Abstract

Every young civil or mechanical engineer is taught that the elastic modulus of steel typically equals 207-210 GPa. It is widely accepted to use such value in design of pipelines and other steel structures without seeking for any confirmation by testing. Assessment of limited available test data of linepipe material for a major gas pipeline showed that the elastic modulus in transverse (circumferential) pipe direction was significantly higher than the expected value of 207-210 GPa. Underestimation of the elastic modulus potentially leads to nonconservative assumptions, in particular when behaviour is displacement controlled or strain governed: loads and stress corresponding to fixed displacements or strain will be underestimated. In order to establish a statistical correlation between elastic modulus in longitudinal and transverse direction a dedicated test program was conducted. This paper presents statistical correlation of the measured data of elastic stiffness. The Young's modulus (YM) in transverse direction is found to be approximately 15 GPa higher than the YM in longitudinal direction, which is close to the typical value of 207-210 MPa. This elastic anisotropy is attributed to the plate rolling and manufacturing process.

INTRODUCTION

South Stream Transport BV (SSTTBV) is developing a major gas transmission system comprising four (4) pipeline strings to be installed in water depths up to 2200 m. The system will have a massive capacity to transport 63 billion cubic metres (bcm) of natural gas per annum, over a distance of more than 900 km through the Black Sea. The pipeline outside diameter (D) will be 32-inch and its wall thickness (ʈ) 39 mm. Material grade of the linepipe is DNV SAWL 450 SFDU and, depending on the supplier, it is manufactured using either UOE or JCOE method. This project can be considered as one of the most challenging pipeline projects ever, stretching the limits of present-day industry.

During production of the linepipe material for SSTTBV, it was required to perform compressive coupon tests on pipeline specimens taken in transverse direction. Analysis of test data of several pipe mills, that are using steel plates from various manufacturers, exhibited a value of elastic modulus consistently higher than expected, independent of plastic material properties. Only little data of the elastic properties in longitudinal direction, which is important in global pipeline design, were available. For displacement controlled (elastic) behaviour, increased elastic stiffness in longitudinal direction leads to higher stress and therefore needs to be assessed properly; if the typical stiffness value is used, stress will be underestimated and results will be nonconservative.

This content is only available via PDF.
You can access this article if you purchase or spend a download.