| Applies To | |||
| Product(s): | AutoPIPE, | ||
| Version(s): | ALL | ||
| Area: | modeling | ||
| Original Author: | Bentley Technical Support Group |
Problem:
AutoPIPE: model pipe on seabead floor
Seabed Piping Not Buried - I looked through your AutoPipe manual, Pipe Soil Properties. Based on your statement, all
those calculations are for buried pipe only. I have a situation that pipes are not buried and just laid on seabed with out any burials, on very soft clay seabed. If you have this case, would you please advise the following?
TR
Number: none
Product: AutoPIPE
Version: 09.04.00.19
Area: General,
modeling
Problem ID#: 27731
Solution:
There is 2 methods of modeling pipelines resting on the seabed floor: use V-stop support or use Soil properties
Method #1: Using V-stop support
If the pipeline is assumed to be completely exposed while resting on the seabed floor, insert a number of node along the pipeline section resting on the seabed floor points spaced at an interval that will not make the pipe fail in sustained stress (i.e. about half the distance needed if above ground). Also, at these points insert a V-stops support with a very large upwards gap to simulate no resistance in the up direction. You can use support friction factor of 0.4 to 1.0 to model the restrain of the pipeline horizontal movement. Unfortunately at this time you cannot set different friction factors for axial and lateral movements.
Method #2: Model with Soil Properties:
Use this modeling approach If a pipeline is assumed to be resting on the seabed floor and want to consider one or more of following:
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soil stiffness under the pipeline
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semi-embedded (soil /debris builds up on the sides of the pipe over time).
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Ideally, a geotechnical engineer should evaluate the specific soils to be modeled in AutoPIPE, and supply design values for the required input variables (K1, P1, & K2). However, these variables are not always included in a soil report. Therefore, AtuoPIPE V8i 9.4 and higher has a soil calculator. This is provided in order to offer the piping designer guidelines for estimating (ranges of) the required input data. Remember, these are guidelines only, soil conditions can vary greatly and each situation should be reviewed by a geotechnical engineer (e.g. frozen soils will require additional consideration).
The difficulty with this approach is calculating a transverse horizontal (lateral) and longitudinal soil stiffness based on assumed friction factors and depth of pipe. The soil equations are based on the pipe's centerline to be below grade. AutoPIPE's soil calculator has built in this limitation by requiring the user to enter in a correct depth. If "Depth to center-line, H [inch]" value is not sufficient to place the pipe centerline below grade the following warning is displayed:
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Edit Soil Properties
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Depth centerline cannot be zero or less than -D/2.
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OK
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Thereby forcing the user to enter in an acceptable value to use the soil calculator. However, the soil calculator may still be used with an appropriate value, but with engineering judgment of the calculated values. Furthermore, there is more information available in the Tools> Model Input Listing> Soil sub-report for values from AutoPIPE's soil calculations.
Additional information for modeling soil properties:
Clay
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a) Transverse Horizontal Soil Properties:
K1 and p1 can be calculated as non-zero with H=0 ( online help, Help > Contents> Contents Tab> Reference Information> PIPE-SOIL).
From the online help example soil model, initially assume a friction factor ( v=0.4 to 1.0) and calculate stiffness based on that. Then P1 for soil will be v*Wp. (Wp is pipe weight per unit length, including coating and contents). K1 can be calculated assuming a small yield disp (e.g. 1 cm). P1 and K1 should be set to higher values if semi-embedded pipe is anticipated.
b) Longitudinal Soil Properties:
The longitudinal K1 and P1 would go to zero with Z=0 but there may be an alternative equation in some textbook which would calculate a non-zero longitudinal K1 and P1. Since only line contact of soil with the pipe is assumed, suggest using a low value to evaluate the longitudinal frictional stiffness effect back on the pipe system. Propose using a lower friction factor, i.e. ½ the value.
c) Vertical Upward Soil Properties:
The vertical up soil stiffness K1 & P1 = 0, but and K2 = 0.1 lb/in/ft (0.006Kg/m/mm) to avoid convergence problems.
d) Vertical Downward Soil Properties:
From the online help reference information, use equation D-16; obtain the initial value of Nc = 5.8 by assuming an H/B of ½ (ref. fig. D-12). Note that this is an iterative process. Observe the vertical displacement and adjust the amount of friction for the transverse and longitudinal properties accordingly as the contact area increases. If the downward displacement reaches the center-line of the pipe, assume values for fully buried pipe for Vertical Downward Soil Properties. Use half of the values for fully buried pipe for the Transverse Horizontal and Longitudinal Soil Properties.
Use AutoPIPE or ASCE 2001 method based on depth to center-line H=0 or 0.01. Change H value based on displacement. If H > D/2 using regular buried pipe option as friction model for longitudinal and transverse directions would not apply.
Loose Sand
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The procedure for loose sand is basically the same as for that of clay. However for vertical downward properties the first 2 terms of equation D-15 should be used as opposed to D-16 (i.e. Nq value is set to zero).
Note: AutoPIPE soil modeling have been in use for 30 years. It is the first stress analysis program to have buried piping. All other programs copied our procedure. The soil stiffness calculation method has been successfully used and is one of the most accepted methods in the industry.
Semi Embed piping
The Buried pipe option can be used to model semi-embedded (debris builds up on the sides of the pipe over time) or non-embedded piping.
The difficulty is calculating a transverse horizontal (lateral) and longitudinal soil stiffness.
a) The vertical up soil stiffness K1, P1 and K2 can be taken as 0.
b) The transverse horizontal K1 and P1 is usually taken as low values. Higher if semi-embedded pipe is anticipated.
c) Transverse vertical down K1 and p1 can be calculated as non-zero with H=0 (e.g. equation D-15 in the appendices under the PIPE-SOIL help files under Example Systems).
d) The longitudinal K1 and P1 would go to zero with Z=0 but there may be an alternative equation in some textbook which would calculate a non-zero longitudinal K1 and P1 although. It may be a low value since only line contact of soil with the pipe is assumed.
Consider entering some non-zero K1 and P1 values to evaluate the longitudinal frictional stiffness effect back on the pipe system. It is recommend to set all final stiffness, K2 = 0.1 lb/in/ft (0.006Kg/m/mm) to avoid convergence problems.
See Also
Model Subsea (Underwater) Piping