Henry M. Yamauchi
Westcoast Energy Inc.
Vancouver, B.C.
Design of the marine-crossing sections of the Vancouver Island, B.C., natural-gas pipeline incorporated measures to preserve or mitigate any damage to the sensitive waters through which it would pass.
This final of two articles on the project (Part 1, July 30, p. 86) discusses the marine crossing construction: the steps in its design and the options for its construction.
Construction on the pipeline system, under discussion for more than 30 years, began earlier this summer (OGJ, June 25, p. 32). It is designed to operate at 14,895 kPa (1,027 psi) over its 590-km (366-mile) length. It crosses a variety of terrain conditions in areas ranging from sparsely to densely populated.
The elevation varies from -425 m in Georgia Strait, an arm of the Pacific Ocean, to higher than 1,150 m on land. The subsea portion of the route represents 15% of the length of the project.
SURVEY
Prior to design of the marine pipelines, a detailed investigation was conducted of sea conditions to determine tidal currents, currents produced by surface winds, and wave-induced water motion. Marine geophysical and geotechnical surveys were conducted to define seabed conditions along the route.
A variety of instruments were utilized to collect data, including (Fig. 1) tethered subsurface current buoys, navigation systems, fathometers, sub-bottom profilers, side-scan sonar, and sediment-sampling equipment.
The result of these surveys and data-gathering programs is a detailed analysis of the water depth, subsea sediment layering, and the known position of all geological features and man-made objects near the selected pipeline route.
It is critical that all data collected be referenced to a known geographic location. Therefore, an accurate navigation system with repeatable results was required.
Typically, for marine surveys conducted close to shore (within 30-40 km), a series of microwave navigation stations are deployed at prominent positions on shore. The geographic location of these units is determined with conventional land-surveying techniques.
Onboard the survey vessel, a computer system continually calculates the location of the ship by transmitting a pulse-coded microwave signal to each of the shore-based slave units which in turn respond and pulse a signal back to the ship-board computer.
The time of travel to the different slave stations allows the position of the ship to be determined mathematically within 2-3 m. The computer provides steering and track data to the helmsman and fixes to the recording systems for the other instrumentation.
The system also incorporates data logging, an ongoing plot of location and track relative to the planned track, and provides a warning of proximity to unusual conditions in the area (such as moorings) which could affect the various items of towed equipment which at times were up to many hundreds of meters behind the survey vessel.
With accurate position information at all times and simultaneous recording of these data on all of the records from the different instruments, it is possible to correlate the data from the different acoustic instruments and to locate where the data came from.
FATHOMETERS, PROFILES
The fathometer is used to gather accurate measurements of the depth of water below the survey vessel and hence to determine bottom topography. The instrument uses high-frequency sound projected in a narrow cone to give as much detail as possible of the relief conditions of the ocean floor in areas where the maximum depths are in the order of 400 m.
Depth is determined by the time for a sound pulse to be reflected back from the bottom. The instrument is run constantly during survey operations to produce a strip-chart profile of the seabed.
Tides in the Georgia Strait (Fig. 2) result in water depth changes up to 5 m. To correct for this, the data are referenced to a tide gauge which records the change of elevation of the water surface over the day.
To delineate subsea sediment layering, three separate sub-bottom profiling systems were used.
These units all function on the same principle: A sound source produces an acoustic pulse at the surface of the water, which is reflected from the sea bottom and from various layers of differing density or materials below the bottom. The reflected pulse is detected at the surface with a string of hydrophones towed behind the survey vessel and recorded on a large plotter.
The end result is an acoustic record that shows the various layers of sediment (sand, silt, gravels, so forth) up to a 100 m or more beneath the seafloor.
The final record looks similar to a slice of birthday cake revealing the layers of cake and icing.
Depending on the instrument used, the frequency output/selection varied between 50 hz and 20 khZ, providing different resolutions and penetration depths of the bottom. The high-frequency signals provide the best resolution (in the order of 0.8-1.2 m vertical), but the greatest penetration is provided by the lower frequency signals, necessitating the use of different instruments to acquire the different types of data required.
SONAR; SEDIMENT SAMPLING
The image from the side-scan sonar (the monograph) resembles an oblique aerial photograph of the seabed created with a narrow-beam, high-frequency sound source (335 khz). The system uses a torpedo-shaped device ("fish") which is "flown" close to the seabed, following the contours of the bottom.
