The project was a 2km, electrically heat-traced, subsea pipe-in-pipe (PIP) system for transportation of a bitumen-like material across a shipping channel. Due to the viscosity of the bitumen, it must be transported at a minimum of 160°C; has a normal pipeline operating temperature of 200°C; and a design temperature is 228°C.
Due to the high operating temperature, pre-stressing and backfilling the PIP was required to lock in stresses at an intermediate pre-stressing temperature. The electrical heat-trace wires (installed to heat up the inner pipe and prevent setting of the bitumen during cooldowns) were used to achieve this pre-stressing during the fabrication process. The heating schedule causes high stress levels and require advanced engineering analyses to model the behaviour of the inner and outer pipe during the fabrication, installation, pre-stressing, and operation. The complex loading history of the inner pipe and the expansion spools was included in the global 3D, finite element (FE) models that were used to validate the pipeline profile, backfilling, pre-stressing temperature, and sequence of operations.
The complex buckling behaviour of the inner pipe is presented and shown to be within DNV GL OS-F- 101 code limits. The end expansion during the various stages of pre-stressing is presented and compared to observed behaviour. The loads and stresses in the bulkheads are presented and shown to be acceptable. The analysis demonstrates that the pipeline system can be safely installed and operated up to the maximum design temperature of 228°C.
The project used pre-stressing by heating the inner pipe to an intermediate temperature before coupling the inner pipe to the outer pipe. The purpose of the pre-stressing was to manage the high axial stresses making it feasible to achieve the high design temperature of 228°C. Pre-expanding of the expansion spools at either end of the subsea pipelines was also used due to the space limitations. Innovative engineering analysis and construction methods were used to ensure the integrity of the inner pipe during the pre-stressing process and operation.