Model Pipe resting on the seabedoor - AutoPIPE

Applies To

Product(s): AutoPIPE,

Version(s): 2004, XM, & V8i

Environment: N/A

Area: modeling

Subarea:

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. Basedon your statement, all those calculations are for buried pipe only. I have a situation that pipes are not buried and justlaid on seabed with out any burials, on very soft clay seabed. If you have this case, would youplease advise the following?

TR Number: noneProduct: AutoPIPEVersion: 09.04.00.19

Area: General, modelingProblem ID#: 27731

Answer:There is 2 methods of modeling pipelines resting on the seabed floor: use V-stop support or useSoil properties

Method #1: Using V-stop support

If the pipeline is assumed to be completely exposed while resting on the seabed floor, inserta number of node along the pipeline section resting on the seabed floor points spaced at aninterval that will not make the pipe fail in sustained stress (i.e. about half the distanceneeded if above ground). Also, at these points insert a V-stops support with a very largeupwards gap to simulate no resistance in the up direction. You can use support frictionfactor of 0.4 to 1.0 to model the restrain of the pipeline horizontal movement. Unfortunatelyat 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:

soil stiffness under the pipelinesemi-embedded (soil /debris builds up on the sides of the pipe over time).

Ideally, a geotechnical engineer should evaluate the specific soils to be modeled inAutoPIPE, 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 andhigher has a soil calculator. This is provided in order to offer the piping designer guidelinesfor estimating (ranges of) the required input data. Remember, these are guidelines only, soilconditions can vary greatly and each situation should be reviewed by a geotechnicalengineer (e.g. frozen soils will require additional consideration).

The difficulty with this approach is calculating a transverse horizontal (lateral) andlongitudinal soil stiffness based on assumed friction factors and depth of pipe. The soilequations are based on the pipe's centerline to be below grade. AutoPIPE's soil calculatorhas built in this limitation by requiring the user to enter in a correct depth. If "Depth tocenterline, H [inch]" value is not sufficient to place the pipe centerline below grade thefollowing warning is displayed:

---------------------------Edit Soil Properties---------------------------Depth centerline cannot be zero or less than -D/2.---------------------------OK ---------------------------

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 engineeringjudgment of the calculated values. Furthermore, there is more information available in theTools> Model Input Listing> Soil sub-report for values from AutoPIPE's soil calculations.

Addtional information for modeling soil properties:

Clay===============================================

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 weightper unit length, including coating and contents). K1 can be calculated assuming a smallyield disp (e.g. 1 cm). P1 and K1 should be set to higher values if semi-embedded pipe isanticipated.

b) Longitudinal Soil Properties:

The longitudinal K1 and P1 would go to zero with Z=0 but there may be an alternativeequation 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 toevaluate the longitudinal frictional stiffness effect back on the pipe system. Proposeusing a lower friction factor, i.e. 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 valueof Nc = 5.8 by assuming an H/B of (ref. fig. D-12). Note that this is an iterative process.Observe the vertical displacement and adjust the amount of friction for the transverseand longitudinal properties accordingly as the contact area increases. If the downwarddisplacement reaches the centerline of the pipe, assume values for fully buried pipe forVertical Downward Soil Properties. Use half of the values for fully buried pipe for theTransverse Horizontal and Longitudinal Soil Properties.

Use AutoPIPE or ASCE 2001 method based on depth to centerline H=0 or 0.01. Change Hvalue based on displacement. If H > D/2 using regular buried pipe option as frictionmodel for longitudinal and transverse directions would not apply.

Loose Sand===============================================

The procedure for loose sand is basically the same as for that of clay. However forvertical downward properties the first 2 terms of equation D-15 should be used asopposed 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 analysisprogram to have buried piping. All other programs copied our procedure. The soilstiffness calculation method has been successfully used and is one of themost accepted methods in the industry.

See Also

AutoPIPE Modeling

Model Subsea (Underwater) Piping - AutoPIPE

Model Dierent Types of PIPING Components - AutoPIPE

Pipes - Modeling in AutoPIPE

Modeling Approaches, Tips, Technique for Bend & Miter PipingComponents in AutoPIPE

Bentley AutoPIPE

External LinksBentley Technical Support KnowledgeBase

Bentley LEARN Server

Comments or Corrections?Bentley's Technical Support Group requests that you please submit any comments you have onthis Wiki article tothe "Comments" area below. THANK YOU!

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