NEW TEMPORAL APPROACH TO EVALUATE THE LATERAL BUCKLING OF ARMOUR WIRES

This paper presents a novel approach to manage irregular external loads through an enhanced design of unbonded flexible pipeline caused by lateral buckling of tensile armour wires. The design challenge is to predict the armour wire kinematics due to random loading conditions. Over the past decades, efforts have been made to simulate and model lateral buckling phenomenon. Based on more than a hundred full scale tests, including deep immersion performance tests and hyperbaric chamber tests, a design approach was established that reproduces lateral buckling. The analyses performed considered regular sinusoidal loading corresponding to extreme sea states, often leading to overconservative combinations of curvatures and axial compressions. Wire displacements and stress states of steel wires were predicted using in-house software with the aforementioned cyclic loading. This methodology has shown room for improvement to decrease the conservatism and make sure we propose the most optimized armour wire geometry for a flexible pipeline designed to lateral buckling. In this paper, it is demonstrated that loading of compression and bending derived from stochastic extreme sea states allows us to bring the migration calculation to a higher level of accuracy. As a result, armour wires achieve their final migration state with lower stress values with stochastic approach with extreme sea states rather than the regular one. Some technical challenges appeared when establishing this new approach. For example, bending cycles order as well as the sea states order impactson wire kinematic and stress state. These observations, shared in this paper, concluded that a temporal approach is required to properly estimate lateral buckling risk when considering irregular loads. A key aspect identified during the development is the need of having two calculation modes when performing lateral buckling calculation with a temporal approach. The first mode allows to manage temporary operation by disregarding sea states order which could be very useful for simulating lateral buckling during installation sequence, whereas the second mode allows to evaluate lateral buckling during service line operation by considering the impact of all the sea states. Calculating armour wire kinematics migration during an entire service life, considering all the sea states, was another challenge requiring significant dedicated software developments. Impact on calculation time has been scrutinized and specific methodology has been implemented to optimize calculation time. Stress comparisons between classical regular approach and this new temporal - two calculation method are presented in this paper to illustrate the improved accuracy. This temporal - two calculation methods also result in improved installation weather windows, and optimized armour wire selection. Finally, this methodology has been reviewed and certified by an Independent Verification Agent.