Semi-empirical equations to predict the bisymmetric dry and wet hydrostatic collapse strengths of flexible pipes

Abstract

This work proposes closed-form equations to calculate the bisymmetric dry and wet hydrostatic collapse strengths of flexible pipes with imperfections. Initially, the bisymmetric hydrostatic collapse is discussed, relying on the classic Timoshenko's equation. Then, the main parameters involved in the collapse mechanism are indicated. Particularly, approaches to account for the effect of the carcass’ cold work and the pressure layers’ residual manufacturing stresses are suggested. After that, an equivalent three-dimensional shell finite element (FE) model is revisited and employed to predict the flexible pipes' wet and dry collapse strengths. Several FE collapse analyses were performed addressing different material properties, initial ovalities, and interlayer gaps. The obtained responses showed that the hypotheses assumed by the modified Timoshenko's equation only hold for the dry collapse condition with no interlayer gaps. However, the calculated collapse pressures with this equation deviate from the FE values as higher ovalities are considered. Hence, an alternative approach was employed to overcome these limitations. In this approach, the FE collapse pressures and the flexible pipes’ characteristics constituted datasets analyzed with a symbolic regression (SR) tool. These analyses led to the closed-form equations, named semi-empirical equations, that best fitted the datasets. Sensitivity analyses of these equations showed that the bisymmetric collapse is significantly affected by the yield strength of the pressure layers’ materials and imperfections. Furthermore, under wet annulus conditions, the confinement provided by the pressure armor also has a substantial impact. Finally, comparisons with experimental tests verified the applicability of the proposed equations.