ROCK DUMPING SECURES TWIN PIPELINES ACROSS MICHIGAN STRAITS

David E. Hairston
Willbros Butler Engineers Inc.
Tulsa

John E. Seng, Lawrence J. Jaskowiec
Great Lakes Gas Transmission Co.
Detroit

Great Lakes Gas Transmission Co., Detroit, in b 199'1 successfully stabilized its two 24-in. natural-gas pipelines crossing the Straits of Mackinac in northern Michigan.

Placing approximately 152,000 tons of limestone rock around and under the lines supported and stabilized them and prevented further erosion of the lake bottom under them.

DUAL-LINE SYSTEM

Great Lakes Gas Transmission transports natural gas through a 1,960-mile interstate pipeline system for delivery to U.S. and Canadian customers.

The pipeline operates 93% as a "dual line" system and extends from an interconnection with TransCanada PipeLines Ltd. at the Minnesota-Manitoba border to a second interconnection with TransCanada at St. Clair, Mich. (Fig. 1).

The pipeline traverses northern Minnesota, northern Wisconsin, and the upper and lower peninsulas of Michigan, which are separated by the Straits of Mackinac.

Great Lakes has two parallel lines crossing the straits about 1 mile west of the Mackinac bridge (Fig. 2).

The lines are 24-in. OD x 0.500-in. W.T. Grade X-60 line pipe with a coal-tar enamel corrosion coating and a 3-in. concrete weight coating for anti-buoyancy. The lines are approximately 1,000 ft apart. They are about 5 miles long from shore to shore and lie in water depths to 240 ft.

The pipelines are buried out from the shorelines to a minimum water depth of 50 ft, with the middle sections lying on the bottom of the straits.

The straits are subject to a wide range of weather conditions. Waves up to 15 ft have been recorded, and shore-to-shore ice can be present in winter.

UNSUPPORTED SPANS

The company first discovered the erosion in October 1992 during an underwater survey with a remotely operated vehicle (ROV), state-of-the-art video equipment, and global-positioning navigation systems unavailable during previous surreys.

The video survey revealed significant erosion in several locations under the pipelines and unsupported pipeline spans of various lengths above the lake bottom.

The camera also observed a variety of debris along the pipelines as well as damage to several sections of the lines' concrete coating.

Despite there being no danger to the pipelines, Great Lakes' management decided that corrective action was essential.

MORE SURVEYS

To determine extent of damage and the best way to remedy it, Great Lakes commissioned additional subsea surveys to establish more accurately the lengths of pipeline that were unsupported and their heights above the sea bottom.

Geophysical surveys in December 1992 gathered data and mapped the bathy-metric profile of the lake bottom adjacent to the pipelines, as well as the centerline profiles of the pipelines.

Two types of pigs were utilized to determine the condition of the pipelines.

Pipetronix' electromagnetic inspection pig determined that the pipelines had no corrosion concerns. Nowsco Pipeline Services' inertial geometry survey pig was used to determine the state of strain of the pipelines.

The results showed that the pipelines were strained well below the allowable Emit. The only required action was stabilization of the lines.

Working from these data, Great Lakes' executive, engineering, legal, and operations departments prepared a comprehensive plan for stabilizing the lines.

Company management then worked with outside marine consultants to develop detailed revetment plans, which called for covering with rock berms the entire underwater lengths of the pipelines while they were in service.

This approach meant that there could be no interference with normal operations and no damage to the pipeLine system.

Following interviews with a dozen contenders, Great Lakes management chose Martech U.S.A., Anchorage, as the primary contractor and Durocher Dock & Dredge, Cheboygan, Mich., as the primary subcontractor.

Durocher provided water craft, including anchor, dive, survey, rock-hauling, and rock-placement barges as well as cranes, front-end loaders, and associated equipment.

Durocher also had primary responsibility for obtaining, transporting, and placing the rock for the revetment berms.

Martech employed other specialist subcontractors to perform survey work--John E. Chance & Associates (now Furgro-West Inc.), Ventura, Calif., and SonSub, Houston--and to mix the grout used to make repairs to the concrete coating: Prepakt Concrete Co., Cleveland.

Willbros Butler Engineers Inc., Tulsa, was assigned responsibility for construction management, inspection, and quality control. Braun Intertec, Minneapolis, had responsibility for environmental inspection during the project.

ENVIRONMENT, SAFETY

The U.S. Corps of Engineers and the Michigan Department of Natural Resources issued the necessary permits for the project after receiving detailed environmental impact studies and descriptions on how the work was to be performed.

All rock fill was precleaned, work crews received environmental training, a spill prevention and control plan was developed, and a zero-discharge policy was observed at all times.

Preproject environmental studies determined the work would have no effect on water quality at the straits, would not affect local water supplies nor cause shoreline erosion, and would have no effect on woodlands, aquatic, or terrestrial life.

Environmental inspectors were on site to enforce and record compliance with the environmental specifications.

Noise levels, as well as air emissions from construction equipment and watercraft, were comparable to levels routinely experienced in the area.

To ensure safety at all times, contractors followed special precautions so that no construction equipment touched or damaged the pipelines. Trained Great Lakes' personnel continuously staffed automatic pipeline shut-off valves on both shores of the straits.

The U.S. Coast Guard was notified and posted boating advisories.

The project was designed to withstand currents up to 5.2 knots (the "100-year" storm), such currents being 37% swifter than the highest ever recorded at the Straits.

