Field Testing Casing above 80% of Burst
11 May 2013
I'm currently facing well design issues related to casing pressure test.
According to planned architecture, Pore & Frac pressure in Hands, and design file, the Maximum expected Wellhead pressure of my casing will correspond to 91 % of casing burst.
Triaxial design analysis on a well full of gas scenario is showing good results.
However, the associated pressure test required would have to be as high as 91% of Yield.
This test (if performed) would have to be performed having a certain number of mitigations measures in place (light fluid inside casing / test packer to test only required area)
Pure desk engineering considerations are showing achievable results.
However, this is going against common "oilfield" standards calling not to go above 80% of Burst (and I must say against my common sense considering the risks doing this operation in case of failure).
Recognised engineering and design companies are also always remaining that Pressure test should not be a dimensionning load case in any architecture.
My past experience in HPHT has also led me not to test to MEWHP at least for drilling (DST case is not what we are talking about here) in almost all cases...
The maximum documented value found in some Operators standard was 85% of Burst.
My question is the following: Has anybody within this community already experienced and performed hard cement pressure test on the rig at values above 80% (or 85%) of casing Burst for an intermediate casing.
If yes, what was the feedback and what would be your recommendations.
If no, I would also be interested in having your arguments.
If you believe I am a fool trying to conduct such a test, say it :-)
Thank you for your view on this one
You're certainly not a fool to question this test; it's not needed (well almost not needed).
What is the objective(s) of the test? Pressure vessel integrity, verification of worst case load case, connection leaks, others?
It might seem radical but why pressure test at all? Do we test also for collapse or tension? No, we trust this element of the design, so why don't we trust the burst design? It's probably because we've always done it, its easy to do and consequence driven since a burst failure will nearly always be at surface having the greatest HSE impact.
If the test if verification of the worst case design load then this will often drive the tubular design, particularly in HPHT (and most certainly in XHP wells) which is asinine. The test conditions can't replicate the load case conditions and the probablity of the test is 1, whereas the likely next worst load is a failure case, having a probability much less than 1. Relability Based Design is a perfect application here and you completely avoid the awkward (read impossible) question of trying to quantify to management about what is the additional risk of lowering a design factor. (Since DFs and working stress design methods are incapable of describing likelihood of failure) RBD can quantify the risk.
Can I suggest limiting the test to a lower, nominal pressure which is an operationally safer test to verify integrity? Make sure the tubular specification and its manufacture is adequate, that the tubulars are competently inspected and handled (both in shipping and running) appropriately, that the correct thread compound (if used) is used, correct MUTs are used, that there are competent TRS casing crews and the connection make-up is verified as run.
In the early days we used to test to MEWHP + BHM for HPHT wells. Sometimes this was right on the limit of 1.1 burst DSF. This called for +10ksi float equipment and nerves of steel. Today we test to the maximum drilling load case (100bbl Kick) anticipated using weighted mud- this is because it has been field proven that pressure testing with Mud is not a guarantee that the casing will not fail a pressure test with Brine or Seawater later. Ensure the correct procedures are taken to ensure the casing is made up properley- this is where big mistakes can be made.
Usually it is the minimum design factors that control our casing design. If certain load case is within the limits of design factor in respect to casing burst rating then it is fine. 80% rule would be too conservative in many cases for the burst load.
Generally hydrostatic pressure test should not drive your casing design, but rather operational load cases. If in this case your load case also results in lower design factor then you should consider changing pipe or doing risk assessment for alternative options.