Standard Connections |
 
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Standard Connections |
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You define standard pipe-in-pipe connections between a pair of nodes when setting up your model, and those two nodes remain connected throughout all subsequent analysis stages. You can define standard connections in two ways: you can identify the pair of nodes directly, or you can connect pairs of nodes from two groups of elements using a generate option. It is important to be clear that even when you use the latter method, a pair of nodes connected initially remain connected thereafter.
Pipe-in-pipe connections can be considered to operate like Spring Elements, whether linear or non-linear. However the springs in this context do not directly connect the two nodes (in many cases the nodes are coincident, at least initially, and so cannot be connected by a spring). What happens is that a spring stiffness is added directly into the stiffness matrix at locations corresponding to the appropriate motions of the two nodes. The direction of the spring is computed as being normal to the primary node.
Pipe-in-pipe connections are primarily designed to resist relative lateral motion of the connected nodes. However, it is also possible to simulate some resistance to motion in the axial/longitudinal direction, which may be useful in certain circumstances.
Note also that the stiffness contribution is always computed and added, no matter how far apart the two nodes move axially. If the degree of relative motion between the inner and outer pipes in the axial direction is significant, then you should define Sliding Connections instead.
Strictly speaking, there are no mandatory requirements regarding which pipe should be designated as primary and which as secondary. Recommended practice is to designate the larger/stronger pipe as the primary pipe, and the smaller/dependent pipe as the secondary pipe. The following examples provide some practical illustration.
•For pipe-in-pipe configurations, the outer pipe is normally designated as the primary pipe, and the inner pipe as the secondary pipe. The outer pipe is naturally larger in diameter and usually has a higher bending stiffness. Top tensioned risers for example, typically have a steel outer pipe with a flexible inner tubing. In this case, the steel outer pipe should be designated as primary, with the inner tubing denoted secondary.
•For pipe-on-pipe configurations, the main/carrier pipe is normally designated as the primary pipe, with the attached pipe as the secondary pipe. Again the carrier pipe is typically larger and stronger than any attached lines. Hybrid riser towers for example, typically have a central steel tendon with a number of attached lines to support production, water injection etc. In this case, the steel tendon should be designated as primary, with any attached lines denoted secondary.
The primary elements are used to determine the orientation vectors of the stiffness connections between the connected pipes, and it is important that these vectors remain perpendicular to the overall pipe geometry in so far as possible. Designating the smaller/dependent pipe as the primary pipe is not recommended, as the consistency of the orientation vectors could be compromised due to the flexibility of the inner pipe. Considering the top tensioned riser for example, if the flexible inner tubing is allowed to set down under self weight inside the steel pipe, the local orientation vectors could be non-horizontal and also vary considerably at different elevations.
Pipe-in-pipe drag and hydrodynamic inertia forces due to the presence annular fluid are dependent on velocity and acceleration solution variables (the fluid is assumed to move rigidly with the outer pipe nodes). For this reason, a direct finite element coupling between the inner and outer pipe-in-pipe nodes must be explicitly specified using pipe-in-pipe connections between the inner and outer nodes for the drag and hydrodynamic inertia forces to be computed. So Flexcom automatically inserts token connections of zero stiffness where required to ensure that hydrodynamic loading on all inner nodes is modelled, even if there is no user-defined connection between all outer and inner nodes. Refer to Hydrodynamic Forces for further details.
•*PIP CONNECTION is used to define pipe-in-pipe connections between nodes of the finite element model.
•*PIP STIFFNESS is used to define force-deflection curves for non-linear pipe-in-pipe connection stiffnesses.
If you would like to see an example of how these keywords are used in practice, refer to A03 - Pipe-in-Pipe Production Riser.