Vancouver Island to the British Columbia mainland is nearing completion.
The project is being conducted by Pacific Coast Energy, Vancouver, B.C., which is jointly owned by Westcoast Energy Inc., Vancouver, and Alberta Energy Inc., Calgary. The project is being managed on behalf of Pacific Coast Energy by Westcoast Energy.
The design challenges of this project have consisted of deep water (maximum 1,360 ft; 415 m), steep and rocky shores, and an irregular seabed topography.1
In addition, the project location is far from an established offshore construction market, necessitating long-distance mobilization of specialized equipment.
CROSSINGS, SHORE APPROACH
The project has involved three pipeline crossings, two within Malaspina Strait, and one within the Strait of Georgia (Fig. 1). Each crossing consists of twin 10.75-in. OD pipelines.
These crossings are characterized by generally rocky and steep shore approaches and deep channels with predominantly glacial till and some soft bottom sediments. Extensive subsea survey work was performed in 1989, before detailed design, to map the seabed topography and determine soil and current conditions.
Pipeline routes were selected to reduce the degree of bottom roughness and potential pipe spanning along the routes, and a design was prepared which took into account these environmental parameters while allowing installation by S and J-lay, reel, and towing methods in order to maintain a competitive bidding climate.
The resulting design details are summarized in Table 1.
The shore approaches at Secret Cove on the mainland and at Anderson Bay and Kiddie Point on Texada Island are rocky and required blasting to achieve an acceptable trench profile.
Of these, the Anderson Bay shore approach was the most challenging. It has an average slope of about 45 consisting of numerous near-vertical rock faces and ledges extending to about 1,000 ft (305 m) water depth. After extensive survey and study, it was concluded that blasting a trench at this location might not be feasible and at any rate would be very expensive.
Thus it was decided to cross this shore by directionally drilling holes from shore over a horizontal distance of about 2,260 ft (690 m), exiting at a point 750 ft (230 m) underwater, and to line these holes with a 16-in. casing pipe cemented in place.
CONTRACTING
It was initially thought that a towing method of installation would be the least costly because it would allow shore make-up of most of the pipe and make maximum use of locally available marine equipment.
An extensive prequalification round was held to encourage the formation of consortia or joint ventures between Canadian and foreign contractors. This process resulted in the selection of eight bidders for the main pipeline installation work, of which six actually submitted bids proposing the S-lay, J-lay, reel, and tow-pull methods of installation.
This is perhaps the first project where all these methods were considered in a truly competitive environment.
The successful bidder was a joint venture of Stena Offshore and Northern Construction. The bid was based on the use of the reelship Apache for most of the route, with a section approximately 2,600 ft (800 m) long at the Vancouver Island shore approach based on installation by bottom pull in order to include concrete coating of that section for pipeline stability.
That towing methods for the entire crossing were not as economical as originally thought was in part due to contractors' proposals to bring in major equipment from outside Canada (long distance) and because pipe tensions associated with tows would lead to a higher number of spans, placing a penalty on the tow-pull method.
The contract for directionally drilling the curved holes at the Anderson Bay shore crossing was awarded to Volker Stevin, while the excavation of trenches at the other shore crossings, including rock blasting, was awarded to Dilcon.
SHORE-APPROACH CONSTRUCTION
The most interesting part of shore-approach construction was the drilling of the pull-tube holes at Anderson Bay.
A slant drilling rig was used, set at a 45 angle (Fig. 2). A 26-in. conductor pipe was first placed at the surface, after which a 9 7/8-in. pilot hole was drilled with a combination of top drive and a mud motor and measurement-while-drilling (MWD) survey equipment to monitor the shaft trajectory.
An acceptable target area for both shafts was achieved, as verified by manned submersible.
Subsequently, each shaft was enlarged to 21 1/2 in. OD, after which 16-in. casings were set and cemented in place with the ends protruding from the submarine rock face by about 6 ft (2 m).
This entire operation took 81 days, 46 of which were used for the actual drilling and hole opening in rock which varied in compressive strength between 20,000 psi (138 MPa) and 30,000 psi (207 MPa).
