For the protection from object drop/fishing trawl impact, flexible flowline is normally trenched or rock-dumped. And hence, upheaval buckling is promoted by the elevated temperatures and high pressures. In order to reduce the rock cover requirement for mitigation of upheaval buckling, rock-dumping or trenching while the flexible pipe are pressurized has been performed successfully in several north sea projects.
The temperature and pressure induced elongation of flexible pipe are design dependent. For high pressure flexible flowline, the pressure expansion is significantly higher than conventional rigid pipelines. Due to the low bending stiffness and high pressure expansion, a flexible flowline will buckle laterally when it is pre-pressurized in hydro-test before trenching or rock-dumping. As a consequence, lateral imperfections are induced and will be kept after trenching or rock-dumping due to lateral resistance and bending stress relaxation of the flowline. In these locations, the flowline tends to deform laterally in operating. On the other hand, when the flowline is de-pressurized after trenching or rock-dumping the contraction of the flowline is restrained by the surrounding soils or rocks, and hence axial tension force can be obtained. When the flowline starts to operate, this tensile force will neutralize part of the compressive axial force, and therefore the required upheaval resistance is reduced.
In this paper, global buckling of a pre-pressurized flexible flowline has been studied, and the influence on the requirement of rock covers is presented.