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What are some typical modeling approaches for subsea (underwater) piping?
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
Concrete Encased Steel Pipe:
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
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.
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: 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 detialed 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)?
Answer: 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"
Answer: 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
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