Flexcom > Data Inputs > $MODEL

Purpose

To define internal and external pipe sections when part of a pipe model is contained within another.

Theory

Refer to Pipe-in-Pipe Sections for further information on this feature.

Keyword Format

Optional line which provides advanced modelling options regarding drag and inertia loads on inner pipe-in-pipe sections:

INNER_HYDRO=Hydrodynamic Solution, AUTO_CREATE=Auto-Create Connections

One line that is then repeated as often as necessary to specify all pipe-in-pipe sections:

OUTER=Outer Element Set Name, INNER=Inner Element Set Name

The inner and outer element sets must be defined under *ELEMENT SETS. INNER_HYDRO may be either LHS (the default) or RHS. AUTO_CREATE may be either YES (the default) or NO. These options are highly specialised and should only be explored by power users of the software. The default options are recommended by Wood. Refer to Drag and Inertia on Inner Pipe Sections for further information.

Table Input

Pipe-in-Pipe Sections

Input:	Description
Outer Element Set:	The element set which represents the external or outer pipe section in the pipe-in-pipe model.
Inner Element Set:	The element set which represents the internal or inner pipe section in the pipe-in-pipe model.

Drag and Inertia on Inner Pipe Sections

Input:

Description

Hydrodynamic Solution:

This options allows you to specify whether drag forces and hydrodynamic inertia on inner pipe-in-pipe sections are modelled as either:

•Mass and damping terms on the left hand side of the equations of motion (capturing the required coupling between the outer node’s velocity/acceleration and the inner node loading), the default, or...

•Force terms on the right hand side of the equations of motion (for compatibility with earlier versions, but theoretically incorrect).

Auto-Create Connections:

This options allows you to specify whether:

•Flexcom should automatically insert token connections of zero stiffness where required to ensure that hydrodynamic loading on all inner nodes is modelled, the default, or...

•The solution should proceed using only the pipe-in-pipe connections which you have defined explicitly (computationally more efficient in some cases, but there may be a possibility that not all inner nodes will experience hydrodynamic loading at all times during the simulation).

Notes

(a)A particular Outer Element Set can be associated with more than one Inner Element Set, for example in the modelling of bundled riser systems. But for obvious reasons a particular Inner Element Set can be internal to only one Outer Element Set.

(b)A particular element set which is the Inner Element Set on one row of the Pipe-in-Pipe Sections table can be an Outer Element Set on another row, as for example in the analysis of a multi-tube production riser.

(c)The use of the Pipe-in-Pipe Sections table is not mandatory when defining a pipe-in-pipe model. For example, in the analysis of piggy-backed systems, it is valid to specify pipe-in-pipe connections without defining internal and external pipe-in-pipe sections.

(d)The Hydrodynamic Solution and Auto-Create Connections options are highly specialised and should only be explored by power users of the software. The default options are recommended by Wood. Refer to Drag and Inertia on Inner Pipe Sections for further information.