The Electrically Trace Heated Blanket: A New Concept for Efficient Subsea Flowline Unplugging

Abstract

Subsea Flowlines blockage due to hydrate or paraffin plugs, resulting from pour point issues or deposition, is a frequent concern in subsea production requiring expensive remediation methods. The expenditures associated with subsea flowlines unplugging can increase very quickly, especially when considering the associated loss of production as well as the various investigations needed to define the appropriate remediation strategy. Such investigations cover the identification of the plug's nature, its location, the assessment of the appropriate dissociation method as well as the flowline restart strategy. In some scenarios, the plug dissociation method, like depressurization for hydrate plug and chemical soaking or pigging for paraffin plug, may take a long time ranging from several days to several months. Often, the remediation cannot be performed from the topside facility and will require the mobilization of an external drillship vessel to carry manifold or Xmas tree work over. Ultimately, the plug removal method may fail and therefore lead to flowline abandonment and/or replacement. From these observations, there is room for the development of a more efficient, predictable and reliable method to unplug subsea flowlines. With this regard, the development a new subsea flowline intervention system named Electrically Trace Heated Blanket (ETH Blanket) has been initiated. The ETH-Blanket is a compact and modular system, with length adaptable typically up to 2km. The system is equipped with trace heating cables relying on Joule's Effect for heat generation to dissociate the plug and distributed temperature sensing (DTS) to monitor the flowline's bore temperature in order to identify the location of the plug, characterize its nature and follow-up the temperature and pressure profiles during plug dissociation. The ETH Blanket can be deployed onto any kind of existing flowlines (flexible or rigid) and in any condition (buried or not) from a Light Construction Vessel. The power required to dissociate the plug is low (typically <1MW) and once the dissociation is completed, the ETH Blanket can be recovered onto the intervention vessel and relocated. This paper describes into details the ETH-Blanket assembly and its operating principles, its anticipated thermal performances determined using CFD modeling, as well as and its deployment method and spread. To illustrate the ETH-Blanket efficiency, a typical multiple hydrate plugs dissociation operation will be presented and compared to a conventional topside depressurization. As a conclusion, the on-going fast-track qualification programme for the development of the Electrically Trace Heated Blanket Technology will be presented.