Cross-country high pressure gas transmission pipeline construction is all challenging in any country and in any terrain be it flat, mountainous, desert, boggy water-logged. However, it is all the more challenging in a country like Bangladesh, which has been crisscrossed by several mighty rivers and their innumerable tributaries, canals and waterways. Any pipeline of significant length faces few river and waterway crossings. At the early stage most of the rivers and waterways were crossed by open cut surface or bottom pull method. But in 1980s the newborn multipurpose company Bakhrabad Gas Systems Ltd first deployed Reading & Bates (the company who claimed to have patented the method) for Horizontal Directional Drilling (HDD) for crossing Karnaphuly river in north Patenga for its city ring main project.
Believe it or not, the 1700-meter long 16-inch diameter river crossing across the busiest river channel, the gateway to ever busy Chittagong Port was accomplished in three weeks off course at a few million dollars. The same river was crossed at Kalurghat by a local company by open cut surface pull method. It took much longer time and BGSL management faced a nightmarish experience. The pipeline was about to be swept away. During pull back the author witnessed both the crossings and promised not to be a party of such open cut crossing ever again. Since then the author has been involved in the following HDD river crossings

- 20-inch 800-meter Halda River crossing
- 30-inch 405-meter Gumti River crossing
- 30-inch 385-meter Buri River crossing
- 30-inch 565-meter titas river crossing
- 30-inch 585-meter Titas River crossing


Apart from above, the author visited sites of the following crossings during implementation of work.

- 20-inch Meghna-Gumti River crossing at Daudkandi
- 20-inch Meghna River crossing at Bausia
- 20-inch Old Brahmaputra crossing at Langolbandth
- 20-inch Shitalakhya crossing at Kanchpur


The objective of this article is to apprise the EP reader of the very interesting world of HDD crossing.
Directional crossing have least environmental impact of any alternate method. The technology also offers maximum depth of cover under the obstacles, thereby affording maximum protection and minimizing maintenance costs. River traffic is not interrupted as the works are confined to the banks. Directional drilling has predictable short construction schedule. Perhaps most significant these days, directional crossings are in many cases less expensive than other methods.
Originally conceived in the 1970s, directional crossings are a marriage of conventional road boring and directional drilling of oil wells. The method is now the preferred method of construction. Crossings have been installed for pipeline carrying oil, natural gas, petrochemicals, water sewage and other products.
Ducts have been installed to carry electric and fiber optic cables. Besides crossing under rivers and waterways, installations have been made crossing under highways, railroads, airport runways, shore approaches, islands, areas congested with buildings, pipeline corridors and future water channels.
The longest crossing to date has been about 6000 feet pipe diameters of up to 48 inches have been installed. Although directional drilling was originally used primarily in the US Gulf Coast through alluvial soils, more and more crossings are being undertaken through gravel, cobble, glacial till and hard rock.
Horizontal drilling requires precise engineering and design. Otherwise the probability of failure is not uncommon. HDI had a failure in its 24-inch crossing of Meghna river at Bhairab. In the recent past Cherrington Asia Limited (CAL), an offshore of Cherrington Corporation, failed to cross Titas River during implementation of Habiganj-Ashuganj section of Rashidpur-Ashuganj loopline project.
The accuracy of the placement of directional crossings is important for public safety. Those involved with directional crossings should be knowledgeable about methods to locate existing underground utilities, survey tool selection, survey tool operation and calculation methods. The directional drilling contractor is often asked to install a new crossing in proximity to a previously drilled installation. The engineer and contractor should be able to interpret all available dates so the new installation can be accomplished safety and without damage to existing utilities.


Pre Construction Design

The planning stages for any successful directional crossing are critical. The preliminary investigation of the project is to include a comprehensive utility compilation to properly plan the proposed drilling alignment. The accuracy of the initial survey of the existing underground utilities is the basis of the subsequent accuracy of installation. In extreme cases, property damage, injury or loss of life can occur from installations.
The emerging field of Subsurface Utility Engineering (SUE) is providing better methods to map underground utilities. Initially, the designer compiles a utility compilation plan from the owner's records and facility maps. During the preliminary design phase the depth prior the designer traces the existing utilities using electronic technologies to determine the surface location and presence of below ground facilities.


Technique

Pilot Hole: A pilot hole is drilled beginning at a prescribed angle from horizontal and continues under and across the obstacle along a design profile made up of straight tangents and long radius arcs. Concurrent to drilling pilot hole, the contractor may elect to run a large diameter “wash pipe” that will encase the pilot drill string. The wash pipe acts as a conductor casing providing rigidity to the smaller diameter pilot string for bit changes. The directional control is brought about by small bend in the drill string just behind the cutting head. The pilot drill strings not rotated except to orient the bend. If the bend is oriented to the right, the drill path then proceeds in a smooth radius bend to the right. The drill path is monitored by an electronic package housed in the pilot drill string near the cutting head. The electronic package detects the relation of the drill string to the earth magnetic field and its inclination. This data is transmitted back to the surface where calculations are made as to the location of the cutting head. Surface location of drill head also can be used where there is reasonable access. The magnetic steering tool is the industry standard for long, deep crossing. The magnetic steering tool allows the drill crew to calculate the position of the bore at any time during drilling. Drilling corrections are made so the bore complies with the crossing specifications.
The magnetic steering tool uses a rugged tri-axis magnetometer to determine the tool position relative to earth's local magnetic field. The magnetometer is an electronic compass. A tri-axis accelerometer package is used to determine the tool position relative to the earth's axis the accelerometer is an electronic level. Together the data output from the instruments are mathematically transformed to yield the tool attitude. The attitude includes the inclination of the steering tool, the direction of the inclination of the steering tool, and the tool face, and reference of the tool body position relative to the bore path.

