HDD Risk Identification
Potential risks that are a part of all HDD projects include a failure to complete the bore, safety of the public and workers, environmental issues, damage to surface structures, and striking other underground structures. Risk reduction is the attempt to identify potential risks during the planning and design phase so that they may be eliminated or reduced. A risk contingency plan is often used by the engineer and/or contractor to help identify the major risks and develop general plans and procedures for resolving them. Unexpected risks often occur. Risk contingency plans are a starting point for determining the best course of action to overcome the problem. The use of experienced contractors is also a benefit when encountering unexpected problems. Their resourcefulness and experience at solving HDD problems will often reduce delays and find alternative methods in order to successfully complete the HDD project.
Failure to complete the bore is often the main concern, as the project would not be attempted if the product pipe could not be installed. The following are some of the frequent problems that can result in a failed bore attempt:
- The HDD can lose circulation by drilling through voids, highly fractured rock, a collapse of the bore hole fast, improper drilling fluids, or a migration of water. An effective drilling-fluids plan and a suitable drilling rate for the subsurface conditions around the bore are required. These are critical factors for maintaining circulation and bore-hole stability. Circulation can be lost very quickly if voids are encountered and cannot be resumed until the void is filled or the path to the void is sealed.
- Obstructions can cause a failed bore if the drill bit, reamer, or product pipe cannot be advanced past the object. If identified during planning, the best approach is to avoid the obstacle. If the bore cannot be changed, an appropriate response can be developed before installation begins. Typical obstructions faced during HDD construction are cobbles and boulders, gravel beds, wood, bedrock, and construction debris or foundations.
- Hydrolock is a condition that may occur when the circulation from the bore hole is lost and the subsurface formation is resistant to fracturing, resulting in a hydraulic cylinder in the bore hole. This problem is common in fine-grained rock, frozen ground, and any formation that is resistant to hydraulic fracturing.
- Line and grade problems occur due to faulty tracking and steering or subsurface conditions that prevent or hamper the proper steering. The improper matching of the downhole tools to the subsurface conditions can also cause line and grade problems.
- Bore hole collapse severely impacts the chances of success on any HDD project. Soft or loose soils present a high risk of collapse during HDD operations. When a bore hole collapses, there is an immediate increase in rotary torque and pressure and a decrease or loss of circulation.
- A failure or damage to the product pipe during HDD installation can occur due to an improper ream bore hole, poor workmanship, and improper HDD design. The best approach to avoid product-pipe failures is to establish conservative design criteria and follow the pipe manufacturer's guidelines. It is also important to make sure that the products specified in the design are actually the products delivered to the work site. Another key factor in maintaining suitable product pipe is to ensure that all weld or fusion connections are properly performed and tested.
- Surface heave or humping is a condition that is usually a result of excess pumping of drilling fluids after a loss of circulation. This condition can quickly pressurize the formation and cause a heave at the surface. Heaving can also arise from reaming with a barrel reamer without enough depth, which can result in a displacement of soil towards the surface. Pulling the reamer or product pipe through the bore hole too rapidly can also result in a surface heave.
- Fortunately large surface collapses are rare. Heaving or humping the surface is a far more common occurrence. Surface collapse is typically a result of a significant overexcavation above the bore hole, which can be caused by large volumes of thin drilling fluid used at high velocities, incomplete filling of the annulus with drilling fluids for a large-diameter bore near the surface, or leaks in high- pressure pipes after installation, which erode the soil above the bore hole.
Experienced and knowledgeable personnel are valuable assets in the successful completion of HDD projects. While unforeseen problems do arise, most problems encountered during HDD installation result from the decisions made during the planning and design phase or from the lack of contractor qualifications for the proposed crossing. Largely due to the rapid increase in HDD construction, the skill and experience of many HDD contractors are questionable. Many contractors are new to the HDD field and have limited experience. This makes it all the more important that owners and engineers make an effort to ensure that the contractors are qualified for the specific HDD project. In most cases a HDD contractor who has experience installing small-diameter pipe or communications cable is not qualified for a large-diameter pipeline crossing. The contract documents and technical specifications are an important part of most HDD projects. A key part of the HDD project's success is the contractor's full understanding of the project requirements. With proper planning and design and the contractor's full understanding of the project, the chances of a successful HDD project are greatly increased.
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