The sound is emitted as a fan-shaped beam on a horizontal plane from the fish. Any object lying on or protruding from the seafloor reflects part of the sound back to the instrument. The signal is transmitted via the tow cable to the surface where the computer processor and display devices are mounted.
The record is corrected for ship speed and acoustic distortion by the computer, and the monograph is displayed on a high-resolution CRT. Computer enhancement combined with the use of color to show the strength of the reflected signal (blue for weak return, red for a very strong return) provides approximately two to three times the resolution of a conventional side-scan wet or dry paper recorder.
The signal is also digitally recorded on tape and simultaneously presented on a thermal recorder for the highest possible paper hardcopy reproduction.
The monograph records consist of a swath of seabed information showing the geologic and topographic features or any man-made objects on the seafloor. Abrupt vertical features show up particularly well because of the shadow which they cast.
In this survey, the instrument was operated on a 150-m scale range (each side of the tow fish) to produce high-definition records with exceptional resolution characteristics.
For example, near the north end of Texada Island, a 40-mm diameter abandoned telegraph cable was defined at the outer perimeter of the record with the 150-m range. It was possible to follow the cable for several hundred meters and to measure its height off the bottom where it bridged between high spots.
The monograph records were laid out finally to produce a mosaic-like image for the operator and geophysicists to define all potential areas of hazard and seabed geologic conditions.
Marine sampling was carried out at locations selected to provide confirmation of the various deposits present and to provide samples for laboratory index testing and detailed logging. The locations of the samples were specified with coordinates from the survey system and were cross checked with water depth.
Some of the locations were targets less than 50 m across selected to obtain specific geological data. In general, the targets were successfully hit with the sampler through water up to 425 m deep.
The samples were taken with a gravity drop corer and a Shipbeck grab sampler. The gravity corer consisted of a core barrel with tail fins, additional weights, and a cutting shoe.
The corer was dropped into the bottom from a selected height above the bottom and penetrated the softer sediments, retrieving a cylindrical sample in a PVC tube. The grab sampler consisted of a spring-loaded bucket which rotated to scoop up a sample of the bottom when it was triggered by contact with the bottom.
Testing of the samples included detailed logging, grain-sized analyses, and various index tests.
Because many of the samples were very soft, extruding them from the sampler tube tended to damage them and destroy the internal structure. This internal structure was formed by the geologic mode of deposition and was critical to the proper interpretation of the geologic environment.
To obtain the maximum amount of information, all of the samples were X-rayed before opening which provided valuable information. In addition, information on the grain-size distribution in the samples was obtained, and the X rays allowed some samples to be logged and kept unopened for future testing.
It was found that some subtle information, important to the geological interpretation, could only be seen on the X ray.
SHORE APPROACHES
The shore approaches of Secret Cove, Kiddie Point, Little River, and Powell River will be excavated with conventional equipment, including backhoes and clams on barges. The approaches at Secret Cove on the mainland and Kiddie Point on the north end of Texada Island will require drilling and use of explosives.
The shore approach at Anderson Bay on the south end of Texada Island is more challenging because it is steep and irregular.
It extends approximately 420 m offshore to a water depth of about 250 m. The average slope is approximately 45 with several vertical or near-vertical rock cliffs and intermediate ledges.
This shore approach will be drilled from land to sea with the slant-hole drilling technique (Fig. 3).
After the drill rig is set up and positioned, a pilot hole will be drilled, followed by reaming to 508-61 0 mm. The enlarged hole will then be cased by the drill rig with 406.4 mm casing. Since the crossings are twinned, a second hole about 30 m away will be also drilled.
INSTALLATION
Various installation methods are available for construction of the marine pipelines.
The three main construction methods are conventional laybarge, dynamically positioned reel ship, and bottomtow.
LAYBARGE
The conventional laybarge method has been used extensively for the installation of offshore pipelines worldwide. Numerous pipelines of various diameters have been successfully laid and are operating in water depths and environmental conditions exceeding those in the project area.
This method involves welding together individual sections of steel pipe on the barge, inspecting each weld joint by radiography to ensure joint integrity, and applying corrosion-coating material to the weld-joint area.
The barge is either winched forward on its anchors or dynamically moved forward. The welded section is lowered down a ramp onto the stinger, which eases the bend of the pipe as it leaves the barge. The pipe forms an "S" curve configuration between the barge and the seabed.
The stresses in the suspended pipe between the stinger and the seabed are controlled by application of tension to the pipe aboard the laybarge. Deepwater pipe laying requires a combination of increased barge tensioner capacity and increased pipe-departure angles.
