DOWN HOLE INSTRUMENTATION HOUSINGS:
WHY A PRESSURE CERTIFICATION TEST ?
Pressure housings, as used by the wireline service companies are subjected to heat and external pressures - so called collapsing pressures. These can reach well over 25,000 psi. Temperatures can reach over 700ºF in geothermal wells but normally do not exceed much over 500ºF in deep oil wells. The pressure is generated by the column of drilling mud within the borehole, while the temperature is generated by the geothermal gradient of the earth. The closer to the center of the earth the hotter the temperature. These pressure housings normally contain sophisticated electronic payloads that measure specific parameters of the surrounding environment.
Pressure housings are rated by their operational limits. For example: 25,000 psi @ ambient temperature.
Since the temperature weakens the material, this same housing may be down-rated to 20,000 psi @ 500ºF.
Pressure housings are routinely designed with a very little operational margin for failure - normally about 10%. This represents the compromise taken between the dimensional constraints and the odds of a collapse incident.
The design and engineering of a pressure housing require a substantial amount of field experience as related to the housing's intended utilization. This is normally done by an engineer experienced in this art. He relies on: a couple of formulas, the inherent strength of the material, the strength of the material when hot, and the housing's wall thickness. Threads, seals, embrittlement, and other mechanical loads have to be considered and incorporated in the stress analysis.
Once this is done, the engineer has to match the calculated stress with the material that will support this stress. Here is where the design calculations enter into a "gray" area: Estimating a realistic value for the variables. For example:
The actual raw material's "proof strength" may vary several percentage points from that which is advertised.
The actual strength enhancement obtained from the heat treating, aging or cold working of the material may vary a few percentage points from that which is advertised. (Mostly due to shape or size and/or process tolerance).
The actual strength of a particular material at a given temperature may not be obtainable from the manufacturer or if provided it may be in a general or vague format.
If not chart-certified, the wall thickness of the machined housing may vary considerably from the specified dimension. This is due to the hole not being concentric with the outside diameter. The ends of the tube may be within the concentricity tolerance, but the rest of the tube may not.
All these variables (whose real value the engineer can only guess at) when combined may add up to be MORE THAN the 10% failure margin designed into the pressure housing. As a result, the theoretic operational limit of a housing is at best a "probable limit." This is true for each individual housing regardless of the manufacture strategy used to produce it.
Collapsed housings are spectacular. They always draw a crowd.
Once the mechanical and structural equilibrium of a housing are overcome by the surrounding hydrostatic pressure (or triggered by an additional shock) it collapses. This produces an implosion which enhances the destructive forces of the pressure. The event is complete in a few milliseconds.
Most of the time, the collapsed housings look like a steel snake after eating its diet of bulkheads, capacitors, transformers, detectors, and things like that. Small bumps in the otherwise shriveled skin show their location. Design engineers debate for days on end about the collapse starting point and direction (they estimate this from the extrusion direction of what is now electronic paste), material fatigue and wear, velocity and peak pressures of the shock wave, and many other things that could explain why a certain fold, or bump, or valley on the skin of this hydraulics' masterpiece has the shape that it has.
Invariably, shortly after one of these unfortunate (although spectacular) events, the service company's reputation may become tarnished and all the affected managers begin a frantic search for an owner of this now "snake."
The managers invariably side with one of the debating camps: the ones who suspect a weak design or the ones who suspect a field abuse of the housing in question. The key arguments are always the same:
1. WHAT IS THE OPERATIONAL LIMIT OF THIS TYPE OF HOUSING ?
2. WAS THIS PARTICULAR HOUSING SOMEHOW WEAKER ?
The answer to the first question is normally dependent on a joint decision between marketing and engineering. On one hand, the deeper the well the more profitable the service. On the other hand, the deeper the well, the closer the failure conditions grow to the operational limit of the housing. Typically, the limit of a premium housing is about 90% of the theoretical collapse pressure as adjusted by actual hot hydrostatic proof tests.
The answer to the second question requires an abundance of personal composure, especially when looking at a contorted flat tube full of little flat things. As a rule, the question is posed by the damaged party. Also as a rule, unless the housing previously endured a hot hydrostatic proof test confirming its operational pressure, any and all of the answers become academic in nature and by default, the answer is assumed to be affirmative. Its direct consequences may be very, very costly.
The reason for a pressure test certification is to dispel any doubts about the soundness of a particular pressure housing. It helps both the service company and the owner of the well in assessing the operational responsibility of a particular service.
ADDITIONAL BENEFITS OF PRESSURE TEST CERTIFICATIONS
Third Party Test: This assures an objective test. It removes the doubt perceived when a company is responsible for it's own "self-evaluation" conclusions. Specially when rating it's own products.
Material cost and supply: It is not uncommon to see Material Specification Sheets for premium housings which are many pages long. In them, there are many limits for every phase of it's steel production. Minimum of this, maximum of that, and so on. As a rule, their limits are extremely narrow, and conform very closely to the manufacturing procedures of one particular steel mill. While a material with similar properties (although produced under different, or more relaxed specifications) may perform as well and be more economical, the engineering department of a company may be apprehensive in authorizing alternate specifications and/or suppliers since the original ones "proved" to be adequate.
By introducing a requirement for a Pressure Test Certification combined with a broader latitude in the material chemistry and/or production, it would encourage the steel mills to develop and produce alternate variations of the materials with the potential for higher properties, or at least less costly.
Within this unconstrained environment, the Purchase Order to a vendor, would read similar to this example: "Supply & Pressure Housings made with ASTM 6226 type steel with an O.D. of 3.625" +/- .010 ; an I.D. of 2.780" +/- .005 ; 178" long ; Individually certified at 25,000 psi external pressure while at 500ºF." This would compel the steel mill, as well as it's processing shops, to guarantee each other's quality and workmanship in the creation of an acceptable product. The buyer benefits from this strategy by promoting the responsibility, as well as the inventive potential of its suppliers, while enjoying a "proved" new product adequately suited for its intended use.
Basic: Cold Test
Load specimen inside the pit, seal, pump to pressure, hold 15 minutes at pressure, release pressure, open the pit, unload the specimen from the pit, inspect the specimen for leaks, issue certification of the pressure cycle and observed leaks, or specimen failure (if any).
Hot Test
Load specimen inside the pit, seal, pump to 2,000 psi to set all the specimen's seals, start the heating cycle, allow the pressure to rise due to the expansion of the water, throttle the pressure when it reaches the test set point, allow the temperature to reach the test set point, throttle both temperature and pressure at their set points, and hold these for 15 min. Start the cooling cycle. Wait until the pit temperature falls below the boiling point of the water, release the remaining pressure of the pit, unload the specimen from the pit, inspect the specimen for leaks, issue certification of the pressure/temperature cycle and observed leaks. or specimen failure (if any).
Certification:
A non-removable paper sticker shall be affixed to the specimen, certifying the type of test cycle performed, and its result. Other information on the sticker:
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