Upheaval Buckling of Unbonded Flexible Pipes: An Alternative Approach

Due to their non-linear bending response, low bending stiffness and potentially high axial expansion coefficients, buried unbonded flexible pipes can be susceptible to Upheaval Buckling (UHB). The traditional industry approach is to prevent UHB by providing sufficient download to limit upwards movement of the pipe to very small levels (normally within the mobilisation distance of the soil). Since unbonded flexible pipes offer little resistance to bending, the required download to prevent UHB of unbonded flexible pipes can be significant resulting in prohibitive rock-dump requirements. However, by design, unbonded flexible pipes can accommodate large bending deformations without compromising the cross-sectional integrity of the pipe. The objective of this paper is to propose an approach which takes advantage of the pipes capability to accommodate large deformations and to minimize download requirements. This paper presents a comprehensive literature review of the state-of-the-art methods currently adopted in industry for the assessment of UHB of buried unbonded flexible pipelines. An alternative approach which permits significant upwards movement of the flexible pipe, taking advantage of the pipes ability to accommodate such movements, is also presented. A comparison of the potential failure modes, mechanisms and consequences associated with these approaches are studied and acceptance criteria are proposed. Parametric finite element analysis studies are performed to compare the flexible pipe behaviour and download requirements using the traditional approach and the proposed alternative approach. This paper presents case studies which demonstrate that significant savings in the volume of rock-dump can be safely made when permitting large deformations when compared to the traditional UHB prevention approach.