The predictive upheaval buckling (UHB) for un-bonded flexibles can be challenging due to their complex multilayer structure and nonlinear properties. A combination of design data preparation and interpretation, in-place analysis methodologies, and installation can result in a varied UHB screening exercise and results. For flexible flowlines prone to UHB, pre-pressurisation is widely used prior to trenching and backfill to improve their UHB performance.

This paper is intended to present a study on the issues associated with the flexible UHB assessment. The solidity and veracity of the current UHB screening and analysis procedure is examined for pre-pressurised flexibles in particular. The initial effective tension induced by pre-pressurisation prior to backfill is investigated in detail. Pitfalls and inconsistencies in the methodologies are reviewed.

It is demonstrated that due to the neglect of nonlinearity, potential misuse of design parameters and assumptions made for the UHB design screening and analysis procedure can be compromised and open to rationalisation. The tangible benefit of initial pressurisation may be inaccurately evaluated for UHB mitigation. It is established that for the same flexible flowline, its UHB resistance performance can be either overpredicted or underestimated in certain conditions. An alternative approach is developed and proposed to redress some deficiencies. A case study is presented to show the key results and conclusions.

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