TECHNICAL INFORMATION

Well Safety

Well integrity is based on a system of "operable" barriers. An obvious example of a safety barrier is a Xmas tree, of which the actuated valve(s) is the "operable" barrier while the well is flowing. In an offshore application this is not enough; the well must still be able to withstand the tree being badly damaged, or even knocked off. A single failure, such as this, should not result in a loss of well control. The second barrier in this situation would normally be a sub-surface safety valve, set at a depth below the influence of any surface effects. This would prevent the single incident, or failure, from adding further danger or causing an environmental problem.

All North Sea wells require at least two operable safety barriers at all times; as stated above, these could be:

Xmas tree plus surface controlled sub-surface safety valve.

Other examples being:

  • Packer elements and tubing head spool
  • Tubing and tubing head spool
  • Annulus safety valve and tubing head spool.

The latter example refers to gas lift wells - a particular problem. The volume of gas contained in many thousands of feet of annular space has an appreciable destructive capability, if suddenly released, for whatever reason. This problem exists even if the reverse flow check valves built in to the gaslift valves effectively seal-off the tubing from the annulus. To counter this, an extra packer may be required, close to the sub-surface safety valve setting depth. The large proportion of gas volume below this point would be contained by an annular safety device linked to the operation of the surface controlled sub-surface safety valve. This limits the volume of gas discharge possible in the event of a surface failure.