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.
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.
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.
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).
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.
Hot tap intervention was considered for this scenario, but other options were deemed to be more suitable.
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.