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What are some typical modeling approaches for subsea (underwater) piping?
Model Pipe resting on the seabed floor - AutoPIPE
Concrete Mattress:
Assuming the pipe is correctly supported by soil properties, select a range of piping where concrete mattress will be laid across, select Insert> Distributed load..> suggest "Load case to combine with " = GR, enter correct value of weight in vertical down direction, and press OK button to apply.Or, add additional weight using insulation thickness and density
Note: To model stiffness of concrete mattresses restraining the pipe. Two approaches are:
A. Insert soil over this area with equivalent stiffness to match rigidity of mattressesB. Insert V-stops with high friction e.g. much greater than 1.0
Model Riser Clamps:
Model Concrete Encased Steel Pipe:
Answer:
There is no accurate way to simulate this analysis. In order to correctly account for the pipe & concrete stiffness, you would need outside program values for total pipe stiffness (pipe & coating), OR, suggest to calculate an equivalent steel thickness by calculating the equivalent moment of inertia. Es*Ie = Es*Is + Ec*Ic Where Es, Ec stand for modulus of elasticity of steel and concrete respectively.Ie, Is and Ic are the equivalent moment of inertia, steel pipe inertia and concrete pipe coating inertia. The inertia can be approximated by Pi * R^3 * t R is the mean radius of the pipe or coating. Assume equivalent radius is same as steel radius and solve for equivalent thickness. You can recompute equivalent radius and solve again for new thickness. Please note you may use 1/2 or so (cannot recall exact number as neutral axis will move) of the concrete inertia as it will crack in tension. AutoPipe will give you stress in the equivalent steel pipe. You may need to evaluate the stress at most stress point by splitting the moment between pipeand coating proportional to their E*I. For example Ms = M * EsIs/EsIe and Mc = M* EcIc/EsIe And calculate stress for pipe and concrete using their actual diameter,thickness and material. So in conclusion, of modeling a concrete encased steel pipe, based on the information above:
1. Insert a pipe property2. Combined Concrete / Steel Stiffness: a. Set Pipe material = Ns (nonstandard) b. Calculate / insert the pipe properties for the combined Concrete / Steel Pipe
3. Account for weight and correct size: a. Set insulation thickness = XXX inches b. Set Insulation material = Other c. Set Insulation density = XXX lbs/cuft ( for concrete only). d. Set Density = XXX lbs/cuft ( for steel only)
Note: setting the correct insulation thickness, insulation density, and pipe density will accurately account for the weight and outside dia of the combined pipe/ concrete for the wind / wave / current loading.
4. Because the material is set to NS, update the data on the Press / Temp/ PipeID tab.
a. Calculate and insert the expansion coeff, hot mod, and hot allowable based on the combined concrete / steel pipe.Notes: Take care not to double up on insulation/lining density and pipe density
Is AutoPIPE software suitable for the analysis of subsea pipeline crossings?
Answer:AutoPIPE is limited to small displacement analysis and elastic pipe material. As long as these limitations are justified, you may use AutoPIPE for pipe stress analysis. Small displacement assumption implies that the pipe deflections will not severely affect computed forces and moments. For example if a simply supported horizontal beam of length L deflects L/4 at the mid span, this will be considered a large deflection since pipe weight is no longer supported by moment alone. Axial force or cable catenary action will contribute more to pipe resistance than moment in this case. AutoPIPE analysis for straight beam will show a zero axial force which is not correct if the vertical deflection is large.
In some cases the user may require a strain limit instead of stress allowable limit, AutoPIPE results will be conservative since we do not allow yielding of pipe material. A material nonlinear model may be required to justify larger deflections in this case.
Ultimately the client has to approve the use of AutoPIPE knowing the limitations ove the program.Bentely's CAE development team is constantly updating the application with new enhancements and abilities. Please review the latest version of AutoPIPE for current capabilities and limitations.
Does AutoPIPE consider external pressure or temperature.
