The Raia field (formerly BM-C-33) is an ultra-deep water (2850 m) subsea development located in the Pre-salt region offshore Brazil. Its field architecture features a series of rigid production, gas lift and gas export flowlines and steel lazy wave risers (SLWRs). Raia is among the deepest projects ever executed by TechnipFMC. The project stringent flow assurance requirements demand high thermal performance for the production pipelines. Therefore, five-layer polypropylene (5LPP) factory applied coating (FAC) combined with injection molded polypropylene (IMPP) field joint coating (FJC) was selected. To achieve the targeted U-value, approximately 80 mm of 5LPP is required for the production pipelines.
It is known that thick insulation coating magnifies the radial pressure seen by the pipe at the interface with coating. This magnified radial pressure exerted on the rigid pipeline typically leads to further requirements on wall thickness (WT), because the additional mechanical resistance provided by the insulation coating is disregarded, as per current industry practice.
The mechanical contribution of a thick wet insulation system is not negligible and can modify the pipeline limit states for system collapse and combined external overpressure and bending. To demonstrate the benefit, a research and development (R&D) study was carried out as part of the Raia project. Mechanical characterization of the 5LPP FAC system and IMPP FJC was performed to develop material models. These models were input to a state-of-the-art finite-element analysis (FEA) model, which was used to simulate both collapse and local buckling response of the coated pipeline. Subsequently, full-scale collapse tests were performed to validate findings from the numerical simulations. The R&D study demonstrated that the 5-layer polypropylene coating significantly improves the resistance of the pipeline to collapse and local buckling limit states.
Subsea rigid pipelines are often designed in accordance with the requirements contained within DNV-ST-F101 (Det Norske Veritas, 2021). Project-specific requirements in terms of in-service operation, installation and fabrication drive the material grade selection and sizing of both pipeline and insulation coating. One of the critical loading conditions experienced by a pipeline installed in ultra-deep water is the combination of bending and external overpressure, particularly for the pipe located in the sag-bend (i.e. pipeline convex-downward curve before contacting the seabed) region of the catenary during subsea installation at ambient temperature. The pipeline may also be subjected to combined bending and external pressure during lateral buckling but in this case, the pipeline is in operation and thus internally over-pressurised in contrast to the installation condition where the pipe bore is empty. In service, the most onerous loading seen by a pipeline when depressurised during a shut-down event, is external hydrostatic pressure as bending moment is less significant. The risk of pipeline failure under such load is exacerbated when the former is still hot after shut-down, because elevated temperature has a negative effect on the steel pipe strength, and even more so on the coating strength. Hence making the collapse pressure the primary design parameter.
