Offshore staff
MONACO -- Attendees today at the DOT conference were given an overview of engineering solutions for the high internal pressures and temperatures (HP/HT) of non-buried pipelines subject to lateral buckling and longitudinal ratcheting. When this occurs, there is a risk of high strains, which could cause the pipe steel wall to fail, and/or excessive axial movements (walking).
According to Daniel Carneiro, João Gouveia, and Rafael Parrilha of the Bureau Veritas Group Rio de Janeiro Technical Center, and Carlos de Oliveira Cardoso of the Petrobras Research Center (Cenpes) of Brazil, these solutions are highly encouraged given the high cost of pipeline systems in deepwater. Their paper summarized the thermal expansion control strategy for two 219 mm (8.625 in.) OD, 25.4 mm (1 in.) thick, 6 km (3.7 mi) length oil production pipelines recently installed offshore Brazilian coast in water depths from 1,443 m to 1,572 m (4,734 to 5,157 ft). The arrangement employs dual sleepers to limit buckling spacing and end anchoring to prevent pipeline walking.
Several studies were performed using non-linear tri-dimensional finite element models considering: pipe-soil interaction with full 3D seabed bathymetry; load history maintained from pipelay to operational cycles, including temperature transient effects; plastic strains (including steel properties de-rating due to high temperature) and section ovalization; mechanical contact between pipe and appurtenances during both installation and operational phases. Pipe-soil interaction comprised embedment considering dynamic pipelay effects and full non-linear response in lateral and axial directions. Strain concentration factors due to field joints were evaluated using detailed solid models considering non-linear response of both steel and insulation materials. Buckle initiator geometry was assessed through critical force sensitivity study, and their adequate locations were determined on seabed roughness analyses.
Extensive engineering workload and tools were invested on finite element models to realistically represent the system relevant parameters (pipe, soil, operational data, etc.) covering several scenarios, keeping the adequate level of conservatism required for subsea systems assets.
Interesting aspects of the pipeline behavior, such as the ratcheting effects induced by operational cycles, were observed throughout the design analyses. An overview of how they were investigated and the foreseen results were also discussed. Adequate monitoring of the pipeline behavior in the course of its operating life would bring noteworthy contribution to the understanding of these phenomena, and was therefore recommended within the design reports.
The close cooperation between operator, designer, installer and certification body was a key success factor to provide a safe and feasible engineering solution in the tight project required schedule, according to the authors.
11/04/2009
