Fatigue analysis: Components data

The components data page is only available when damage is calculated using stress factors or using externally-calculated stress. The components are those for which damage is to be calculated. Components can represent, for instance, different layers of, or elements in, the cross section of an umbilical or a flexible.

Component name

This is used to identify the component in the results.

In the case of externally-calculated stress, this name is also passed to the external function that calculates stress, in the ExternalResultText of the ObjectExtra structure, allowing you to pass parameters to the external function. The most important use of this facility is to pass the $\theta$ value to the external function. The convention is that, before the external function is called to derive a stress result, any occurrence of the text %theta% is replaced with the actual value of theta, in degrees. So suppose that the component name was specified to be theta=%theta%. The actual ExternalResultText strings passed to the external function would be of the form theta=0.0, theta=45.0 etc.

Tension variable (stress factors only)

Specifies which tension variable, wall tension or effective tension, is used to calculate stress.

Tension and curvature stress factors (stress factors only)

The stresses used to calculate damage are calculated according to the formula \begin{equation} S = \Kt T + \Kc(\Cx \sin \theta - \Cy \cos \theta) \end{equation} where

$S$ is stress,

$\Kt$ and $\Kc$ are the tension and curvature stress factors, respectively

$T$ is either wall tension or effective tension, as specified by the tension variable

$\Cx$ and $\Cy$ are the components of curvature in the line's local $x$ and $y$ directions, respectively

$\theta$ is the circumferential location of the fatigue point

In effect, this formula defines stress to be the sum of contributions due to direct tensile strain and bending strain. The circumferential variation (i.e. the terms which refer to $\theta$) is to account for the fact that bending strain varies with $\theta$. So, for a point in the plane of bending, stress is given by $S = \Kt T \pm \Kc|C|$, where $C$ is the curvature vector $(\Cx,\Cy)$. Similarly, for a point at 90° to the plane of bending, stress is given by $S = \Kt T$.

The stress factors will typically be calculated from experimental data or from detailed analytic models of the umbilical or riser cross section. Suppliers of such products are usually able to provide the necessary stress factors.

Stress result (externally-calculated stress only)

The name of the externally-calculated result that provides the stress values.

S-N curve

Specifies which S-N curve is used for damage calculations for this component.