Wall Thickness Design14 August 2006
August 2 – August 28, by Mr. Eng Bin NG
Applicable Codes
Pipe Expansion Calculations
Flexibility Analysis Methodology
5
Pipeline Construction - Conventional & Unconventional
Weekending
Pipe Expansion Calculations, Flexibility Analysis Methodology
Allowable Free Span Calculations, On-bottom Roughness Analysis
Pipeline Protection against Anchors, Wave Liquefaction & Earthquake
Cathodic Protection Design
Pipeline Construction - Conventional & Unconventional
Installation Engineering
1 2 3 4 7 8 9 10 11 14 15 16 17 18 21 22 23 24 25 28
Workshop, revision, exercise
General
Wave-induced liquefaction is likely to affect the integrity of the pipeline. Examples include:
Pipeline may float out of the liquefied soil if the SG of the pipe is lower than the SG of the liquefied soil. Once exposed, the pipe may deflect laterally till failure occurs.
If SG of pipeline is much greater than SG of liquefied soil, then the pipe will sink further.
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General (cont’d)
Protection against soil liquefaction may be carried out by one of the following manners:
Burying the pipeline below the influence depth of liquefaction.
Backfilling the pipeline with suitable materials that will not liquefy
Designing the pipeline such that the SG of pipeline is almost equal to the SG of liquefied soil
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Backfill Design
“Manual on the Use of Rock in Coastal and Shoreline Engineering” (Ref. [1]). This manual provides theoretical basis for designing rock berm layers in offshore areas.
The backfill design in the offshore areas is based on the “Critical Shear Stress Method” as described further below:
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Backfill Design (cont’d)
The critical shear stress to initiate movement for a given material is calculated using Shields’ criterion (Ref. [1]), where:
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ρr = Berm material oven-dried density (kg/m3)
ρw = Seawater density (kg/m3)
ψCR = Critical shields’ number
=
j = Angle of repose of the backfill layer (degree)
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Backfill Design (cont’d)
The critical shear stress is equated to the actual applied shear stress due to wave and current action in order to find the backfill size.
The actual applied shear stress is calculated using the following equation (Ref. [1]):
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C = Chezy coefficient (m1/2/s)
≈ (1-3)D90
U = Wave-induced velocity (m/s)
fw = Wave-associated friction factor
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Backfill Design (cont’d)
The nominal backfill layer diameter (Dn50) is related to the median sieve diameter (D50) by the following relation:
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Backfill Design (cont’d)
The grading distribution curve for the backfill layer shall satisfy the requirement from Shore Protection Manual (Ref. [2]) as given below:
where: D50max = Maximum diameter exceeded by
50% of the rock berm material (mm)
D50min = Minimum diameter exceeded by
50% of the rock berm material (mm)
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Reference
[1] “Manual on the Use of Rock on Coastal and Shoreline Engineering”, CIRIA Special Publication 83, 1991.
[2] Shore Protection Manual, US Army Coastal Engineering Research Centre.
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