Risers Static Analysis Using ANSYS

2010 ANSYS SOUTH AMERICAN

CONFERENCE & ESSS USERS MEETING

Alfredo Gay Neto

alfredo.neto@gmail.com

Risers are structures used in offshore applications for the

transportation of oil and gas from the vessel on the water surface

to the seabed, and also in the opposite way. These structures can be installed in various typical configurations:

2Examples of riser configurations

• Structural analysis of these elements may be performed in a global way.

• Risers are idealized as very long beams subjected to static and dynamic

loadings.

• No ovalization or warping of the cross section is considered in this work.

• The whole cross section is supposed to have equivalent EA, EI and GJ, for

composing the constitutive equation in the model.

• The actual cross section may be complex, and may have non-constant

stiffness.

Riser Analysis

3

(virtual prototype developed by Numerical Offshore

Tank – USP)

Once the riser is installed, its static configuration depends on some static

loading, as:

• The riser weight

• The drag due to sea currents

• The load resultant from both external and internal pressures

– The riser contains a pressurized fluid

– The riser is submerged, being subjected to buoyancy force

• The contact between the riser and the seabed

– Normal component

– Tangential component

(friction)

Static Loads

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The animation shows a riser laying in the seabed

and, after, being loaded by a lateral constant

sea current. The frames represent the load steps

up to total loading application.

• The lift due to sea currents, that can cause a periodic loading in the transversal

direction of the riser (causing vortex induced vibration VIV)

• The movement imposed from the floating unit oscillations to the riser (can be

approximated as harmonic in a first model)– Actually this load is not deterministic and have to be considered using a statistical approach. Using

sea waves energy spectra associated to their probability of occurrence, one can predict a more

realist excitation in risers due to sea waves.

– OBS: dynamic loads are not considered in this work. They are being included

in a future work.

Dynamic Loads

5

• This work presents an ANSYS procedure to analyze free-hanging risers

statics. The loads considered are the riser effective weight and the

contact between the riser and the seabed.

6Example of a static riser configuration (free hanging)

• The riser is modeled as a very long beam (using BEAM188

elements)

• The riser cross-section can be very complex and is not

detailed in the model

• Global stiffness behavior is provided by EA, EI and GJ

(known)

• The riser is assumed to be unstressed on its straight (initial)

configuration

Assumptions

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• APDL code was used to:

– General data entry

– Construct riser geometry;

– Mesh the geometry;

– Construct contact pairs;

– Do the loading sequence to solve the problem.

• Each step is discussed next...

Procedure

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APDL – General data entry

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EA = 6080489749 !Axial stiffness

EI = 110821607.7 !Bending stiffness

GJ = 85247390.5 !Torsion stiffness

ndiv = 800 !Number of divisions (mesh)

Length = 1600 !Riser Length

valuex=-800 !Projection of the riser in x axis (in final configuration)

valuey=0 !Projection of the riser in y axis (in final configuration)

valuez=1000 !Depth (z) (in final configuration)

rho = 237.14 !Specific effective mass per unit lenght of the riser

gravity = 9.81 !Gravity acceleration

These commands define some scalar parameters in ANSYS

• Seabed is defined as a flat plane located in z=0

• Riser is defined as a straight line aligned with x axis.

APDL – Geometry construction

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!!!!!!!!!!!!!Seabed Definition!!!!!!!!!!!!!!

k,1,0,500,0

k,2,1000,500,0

k,3,1000,-500,0

k,4,0,-500,0

A,4,3,2,1

!!!!!!!!!!!!!!!!RISER Definition!!!!!!!!!!!!

k,10,0,0,0

k,11,Length,0,0

L,10,11

• The cross section is defined using stiffness data EA, EI

and GJ

• Material Young Modulus is set to a unit value in order to

keep the stiffness values entered

APDL – Riser properties

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ET,1,BEAM188 !Defines element type

!!!!!!!!!!!!Cross Section!!!!!!!!!!!!!!