DETERMINING WORK SCOPE

Before rock placement, additional underwater surveys were made to determine the precise scope of the work. These surveys began on May 28, 1993, and included continuous video recordings made by a camera attached to the ROV.

The 3,000-lb ROV was attached to a cage connected by steel cable to a survey barge and lowered by winch to the bottom.

Attached to its cage by a 400-ft umbilical cord, the ROV pinpointed the exact location of the entire length of the two pipelines. The ROV also determined the condition of the concrete coating and the state of the cathodic-protection system that guards the pipelines against corrosion.

The cameras showed that 320 ft of concrete coating had broken away, resulting in minimal scarring of the exterior pipe coating. Additionally, the cameras provided detailed digital centerline and cross-sectional surveys of each pipeline.

Data from these surveys made it possible to determine the minimum quantity of rock fill required for the project.

The video cameras also located an assortment of debris--a Maytag washer, empty 55-gal drums, a ship's anchor, steel cables--resting under or against the pipelines.

Horizontal control for the survey work was provided by Starfix and Differential Global Positioning systems--satellite networks developed by the U.S. Navy to ensure accurate navigation for ships, including missile submarines.

The positioning systems, accurate to within 3-5 ft, were also used to place the 10,000-15,000 lb anchors along the full underwater length of the two pipelines (Fig. 3).

Precise placement and monitoring of the anchors were critical: Great Lakes Transmission insisted that no anchor come within 200 ft of either pipeline (Fig. 4).

The rock-placement barge and the diving barge used these anchors to position themselves.

Attached to the anchors in a four or six-point system, the barges (riding cross-wise to the pipelines) could move ahead, back, or sideways by pulling or releasing the cables.

CONCRETE REPAIR

Divers made repairs to the concrete coating at ten locations on the two pipelines. They wrapped fiber glass grout blankets, averaging 12 ft long, around the pipes at these locations. They secured the blankets to the pipes with straps.

Premixed grout was then pumped through hoses from the diving barge to fill the grout bags and complete the repairs.

When this work was complete, the divers visually inspected areas where the concrete was damaged and where debris was found to be certain the pipeline was free of damage.

Water temperatures, 34-35 F., required that the divers, to prevent hypothermia, wear wet suits equipped with hot water-circulation systems.

Length of bottom time depended on the depth of each dive.

Breathing a mixture of helium and oxygen while submerged, a diver was allowed to be on the bottom for approximately 45 min at the deepest depths of 240 ft.

Total time for descent, work, ascent, and decompression in the surface chambers on the diving barge was 3-4 hr, depending on depth of dive.

A second diver aboard the diving barge was in full diving gear, ready to enter the water at any time in an emergency. the divers began operations on June 25, 1993, and completed their work on July 17, 1993.

An ROV also supported the diving operations by taking video pictures of subsurface work, such as the divers removing debris.

ROCK-PLACEMENT PROGRAM

The project consisted of placing crushed limestone under the spans and next to and over the pipelines. The rock fill was mined and crushed to an average size of 5 in. in diameter at the Michigan Limestone Operations quarry near Cedarville, Mich.

A dedicated railroad delivered the rock to a port on Lake Huron, approximately 5 miles from the quarry.

Seven 1,200-ton barges dedicated to the Great Lakes' project transported the rock to the project site. Average barge transit time to the site was about 4 hr.

At the site, a 150-ton crane on the rock-laving barge transferred the rock to a steel hopper where it entered "fall pipes" specially engineered for this project (Fig. 5).

Used around pipelines and production platforms in the North Sea, fall pipes consist of telescoping tubes of varying lengths and diameters.

Two fall pipes were constructed for the job, one for water depths from 50 to 150 ft and the second for depths from 90 to 240 ft. Each fall pipe had three tubes, 30, 36, and 42 in. diameter.

Holes were cut in each fall pipe to draw water into it as the rock fell through. This design caused the crushed rock to flow through the fall pipes hydraulically, with the velocity of the rocks' descent decreasing as the diameter of the fall pipe increased (Fig. 6).

Contractors maintained the tips of the fall pipe approximately 15 ft above the pipelines by adjusting the length of the telescoping sections with wire lines.

Additionally, two winch lines permitted the operator to swing the fall pipes a few degrees in either direction from vertical to make placement of the rock in the desired locations easier.

The observed terminal velocity of the rocks being deposited on the bottom of the straits compared to the prevailing currents in the straits: 2-3 fps.

The resulting impact of the rocks striking the pipes was comparable to a brick being dropped approximately 6 in. in air.

The slope of the rock berms covering the pipelines was a maximum of 1:3 (vertical to horizontal) for the shallow spans and a maximum of 1:5 for the deep spans.

Where there was erosion that had created unsupported pipeline spans, three rock-placing passes over the eroded areas made the rock flow under the pipelines.

The first and second passes formed pyramid-shaped, continuous lines of rock mounds on either side of each pipeline, as well as allowing rock to tumble down under the pipelines.

A third pass, centered between the mounds, placed the rock directly over the pipeline, building rock up, around, and over the pipelines until 18 in. or more of rock cover had been established. A sonar device from Simrad-Mesotech Inc., Vancouver, placed near the end of the fall pipes helped in placement of the rock.

The device sent signals through cable to equipment in the control room on the rock barge. This equipment displayed an image of the lake bottom, the pipes, and the rock on a color monitor. The image showed placement of the rock relative to the pipelines.