AutoPIPE does not consider external pressure or temperature. However, the external pressure can be approximately accounted for by subtracting it from internal pressure. External temperature can be approximated finding the average pipe temperature using heat transfer concepts. Other wise use bounding values, like higher and lower temperatures as two separate load-cases.See AutoPIPE FAQ#36: How do I calculate the DNV 2000 tension terms.
Can you then confirm that AutoPIPE is not suitable for deep offshore analysis as regard to calculating true pipe wall force which will take into consideration the end cap effect and external pressure?
Answer:AutoPIPE does apply the axial force created by hydro static forces. This is included in the buoyancy for the GR load case. External pressure can be approximately accounted for by subtracting it from internal design pressure. AutoPIPE has been successfully deployed on many deep water riser analyses. Usually the pipe is close to vertical or lies on the sea bed. Stinger pipes cannot be analyzed in AutoPIPE as we do not handle catenary action.
Is it possible to simulate the free catenary for subsea pipeline?
Answer:Our software AutoPIPE does not support large displacement and tension stiffening and hence should not be used for free catenary systems. It has been successfully for riser piping which is close to vertical for which such action is insignificant.
We are modelling offshore pipeline and Riser at 50' water depth in sandy soil. Pipeline is buried 2' below sea bed and close to platform it is exposed. Which method we shall adopt for calculating soil spring? AUTOPIPE Method or ASCE 2001 Method. our client wants to know which one is realistic and why?
Answer:Answer: Due to liability lawsuits Bentley Technical support cannot provide specific or settings for your analysis. Please see the following AutoPIPE help section: Help > Contents> Contents Tab> Reference Information> PipeSOIL overview> Comparison of Soil Restraint Properties for detailed difference between these soil calculations. The user must determin which soil method is appropriate for their analysis.
How many nodes do we need to put on the pipeline in order to simulate the hydrodynamic forces same as while we are defining the CD and CM in the wave load interface (from Wave/Load)?
One approach duplicated the same number of node points per span equal to the number of mass points per span calculated by AutoPIPE.
Regarding hydrodynamic data, in the help file it stated that:
"Insert/Xtra Data/Hydrodynamic data can be used to set user drag and inertia coefficients for different points. However these coefficients cannot be set for mass points. User would need to enter actual nodes for setting these hydrodynamic factors."
Since the wave load interface doesn't has the space that we can input the lift coefficient, how can we define the lift coefficient if we don't want to use the function defined in "Insert/Xtra Data/Hydrodynamic data"
The Lift coefficient for all points is assumed zero unless identified by Insert> Xtra Data>Hydrodynamic Data. You may still have to amplify the lift factors (possibly by a factor of 2, since soil points are placed at the midpoint between actual points) to get the correct force.
Typical hydrodynamic data defined for the seabed piping including non-zero lift coefficient.
Typical Hydrodynamic data defined for the riser, zero lift coefficient
How can I verify if hydrodynamic data is being applied to my seabed piping model?
Hydrodynamic load case is added to a model by using Insert> Xtra Data> Hydrodynamic Data... command. Lift coefficient (Cl) value is used to calculate the lift forces due to the motion of a fluid body relative to the structure. Enter the lift coefficient to be used at the current point. If the default (0.0) lift factor is accepted, lift forces will not be calculated at the current point.
The lift force is calculated at each point using the Morison equation (see online help for details) in conjunction with the defined Wave loading(s). Each wave loading is applied during a specified Load case (ex. U1).
There are 2 approaches for modeling seabed piping, support the pipe with Soil properties or V-stops with large gaps above the pipe. This topic will focus on the V-stop methodology.
