The pipeline industry is seeing a constant growth in the use of trenchless methods for pipeline installation. Horizontal directional drilling (HDD) has become a common method adopted by many contractors thanks largely to the fact that trenchless methods cause the least amount of surface damage and almost no traffic disruption. It has been found that the maximum lifetime load experienced by the pipe in most cases is during the pullback process when the pipe is pulled into the borehole.
Now that sturdier and elastic materials with better load-bearing capacity, such as high-density polyethylene (HDPE), polyethylene (PE), and glass reinforced plastic (GRP), pipes are being used for installation of new lines, the service life and long-term performance of these pipelines have increased greatly. (Learn more in "The Lifespan of Steel, Clay, Plastic & Composite Pipes.")
HDD combined with better pipe materials has made it easier to install pipelines through environmentally sensitive areas such as wetlands, estuaries and river crossings. The fused jointing that is used for PE pipes is excellent for these places, as it provides a leak-proof joint and ensures that sensitive areas do not get contaminated by whatever the pipeline is conveying. However; as with all construction techniques, trenchless methods are also prone to faults and failure.
There are parameters that need to be followed to ensure that the drilling procedure matches the geographical conditions prevalent in the area. Inadequate information about site conditions or improper geotechnical investigation and soil sampling can doom the project from the beginning. An experienced and knowledgeable contractor equipped with accurate information is vital to ensuring successful completion of any trenchless project.
One of the major problems that a trenchless drilling project can encounter is oversizing of the borehole, i.e. drilling or back reaming that exceeds the designed limit. This is especially common in large diameter boreholes because of the scope of work and unpredictable geographical conditions present. An oversized borehole can collapse, deviate from the planned path and cause settlement or heaving. To determine if any deviation during drilling is necessary, the ground movement is carefully monitored because timely intervention can prevent costly errors from taking place. Nominal oversizing of the borehole is essential for conveying cuttings and also for the back reaming process, but is limited between 1.2 to 1.5 times the diameters of the pipes to be installed; however, if cobbles and boulders are expected during the drilling process, or the soil has a tendency to swell, the oversize is increased to 25%. While this oversize is essential, a borehole that has a size greater than the required oversize can cause problems such as collapse, upheaval or settlement.
Causes of Borehole Instability in Large Diameter Boreholes
Boreholes can become unstable for a variety of reasons and usually are caused by a combination of circumstances, which could be unnatural or natural. Natural factors, also known as uncontrollable factors because they occur in nature include:
- Geologic formation with faults
- Fractures or cracks
- Highly stressed natural formations
- Induced or natural shale collapse
- Loose formations
- Mobile or unstable formations etc.
Unnatural factors also known as controllable factors include:
- Drill string vibrations
- Bit design
- Transient pore pressure
- Inclination or azimuth of borehole
- The density of circulating mud etc.
While the uncontrollable factors cannot be subdued, they can definitely be avoided by undertaking a professional geotechnical investigation of the proposed site before the bore path is planned. This helps to avoid any natural formations that can cause problems during the drilling process and to find an alternate path for the borehole.
Controllable factors can be kept in check by ensuring that the equipment used is as per design and is in excellent working condition. (Read on in "Proper Maintenance for Drill Rig Equipment.")
Indicators of Borehole Instability
A borehole can become unstable either due to direct causes or indirect causes. Direct indicators of an unstable borehole include the excess volume of drill cuttings, caving or heaving at the surface, cement required in excess over designed quantity, etc.
Indirect indicators of an unstable borehole include a hung up drill string or casing, increased pressure of circulating lubricant, excess drill string vibration, failure of drill string, poor logging response, problems with controlling deviation, experiencing high drag and torque, etc.
When a borehole is drilled, the equilibrium of the soil or rock mas is disturbed and redistributed because the original support is now replaced by hydraulic pressure from the circulating fluid. Instability results when this redistributed stress exceeds the rock or soil strength in compression or tension. Tensile failure can occur when the stress exerted by the drilling fluid, due to excess weight of the mud, causes a hydraulic fracture, resulting in loss of circulation. This can cause the tensile stress of the drilling mud to exceed the tensile strength of the geologic formation through which it circulates.
Problems Encountered with Large Diameter Boreholes
Collapse of the Borehole
When a borehole is drilled, the surrounding soil loses its support base and transfers its stress to the surrounding soil. The risk of borehole collapse is directly proportional to the diameter of the borehole. If the bedding planes are weak and intersect the bore at unfavorable angles, it can cause fractures that will allow the drilling fluid to find an escape route, thus decreasing the circulating pressure and disturbing the equilibrium. The resulting loss in pressure and softening of soil due to the invasion of fluid can lead to the collapse of the borehole.
Deviation of the Borehole
The allowable deviation for a borehole is +/- 1% of the bore length and the angle of bore should be between 8° and 20°, with 12° considered to be optimal. When a borehole exceeds the designed diameter it can cause the drill bit to deviate and bore at an alternate angle that can miss the target by a large difference. Any difference in mud pressure and variation in the vibration of the drill string should be taken seriously and corrected before failure occurs.
Settlement of the Borehole
Settlements that occur with boreholes are either large or systematic in nature. Large settlement commonly occurs when the borehole is over-excavated or the bore diameter exceeds the designed limit subject to the geological condition of the soil, or due to operator error. This can result in voids or sinkholes that appear on the surface. Systematic settlement occurs when the annular space between the installed pipe and the excavation collapses.
Heaving occurs when the depth of the borehole from the surface decreases due to oversizing or when there is over-pumping of drilling fluid after the loss of circulation in the borehole. The resulting pressure can cause heaving that is visible on the surface of the ground.
Nominal oversizing is essential to allow the drilling fluid to circulate effectively to lubricate and stabilize the borehole, help in the removal of soil cuttings and pull the pipe back for installation through the borehole safely. As the borehole diameter increases, the chances of collapse or deviation also increases, hence care should be taken to ensure that designed parameters based on the geotechnical investigation and report of the site are followed to the letter. This will help prevent costly reworking or failure of a pipeline project.