When pipelines are abandoned, they don't just sit there in the ground. They reverse the DC charge and it makes the pipeline basically dissolve into the ground.
Probably a valve leak...(no proof)....17000 gals is just a drip...a splash....400 bbls....not much more.... 
I have a 4' section of 20" pipe that I welded ends on to make a tank for an air compressor in the shop. It still sports the green.
Here's how you clean it up:
1. Setting firebricks underneath (to allow airflow), fill with wood.
2. Ignite.
3. Keep an eye on the fire, and keep the hose handy.
4. Eventually, the paint peels, then catches fire in great sheets!
5. Douse fire thoroughly. I thought I had, and even though there was still standing water on the driveway the next morning, the wood inside the barrel had caught again.
Too late for me now, but I'll back up the flammability of the coating. When welding the ends on my "tank", the stuff was on fire around the weld, dripping little fireballs on to the floor.
Williams made good use of that pipeline, and right of way, to make billions in the fiber industry.
If I had to hazard a guess on the cause of the leak, I'd say a blown seam, but a blown seam would probably have a bigger leak than 17k gallons unless it was caught very early and valves shut down quickly.
The inspection process is fairly simple. When a weld is made, it is visually inspected and then x-rayed. The X-Ray should show any defects inside the weld. On jobs using mechanized welding, Ultrasonic testing is used a lot in place of X-Ray. I'm not terribly familiar with UT, but I know that it uses sound waves to find defects and IMO is more open for interpretation than X-Ray is. If the band on the pipe that the UT machine travels around isn't straight, it can miss defects from my understanding. I may simply be biased against UT because I don't fully understand it. It is an accepted form of inspection, so it's probably fine.
Once the weld is accepted after X-Ray or UT, it is sandblasted and coated with the same green coating used on the pipe. Sometimes that is done using a machine that heats the pipe and applies the dope, and sometimes it is done by hand and basically painted on. After the coating is done, the length of pipe is checked with what is called a jeep, basically a charged spring that goes around the pipe and makes a sound that sounds like *Jeeeppp* if bare metal is found. This allows any damaged coated to be repaired to insure the pipe is protected from the outside.
Once the coating is checked, the pipe can be lowered into the ground and covered up. There should be someone watching this process to insure that the pipe is covered properly and not set on rocks or covered with rocks that can damage the pipe, but gas companies are starting to use less and less utility inspectors and rely on the weld inspectors for everything, and they can't be two places at once. Usually when pipe is being lowered in and covered up, there is welding going on up ahead, which is where the welding inspector can be found.
After so much pipe is lowered, usually many miles worth, it is hydro tested to check the rest of the pipe, and a "smart pig" is ran though to check for anomalies. Sometimes dents/etc are caught by the smart pig and sections need to be dug up and replaced, but smart pigs don't seem all that smart to me. I've done dig-ups and seen defects that were missed by the smart pig.
Once the pipe has been lowered in, covered up, tested and pigged, that section is ready for service.
Typically this process insures that the pipe will have many years of service without leaks, but it is still done by humans so there is always the potential for human error. Pipelines are still the safest and most effective way of transporting oil. Even accounting for an occasional leak, they are still the most environmentally sound way to transport gas and oil.
The inspection process is fairly simple. When a weld is made, it is visually inspected and then x-rayed. The X-Ray should show any defects inside the weld. On jobs using mechanized welding, Ultrasonic testing is used a lot in place of X-Ray. I'm not terribly familiar with UT, but I know that it uses sound waves to find defects and IMO is more open for interpretation than X-Ray is. If the band on the pipe that the UT machine travels around isn't straight, it can miss defects from my understanding. I may simply be biased against UT because I don't fully understand it. It is an accepted form of inspection, so it's probably fine.
Once the weld is accepted after X-Ray or UT, it is sandblasted and coated with the same green coating used on the pipe. Sometimes that is done using a machine that heats the pipe and applies the dope, and sometimes it is done by hand and basically painted on. After the coating is done, the length of pipe is checked with what is called a jeep, basically a charged spring that goes around the pipe and makes a sound that sounds like *Jeeeppp* if bare metal is found. This allows any damaged coated to be repaired to insure the pipe is protected from the outside.
Once the coating is checked, the pipe can be lowered into the ground and covered up. There should be someone watching this process to insure that the pipe is covered properly and not set on rocks or covered with rocks that can damage the pipe, but gas companies are starting to use less and less utility inspectors and rely on the weld inspectors for everything, and they can't be two places at once. Usually when pipe is being lowered in and covered up, there is welding going on up ahead, which is where the welding inspector can be found.
After so much pipe is lowered, usually many miles worth, it is hydro tested to check the rest of the pipe, and a "smart pig" is ran though to check for anomalies. Sometimes dents/etc are caught by the smart pig and sections need to be dug up and replaced, but smart pigs don't seem all that smart to me. I've done dig-ups and seen defects that were missed by the smart pig.
Once the pipe has been lowered in, covered up, tested and pigged, that section is ready for service.
Typically this process insures that the pipe will have many years of service without leaks, but it is still done by humans so there is always the potential for human error. Pipelines are still the safest and most effective way of transporting oil. Even accounting for an occasional leak, they are still the most environmentally sound way to transport gas and oil.
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