When reviewing the output report, remember in order to consider Gaps/Friction/Soil in a model it must be analyzed as a non-linear analysis. As a non-linear analysis load sequencing applies (see Non_Linear_Load_Sequencing.pdf, located in the AutoPIPE folder). With that said, concentrate on the report's operating condition / combination (ex. GrT1P1U1, ). If more understanding about the combination is required then review the individual results from the load case results (ex, Gr, T1, P1, U1) where due to load sequencing the result of the individual load cases are added together to arrive at the combination's results.
For example, provided the following model of seabed piping:
Wave loading is applied to load case (U1).
Hydrodynamic loading is applied to the entire pipe segment.
Point Load FORCES (N )
name combination X Y Z Result
------ ----------------------- ------ ------ ------ ------
E07 V - Stop Tag No.: <None> [ID: E07 1]
Gravity{1} 0 -95 0 95
Thermal 1{1} 0 0 0 0
Pressure 1{1} 0 0 0 0
User 1{1} 0 84 0 84
GT1P1U1{1} 0 -11 0 11
Note:
1. The operating condition due to Gravity +Thermal+Pressure+U1 (Wave Load) = 11 N acting vertically down.
2. All v-stops have a gap above the pipe, but under U1, there is a vertical Up (+y) load acting on the support. This is correct. Remember Load sequencing.
3. Do not for get about Buoyancy, the results above includes Buoyancy. If you are concerned about knowing the results of pipe movement due to only hydrodynamic data, be sure to uncheck Buoyancy on the Segment tab of the input grid.
Re-run the analysis, and review the results:
R E S T R A I N T R E A C T I O N S
Gravity{1} 0 -165 0 165
GT1P1U1{1} 0 -81 0 81
Here we can see the true affect of hydrodynamic data on the pipeline, operating condition GrT1P1U1 = 81 N acting down. Notice how the Gravity load without Buoyancy was updated to 165 N from 95 N, again acting downward.
Is marine growth thickness on a riser pipe included for Buoyancy loads and Wave loads?
Marine growth thickness usually varies with depth. Also, from AutoPIPE online help:
Marine Growth: Enter the amount of marine growth attached to the submerged structure. This value is used to calculate the effective diameter to be used in force calculations. Marine growth is assumed zero above still water level.
No AutoPIPE does not consider marine growth thickness (under Load> Wave) in the calculation of buoyancy.
Recommend one of the following modeling approaches:
Options #1
For a given PipeID apply marine growth as insulation of the correct thickness and density, and enter a small value for marine growth on the Wave Loading dialog.
AutoPIPE does consider insulation around the pipe in the buoyancy load which can be used to simulate marine growth over a section of pipe and also capture additional weight of riser pipe marine growth. However, multiple pipeID's are needed with different insulation thickness to capture the marine growth as the thickness varies with depth.
Also, by entering a small marine growth value on the Wave Loading dialog (where the pipe + marine growth OD is mostly accounted for by the insulation settings on the pipe property dialog) a current profile can still be applied.
Option #2
Account for marine growth weight with Insert> Distributed load and enter the correct value for marine growth on the Wave Loading dialog.
Assuming marine growth is linearly increasing with depth, Suggest modeling as distributed load down the riser pipe. After selecting the riser pipe from the surface to the ocean floor, press Insert> Distributed load> enter a smaller distributed load near the surface and a larger distributed load near the ocean floor, all points between them will be linearly adjusted accordingly.
On the Wave dialog, enter the correct marine growth profile to account for the wave forces.
Only drawback to this approach is that there is no method of correctly accounting for the buoyancy of the marine growth on the pipe. If buoyancy due to marine growth is believed to be significant, model with option #1 above.
On long risers supported at the bottom does auto pipe calculates the compressive stresses due to selfweight?
Yes AutoPIPE will calculate the axial load due to gravity and pressure and transfer to top flange or bottom supportBut this relates to following AutoPIPE FAQ 36: How do I calculate the DNV 2000 tension terms?
Please see the following AutoPIPE help section: Help > Contents> Contents Tab> Bentley AutoPIPE> Frequently Asked Questions> FAQ document>
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