Pipe-in-Pipe Sections |
 
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Pipe-in-Pipe Sections |
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You define pipe-in-pipe section data very simply by defining a pair or pairs of outer and inner element sets, the composition of the sets being defined in the normal fashion. Pipe-in-pipe section data is used in the calculation of buoyancy and hydrodynamic forces on the relevant sets.
With regard to buoyancy forces, the internal fluid of the outer set of a pair becomes the external fluid for the inner set, and pressure forces are calculated on that basis. In terms of hydrodynamic loading, the annular fluid acts as the external fluid for the inner pipe. This means that the inner pipe is not subjected to any hydrodynamic loading from the external ambient environment. Specifically, wave and current loads are experienced by the outer pipe only, and do not (directly at least) apply any hydrodynamic loading to the inner pipe.
Note also that the annular fluid is assumed to move rigidly with the outer pipe as it is entrained within it. So the velocity and acceleration terms for the annular fluid are assumed equal to those of the outer pipe. Therefore, the drag and hydrodynamic inertia forces are themselves dependent on the unknown velocity and acceleration solution variables. For this reason, Flexcom automatically creates a direct finite element coupling between the inner and outer nodes (using token pipe-in-pipe connections of zero stiffness) so that the relevant drag and hydrodynamic inertia forces may be computed for any inner nodes which are not already connected to outer nodes via Standard or Sliding Connections. In earlier versions, it was the responsibility of the user to create these additional connections, but this is no longer necessary. Hydrodynamic forces on the inner pipe are then computed in the normal fashion via Morison’s Equation. Recommended practice is to specify the hydrodynamic properties of both pipes as normal, and allow Flexcom to handle the pipe-in-pipe modelling aspects of the configuration. Refer to Hydrodynamic Forces for more detailed information on this topic.
If you are defining a pipe-on-pipe (or piggy-backed) system, rather than a genuine pipe-in-pipe one where one set is contained within the other, then the specification of pipe-in-pipe sections is unnecessary. Buoyancy and hydrodynamic forces on all elements will then be calculated in the normal way.
A particular outer element set can contain more than one inner element set, but naturally a particular inner element set can have only one associated outer element set. Also, the inner element set of one pair can also be the outer element set of another pair. An example might be a multi-tube production riser, consisting perhaps of outer and inner risers and production tubing. The inner riser is contained within the outer riser, but also contains the tubing, so it would be both an inner element set and an outer element set. In the likely event of the inner riser sliding axially out of the outer riser while the tubing remains in place, then the fluid contents of the outer riser would define the hydrodynamic loading on the tubing.
•*PIP SECTION is used to define internal and external pipe sections when part of a pipe-in-pipe model is contained within another.
If you would like to see an example of how this keyword is used in practice, refer to A03 - Pipe-in-Pipe Production Riser.