Comparison of Pipeline Drilling Methods: Bores, HDDs & Direct Pipes [Gaille Energy Blog Issue 75]
- Posted by scottgaille
- On September 24, 2018
- 0 Comments
- Boring, Direct Pipe, HDD
Over the course of their construction, pipelines encounter many obstacles that require the use of subsurface tunnels. Such tunnels enable the pipeline to pass beneath roadways, railroads, rivers, and environmentally sensitive areas without disturbing them. There are three principal methods of constructing pipeline tunnels: (1) conventional boring, (2) horizontal directional drills, and (3) direct pipe drills.
- Conventional Boring
A boring machine is lowered to the bottom of the bore pit to tunnel using a cutting head mounted on an auger. The auger rotates through a bore tube, both of which are pushed forward as the hole is cut. The pipeline is then installed through the bored hole and welded to the adjacent pipeline. . . . Major factors limiting the success of a boring operation include the crossing distance, subsurface soil and geologic conditions, and existing topography. Boring operations typically occur over crossing distance of 50 to 60 feet. The maximum length a bore could achieve in ideal soil conditions typically does not exceed 400 feet. The Williams Companies, Inc., Subsurface Pipe Installation (2014).
The below figure illustrates how boring is used to pass beneath a roadway.
- Horizontal Directional Drilling
Unlike boring, an HDD can be used to install subsurface pipe over great distances. The longest successful land-based 30-inch HDD is just under 7,000 feet in length. . . . Factors which affect the success of an HDD operation include crossing distance, subsurface soil and geologic conditions, and existing topography. Risks associated with a HDD crossing technique include potential inadvertent returns of fluids during HDD drilling operations and potential hole collapse during construction. The longer the length of the HDD, the more forces are applied to the pipe and the larger potential for failures. The Williams Companies, Inc., Subsurface Pipe Installation (2014).
Earlier this month, I had the opportunity to visit an HDD that was underway. The below image shows two different sized reamers that are used to expand the diameter of pipeline tunnels. The wider the tunnel, the more reamers are required. Each tunnel begins with a small diameter pilot hole, which is then widened with sequential passes down the hole—each using a larger diameter reamer.
These reamers are attached to the end of a drill string. The video below shows the rig spinning the drill string to advance the reamer through the earth. The entrance to the HDD tunnel is directly behind the photographer.
The shavings from the drilling progress and related mud flow back out of the hole in the video below.
The returning mud is then processed and disposed of in the following video.
When the tunnel is complete, the pipeline is inserted and pulled through the hole as depicted below.
HDD risks include (i) the hole collapsing before the pipeline can be pulled through or (ii) the pipeline getting stuck during the pull through.
- Direct Pipe Installation
Direct Pipe is a trenchless method that combines the advantage of established pipeline installation methods of microtunnelling and HDD. Direct Pipe installations may be much shorter and shallower than HDD installations because the excavation is continuously cased, reducing the risk of hole collapse and subsequent settlement. The Williams Companies, Inc., Subsurface Pipe Installation (2014).
As described above, the HDD method creates the tunnel first and then, as a second step, the pipeline is pulled through it. With direct pipe installations, these two steps happen simultaneously. The direct pipe drilling head is attached to the end of the pipeline, such that as the drill head advances through the earth, the pipeline is pushed/pulled along behind it. The returns from the drilling are delivered back to the surface via umbilicals inside the advancing pipeline. The below figure illustrates how the drilling mechanism (far left) is attached to the pipeline.
The following figure depicts a direct pipe installation passing beneath a river.
One risk of the direct pipe method is a prolonged mechanical breakdown in the midst of the drill. If the drilling is halted for too long, the pipeline can adhere to the sides of its own tunnel, thereby making it difficult to continue the pipeline’s advance.
- Drilling Issues in Construction Agreements
The principal contractual issue that arises from drilling is which party bears the risk of a failed hole—or one that is more expensive than anticipated. Pipeline construction contracts often include “no hole, no pay” clauses, which (i) make the drilling contractor’s compensation contingent on success, and/or (ii) only pay the drilling contractor a fixed fee irrespective of how much it actually costs to complete the hole (or how many times it has to be drilled). Such clauses can increase the cost of underground installations, since the drilling contractor will build into its price the risk of failure.
There also is the related question of under what circumstances, if any, the drilling contractor’s compensation can be increased. What happens if, for example, the drilling contractor encounters unanticipated subsurface conditions? Even if sample cores are taken and studied, the conditions may vary between the sampling locations. The agreement should make clear whether the owner or the contractor bears the risk of unknown conditions.
While circumstances vary by project, it is better for such drilling risks to be discussed in advance and addressed in the construction agreement—rather than after a problem develops.
About the Gaille Energy Blog. The Gaille Energy Blog (view counter = 80,240) discusses issues in the field of energy law, with periodic posts at http://www.gaillelaw.com. Scott Gaille is a Lecturer in Law at the University of Chicago Law School, an Adjunct Professor in Management at Rice University’s Graduate School of Business, and the author of three books on energy law (Construction Energy Development, Shale Energy Development,and International Energy Development).
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