Maximum Allowable Operating Pressure (MAOP) calculation / Barlow's Formula
In the Corrective Order issued by PHMSA to ExxonMobil Pipeline Company (EMPCO) dated Apr 2 2013, it is stated that
- the pipeline was 20" O.D.,
- 0.312" wall thickness,
- made of API-5LK-42 pipe. (This pipe has a yield strength of 42,000 psi)
Using a minimum safety factor of 1.5, this yeilds a MAOP of 873.6 psi.
Note well that this MAOP would be valid assuming the pipeline was brand new, without any damage or corrosion. Note well also this pipeline is in fact 66 years old.
There are several basic questions which have not answered.
- Where has the failed pipe been taken? What is the name of the lab?
- Who is lead agency in the investigation? PHMSA? / DOT? / NTSB? / EPA?
- Get the operational records for the East Conway Pumping Station and MAKE THEM PUBLIC.
- Was there cathodic protection? When was it last inspected? Was cathodic protection maintained during the years when the Pegasus pipeline (formerly known as the Magnolia Pipeline) sat idle?
- Has that pipeline ever been cited for safety violations or corrosion anywhere along the line?
- HINT: The cited operating pressure of 708psig at time of failure and MAOP of 820psig is NOT CONSISTANT with other Dil-bit pipelines which nominally operate in the range of 1300-1440 psig.
- What is the nominal pipeline thickness? What was the thickness at the point of failure?
- What were the DETAILED RESULTS of the last 10 years of in-line inspection data? Don't accept the simple answer, "it passed". It's not that simple.
- What is the kind of steel used in the pipe? What is the yeild strength (can only be determined by metallurgical analysis).
- Where are the nearest isolation valves? When exactly where they closed?
- What is the timetable relating to pipeline startup or shutdown events? (Need records for compressor stations 100 miles in both directions for last 5 years)
POSSIBLE CONTRIBUTING CAUSES OF FAILURE:
- Corrosion due to age
- Corrosion due to lack of cathodic protection
- Corrosion to due to properties of product being transported (dilbit synthetic crude)
- External damage due to construction, and/or local seismic activity.
- Sustained overpressure due to market demands, and the physics of high-viscosity pipelines (dilbit pipelines have been widely reported to run at 1300-1440 psig, which is far different from reported MAOP=820 psig and O.P.=708 psig at time of failure)
- Over-pressure events (impulse) caused by cavitation, which is known to be an issue with high-pressure, high-viscosity pipelines.
- Over-pressure events (impulse) caused by fluid-hammer associated with compressor startup/shutdown, or valves opening/closing.
(Repeated overpressure events can stress pipelines and cause small cracks which can lead to failure.)
- Mayflower is in the middle of two opposing forces. a) the Conway compressor/pumping station, and a b) gravity from forcing heavy dilbit up a big hill ~7 mi SE of Lake Maumelle just east of where it crosses under Hwy-9. Another way to say this is that it is at the bottom of a "pressure well". It is the most likely place of failure b/c that's where the highest pressure will be.
- Operator Error and/or Instrument Failure.
One curious thing.. ExxonMobil says they detected the pressure drop and shut the pipeline down as early as 1:15 p.m., yet the first 911 call was not until 2:45.
I believe it is possible that there was operator error, that the apparent pressure drop may have been due to cavitation (prone in high-viscosity pipelines), not a leak. If they shut a valve downstream first, then this could have caused a pressure wave backwards towards Mayflower. This could have actually caused the rupture.
Tell the media, the attorney general, the private attorneys, to get these questions answered!
GET THE OPERATIONAL LOGS FROM THE CONWAY COMPRESSOR AND ANY MONITORING STATIONS WITHIN 100mi and MAKE THESE DATA PUBLIC.
If the journalists, government officials, and private attys fail to get this information, then YOU (private citizens) must get this information!
I hope this helps-- BH