12. SHALLOW GAS DIVERTER OPERATIONS - Is potential chemical use and discharge associated with shallow gas diverter operations subject to the Regulations?

When drilling wells at some locations, it will be normal practice to operate a shallow gas diverter that uses bursting discs to alleviate excess pressure and ensure the safety of the facility.  The diverter is only used for the well section where the shallow gas is expected, and the bursting discs are thin flexible metal and very sensitive to defects and mechanical damage.

 Prior to drilling operations, the discs are normally tested with seawater, and they may burst with discharge to the sea.  Providing the tests are undertaken using seawater and do not involve the use of any chemicals, the tests do not require to be permitted.  During the drilling operations, the discs are designed to rupture if there is excessive pressure, which could result from infrequent and unforeseen operating conditions, such as cuttings packing-off in the hole, or from shallow gas incidents.  Rupturing of the discs will result in the discharge of drilling fluids to the sea.  At the end of the relevant well section, there will also usually be a further discharge of drilling fluids, when the riser is disconnected to replace the shallow gas diverter with a BOP.  The latter is normal operating practice, as it is not possible to recycle the mud from the base of the riser, but the discharge is usually made at the sea surface rather than via the usual sub-surface cuttings disposal chute

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