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Tech Tidbits, vol. 10 - Controlling Fluids in the Control System
Are the O-rings in your drilling control system getting a swelled head? If control system O-rings are swelling, it is possible that someone has put the wrong fluid into one or more of the fluid reservoirs. Some might say, “Why is this a big deal, doesn’t it happen all the time?” Well, in the case of a drilling control system, it can cost big money to fix. In some cases it has cost millions of dollars to remedy a single incident, and the potential for loss will only increase with the influx of new, inexperienced personnel into the drilling industry and today’s expensive deepwater programs.
Description of Issue
Control fluid cleanliness and condition is considered essential by all control system manufacturers, yet access to the control fluid tanks is uncontrolled on many rigs. In addition, newly hired, untrained people are often assigned the job of keeping the storage tanks full.
Here is a description of an incident that cost millions of dollars and months of time to repair:
- When a subsea BOP stack was retrieved for standard maintenance, multiple pod functions were found with swollen and damaged Nitrile O-rings.
- The three control pods were shipped to the OEM for repairs. The repairs consisted of replacing 3,000 O-rings. Cost of the work at the OEM was $3,000,000, and it required over three months to repair all three pods.
- The bladders in the accumulator bottles were found to have degradation. All 120 had to be replaced.
- No traces of contamination were found in the existing control fluid.
- Some swollen O-rings shrank back to near their original size when left in the open air.
- Traces of aromatic compounds, possibly a diesel-type fluid, were found when the O-rings were analyzed. Aromatic compounds will cause O-rings and other elastomers to swell and/or disintegrate.
- Despite the latest laboratory test techniques, there was no way to definitively identify the source of contamination. The contaminant was probably introduced and then flushed through normal operations before the BOP stack was pulled. This single incident reportedly resulted in a total, gross charge to the program of about $19,000,000.
In another case, fire-fighting fluid was inadvertently added to the control fluid concentrate tank. By the time it was found, the fire-fighting fluid had contaminated the entire system and a full flush was required.
Some Solutions
Some suggestions for avoiding this issue include the following:
- Label hoses and tanks – Clear labels and color coding of the hoses and tank openings will make it easier for personnel to keep things in order.
- Control access to reservoirs - Lock the control fluid tanks and valves to limit access to those trained to handle the fluid.
- Train personnel - Have subsea personnel trained by the fluid OEM to ensure proper handling and inspection.
- Test the BOP control fluid on a daily basis – Train subsea personnel to test the fluid for proper mixture, if testing not being done already.
- Have fluid samples regularly analyzed – Have samples of the BOP control fluid thoroughly analyzed at regular intervals by trained in-house personnel, by a lab or by the OEM. Ensure that cleanliness (per NAS 1638) and conductivity (measurement of dissolved solids) are included in the analysis, in addition to the standard pH, concentration, alkalinity and viscosity.
- Ensure compliance of water supply – In at least one case a poorly maintained rig water system was producing unsuitable water. Have the water supply monitored and remedied if necessary. An often overlooked issue is the calcium content of water from boats. Calcium is very harmful to modern control systems. Dissolved solids like calcium may precipitate out, especially after mixing with control fluid additives. Note: If the water supply is via work boat, always verify the condition of the water. Tanks are often not cleaned correctly, and water may be contaminated with completion fluids or other substances.
- If the water supply or related equipment is presently unsuitable, have pre-mixed fluid sent to the rig – A (hopefully) temporary measure until the issue is resolved or the system is flushed.
For more information or technical questions, please contact WEST Engineering Services at west@westengineer.com or by phone at 1-281-375-5515.
