Page 61 - Pipelines and Risers

34 Chapter 2

Safety factors f should be determined by the designer with appropriate consideration of the
magnitude of increases that may occur for installation bending strain or in-place bending
strain. A value of 2.0 for safety factors f is recommended if no detailed information on the
uncertainties of load effects is available. Safety factorfmay be larger than 2.0 for cases where
installation bending strain could increase significantly due to off-nominal conditions, or
smaller than 2.0 for cases where bending strains are well defined (e.g. reeling) or in-place
situation.
A lower safety factor may be allowed for installation phase provided that potential local
buckling can be detected, repaired and buckling propagation can be stopped through use of
buckle arrestors.

2.6 Wall Thickness and Length Design for Buckle Arrestors
During the installation the risk of local buckling initiating a propagating buckle will be
considered to be high, hence buckle arrestors will be designed to limit the extent of the
damage of a propagating buckle, see JPK (1997).
(;r
Equation used to determine whether buckle arrestor is required, may be taken as:
Ppr = 24 * SMYS * - (2.22)

where:
Ppr = Propagating pressure for the pipeline
SMYS = Specified Minimum Yield Stress
t = Pipe wall thickness
D = Pipeline outer diameter

Upon solving the following equation, feasible buckle arrestor wall thickness and length
combinations is obtained. This equation is valid for thick-walled cylindrical buckle arrestors
(Sriskandarajah and Mahendran, 1987).

(2.23)
L J
where:
P, = Crossover pressure = SF*Ph (2.24)
LBA = Buckle arrestor length
SF = Safety factor = 1.5
Ph = pw*g*(hm+ht+hs) (2.25)

and where:
Ph = Hydrostatic pressure