SECTYPE, 1, BEAM, ASEC, , 0

SECOFFSET, CENT

SECDATA,EA,EI,0,EI,0,GJ,0,0,0,0

!!!!!!!!!!!!Material!!!!!!!!!!!!!!!!!!!

MPTEMP,1,0

MPDATA,EX,1,,1

MPDATA,PRXY,1,,0

(Extracted from ANSYS 12.1 help)

• The mesh in the riser line is very simple

• All elements have the same length

APDL – Riser meshing

LESIZE,5, , ,ndiv, , , , ,1 !Defines number of divisions in the line number 5

(riser line)

LMESH,5 !Meshes the riser line

• Node 1 is fixed (to represent the anchoring location)

• A Node numbered 10000 is created and fixed (will be a pilot node used in

the contact pair)

APDL – Boundary Conditions

!Boundary Conditions

D,1,UX,0

D,1,UY,0

D,1,UZ,0

D,1,rotx,0

D,1,roty,0

D,1,rotz,0

n,10000,1,1,1 !pilot node

d,10000,ux,0

d,10000,uy,0

d,10000,uz,0

d,10000,rotx,0

d,10000,roty,0

d,10000,rotz,0

• The contact between riser and seabed is assumed to be a “nodes

to surface” model

• The seabed is modeled as a rigid target (connected to node 10000

– pilot node)

• Contact is assumed to be frictionless

• Contact characteristics are:

Contact definition

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• The loading sequence solved in the model is:

1. Tensioning of the riser due to a displacement imposition in one

of the tips

2. Loading of riser effective weight

3. Displacement imposition in the top, leading the riser to go to its

prescribed position given by:

• Top position (vessel)

• Anchoring position (seabed)

Loading Sequence

• A displacement value is imposed to the node 2 (tip of the

riser)

• This load step makes the system artificially tensioned (with

very high geometric stiffness)

Loading Sequence - 1

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/solu

D,2,UX,10

D,2,UY,valuey

D,2,UZ,valuez

ANTYPE,0 !Static analysis

NLGEOM,1 !Geometric stiffness

NSUBST,1,100000,1!Number of substeps

outres,all,all !Saves all the results

SOLVE !Solves the load step

• The nodal contribution of the effective weight of the riser is

calculated by dividing it by the number of nodes

• The same load value is applied to each node

Loading Sequence - 2

!!!!!!!!!Riser Weight!!!!!!!!!

lsel,s,line,,5 !Selects the line of the riser

nsll,s,1 !Selects the nodes of the line 5

F,all,FZ,-rho*Length*gravity/(ndiv+1) !Imposes nodal loads

allsel,all !Select all the nodes and geometric entities

SOLVE !Solves the model

• The displacement imposition is performed in node 2 (riser top)

• It makes the whole tension distribution to decrease its magnitude and the

riser achieves the free hanging configuration

• During the process contact between riser and seabed occurs, turning the

convergence a difficult task

Loading Sequence - 3

NSUBST,100,10000,40

D,2,UX,valuex

D,2,UY,valuey

D,2,UZ,valuez

SOLVE

• The bending moment

distribution is an important

issue for studying riser

behavior (bending stresses)

• The maximum bending

moment is located in the TDP

(touch down point) region

• TDP is one hot spot for

studying riser life

Results – Free hanging configuration and bending

moment distribution

Riser configuration colored by bending moment distribution

• As expected, the

maximum tension is

located at the top position

• The riser top position is

also a hot spot for

designing a possible

configuration

Results – Free hanging configuration and tension

distribution

Riser configuration colored by tension distribution

• This work shows that it is possible to deal with the catenary static

riser configuration using ANSYS

• An APDL code was done for this purpose and beam elements

showed do be good for predicting maximum bending moment and

tension values. It was possible to see the expected hot spots for

these results

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• The work can be extent to consider sea current effects,

causing a 3D riser configuration

– Morison Model

• Dynamic loads can be considered, for example by a harmonic

excitation of the top position

Future works

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Risers Static Analysis Using ANSYS

2010 ANSYS SOUTH AMERICAN

CONFERENCE & ESSS USERS MEETING

Alfredo Gay Neto

alfredo.neto@gmail.com