1. Pipeinpipe contact
It enables the user to model pipeinpipe 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.
A pipeinpipe pair consists of a master pipe and a slave pipe. Both the master and the slave pipe may consist of beam or bar types of elements and must be of cross section types CRS0 or CRS1. 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.
Note that:  If using the sheltered closed option, the master and slave pipes may not share supernodes.
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 pipinpipe 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 nonpipeinpipe 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.
Input parameters

Fluid loading on inner pipe: Determines if the inner pipe is exposed to environmental loading or kinematics based on relative movement between inner and outer pipe.

Master position: The position of the master pipe in the pipeinpipe contact pair. This option decides if the outer or inner pipe should be the master pipe. 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 number of artificial contact elements is equal to number of nodes in the master pipe.

Stiffness type: Either constant (linear), or nonlinear contact compression stiffness between the master and slave pipe. In the latter case, a table of force/displacement pairs must be entered. The contact stiffness, specified as stiffness per unit length along the master pipe, should be chosen large enough, but not stiffer than necessary. Very high stiffness values may lead to long runtimes, instability and highfrequency numerical noise. When selecting the stiffness it may be useful to consider penetration for a characteristic force and also the convergence of the results that are of interest.

Relative damping level (RELDAM): Relative damping at estimated eigenperiod in the master, slave and contact spring system.

Damping: damping coefficient per unit length of master pipe

Spring stiffness (STIFF) associated with static friction coefficient. The stiffness used when loading friction forces.

Axial friction switch: This swith controls if the axial friction between master and slave is included.

Rotational friction switch: This swith controls if the rotational friction between master and slave is included.

Velocity limit for change from static to dynamic friction
Based on specified damping level the stiffness proportional damping coefficient is calculated by
\(a_2=2\times \mathrm {RELDAM}\times \sqrt{\frac{\mathrm {AMS_M+AMS_S}}{\mathrm {STIFF}}}\)
where \(\mathrm {AMS_M}\) and \(\mathrm {AMS_S}\) are structural mass per unit length of the master pipe and the slave pipe respectively and \(\mathrm {STIFF}\) is contact spring stiffness per unit length.
1. Recommended default values
The following are recommended default values for setting up a typical pipein pipe analysis. It is recommended to always do a parameter variation after setting up a model to ensure convergence of results and that damping does not affect modes of interest. Only a small amount of axial friction is included in the suggested input to improve convergence (compared to turning axial friction off).
Parameter  Recommended value  Unit 

Fluid loading on inner pipe 
Sheltered Closed 
() 
Master position 
Outer 
() 
Stiffness type 
Linear 
() 
Linear spring stiffness 
1e+5 
(N/m^2) 
Relative damping 
0.71.0 
() 
Damping 
0.0 
(Ns/m^2) 
Friction stiffness 
2.0e+6 
(N/m) 
Static friction coefficient 
1e3 
() 
Dynamic (sliding) friction coefficient 
1e3 
() 
Axial friction switch 
ON 
() 
Rotational friction switch 
OFF 
() 
Velocity limit 
0.1 
(m/s) 
Global input in dynamic calculation parameters (procedure tab):
Parameter  Recommended value  Unit 

Global stiffness proportional damping factor 
0.001 
() 
2. Recommended static loading sequence
To improve convergence in the static analysis, it is not recommended to include pipeinpipe contact forces as the first load group. Loading Volume forces, initially prestressed segments etc. before pipinpipe forces will in most cases improve static convergence.
3. Tips for efficient analysis with pipeinpipe

Set Pipeinpipe contact to entered in static calculation parameters. The internal pipe does not have to be placed in the exact centre, or even inside of the external pipe, it will be pulled towards the centre when the contact forces are applied.

Load Volume forces before pipeinpipe forces. This will in many cases make the application of contact forces much more stable due to the additional drag forces and weight.

Setting the external pipe as master will often improve convergence and runtimes.

It is recommended to use shorter elements in the master pipe. The opposite may lead to slower convergence and numerical noise.

Note that the element mesh of the master pipe controls the number of contact elements and should therefore be chosen carefully with regards to convergence of results and speed. Use a refined mesh where necessary, for example in areas where large curvatures and contact is expected, inside bellmouths/guidetubes, flexjoints etc. Run times increase exponentially with number of elements in pipeinpipe pairs, so it is important to keep number of elements to a minimum, but sufficient to ensure convergence of key results.

Use linear contact stiffness and start off with a low stiffness for stability and increase gradually if needed.

For a typical application, drillingriser, an element length of 23 m for both the drillpipe and marine riser will be sufficient. In areas where the drillpipe is bent and large contact forces are expected (for example around flexjoints/balljoints), the element length should be reduced.

External contact radius of inner pipe must be larger than zero

If the master position is outer, all segments in the slave(inner) pipe segments must have external contact radius which is smaller than the internal contact radius of the outer(master) pipe and larger than zero.

If the master position is inner, all segments in the master (inner) pipe segments must have external contact radius which is smaller than the internal contact radius of the outer(slave) pipe and larger than zero.
4. Input reference
You can find input reference here