Heat Trace Ltd - Subsea Project
Heat Trace Ltd has teamed up with Technip to deliver the world's first subsea electrically trace heated pipe-in-pipe (ETH-PIP) system. The end client is TOTAL E&P UK.
The Islay project consists of the natural depletion extraction of gas condensate from the Islay reservoir located in block 3/15 of the Northern North Sea, approximately 6km North East of the existing Forvie Manifold. It is this manifold to which the Islay pipeline will be connected. The full Islay project work scope covers installation of a 6 kilometre ETH-PIP, control umbilical, subsea structures and seabed preparation, including detailed design, engineering and project management.
The maximum wellhead shut-in pressure is 620 barg, with a wellhead flowing temperature of approximately 110 deg.C. The water depth at the Isay X-tree location is 122m. The Islay development has a design field life of 20 years.
The pipeline itself is a 6km long insulated Pipe-in-Pipe, comprising a 6" flowline pipe inside a 12" carrier pipe. The flow line is made from Super Duplex Stainless Steel, a material best suited for aggressive environments due to it's resistance to erosion, corrosion and corrosion fatigue. The 6km ETH-PIP pipeline is to be reeled onto the Apache II vessel in order to lay it on the seabed.
The Islay project is the World's first subsea electrically trace heated pipe-in-pipe technology using Heat Trace Ltd's LONGLINE series resistance heating cable. The LONGLINE is a cable featuring 3 individually insulated metal conductors encased within a metal braid and outer insulating jacket.
The LONGLINE heating cable maintains the temperature of the gas condensate to ease flow conditions and enhance production operability of subsea pipelines that can become blocked by the formation of hydrates or wax.
Successful application of this technology reduces capital and operating costs for fields with challenging flow assurance conditions. The Islay project is a pilot project for the use of this innovative pipeline technology, which is suited to both shallow and deepwater environments.
Flowline Pipe, Trace Heating Cable,
Flowline Pipe within the Carrier Pipe
Design & Engineering
The heat tracing cable has been specifically designed for the Islay project. The cable is constructed from three metal heating conductors, each individually insulated and then brought together within a braid and outer jacket. This specially designed construction increases flexibility of the cable.
Manufacture of the LONGLINE cable was carried out completely in house at Heat Trace Ltd's Helsby and Bredbury facilities. The project comprises six pipe "stalks" each approximately 1km in length. Each "stalk" requires a heating element of a single length in order to minimise cable joints and ease installation and handling. To achieve these cable lengths Heat Trace Ltd's metal extrusion facility produced conductors in infinite lengths which were then cut down to the required 1km lengths.
Whilst the Islay EHT-PIP system is a test project it is Heat Trace Ltd's intention to use the same metal extrusion technology for future subsea, above and below ground, multi-kilometre pipeline projects, to minimise the number of electrical power supply points.
In addition to the specialist metal extrusion process and cable construction, the heater for this particular project is connected to a single power supply of around 2000v, three phase. This is believed to be the highest voltage ever applied to a conventional heat tracing system. Another WORLD FIRST for Heat Trace Ltd.
Other areas of specialist design include the terminations and splice assemblies. Each power termination, splice and star connection method were specifically designed by Heat Trace Ltd and approved by Technip and Total.
All terminations use fast curing materials specifically designed for these terminations. Throughout the Design and Manufacture phases, Heat trace Ltd have continued to liaise with Technip to improve elements of the design where possible, working with all involved parties towards the common goal of providing this WORLD FIRST.
2km Drums of Longline awaiting
Heat trace cable on winding machine
Heat Tracing & Optical Fibre Cables
The 6km long pipeline is manufactured in approximately 1km stalks, after which each 6" flowline pipe is inserted within the 12" carrier pipe. Whilst this process takes place, a cable application machine applies both the heat tracing and fibre optic cables. After each push, centralisers are applied to ensure the flowline is centred within the 12" carrier pipe, then, insulation material is applied between centralisers.
The next stage of the project is the pulling of each length on board the Apache II vessel. After the first length is reeled on board, the 6" flowline is welded to the next 6" flowline stalk. At this point the heat tracing cables are spliced together, the fibre optical cables are also spliced together, centralisers and insulation applied, and then the 12" carrier pipe is pulled over and finally welded.
During pipelay Heat Trace Ltd will have a member of staff on board the Apache II. This caters for the unlikely event of emergency pipe cutting being required during pipelay in the North Sea.
Apache II Pipe Laying Vessel
“Heat Trace explain how 3rd generation heat trace cables improve safety of heat tracing circuits and help to reduce overall system operating cost, without sacrificing high temperature performance”.