CAESAR II STATIC LOAD CASE
EDITOR
Loren Brown
Senior Engineer/Developer
CADWorx & Analysis Solutions
Intergraph Process, Power, & Marine
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TYPES OF LOADS
• Primary Loads – Force driven, cause
catastrophic failure.
– Weight, Pressure, Point Loads, Uniform Loads,
Hanger Loads, Wind and Wave loads.
• Secondary Loads – Strain based, cause fatigue
failure.
– Temperature, Displacements.
AVAILABLE LOAD TYPES IN CAESAR II
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W (Weight), WNC (Weight No Contents)
WW (Water-filled Weight)
P (Pressure), HP (Hydrotest Pressure)
T (Temperature), D (Displacement)
H (Hanger Pre-loads), F (Concentrated Loads)
U (Uniform Loads)
Win (Wind), Wav (Wave and Current)
CS (Cut Short or Cut Long)
Available Stress Types in CAESAR II
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OPE – Operating
SUS – Sustained
EXP – Expansion
OCC – Occasional
HYD – Hydrotest
HGR – Hanger Design
FAT - Fatigue
Load Case Definition
• Operating case contains all loads in the
system.
– L1 = W+P1+T1+H (OPE)
this is called a basic load case
• Sustained Case contains only primary loads.
– L2 = W+P1+H (SUS)
another basic load case
• Expansion Case is the difference between the
operating and sustained cases.
– L3 = L1-L2 (EXP)
this is called a combination load case
Combination Load Cases
• Used to add or subtract results from
previously defined primitive load cases.
• Necessary for proper EXP and OCC code stress
definition.
• Not used for restraint or equipment load
definition, nor for displacement reporting.
Why subtract SUS from OPE?
• Why not simply use L3 = T1 (EXP)?
– Because the restraint configuration may result in
an incorrect solution.
– Nonlinear restraints drive the restraint
configuration.
– Other loads in the system combine to change the
restraint configuration.
Nonlinear Restraints
• Stiffness of Restraint changes depending on
position of pipe or forces on restraint.
• Examples:
– Uni-directional Restraints (+Y)
– Gaps in restraints
– Friction
– Large-rotation rods
– Bi-linear Restraints
Force vs. Distance in Nonlinear
Restraints
Example 1:
T1 (EXP)
L3 = T1 (EXP)
This is how the line is modeled in
Caesar II. The gaps are equal on
both sides of the pipe. No loads are
yet applied.
The thermal forces have closed
the gap on the right side.
Total Displacement for T1 (EXP) = 1 x Gap
Example 2: L1 – L2 (EXP)
L2 = W+P1 (SUS)
L1 = W+P1+T1 (OPE)
Weight has caused the pipe to close
the gap to the left. This can happen
when the pipe pivots about a
different restraint.
Operating conditions have caused
the pipe to close the gap to the
right, even against the weight force
trying to hold it on the left.
Example 2 (con’t)
• If we subtract the displacements of the SUS
case from OPE we get:
– Total Displacement for L1-L2 = 2 x Gap
– In a linear system T1 (EXP) = L1 – L2 (EXP)
– In a nonlinear system this is not guaranteed.
– This represents the effect of temperature in the
presence of other loads.
– This is a displacement stress range, not starting
from the neutral position.
Occasional Load Cases
• For most piping codes (not the offshore
codes):
– Set up an OPE case that includes the occasional
load
– Subtract the standard OPE case from the OPE that
includes the occasional load. We call this the
segregated occasional load case.
– Add the above load case results to the SUS load
case results for the code stress check
Example 3: Occasional Load Cases
• Assume we have a uniform load representing a
seismic load, U1.
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–
–
–
–
–
L1 = W+P1+T1
L2 = W+P1
L3 = W+P1+T1+U1
L4 = L1-L2
L5 = L3-L1
L6 = L2+L5
(OPE) standard operating
(SUS)
(OPE) operating with occasional load
(EXP)
(OCC) segregated occasional
(OCC) * occasional code stress case
* use scalar combination method.
Combination Methods
• Algebraic:
– Used for subtracting two load cases.
– Takes the displacements from the referenced cases
and subtracts them.
– Then computes forces, moments, and resultant stress
from these displacements.
• Scalar:
– Used for adding two load cases.
– Adds the stresses from the two referenced load cases.
– Unlike algebraic the stresses are not recomputed from
displacements.
Notes on combination methods
• Don’t use algebraic for adding two load cases.
– You can’t take credit for occasional loads acting
opposite to operating loads.
• Don’t use scalar for subtracting two cases.
– This results in a lower code stress than actual.
Output Types
• Displacement
– Usually reported only for basic load cases
• Force
– Usually reported only for basic load cases
• Stress
– Reported based on code requirements.
Example 4 – Restraint Loads
The algebraic difference between these two conditions will result in a positive
force on the restraint. This is an impossible condition. But the EXP code stress is
correctly computed for this condition.
What to report
• Suppress the HGR cases and the segregated
occasional load cases.
• Report displacement, force for all primitive
load cases.
• Don’t report stress for the operating load
cases.
– This is not true for offshore codes, nor FRP codes,
nor buried pipe codes.
• Report only stress for combination load cases.
Using the Hot Modulus of Elasticity
• It is required to use the cold modulus of
elasticity for stress computation.
• You can reduce restraint loads by use of the
hot modulus of elasticity.
• Create identical OPE cases, one with hot
modulus for restraint loads, and one with cold
modulus for use in the combination with SUS
for determining EXP stress.
Using the Friction Multiplier
• Friction Multiplier acts on the Mu value
entered on each restraint in the model.
• Input 0.0 for no friction and 1.0 for full
friction.
• Create identical load cases, but change the
value of Friction Multiplier on one of them.
• Compare the results in the Restraint Summary
and report the worst-case results.