
	Applies To

	Product(s):	AutoPIPE,
	Version(s):	2004, XM, & V8i
	Environment:	N/A
	Area:
	Subarea:
	Original Author:	Bentley Technical Support Group

Problem:

What are some typical modeling approaches for subsea (underwater) piping?

Answer:

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.
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 pipe
and 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 property
2. 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:

1. Take care not to double up on insulation/lining density and pipe density
Is AutoPIPE software suitable for the analysis of subsea pipeline crossings?

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 AtuoPIPE 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?

AutoPIPE does apply the axial force created by hydrostatic 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?

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: 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.