The Electrically Trace Heated Blanket (ETH-Blanket) is a new offshore intervention system currently in development by TechnipFMC for the efficient remediation of plugs due to hydrates or wax deposit in subsea production and injection flowlines. The ETH-Blanket consists of a network of heating cables placed underneath an insulation layer which is laid onto the seabed above the plugged flowline. By applying electrical power to the cables, heat is generated by Joule effect which warms up the flowline content until hydrate dissociation or wax plug remediation through softening or complete melting. The ETH-Blanket is currently developed within a Joint Industry project (JIP) between TechnipFMC and Total.
The dissociation of hydrate plugs using active heating incurs a number of risks for the integrity of the flowline and for the restoration of production to nominal conditions. As the flowline content is warmed up from ambient to hydrate dissociation temperature and during the dissociation of the hydrate plug, the pressure inside the flowline may potentially increase above design limits due to hydrate degassing and fluid volume expansion. Also, plug run-away scenarios may occur if a large pressure difference exists between both sides of the plug. The remediation operation may fail because of insufficient power or misplacement of the ETH-Blanket. Lastly, even following successful operation of the ETH Blanket, new flowline blockage may occur during subsequent operations such as cold re-start.
To mitigate those risks, a hydrate remediation philosophy has been developed specifically for the ETH-Blanket Service. It is based on the development of in-house tools and procedures and builds upon experimental and modelling work performed as part of a previous JIP focusing on the dissociation of hydrate plugs using an ETH-Pipe-in-Pipe . This paper introduces the different elements of the hydrate remediation philosophy, including the development and experimental validation of the dedicated tools used to define the appropriate heating sequence for the safe and efficient dissociation of hydrate plugs.