Pipe-in-pipe

Pipe-in-pipe enables the user to model pipe-in-pipe contact effects where each of the pipes is defined as a single line or as a main riser line. For normal riser systems this data group is normally not necessary.

1. Pipe pair definition

A pipe-in-pipe pair consists of a master pipe and a slave pipe.

Both the master pipe and a slave pipe may consist of beam or bar types of elements. They must be of cross section types Thin-walled cross section or Axissymmetric cross secion.

See also Recommendations regarding pipe-in-pipe modelling

2. Contact behaviour

The contact between the master pipe and the slave pipe will be applied between a node on the master pipe and an element in the slave pipe. This results in nodal loads along the master pipe and discrete element loads along the slave pipe. The discretization of the master pipe must therefore be fine enough to model the contact.

3. Fluid loading

The outer pipe will have the inner fluid as contents regardless of elevation and movement. The inner area of the contents will be reduced by the area taken up by the inner pipe where it is present.

Buoyancy and hydrodynamic loads for an inner, sheltered closed pipe are calculated in the same way as if it was not in a pip-in-pipe pair, with the following exceptions:

  • The load coefficients (for instance drag and added mass coefficients, wet weight) are corrected for the change in fluid density, i.e. with the ratio of the density of the inner fluid to the density of sea water (given in the environmental description)

  • The velocity and acceleration of the surrounding fluid are set to the transverse motion and acceleration of the outer pipe

The different buoyancy, drag and added mass contributions are calculated in the same way as for non-pipe-in-pipe elements; some to the mean water level and some to the specified wave elevation given by sea surface definition.

The inner pipe will therefore not be affected by the fluid when it is above the wave crest.

um pipeinpipe fluidload
Figure 1. Fluid loading on inner pipe

4. Limitations

If using the sheltered closed option, the master and slave pipes may not share supernodes.

5. Example

See Help → Examples → Riflex → Guide Tube