This article shines light on the isolation feasibility processes required when accommodating a Shutdown & Turnaround (TAR). In this scenario an isolation of a 10-inch export pipeline was required to conduct topside maintenance during the TAR. The following article, details various methods you can adopt when performing a pipeline isolation and discusses real world barriers taking into consideration reasons for and against each method.
Pipeline Brief
Each scope is different, and the methods used to complete a scope are influenced by various factors, including but not limited to, the length of the TAR, the number of flanges and their locations in the pipeline as well as the work scopes included within the TAR, in this case, work on an Emergency Shutdown Valve (ESDV).
In this scenario it was determined that existing infrastructure was such that a double block was deemed unsuitable, therefore isolation options were explored.
The pipeline has a diameter of 10 inches and included a limited number of flanges. Additionally, it is recommended that the scope was conducted topside due to the costs and risks associated with subsea scopes.
Primary Methods
Pipeline Isolation Plug/Tool
A pipeline isolation plug/tool is a non-intrusive isolation method that could be launched and recovered from the Pig Launcher & Receiver (PLR) and could be set in place upstream from the ESDV in such a way that it would not interfere with TAR operations. Once set in place the topside pipeline would be vented and work could be carried out safely during the TAR.
Reasons for choosing a Pipeline Isolation Tool/Plug
- No modification required to topside pipework to complete isolation.
- Pipeline system downwards of the isolation tool does not have to be depressurised.
- Use of pipeline isolation plug / tool does not impact on any of the proposed TAR scopes.
- Allows for full maximum allowable operating pressure (MAOP) test on the disturbed connections.
Risks to consider
- Low risk of previous isolation indents in the desired isolation position preventing a strong seal.
- Risk of tool getting stuck if pigging route not properly assessed against tooling dimensions
- Additional third-party pumping equipment spread required
DBB Saver (DBB) on the ESDV
This procedure would work by creating and maintaining a vacuum across the ESDV for a temporary basis while a mechanical isolation is conducted using a spade/blind. Before being repeated to remove the manual isolation upon completion of the scope.
It’s important to detail that you must check the ESDV testing data to ensure an isolation can be maintained while succumbed to full riser pressure.
Alternatively, If required to manage for the full TAR duration, DBB-SAVER could be applied by closing the Subsea Isolation Valve (SSIV), depressurise the topside pipeline, close the ESDV, the DBB saver would then manage the leak rate across the ESDV. In this scenario, significant volumes of Nitrogen (N2) would be required to manage (~ 1.5 x 16 bottle N2 quad per day).
Reasons for choosing a DBB on the ESDV
- Use of existing topsides connections/equipment.
- Completely controlled by topside team.
- Could be trialled offshore ahead of TAR if time available.
Risks to consider
- Duration of isolation may require significant volumes of N2 (~1.5 x 16 Bottle N2 quad per day (subject to leak rate)).
- Not considered a mechanical isolation for break and containment scopes.
- Detailed risk assessment required to ensure isolation suitable.
- Contingency plan required in event of any issues.
- Isolation can’t be quantified until execution.
- Work scope on ESDV may not be achievable, which may increase risk on future scopes/CoP.
Secondary Considerations
Add On Gate Valve (AOGV)
Inboard of the ESDV, the AOGV provides a retro fitted gate valve by clamping over an existing flange connection to create a pressure envelope before separating the flanges replacing the gasket with a fully line rated gate valve.
Consideration must be given to a suitable location for the isolation due to the size of the assembly and rigging support would be required due to the weight.
Reasons for choosing an AOGV
- Could possibly be installed and tested prior to TAR (if ready and manufactured in time)
- Allows for full MAOP test on the disturbed connections.
Risks to consider
- In this case there is a lack of Flange options to consider.
- Manufacturing risk – #1500 version would have to be built specifically for this scope.
- Unlikely to work on the ESDV valve flange.
- Long lead times with further qualification work require to prove suitability.
Hot Tap Isolation Tool
Hot tap intervention was considered for this scenario, but other options were deemed to be more suitable.
Recommendations
Following a review of the available options the isolation feasibility study concluded and recommended that a pipeline isolation plug / tool be deployed to provide the client with a suitable isolation to allow the TAR scope on the 10’ gas export system.
It was also recommended that the use of the DBB saver was investigated in parallel, to ensure a backup option is considered in detail.
