The structural performance of flexible pipes

Abstract

<p>High density polyethylene pipes have been used for over a decade but little information is available on the in-service behavior of these pipes. An investigation to evaluate the field performance of HDPE pipes was undertaken using visual information obtained from a remote video camera. The performance characteristics considered were cross sectional deformations, wall buckling, wall crushing, wall cracking, and joint separation. Few major structural problems were noticed due to the effect of granular material used as a backfill.;CANDE is one of the commonly used programs for buried pipe analysis; however, the limitations of CANDE include application only to small deflections. The recent tendency of using thermoplastic pipes for deep applications increased the need for investigating the effect of large deflections which led to the use of ANSYS. The main advantage of CANDE relative to ANSYS is the use of the nonlinear soil models. A computer code using ANSYS programming language was written to model the soil behavior using hyperbolic tangent modulus with power and hyperbolic bulk modulus models. CANDE and the small and large deflection theories of ANSYS were compared with Moser's (1994) results. This comparison showed that CANDE over-predicts the pipe deflections as the soil approaches the shear failure and that ANSYS better describes the pipe behavior. CANDE can be used as long as the shear failure of the soil was not reached. Considering large deflections for flexible pipe analysis becomes significant for pipe deflections of 4% or more.;Since the parallel plate test deflection rate is not related to practical loading rates, new mathematical constitutive models were developed for HDPE material using the compression tests results performed by Zhang and Moore (1997). These constitutive models consider the effect of strain rate and time on the HDPE modulus. These mathematical models were programmed in the code written in ANSYS and a finite element analysis was used to validate its use and to compare the pipe deflection using small deflection theory with linear elastic HDPE modulus and large deflection using strain rate dependent HDPE modulus. This comparison showed differences up to 32% at a depth of burial of 15 m.</p>