Page 50 - Pipelines and Risers

23

Chapter 2

Wall-thickness and Material Grade Selection

2.1 General

2.1.1 General
In this section, the basis for design of wall thickness is reviewed and compared with industry
practice. The codes reviewed are ABS, API, ASME B31, BS8010, DNV and ISO.
Wall thickness selection is one of the most important and fundamental tasks in design of
offshore pipelines. While this task involves many technical aspects related to different design
scenarios, primary design loads relevant to the containment of the internal pressure are as
follows:

- the differential pressure loads
- longitudinal functional loads
- external impact loads

The current design practice is to limit the hoop stress for design against the differential
pressure, and to limit the equivalent stress for design against combined loads. This practice
has proved to be very safe in general, except when external impact loads are critical to the
integrity of the pipeline. Nevertheless, this practice has been used by the pipeline industry for
decades with little change, despite significant improvements and developments in the pipeline
technology, see Sotberg and Bruschi (1992) and Verley et al. (1994).
Considering the precise design and effective quality and operational control achieved by
modern industry, and with the availability of new materials, it has been realized that there is a
need to rationalize the wall thickness sizing practice for a safe and cost-effective design, see
Jiao et al. (1996). New design codes provide guidance on application of high strength and new
materials, as well as design of high pressure and high temperature pipelines.

2.1.2 Pipeline Design Codes
- ASh4E B31 Codes
The early history of pipeline design codes started in 1926 with the initiation of the B31 code
for pressure piping followed by the well-known ASME codes B31.8 for Gas Transmission
and Distribution Piping Systems and B31.4 for Oil Transportation piping in the early 1950’s.