cae oil & gas 2011 - essscae oil & gas 2011 [email protected]. risers are structures...
TRANSCRIPT
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
Riser AnalysisRiser AnalysisRiser AnalysisRiser AnalysisRiser AnalysisRiser AnalysisRiser AnalysisRiser Analysis
� 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.
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(virtual prototype developed by Numerical Offshore Tank – USP)
Once the riser is installed, its static configuration depends on some Once the riser is installed, its static configuration depends on some Once the riser is installed, its static configuration depends on some Once the riser is installed, its static configuration depends on some static loading, as:static loading, as:static loading, as: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
Static LoadsStatic LoadsStatic LoadsStatic LoadsStatic LoadsStatic LoadsStatic LoadsStatic Loads
◦ The riser is submerged, being subjected to buoyancy force
� The contact between the riser and the seabed
◦ Normal component
◦ Tangential component
(friction)
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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 VIVVIVVIVVIV)
� 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
Dynamic LoadsDynamic LoadsDynamic LoadsDynamic LoadsDynamic LoadsDynamic LoadsDynamic LoadsDynamic Loads
◦ 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.
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� 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
AssumptionsAssumptionsAssumptionsAssumptionsAssumptionsAssumptionsAssumptionsAssumptions
� Global stiffness behavior is provided by EA, EI and GJ (known variables)
� The riser is assumed to be unstressed on its straight (initial) configuration
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� APDL code was used to:◦ General data entry
◦ Construct riser geometry;
◦ Mesh the geometry;
◦ Construct contact pairs;
ProcedureProcedureProcedureProcedureProcedureProcedureProcedureProcedure
◦ Construct contact pairs;
◦ Do the loading sequence to solve the problem.
� Each step is discussed next...
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APDL APDL APDL APDL APDL APDL APDL APDL –––––––– General data entryGeneral data entryGeneral data entryGeneral data entryGeneral data entryGeneral data entryGeneral data entryGeneral data entry
EA = 6080489749 !Axial stiffness
EI = 110821607.7 !Bending stiffness
GJ = 85247390.5 !Torsion stiffness
ndiv = 800 !Number of divisions (mesh)
Length = 1600 !Riser Length
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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
APDL APDL APDL APDL APDL APDL APDL APDL –––––––– Geometry constructionGeometry constructionGeometry constructionGeometry constructionGeometry constructionGeometry constructionGeometry constructionGeometry construction
!!!!!!!!!!!!!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
� Seabed is defined as a flat plane located in z=0
� Riser is defined as a straight line aligned with x axis.
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k,10,0,0,0
k,11,Length,0,0
L,10,11
APDL APDL APDL APDL APDL APDL APDL APDL –––––––– Riser propertiesRiser propertiesRiser propertiesRiser propertiesRiser propertiesRiser propertiesRiser propertiesRiser properties
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
� 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
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MPDATA,PRXY,1,,0(Extracted from ANSYS 12.1 help)
APDL APDL APDL APDL APDL APDL APDL APDL –––––––– Riser meshingRiser meshingRiser meshingRiser meshingRiser meshingRiser meshingRiser meshingRiser meshing
LESIZE,5, , ,ndiv, , , , ,1 !Defines number of divisions in the line number 5
(riser line)
LMESH,5 !Meshes the riser line
� The mesh in the riser line is very simple
� All elements have the same length
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APDL APDL APDL APDL APDL APDL APDL APDL –––––––– Boundary ConditionsBoundary ConditionsBoundary ConditionsBoundary ConditionsBoundary ConditionsBoundary ConditionsBoundary ConditionsBoundary 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
� 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)
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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 definitionContact definitionContact definitionContact definitionContact definitionContact definitionContact definitionContact definition
� Contact characteristics are:
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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 to the riser go to its prescribed position given by:
Loading SequenceLoading SequenceLoading SequenceLoading SequenceLoading SequenceLoading SequenceLoading SequenceLoading Sequence
go to its prescribed position given by:
� Top position (vessel)
� Anchoring position (seabed)
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Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence -------- 11111111/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
� 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)
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SOLVE !Solves the load step
Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence -------- 22222222
!!!!!!!!!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 nodal contribution of the effective weight of the riser is calculated by dividing it by the number of nodes
� Each node has the same load value
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Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence Loading Sequence -------- 33333333
NSUBST,100,10000,40
D,2,UX,valuex
D,2,UY,valuey
D,2,UZ,valuez
SOLVE
� 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
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� The bending moment distribution is an important issue for studying riser behavior (bending stresses)
� The maximum bending
Results Results Results Results Results Results Results Results –––––––– Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and bending moment distributionbending moment distributionbending moment distributionbending moment distributionbending moment distributionbending moment distributionbending moment distributionbending moment distribution
� The maximum bending moment is located in the TDP (touch down point) region
� TDP is one hot spot for studying riser life
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� As expected, the maximum tension is located at the top position
Results Results Results Results Results Results Results Results –––––––– Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and Free hanging configuration and tension distributiontension distributiontension distributiontension distributiontension distributiontension distributiontension distributiontension distribution
� The riser top position is also a hot spot for designing a possible configuration
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� 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. Is was possible to see the expected hot spots for these resultsexpected hot spots for these results
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� The work can be extent to consider sea current effects, causing a 3D riser configuration
Future worksFuture worksFuture worksFuture worksFuture worksFuture worksFuture worksFuture works
� Dynamic loads can be considered, for example by a harmonic excitation of the top position
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