Allowable Stress for offshore pipeline

1.      Pipe stress criteria

The pipe stress analysis is subject to the requirement of codes such as CSA Z662, ASME Code for Pressure Piping B31.8, IGE/TD/1, BS 8010, DnV, IP6, etc. Based on the nature and duration of the imposed loads, pipeline stresses are categorized as hoop, longitudinal, shear, and equivalent stresses.

1.1  Pipeline Allowable Stresses

Hoop, longitudinal, shear, and equivalent stresses will be calculated considering stresses from all relevant load combination. Allowable stresses as indicated in Table 1-1 may be adopted in accordance with Institute of Petroleum Pipeline Code - IP6, ASME B31.8, DnV, CSA Z662, BS8010, client requirement, etc.

Table 1‑1: Allowable stresses

Operation
Condition	Code	Stress	Allowable Stress (% SMYS)
Functional loads during operation	IP6, CSA Z662, ASME B31.8	Hoop (4)	0.72
	IP6, ASME B31.8, CSA Z662	Thermal expansion stress	0.72
	IP6, CSA Z662, ASME B31.8,	Longitudinal stress	0.72(5), 0.90(6)
Functional plus Environmental load during operation	IP6, CSA Z662, ASME B31.8,	Equivalent tensile stress	0.90
Hydrotest
Condition	Code	Stress	Allowable Stress (% SMYS)
Hydrotest loads	IP6	Hoop(3)	0.90
	IP6	Von Mises	0.96
Installation / construction
Condition	Code	Stress	Allowable Stress (% SMYS)
Functional load during construction	DnV	DnV Equivalent	0.72 (1) or 0.96(2)
Functional plus Environmental load during construction	DnV	DnV Equivalent	0.96(1) (2)
Functional load plus Environmental spanning	DnV	Von Mises	0.85
Note:             (1)   Sagbend (2)   Overbend (3)   Test pressure is to be based on this requirement (4)   Hoop stress for minimum wall thickness ( nominal less manufacture tolerance) (5)   Unrestrained pipeline (6)   Restrained pipeline