Reaming: Once the pilot hole is complete the hole must be enlarged to a suitable diameter for the product pipeline. For instance, if pipeline to be installed is 36-inch diameter, the hole may be enlarged to 48-inch diameter or larger. This is accomplished by “pre-reaming” the hole to successively larger diameters. Generally, the reamer is attached to the drill string on the bank opposite the drilling rig and pulled back into the pilot hole. Joints of drill pipe are added as the reamer makes its way back to the drilling rig. Large quantities of slurry are pumped into the hole to maintain the integrity of the hole and flush out cuttings.

Pullback: Once the drilled hole is enlarged, the product pipeline can be pulled through it. The pipeline is prefabricated on the bank opposite the drilling rig. A reamer is attached to the drill string, and then connected to the pipeline pull head via a swivel. The swivel prevents any translation of the reamers rotation into the pipeline string allowing for a smooth pull into the drilled hole. The drilling rig then begins the pull back operation, rotating and pulling on the drill string and once again circulating high volume of drill slurry. The pull back continues until the reamer and pipeline break ground at the drilling rig.

Access: Heavy equipment is required on both side of the crossing. To minimize cost, access to either side of the crossing should be provided with the least distance from an improved road. Often the pipeline right of way is used for access. All access should be provided by the owner. It is not practical to negotiate such agreements during the bid process.

Work Space: The rig spread requires a minimum 100 feet wide by 150 feet long area. This area should extend from the entry point away from the crossing although the entry point should be at least 10 feet inside prescribed area. Since many components of the rig spread have no pre-determined positions, the side can be made up of smaller irregular areas. Operations are facilitated if the area is level, hard standing and clear of overhead obstructions. The drilling operation requires large volumes of water for the mixing of the drilling slurry.

Pipe Side: Strong Consideration should be given to provide a sufficient length of work space to fabricate the product pipeline into one string. The width will be as necessary for normal pipeline construction although a work space of 100 feet wide by 150 feet long should be provided at the exit point it self. The length will assure that during pullback the pipe can be installed in one continuous pull.

Stress Analysis: In finalizing design, the stresses imposed during construction and in service must be calculated and checked to remain within allowable limits for the grade of steel. The stresses at each stage must be considered acting individually and in combination. Stresses result due to spanning between rollers prior to pull back, the hydrostatic testing pressures, pulling forces during installation, radius of curvature as the pipe enters the ground, the drilling profile curvature, external pressures in the drilled hole, and the working pressure.

Pre-installation:
a. Hoop and longitudinal stresses resulting from hydrostatic testing are calculated.
b. Using the known distance between rollers as the free spanning stress the maximum hogging and sagging moments can be calculated. Considering the greater of these two moments, the maximum spanning stress is calculated. During hydrostatic testing the pipeline will be full of water therefore additional weight induced stresses must be taken into account.

Installation:
a. The spanning stress. Also apply in the installation phase.
b. The theoretical pulling force must be determined in order to provide the stresses that will result. An assumed down hole friction factor of 1.0 is recommended to provide conservative results and to include the effect of the pipeline being pulled around a curve. The maximum predicted pulling force should then be used in calculating the resulting longitudinal stress.
c. Allowing for a 10 percent drilling tolerance, leads to the use of a radius of curvature 90 percent of the design radius when calculating the longitudinal curvature of stresses.
d. External pressure from static head in the drilled hole and or overburden pressures must be considered. It is recommended that the static head resulting from the maximum envisaged drilling fluid density should be used with a factor of safety of 1.5 to provide conservative estimations of resulting hoop and longitudinal stresses.

Post installation:
a. The longitudinal curvature stress are used again
b. External pressure stresses also apply
c. Hoop and longitudinal stresses resulting from the final hydrostatic test are calculated

In-service:
a. Curvature induced stress
b. External pressure induced stress
c. The maximum allowable working pressure of the pipeline is used in calculating longitudinal and hoop stresses that will be imposed during service

Allowable Stresses: Having determined the individual and combined stresses at each stage of construction and those for the in-service condition, they must be compared with allowable limits

1. ASME B 31.8, Table A842.22 provides the following limits
2. Maximum allowable longitudinal stress: 80 percent SMYS
3. Maximum allowable hoop stress: 72 percent SMYS
4. Maximum allowable combined stress: 90 percent SMYS (SMYS Stands for minimum yield strength of the pipe material)