Session: 04-01-03 Flexible Pipes & Umbilicals III

Paper Number: 123795

123795 - A Methodology to Assess Craze Initiation in Pvdf Pressure Liners 

Crazing has been observed in flexible pipe inner sheaths in operation. Whilst no structural consequences have been observed, Baker Hughes and Equinor have collaborated with Enodo and NTNU to investigate the root cause of the mechanism and develop a predictive framework.

 

Pressure is the only measurable quantity from experiments under the severe loading conditions that are necessary to generate crazes. But to generate a material capacity curve we need to also measure the deformation field of the test specimen. To solve this, we developed a methodology involving a combination of finite element (FE) simulations and high-pressure tests to establish a craze capacity curve for the PVDF material.

 

Crazes were generated in a controlled setting using a custom-built pressure rig. Circular disks of the PVDF material were pressurized with water from one side up to pressures of 2000 bars. We varied the stress and strain state in the disks by changing the disk thickness, disk radius, backing plate geometry, and the applied pressure. Contrast fluid was added to the water to obtain an indication of crazing. After each test, the deformed disk was scanned using a laser measurement rig.

 

We implemented a custom polymer material model in Abaqus (UMAT/user subroutine) to simulate the high-pressure tests. The material model was calibrated from a multitude of material tests involving tension and compression tests with different triaxialities, temperatures, and strain rates. The calibrated material model was then used to simulate the high-pressure test to quantify stresses and strains during testing. The deformed profile of the disk was compared to the profile observed in the high-pressure test for validation. Having established that the FE simulation was sufficiently accurate, we used the stresses and strains from the simulation to establish a material capacity curve describing the onset of craze initiation in the PVDF material. Although several parameters were evaluated, the experimental-numerical procedure revealed that the applied pressure and maximum shear strain could predict the onset of crazing most accurately.

 

The material capacity curve for the onset of craze formation could then be translated into a capacity curve for a given flexible pipe geometry by performing FE simulations where global bending, pressure, and pressure holding time were varied. The capacity curve for that specific flexible pipe geometry could then be expressed as a function of internal pressure and global bending.

Presenting Author: Joakim Johnsen Enodo AS

Presenting Author Biography: Principal engineer and co-founder of Enodo AS. PhD and MSc in Computational Mechanics from the Norwegian University of Science and Technology (NTNU).

Authors:

Henrik Granum Enodo AS
Jens Kristian Holmen Enodo AS
Michele Bonanni Baker Hughes
Majeed Al-Zubaidy Baker Hughes
Peter Kirton Baker Hughes
Andrew Roberts Baker Hughes
Per Nygård Equinor Energy AS
Petter Holmström Equinor Energy AS
Erling Østby DNV
Jonas Hund Norwegian University of Science and Technology – NTNU
Giovanni Perillo Equinor Energy AS
Joakim Johnsen Enodo AS