The out-of-roundness (OOR) for pipe ends is determined as the difference between the largest outside diameter and the smallest outside diameter, as measured in the same cross-sectional plane:
OOR = Dmax - Dmin
Dmax is the largest measured inside or outside diameter of the pipe, Dmin is the samllest measured inside or outside diameter of the pipe,
The DNV definition of ovality:
f0 = OOR/Dnom
Dnom is the nominal inner or outer diameter of the pipe
It is different from the ovality defined in API RP-1111, which is defined as:
δ = (Dmax - Dmin)/(Dmax + Dmin)
The maximum allowable ovality of line pipe is limited by the pipeline specification, in which, in most cases, the pipeline ovality during the fabrication process is not to be more than 1.5% (f0) or 0.75% (δ). The ovalilty of a reeled pipeline may be calculated by using Brazier formula, which was developed based on elastic tubes and a conservative estimate in the plastic region:
Rreel = reel radius
rp = mean pipeline radius
t = wall thickness of the pipe
The ovality of the pipe may increase when the pipe is subject to reverse bending, and the effect of this on subsequent straining is to be considered. For a typical pipeline, the following scenarios influences the ovality:
The ovality may increase during the installation process when the pipe is subject to reverse inelastic bending.
Cyclic bending due to lateral buckling may occur as a consequence of shutdowns during operation, if global buckling is allowed to relieve temperature- and pressure-induced compressive forces.
The ovality due to point loads should be checked. Critical point loads may arise at free-span shoulders and artificial supports. The accumulative ovality, f0, through the life cycle should not exceed 3%. This ovality requirement may be relaxed if the effect of ovality on moment capacity and strain criteria are included, the pigging requirements and repair systems are met, and cyclic-load-induced ovality have been considered.
Finite element analysis may be performed to calculate the increase in ovality during the life cycle of a pipeline. The analysis is to include fabrication tolerances and all loads applied through the pipeline’s life cycle, such as point loads, bending against a surface, axial load, and repeated pressure, temperature, and bending cycles.
[Qiang Bai, Yong Bai, in Subsea Pipeline Design, Analysis, and Installation, 2014: https://www.sciencedirect.com/topics/engineering/roundness#:~:text=Out%20of%20roundness%20is]
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