Page 103 - Pipelines and Risers

76 Chapter 4

The general requirement of the accumulated plastic strain is that it should be based on strain
aging and toughness testing of the pipe material. It is stated that due to material considerations
a permanent/plastic strain up to 2% is allowable without any testing. In practice, this is valid
also for the operational case.

If the pipeline is to be exposed to more than 2% accumulated plastic strain, as is often the case
for reeling installation method, the material should be strain aging tested. However, recent
testing of modem pipeline steel has shown that plastic strain up to 5% or even 10% can be
acceptable.

In order to have an extra safety margin, it is also desirable to have a certain ratio between the
yield stress and the ultimate tensile stress. A requirement to this ratio is given in DNV’81,
paragraph 5.2.6.2, where the yield stress is determined not to exceed 85% of the ultimate
stress. Accumulated plastic strain will increase the yield stress of the material and also
increase the yieldultimate stresses ratio.

4.10 Strain Concentration at Field Joints Due to Coatings

It is necessary to evaluate effects of the concrete coating on strain concentrations at field
joints. It is found reasonable to assume that the SNCF (Strain Concentration Factor) is 1.2.
This value is mainly selected due to an allowable strain as high as 0.4% from the fracture
criterion and the technical information from Ness and Verley (1996).

4.11 References
1. Bai, Y. and Damsleth, P.A. (1997) “Limit-state Based Design of Offshore Pipelines”,
Proc. of OMAE’97.
2. Chen, M.J., Dong, G., Jakobsen, R.A. and Bai, Y. (2000) “Assessment of Pipeline Girth
Weld Defects” Proc. of ISOPE’2000.
3. Denys, R.M., (1992) “A Plastic Collapse-based Procedure for girth weld defect
Acceptance” Int. Conf. on Pipeline Reliability, June 2-5, 1992, Calgary.
4. Hauch S. and Bai Y., (1999). “Bending Moment Capacity of Pipes”, OMAE99.
5. Igland, R.T., Saerik, S., Bai, Y., Berge, S., Collberg, L., Gotoh, K., Mainuon, P. and
Thaulow, C. (2000) “Deepwater Pipelines and Flowlines”, Proc. of OTC’2000.
6. IS0 13623 (1997) “Petroleum and Natural Gas Industries; Pipeline Transportation
Systems”, International Standard Organisation.
7. Koets, O.J. and Guijt J. (1996) “Troll Phase I, The Lessons Learnt”, OPT’96.
8. NEN (1992), NEN 3650, “Requirements for Steel Pipeline Transportation System”, 1992.
9. Ness, O.B. and Verley, R., (1996) “Strain Concentrations in Pipeline With Concrete
Coating”, Journal of Offshore Mechanics and Arctic Engineering, Vol. 118.
10. Nyswm P., T@mes K., Bai Y. and Damsleth P., (1997). “Dynamic Buckling and Cyclic
Behavior of HP/HT Pipelines”, Proc. of ISOPE’97.