The next step was installation of a bellmouth on each casing end to facilitate the entry of the pipe during the shore pull. The bellmouth design was based on ROV installation and incorporated means to guide it over the casing and secure it in place.
Preparatory construction work at the other shore crossings included excavation by clamshell dredge and drilling and blasting in areas with rocky seabed conditions. Rock trenches were provided with gravel bedding to prevent damage to the pipe coating during the shore pulls.
The trench depth was selected so that the pipe would have 3 ft (1 m) to 6 ft (2 m) of cover after backfill. Trenching extended to a water depth from 40 ft (12 m) to 130 ft (40 m) depending on shore approaches.
In some areas, additional stabilization by rock backfill or concrete mats was found necessary in depths beyond 65 ft (20 m) to withstand high currents or to provide additional protection against anchor dragging.
PIPE MAKEUP
A pipe makeup and spooling yard for pipe strings weldment for later reeling onto the Apache was established on the Fraser River in Richmond, south of Vancouver.
The site layout allowed the welding of 3,770 ft (1,150 m) long "stalks" with 60-ft (18 m) pipe joints trucked in from the Stelpipe mill, Camrose, Alta.
The joints were manually welded inside a sheltered area, then a field-joint coating of fusion-bonded epoxy (FBE) was applied, and the welds radiographically inspected as the pipe string was pulled out on a roller track.
Then side booms cradled each completed stalk over onto sleeper beams. The entire 57 miles (92 km) of 10.75-in. OD pipe was made up in 71 days.
Stalks designed for water depths greater than 985 ft (300 m) were subjected to hydrostatic pressure testing in the spooling yard to verify their integrity.
A material-control procedure provided complete traceability of each step in the fabrication and welding of the pipeline system and was continued throughout the installation phase.
On Sept. 17, 1990, the Apache reelship arrived at the spooling yard, docked at a temporary mooring, and started to spool the first stalk onto its 54-ft (16 m) diameter reel drum (cover photo). As a result of intermediate tie-in welds between successive stalks, enough pipe was wound onto the reel to complete the dual crossing of the Northern Malaspina Strait.
On Sept. 20 the Apache set sail to Kiddie Point on the north end of Texada Island to initiate the first shore pull.
A pull cable was connected between a pullhead on the pipe and a shore-based winch. Then the pipe end was pulled to shore inside the pre-excavated trench, after which the reelship laid away.
Close to shore the Apache had to negotiate a tight 90' curve as all four lines departed from that location which is an area with known rock outcrops and boulders (Fig. 1). The Apache accomplished this maneuver by closely following the presurveyed route, using the lowest possible pipe tension so that a short radius of curvature of 1,640 ft (500 m) could be maintained.
The touchdown point was closely monitored by ROV. In several cases the barge had to be maneuvered and the pipe tension increased to prevent the pipe from resting on boulders and creating unnecessary spans.
Near the opposite shore the pipe was capped and laid down. The barge was then turned around and another shore pull executed.
STRAIT OF GEORGIA CROSSING
After reeling both lines of the Northern Malaspina Strait crossing, the Apache returned to the spooling yard to reload the reel, this time for the first 14 miles (22 km) of the Strait of Georgia crossing.
At the Vancouver Island side of this crossing, environmental conditions required the pipe to be concrete coated for stability. As concrete-coated pipe cannot be reeled, a 2,620-ft (800 m) section of each of the dual lines was installed by bottom pull.
The two pipe strings were welded up ashore and pulled into place in a pretrenched ditch to a water depth of 65 ft (20 m) by a 30-ton pull barge. This section was subsequently backfilled.
Later, 3.5-ton concrete mats placed over the pipeline for a length of 3 miles (4.9 km) (Fig. 3) stabilized the shallow water shelf area beyond the concrete-coated pipe out to a water depth of 295 ft (90 m).
Installation of the lines across the southern end of Malaspina Strait required a pipe pull into the predrilled pull tubes at Anderson Bay.
Because of the downward slope of the underwater pulltube entry, the steep incline of the seabed below this point, and the more than 1,000 ft (305 m) water depth under the vessel, the pipe had to be suspended between the vessel and the pull tube entry without its touching bottom.
