ITP Interpipe has designed a recently-installed subsea electrically heat-traced pipe-in-pipe (EHT PIP) pipeline to transport a highly viscous bitumen below a 1.8 km marine channel. The fluid requires temperatures above 200°C in operation and makes it solid-like at ambient conditions. To avoid plugging the line, excellent passive thermal performance with U-values 0.5W/(m²-K) was specified to ensure long cooldown times and allow the line to be flushed with a lower viscosity fluid. An active heating system capable of reheating the line provides both an additional layer of protection and was instrumental to pre-stressing the line during construction.

The pipeline consists of a buried subsea section across a navigable channel and its design temperature of 228°C, the highest value seen for a subsea pipeline to date, created technical challenges both for the design and the material selection:

• the mechanical loads induced by the line thermal expansion and contraction across an extensive temperature range needed to be managed. The solution combined rock backfill to restrain the subsea section expansion with use of the built-in EHT system to heat the PIP system for pre-stressing during installation.
• to ensure that the system would be capable of sustaining all thermal, mechanical and electrical load over a specified design life of 50 years, an extensive qualification campaign was developed, with a hierarchy of tests from small scale, focused on individual components, to full scale tests evaluating the full PIP system behaviour including thermal insulation, electrical cabling, connectors and instrumentation.

Key to overcoming these challenges was ITP Interpipe’s extensive experience with subsea electrically heat traced systems. This led to a focus on collaboration with suppliers, contractors and clients and design of a bespoke qualification programme to validate the system integrity for the severe and complex loading schedule.

The pipeline was installed in 2019 and pre-commissioned in 2020.