Page 53 - Pipelines and Risers

26 Chapter 2

meeting operating requirements. As the selection of material grade will have a significant
impact on the operating life of the pipeline, the operator is normally involved in the final
selection of material grade.

2.3 Pressure Containment (hoop stress) Design

2.3.1 General
The hoop stress criterion limits the characteristic tensile hoop stress, oh due to a pressure
differential between internal and external pressures:
oh<qh SMYS kt (2.1)
where T)h is the design usage factor, SMYS is the Specified Minimum Yield Strength, and kt is
the material temperature derating factor. The hoop stress equation is commonly expressed in
the following simple form:

where pi and pe are the internal and external pressure respectively, D is the diameter and t is
the wall thickness.
For offshore pipelines located in the off platform zone, the design (usage) factor is specified
as 0.72 by all major codes. For pipelines in the near platform zone (safety zone), the usage
factor is specified as 0.50 by ASME B31.8 (1992), or 0.60 by NPD (1990).

The origin for design factor 0.72 can be tracked back to the (1935) B31 codes, where the
working pressure was limited to 80 % of the mill test pressure which itself was calculated
using Equation (2.1) with a design factor up to 0.9. The effective design factor for the
working pressure was thus 0.8 x 0.9 = 0.72. Verley et al. (1994). Since the 1958 version of
B31.8 codes, the factor 0.72 has been used directly to obtain the design pressure for land
pipelines.

Furthermore, definition of diameter and thickness used in Eq. (2.2) varies between the codes,
see Table 2.1. In recent codes, such as NPD (1990) and BS 8010 (1993), the minimum wall
thickness is used rather than the nominal wall thickness while the usage factor remains
unchanged. This may result in a considerably higher steel cost, indicating such codes are
relatively more conservative despite of the significant improvements and developments in
pipeline technology.
In most codes the maximum SMYS used in Equation (2.1) is limited to 490 MPa and the yield
to tensile strength ratio to 0.85. This limits the use of high strength carbon steel such as steel
grade X80 or higher. The yielding check implicitly covers other failure modes as well. To
extend the material grade beyond the current limit, explicit checks for other failure modes
may be necessary. See Chapter 4.