von Mises Combined Loading Analysis

1. Purpose

Calculate combined loading for steel risers.

2. Description

Von Mises combined loading analysis, see e.g. API 2RD.

3. Required results

To perform the code-check, element forces and nodal displacements need to be stored from the simulation.

4. Description of input parameters

The following user input is required.

4.1. Input for stress and usage factor

  • Stress

    1. Mid wall- Mid wall von Mises stress

    2. Maximum - Maximum von Mises stress at inner or outer wall, Sparks (1984).

  • Design factor, \(F_{D}\) , e.g. according to API 2RD.

4.2. Input for fluid properties

  • pd - Design pressure at reference point

  • Reference point - Vertical reference position for design pressure, given in global coordinate system

4.3. Structural properties

Structural properties is given either for a single element, all elements in a given segment or for the whole line.
Combined loading will be calculated for all elements that have specified structural properties.

4.3.1. Input for Riser geometry properties

  • Nominal diameter - Outside diameter of the pipe

  • Nominal thickness - Nominal pipe wall thickness (uncorroded), as given on drawing/specified

  • Ovality

  • External corrosion - External corrosion allowance

  • Internal corrosion - Internal corrosion allowance

By default the nominal diameter and thickness is calculated based on cross section parameters.

4.3.2. Input for Riser material properties

  • E - Young’s modulus

  • \(\nu\) - Poisson’s ratio

  • \(f_{y}\) - Characteristic yield strength

  • \(f_{u}\) - Characteristic tensile strength

4.3.3. Statistics

The response extreme value are estimated using either

  • observed maximum

  • use distribution fitting for a selected return period

4.3.4. Analysis Time Window

The time window can be set using

  • start time

  • end time

4.3.5. Output

Option to add intermediate results.

The intermediate results are stored together with the element results.

5. Calculation of utilization

The capacity is expressed by the utilization factor, \(u\):

\[u = \frac{\sigma_{vM}}{\sigma_{y}\cdot F_{D}} \text{ where } u < 1\]

  • \(\sigma_{vM}\) is the characteristic von Mises stress

  • \(F_{D}\) is the design factor, \(F_{D} < 1\)

6. References

Sparks, C.P. (1984): The influence of tension, pressure and weight on pipe and riser deformations and stresses. Journal of Energy Resources Technology, Vol. 106, pp. 46-54.