Evaluation Qualification of Electrical Heat Trace Pipe In Pipe for a SS Flow Line and Selection for an Application on a Subsea Field in UK, ISLAY
Authors: De Naurois Herve J. , Delaporte Dominique , Helingoe Marc Andrew , Greder hugues 
Published: May 2011

Long tie-backs are increasingly being considered for deepwater field developments with intermediate or smaller reserves and flow assurance issues. The architecture of the Flowline then becomes a key feature of the development. The Total group has reviewed several solutions for flowline optimizion and, among the different technologies available, has identified the Electrical Heat Trace Pipe In Pipe (EHT-PIP) as an interesting option, with higher energy efficiency, which could limit the impact on the host platform. This paper presents the technology of EHT-PIP, and its evaluation-qualification prior its selection for a first application, Islay field, offshore UK.

EHT-PIP technology involves the inclusion of heat tracing cables in a standard PIP, between the outer wall of the inner pipe, and the thermal insulation. Once installed, it can be used safely and with good operability in the different required modes (maintaining the fluid temperature above critical points, or as a controlled warm-up, or for heating the fluid during production).

For flow assurance reasons (hydrates, wax, pour points), subsea flowlines generally require highly effective insulation but long tie-backs push to the limit the conventional passive insulation solution, unless a dual loop configuration is adopted. Another solution is to implement an active heating system. Various alternatives for this, such as the direct electrical heating systems (DEH) and hot water circulation (HWC), have been analyzed, but they showed poor energy efficiency. They also presented limits for long term usage, either linked to the higher electrical current required - heavy qualification of connection and insulation material, for example- or operational issues-, although past applications include some positive experiences, on short term usage. In comparison, the EHT-PIP, requires significantly lower voltage and offers greater efficiency. However, although it has been proven in surface, the solution is less mature for subsea flowline. Total has undertaken an " Evaluation Qualification?? on this emerging technology. This in-house method is used to improve Total's capacity to select innovative technology, while adequately addressing the risks and uncertainties before an implementation. Several future applications of EHT-PIP were being considered, in the UK (one with a 6-km flowline presenting a hydrate issue, Islay, another 27 km long, etc.) and in another deepwater areas. The assessment resulted in a list of 12 technical uncertainties to be resolved. The corresponding program - RMP (Resolution Management Plan) - was discussed with 2 contractors developing this technology (ITP and Technip), and subsequently executed. The results (with full validation of the thermal model, demonstration of reliability and long-term cable ageing test, etc.) were positive and the technology was considered valid with both contractors design for application on the first full-scale industrial project, ISLAY, for the 6-km flowline. Here, the EHT-PIP will be deployed as a secondary hydrate prevention method. During the life of the field, the system will be operated and monitored at regular intervals. Once proven on this subsea flowline, it will be deployed on 6 similar potential subsea tie-backs, where it could improve the economics, and eliminate methanol injection. The validation of this technology therefore offers to TOTAL a technically and economically optimized solution, for future long subsea tie-backs with greater operability, even with difficult fluids and isolated, distant reservoirs.