04. How to model Concrete Encased Steel Pipe for subsea / underwater piping using AutoPIPE?

Applies To
Environment: N/A
Area: Modeling
Original Author:Bentley Technical Support Group
Date Logged
& Current Version
Dec. 2016


How to model Concrete Encased Steel Pipe for subsea / underwater piping using AutoPIPE?


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 one could 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.

Note: Take care not to double up on insulation/lining density and pipe density

See Also

Model Subsea (Underwater) Piping

Bentley AutoPIPE