Page 225 - Well Control for Completions and Interventions

Well Barriers 219

the fluid barrier becomes the priority, and must be carried out before
operations resume. Failure of the secondary barriers is contained by acti-
vating shear and blind rams (tertiary barrier).

6.2.1 Mechanical barriers
Mechanical barriers must be verified by pressure testing. Where possible,
the test should be in the direction of flow. Closed barriers are usually
tested when they are first installed. Normally, open barriers should be
tested at the time of installation and then at intervals in accordance with
regulatory requirements. For example, in the United Kingdom and
Norwegian sectors of the North Sea, Christmas tree valves are normally
integrity tested every six months.
There are several types of mechanical barrier elements, these include:
• Production packer (annulus barrier).
• Tubing hanger seals (annulus barrier). Hanger seal integrity is tested
via ports in the wellhead and by performing an annulus pressure test.
• Wellhead annulus valves (or Valve Removal (VR) plugs in the side
outlet bore).
• Drilling, coiled tubing, and wireline BOPs.
• Plugs. Some operating companies will only consider a mechanical
plug equipped with chevron seals to be a barrier if it can be tested in
the direction of flow. This is because a chevron seal (V-packing) can
only hold pressure in one direction. Most plugs are dressed with two
sets of seals, one for each direction of flow. Testing in one direction
only tests part of the seal stack. Solid slab seals solve this problem, but
are not available for all wireline set plugs.
• Tubing or wireline retrievable SCSSSV. There are many conflicting
views about the use of downhole safety valves as a well control barrier.
However, many operating companies permit the use of the SCSSSV,
providing it can be inflow tested in the direction of flow. Safety valve
acceptance is normally based on a zero leak rate rather than the API
permissible leak rate of 400 cc/minute for liquids or 15 scf/minute
2
gas. Ideally, a small differential pressure from below is needed to keep
the flapper seated. However, the risk of dropped objects must be con-
sidered, since the closed flapper valve in a safety valve does not have
the resilience of, e.g., a wireline set bridge plug.
• Annulus SCSSSV.
• Surface controlled downhole “lubricator valve.”