Sunoco is struggling to complete a section of the Dragonpipe (Mariner East pipeline system) that runs under Valley Creek near the Chester County Library in Exton. They have had a long list of problems at the site, culminating in a problem that has stopped drilling for over a month. It remains to be seen how the company plans to complete that section.
Sunoco’s initial plan for the area was to use “horizontal directional drilling” (HDD) to go deep underground, but that threatened the local drinking water aquifer, so Sunoco switched to the use of open trench construction where possible, combined with flex-bore (aka “HDD lite”) under Lincoln Highway and auger boring under Valley Creek. Auger boring involves installing a casing (basically a large steel pipe through which the operational pipes can later be pulled) immediately behind the drill bit, as part of the boring process. The process is not going well.
Earlier flex-bore drilling under Lincoln Highway (Business Route 30) triggered a series of about a dozen sinkholes and the auger bore process at Valley Creek has already triggered three more subsidences. Sunoco has gone to great lengths to hide what is happening from the public, to the extent that when a property owner put up a camera in his backyard to monitor Sunoco’s activities, the company responded by putting up a makeshift 18-foot fence of black plastic and 2x4s, which promptly blew down. But the sinkholes and the watchful neighbors are probably the least of Sunoco’s problems right now.
Troubles from the start. The distance Sunoco needs to bore under Valley Creek is only a few hundred feet. But as soon as the boring process started, sinkholes began forming at the drill location. In addition, Sunoco has had trouble keeping ground water out of the drill pit in spite of an array of pumps and hoses. A containment area for the pumped water keeps silting up and overflowing. And now, after only about 25 feet of progress, construction has been halted (due, as I understand it, to the presence of rock that resulted in a broken drill). There has been no drilling at the site since February 27.
With HDD, you have choices if you run into trouble. We can now see that the choice of auger boring at Valley Stream carried with it a greater risk of problems than the HDD approach that Sunoco has generally used. (Here, a disclaimer is needed: in this section, I will be discussing issues involving auger boring, geology, hydrology, and HDD in which I don’t claim expertise. The information I have comes from Sunoco’s regulatory filings and from learning what I can from the internet sites of companies that provide the equipment used by Sunoco’s contractors. I encourage readers who know more to set me straight if my analysis is wrong.)
When installing a pipe using HDD, you start by drilling a small pilot hole. Once it is completed, it will ultimately be successively reamed to the desired diameter before the pipe is pulled through. In the course of drilling the pilot hole, you have a variety of options if you run into drilling problems.
If the drill breaks or becomes dull, you can pull it out, replace it, and send it back down the pilot hole. If you have problems with tough rock or “voids” (fissures), you can pull the drill back part way and (because the drill is steerable), try drilling at an angle to your original path, taking the drill deeper or to one side. If you still can’t save the pilot hole, you can fill it with grout and start over, on a new path.
Auger boring provides few options. With auger boring, the process is quite different and you lose most of the options you have with HDD. (A short video showing how the process works is here.) You start with a huge drill in a bore pit. There is no pilot hole. From the start, you are drilling at the final size and pushing in a casing right behind the drill head. A spiral auger inside the casing pulls the cuttings back to the entrance pit. As drilling progresses, new sections of casing are lowered into place and welded onto the casing that is already in the ground. For a large casing, this process is basically non-reversible.
And, for Mariner East, the casing is very large. It is a pipe with an internal diameter big enough to enclose both a 16-inch and a 20-inch pipeline, plus spacers, which means it must be well over three feet in diameter. To create a hole for the casing, the drill must be as wide as the casing is. Once you have that much steel casing in the ground, you can’t pull it back out. You can only push forward.
What can Sunoco do if the problem is serious? From what I hear, the drill at the front of the auger is broken and has become stuck in place. That’s a problem. It might be possible to dig down to the broken drill, retrieve it, and replace it. If that were simple to do, it would already have been done. I have to assume it can’t be done, perhaps because of difficulties in reattaching the new drill to the front of the auger. Even if this approach were to be attempted, it would be very difficult to keep ground water out of the hole. I believe that the present location of the drill is below the local water table.
Another option might be digging a huge pit to expose the entire casing, cutting the casing into pieces, and pulling the pieces out with a crane. That would require operating heavy equipment in close proximity to the other operating pipelines in the easement, a dangerous proposition. And again, ground water would be a massive problem. Once the casing and auger were removed, the pit could be filled and Sunoco could try again.
Abandoning the partially-installed casing and starting again nearby would require finding a new drill path in a 50-foot easement that already has several active pipelines in it. It seems unlikely that a practical alternative path for such a large casing could be found.
What about rerouting? Finally, Sunoco could find an alternative route for this section of Mariner East (as the DEP is asking Sunoco to do at Marsh Creek). That would add a year or more to the project. And finding a new route in a densely-populated area like Exton would be very difficult.
In the “re-evaluation” process required for this stretch of pipeline, Sunoco was required to examine alternative routes. In its filing, the company wrote: “… due to the developed congestion of the area including residences, businesses, roads, and other utilities, there are no practical reroutes for the pipeline in this area.” Although Sunoco’s examinations of alternative routes are not to be taken seriously (as we learned at Marsh Creek), it would indeed be challenging—though not impossible— for Sunoco to find a good alternative route.
To conclude, if my interpretation of Sunoco’s situation turns out to be correct, the company has a major problem on its hands. I want to emphasize again: I am writing about technology that I do not have first-hand experience with. I encourage comments from readers who are more knowledgeable. But the fact that the stoppage at Valley Stream has now dragged on for more than a month suggests that problems along the lines that I am describing may well have occurred there.