Pipe-in-Pipe Configurations |
 
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Pipe-in-Pipe Configurations |
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A further level of complexity afforded by the line modelling facility is the ability to model pipe-in-pipe (or pipe-on-pipe) scenarios. You model the inner and outer pipes using separate lines as described in the preceding sections, and then specify that you would like both pipes to be connected in a pipe-in-pipe type configuration. One line is designated the primary line (typically the outer line), and the other termed the secondary line. Note that while the remainder of this section refers to dual pipe scenarios, it is also possible to model riser bundles (with one primary line and several secondary lines) or pipe-in-pipe-in-pipe configurations (with a primary line whose secondary line also serves as a primary line for another secondary line).
The interaction between inner and outer pipes falls into three categories – linear, non-linear and rigid connections. Both linear and non-linear connections may be considered to operate in a similar manner to spring elements, providing resistance to relative motion of the inner and outer pipes in the transverse plane. Linear connections are typically used to model centralisers, which are characterised by a relatively high linear stiffness. Away from centralisers, the inner and outer pipes are free to move radially relative to one another. Generally speaking, the modelling arrangement should be such that when inner and outer pipes come into contact, there is no penetration of the outer by the inner, and vice versa, and this is achieved by means of non-linear connections. The (user-defined) force-deflection relationship typically have a very low stiffness when the pipes are in their respective un-deformed positions, but the stiffness increases exponentially as the gap between pipes approaches zero. Note that neither linear nor non-linear connections provide any resistance to relative axial motion of the inner and outer pipes. Rigid constraints are typically used to model bulkheads, where the inner and outer pipes are rigidly connected to each other. In the case of pipe-in-pipe model, this is achieved via the nodal equivalence facility discussed previously, but in this case the process is performed automatically by Flexcom, so you are not required to explicitly define any nodal equivalence. For pipe-on-pipe cases, rigid massless elements are inserted between the pipes at appropriate locations.
All connections (whether linear, non-linear or rigid) are defined in terms of distances along the line. You specify a start length, and end length and a spacing when generating connections along the line. As mentioned previously, the automatic meshing algorithm allows you to request that nodes be positioned exactly at specific lengths along a line (in addition to the guidelines you provide in relation to desired maximum and minimum element lengths). When you specify pipe-in-pipe connections, you are effectively requesting that nodes be placed at appropriate locations, and although the process may appear seamless, this is how your request is handled internally.
Refer to Pipe-in-Pipe for further details on pipe-in-pipe modelling in Flexcom.