This required development of detailed procedures and careful planning and coordination of this operation.
This preparation paid off when ROV monitoring showed the pipe in both cases entering the casing pipe on center, causing no bending load whatsoever.
A specially designed rubber plug was clamped on the pipe at an appropriate location to close the annulus between pipe and casing once the pullhead had exited the shore end. Later, the annulus was filled with a corrosion-inhibiting gel injected from the surface to expel water from the pull tube.
The Apache installed 56 miles (90 km) of the marine pipeline crossings in 37 days, and the vessel was demobilized on Oct. 24,1990. Of this total time, 12 days were spent in spooling pipe, 4 in transit, 4 waiting on weather, and 17 days installing pipe.
Weather downtime resulted only when storm winds exceeded 45-50 knots, which prevented the vessel from maintaining position.
The installation phase of the pipelines went very smoothly, without any appreciable downtime of the main installation vessel or the auxiliary equipment such as ROV and tender. This can be attributed to the Apache's being well suited for this type of project and to the careful planning which preceded installation work.
After installation of all the crossing segments, abovewater tie-ins were made at three locations, in water depths reaching 150 ft (45 m), with several small barges linked together and equipped with pipe-lift davits.
Where the tie-in locations were near shore, the slack in the line was taken up by an additional pull from the shore-based winch.
SPAN CORRECTION
The known irregularity of the seabed was expected to lead to several unsupported spans longer than allowable in view of vortex-induced dynamic response. Based on the as-laid position of the pipelines, 74 spans were identified as requiring intermediate supports, which amounted to 92 supports.
Construction bids indicated that grout-bag support was the most economical correction method, compared to rock dumping or other solutions. This method was selected for support heights of up to 10 ft (3 m) in water depths of as much as 1,180 ft (360 m).
The diving subcontractor for this work was Can Dive of Vancouver, which installed the grout bags with a newly developed atmospheric diving suit manufactured by International Hard Suits (Fig. 4).
Despite some equipment difficulties, this proved to be a workable method, avoiding the long decompression times normally associated with saturation diving.
The grout bags provided good pipe support, but the bottom slopes, naturally associated with an irregular seabed, required careful sequencing of grout pours and sometimes special measures to prevent the bags from rolling before the grout had set.
In several locations the elevation of the pipe off bottom exceeded 10 ft (3 m), the maximum practical height of a grout bag. In these cases, a different type of support had to be devised which could be installed in more than 1,000 ft (305 m) water depth with only ROV support, function on a sloping pipe, and rest on an uneven seabed with greatly varying soil properties.
A design developed to meet these demanding criteria uses a two-legged steel support frame with extendable legs. The frame incorporates a self-closing clamp which can be lowered onto the pipe, previously located by ROV, and legs which can be jacked down hydraulically until the pipe is positively supported (Fig. 5).
The design also allows adjustment at a later date should the support or pipe be found to have shifted or settled. In view of the relative simplicity of installation of this design, the contractor elected to use this in lieu of grout bags in several additional locations, resulting in 21 steelframe supports.
TESTING, PRECOMMISSIONING
Once the pipe has been sufficiently supported along the full length, it is filled with water and hydrostatically tested. Test pressure is set at 125% of the maximum operating pressure for a duration of 24 hr.
Subsequently, the marine sections are evacuated and dried with a combination of pigging and vacuum drying.
The pipeline system is scheduled to be put into service in the third quarter 1991. It will include marine pipeline crossings which were successfully installed despite considerable technical challenges.
ACKNOWLEDGMENT
The authors wish to thank Pacific Coast Energy and Westcoast Energy for permission to publish this article.
REFERENCE
- Yamauchi, Henry M., "VANCOUVER ISLAND PIPELINE-1: Long-awaited system to expand Canadian gas network," OGJ, July 30, 1990, p. 86; "VANCOUVER ISLAND PIPELINE-conclusion: Marine-crossing sections require extensive surveying," OGJ, Aug. 13, 1990, p. 47.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
