For isolation purposes, all subsea facilities, including structures, jumpers, spools, flowlines, risers and umbilicals, should be risk assessed for the following activities:
- Subsea equipment installation (excluding well completion)
- Hook-up and pre-commissioning of the subsea system
- Tie-in of future development phases including infill wells with no or minimum production deferment
- Inspection, maintenance and repair activities (excluding well workover, but may include riser changeout) with no or minimum production deferment (review Subsea Inspection, Monitoring, Maintenance and Repair Philosophy for additional information)
- Operation and Flow Assurance requirements
- Commissioning and decommissioning of the subsea system (review Subsea Commissioning Philosophy and Subsea Decommissioning Philosophy for more information)
As a minimum two independent, proven effective barriers between the reservoir (any material source of hydrocarbons or other harmful fluids) and the working environment should be adopted for the design of the subsea components and systems. For simultaneous production operations, a proven single barrier to the producing / injecting flowline and the working environment should be adopted with a back-up means of isolation that minimizes disruption to production.
The design of subsea equipment and proposed isolation philosophy should be in accordance with:
- API RP 17P - Recommended Practice for Subsea Structures and Manifolds
- DNVGL-RP-0002 - Integrity Management of Subsea Production Systems
As a minimum, each barrier must be designed such that:
- Its integrity can be verified remotely (e.g. use of the chemical injection, shut-in tubing head pressure monitoring, tree mounted instruments) or locally (e.g isolatable ROV hot stab, ROV instrumentation)
- It has the ability to measure pressure loss and/or pressure build up in the direction the barrier is to be proven
- Its position / location and status can be established
- Its effectiveness can be re-established as quickly as possible
- It is independent of other barrier elements
- It can operate securely in the actual environment (e.g. pressure, temperature, fluids)
- It is capable of being locked in the safe position (if required)
- No failure of a single barrier, whether caused by human error or equipment failure, must lead to a loss of well control
Electrical equipment must be isolated from power and communication prior to any intervention. Isolation must be via a circuit breaker and by physical disconnection of supply cables.
Where HP recoverable pressure caps are utilized, ROV hotstabs with isolations must be fitted to avoid additional penetrations into the main production bores, where practicable.