Page 411 - Pipelines and Risers

378 Chapter 19

For combined internal pressure and bending, hoop SMTS;
Longitudinal SMYS & SMTS;
For combined external pressure and bending, hoop SMYS;
Longitudinal SMYS & SWS.

Pipe strength under combined internal pressure and bending is an important design case, if
fishing activities are frequent.

It is difficult to compare the requirements of the material property in hoop and longitudinal
directions. Rather the following is a discussion on cost-effectiveness of raking material's
performance in hoop and longitudinal directions.

Raising hoop SMYS will directly result in a proportional reduction of the required wall-
thickness of the line pipe for water depth shallower than 350 mm. However, if the design
codes, on bucklingkollapse for external-over pressure case, are further upgraded, this water
depth may be extended from 350 m to 450 m. It is the author's opinion that the existing
design equations for external-over pressure situations are rather conservative. To achieve
yield and tensile strength values that conform to the requirements, as specified for the
transverse direction, a corresponding increase in the strength in the longitudinal direction is
needed. This in turn leads to increased production costs and may lead to difficulties in
meeting the requirements for yield-to-tensile ratio, toughness and sour service suitability, etc..

As a conclusive remark on materials property requirements, it is believed that:

rn The minimum CTOD values in both hoop and longitudinal directions typically should be
0.1 mm; the applicability of lower CTOD values can be validated by ECA methods.
It is economically beneficial and technically justifiable that for pipe grades X60 to X80
yield and tensile strength in longitudinal direction can be lower by up to 10% than those in
the transverse direction for water depths shallower than 450 m.
0 For fracture and locallbuckling failure modes, the Y/T value requirement can be removed
if the strength analysis explicitly account for the difference of strain-hardening whose
parameters (UR and n) are a function of SMYS and SMTS as the equations given in Bai et
al. (1994).

As a further study, it is proposed to compare the Y/T ratio requirements from alternative
codes (e.g. 0.93 from MI for onshore pipelines, 0.85 from EF'RG). It is perhaps possible to
find some other rational criteria that can replace the Y/T ratio requirement in strength design.
In order to develop alternative criteria, it is necessary to understand the reasoning of using
YTT ratio as a design parameter.

Criteria for bucklinglcollapse calculations of corroded pipes with yield anisotropy were
derived by Bai et al. (1999).