Pipelines - How Deeply Should They be Buried?
One thing we’re doing more of these days is studying older pipelines, and trying to figure out how deeply they are buried. Some companies have no records of this type of measurement, and need to create good files that tell the story.
How deep below the ground surface should a pipeline be buried?
The first answer, as in many things with asset integrity management and corrosion control, is often, “It depends.” But there are some “minimums” that certainly must be met.
Rules governing the oil and gas pipeline industry call for at least 2.5 feet of “cover” over the typical oil or gas pipeline. And what is cover? It’s the soil that was dug out of the ditch in the first place, unless some other factor is involved. If your pipeline crosses a drainage ditch, a stream, or maybe even an existing pipeline, then it must be buried much deeper than that minimum. If a flowing stream is crossing the pipeline route, then concrete, or rock cover, or other dense material has to be used to hold that pipeline in place over time.
For pipelines installed in the last 10 to 20 years, it is likely that good files of “as-built” measurements were built, about a specific line. Those files should show the route of the pipe, where it changes direction as seen from the air, and hopefully those changes of direction for the pipe to run uphill or downhill. The depth of the pipe, at very regular intervals, should be recorded. Why is this important? When some new construction activity is going to be done near, or across, that pipeline, the technicians for that company must be able to go accurately locate that pipe – BEFORE someone else starts digging next to it! All 50 states have laws in place that require “One Call” locating of buried structures to be done, before new subsurface construction starts. That construction might even be repair of an existing structure. But all underground “utilities” must be accurately marked before new digging gets done. It’s for the safety of all people in the general area, and to preserve the integrity of existing structures and assets.
What about older pipelines? Sometimes the file information is good, and has been kept in a known place by the company which owns the line. But what if a pipeline was sold from one company to another? Did hard-copy records get transferred from one entity to the next? What if it got sold more than once? Many times such financial transactions lead to big disruption in records being maintained, knowledgeable people not staying involved in the operation, and the “corporate memory” being lost.
So there are many pipelines that don’t have a good “book of data” which tells their story. We’re often asked to help build that data set about an existing line. And to prove up where it physically runs in “plan view” (looking down from the sky) is one important part. Then we need to establish how deeply it’s buried, at many different points along the route. And then we may also study the easement or right-of-way description, to map that in and compare those legal “boundaries” with where the pipeline physically lies.
Very important are the places where the pipeline crosses driveways, roads, railroads, streams, rivers, lakes, and any other “out of the ordinary” places. Sometimes the pipe may run through another, larger pipe called a casing. Casings were used to help protect a pipeline from heavy loads that may run across it, on a road or highway, or on railroad lines. Identifying the locations of casings is yet another very important part of pipeline surveying.
It’s also vital to look for other pipelines, water lines, sewer lines, fiber-optic cables and other buried structures, which cross the line we are studying. Those other utilities could cause impact to our customer’s pipe, at some point in the future. In some instances, there is electrical interference between two pipelines, which can make corrosion of one pipe occur much faster. Even things like electric train or trolley systems may pose a lot of corrosion risk to buried steel pipelines.
Some field conditions make it clear that a pipeline should be buried much deeper than 30 inches. If there are a lot of houses or commercial buildings close by, depth of burial gets increased for a new structure. But what if the neighborhood gets built up AFTER the pipeline was installed? The risk of a pipeline being there is probably greater than it was. But who needs to assess that risk? What steps can be taken to monitor the pipeline better, and reduce the risks of operating it in that more crowded environment?
And finally, what if you are in farming country? What is the deepest part of the soil that a farmer may be tilling and turning over, to make crops grow better? If a pipeline is not buried deeply enough, or if Mother Nature erodes away some soil with time, a farmer may till up the pipe! Or, just as bad, the farm implement may cut open the protective coating on the pipe, gouge the metal, and cause weakening. Whether slowly or quickly, this kind of damage may hurt somebody!
We want to help responsible pipeline owners and operators maintain good records, especially for those pipelines which pose higher risk levels. And sometimes we help build records pretty much “from the ground up,” to the degree that this even can be done.