Fatigue analysis: Fatigue points

Damage is calculated at a number of fatigue points along the line. These are defined by their arc length $z$ and, depending on the damage calculation, their radial and circumferential position $(r,\theta)$ in the cross section at that arc length, where $r$ is the radial distance from the centre-line and $\theta$ is measured from the line's local x-axis towards its y-axis.

Arc length is determined by the analysis data arc length intervals. Cross-sectional position is specified as follows.

Damage calculated by homogeneous pipe stresses

OrcaFlex calculates the damage at either the inner, outer or mid-wall point of the pipe, so $r$ is $\frac12\IDs$, $\frac12\ODs$ or $\frac12(\IDs + \ODs)$, respectively.

For $\theta$, we calculate damage at equally spaced intervals determined by the specified number of thetas. For example, if you specify $16$ theta values, then they will be equally spaced at $360\degree/16 = 22.5\degree$ intervals $0\degree$, $22.5\degree$, $45°$ etc.

See the pipe stress calculation topic for a diagram and definitions of these terms.

Damage calculated by stress factors

For stress factor fatigue, the radial position $r$ is implicit in the stress factors, so is not explicitly required by the calculation. Damage is calculated at circumferential locations $\theta$ determined by the specified number of thetas, as described above for homogeneous pipes.

Mooring fatigue damage

In mooring fatigue, radial and circumferential variation are not considered at all: there is simply a single fatigue point at each arc length considered.