Page 231 - Well Control for Completions and Interventions

Well Barriers 225

Note: Some coil tubing operations use reverse circulation to clean out the well-
bore. This means no check valves are included in the BHA. In these circumstances,
the primary internal barrier is provided by maintaining overbalance on the forma-
tion to prevent hydrocarbons from entering the reel.

6.8 HYDRAULIC WORKOVER (SNUBBING) UNIT: LIVE
WELL OPERATIONS
6.8.1 External pressure control

Primary Barrier.
• Stripper rubbers or annular preventer.
• If rigged up on top of a Christmas tree, the tree gate valves provide
the primary barrier when rigging up and when lubricating tools in
and out of the riser.
• If no Christmas tree is installed, tools are lubricated in and out of the
well above closed BOP blind rams.
Secondary Barriers (pipe in the well).
• 2 x BOP pipe rams.
Tertiary Barrier.
• BOP shear and blind rams or a shear seal ram.

6.8.2 Internal pressure control
Primary Barrier.
• Check valves (nonreturn valves) in the BHA.
Secondary Barrier.
• A full opening stab-in safety valve (Kelly Cock).
Tertiary Barrier.
• Shear ram and blind ram or shear/seal ram on the BOP.
• Facility to kill or plug the well: pumps, kill weight fluid, LCM,
cement, etc.

6.9 WELL BARRIER SCHEMATICS

Well schematics have been used by the oil industry for many years
to provide a simple overview of the well architecture, and list critical