In the last decade new standards or revisions of existing guidelines have been launched on the subject. Among those, BS 7910 (now in edition 2013) [1] and DNV-OS-F101 (now in edition 2013) [2] are considered as reference in many world offshore districts of the Oil & Gas Industry.
What is peculiar in offshore pipelines with respect to pressure vessel or nuclear plants, for which an engineering criticality assessment (ECA) was first established, it is the fact that in many circumstances offshore pipelines exceed the elastic limit (global pipe bending is a primary stress that causes mainly membrane stress through the pipe wall). This implies the extension of a stress based ECA into a strain based ECA, further including the bi-axial state of stress, caused by the presence of internal pressure and hoop stresses.
An important step of ECA is the definition of loads and load effects at pipe girth weld, from global applied loads on the pipeline to the local effects at the crack.
Finite elements (FE) are currently used to develop the relevant bending moment stress vs. strain relationship for the given pipe diameter, wall thickness and materials, both parent pipe and weld. Related longitudinal stress distribution on the pipe cross section without flaws in the weld is calculated for different pipe life stages (installation, pressure test and operation). The calculated global (or far from the flaw) longitudinal stress distribution is an input for the ECA analysis.
For this aspect the new DNV OS-F101 (2013) has reviewed the appendix A requiring the use of 3D FE analyses to account for the effect of the internal pressure on the Crack Driving Force (CDF).
In this paper it is discussed an analytical approach both to assess the pipeline strength in presence of flaws in the girth welds of offshore pipelines and to define defect acceptance criteria for specific new projects. The approach follows the framework of BS7910 and of DNV OS-F101 and includes load conditions under both installation and operation. In particular specific 3D FE analyses are presented to enforce the applicability of the proposed analytical approach.