A laybarge has an array of anchor lines which are continuously relocated by anchor-handling tugs as the vessel moves forward. The forward anchors provide sufficient horizontal force to accommodate pipe tension, move-up loads, and environmental forces acting on the vessel. The breast anchors resist the broadside loads resulting from beam seas, winds, and current.
The laybarge method is the most commonly used of pipeline construction methods throughout the world and numerous contractors are capable of doing the work. However, there are generally no offshore pipelaying contractors on the West Coast of North America, particularly in the Pacific Northwest area.
The cost of mobilizing equipment and personnel from other parts of the world is considered a major economic factor. A laybarge is, however, expected to be operating off the California coast this winter.
REEL BARGE
The reelbarge method of installation utilizes a large reel mounted in a horizontal or vertical position on a barge or ship.
Before assembly of the corrosion-coated pipe on the reel, it is welded into long strings on land. The individual strings are in turn welded together to form a long pipeline segment as it is wound onto the reel. Depending on the size of the reel and the diameter of the pipe, large quantities of pipe can be wound onto the reel.
This installation method allows all of the welding, inspection, and corrosion coating of the weld joints to be completed onshore. Once the reel vessel is loaded, it moves to the offshore site and commences to lay pipeline in one rapid continuous operation.
Vessel positioning is done by dynamic means, rather than an anchor system, allowing a much greater rate of advance than for a laybarge.
The reel method of installation has been used successfully worldwide for numerous small-to-medium diameter pipelines. Pipe diameters up to 323.9 mm (12.75 in.) may be laid using the method.
The main advantage is that the pipe is welded onshore, where the work is not affected by adverse climatic considerations. Since all of the pipe is loaded onto the installation vessel at one time, the offshore lay operation may be completed relatively quickly.
While this installation method appears to be very sound, the required construction equipment has limited availability. Very few offshore pipelaying contractors have pipe-reel capability. Mobilization costs to bring a reel vessel to the Vancouver area will likely be high.
Nevertheless, preliminary economic comparisons run earlier this summer indicated the reel-barge method as the most likely method of installation for the marine-crossing sections of the pipeline.
BOTTOM TOW/PULL
Installing offshore pipelines by the bottom-tow method has been done in locations where conventional pipelaying equipment is not readily available, or not technically suitable. For this method of installation, the pipe is welded into convenient sections onshore and pulled into the required offshore location by the use of a moored barge or tow vessel.
If this method is selected, pipe make-up sites at Secret Cove, Little River, and Kiddie Point would be prepared by installing pipe-support sleepers, launchways, and holdback winches.
While the factory-applied, anti-corrosion and abrasion-resistant coated pipe is being aligned and welded into strings of approximately 300 m in length, the pipeline trenches and directionally drilled holes with casings will be prepared at respective shore-crossing sites.
Welds will be radiographed, and each string of pipe section will be hydrostatically tested prior to being placed in a storage area for use in the tow.
During installation, the first section of pipe section is towed by the towing vessel, followed by successive strings of pipe which are welded together as the pipe string is slowly towed. As each string is successively welded, the weld joint is radiographed and the anti-corrosion coating applied.
When the full pipe string is assembled, towing is continued to get the pipe to the beach in the landfall area. At this time the tow cable is transferred to an onshore winch which continues to pull until the lead pipe and trailing end are positioned in the specified target areas.
After tow completion, divers inspect and correct seabed spans, and backfill process for the shore crossings and approaches.
The bottom-tow method enables a greater use of local equipment and labor.
TOW-ABRASION TEST
The bottom-tow method, which consists of towing the pipe string on the bottom, requires a barrier or abrasion-resistant coating to protect the anti-corrosion coating during towing over rocky areas of the seabed.
To select the best abrasion coating, a tow test was conducted to evaluate 10 different coatings from four generic abrasion-resistant materials: solid-filled epoxies, urethanes, concrete, and polymer cement. The test employed 24 joints of coated pipe welded into two strings, each measuring approximately 220 m in length.
Two separate tows were conducted, the first for a distance of 15 km out and back. The sea bottom material in this case was mostly soft sediment, sand to cobbles, and some boulders.
The second was over a distance of 24 km, out and return. In this case the strings were pulled over the rocky sea bottom near the north end of Texada Island.
The tow test confirmed original coating assumptions. Selection of the abrasion coating is currently being made, based on an overall evaluation including technical performances during application, tow, and cost. The cost includes coating materials, application, and other installation impacts due to coating weight, field joint material, and application schedule.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
