BENDING STIFFNESS OF UNBONDED FLEXIBLE PIPES AND ITS APPLICATION IN INSTALLATION

The bending hysteresis behavior of unbonded flexible pipe is well known and fully implemented in flexible riser system design and analysis, for example, extreme, interference and fatigue analysis. However, in some quasi-static applications, the inclusion of the full bending hysteresis loop sometimes leads to noticeable deviations from configurations observed in the field. To address the issue, this paper intends to interpret these differences from the nature of the internal friction, which is the main mechanism governing the pipe bending hysteresis behavior. Two operation cases are presented for the study: 1) a flowline installation, and 2) a flexible riser wet storage. The pipes in both cases are not subjected to any significant wave excitations. Therefore they are considered quasi-static applications. The flowline installation involves a camelback configuration at the end fitting-gooseneck connection. The riser wet storage case involves wave configuration with installation ballasts, considering about a 1-year wet parking period. The paper presents a configuration sensitivity study with different bending stiffnesses and identifies the one that produces the best agreement between the predicted and measured configuration in the survey. Based on the findings, the paper also recommends the most appropriate use of pipe bending stiffness in designing quasi-static jumpers, which predicts accurate configuration and associated support interfacing loads thus avoiding excessive conservatism.