Chance Technical Design Manual

DESIGN EXAMPLE 12: HELICAL ANCHORS FOR PIPELINE BUOYANCY CONTROL

PURPOSE This Design Example provides an aid in the selection of appro priate helical anchors for pipeline buoyancy control.

Sample Problem - Natural Gas Pipeline Borehole BH-1 HeliCAP® Software Input Values

Depth (ft)

Clay Cohesion (psf)

Sand N-Value (SPT)

Soil

ASSUMPTIONS • Pipe contents: Natural gas

0

7

Sand

DESIGN EXAMPLES

• Pipe Outside Diameter (OD): 42” • Pipe Wall Thickness (T W ): 0.938” • Grade of Pipe: API 5L, Grade X65 • Minimum Yield Strength Of Pipe (F y ): 65,000 psi • Pipe design pressure (P d ): 1,440 psi • Maximum Operating Pressure (P m ): 1,440 psi • Maximum Operating Temperature (T m ): 85° F • Construction type design factor (F): 0.50 • Longitudinal joint factor (E): 1.0 • Temperature Factor (T): 1.0 (T m < 250°F) • Coating: Fusion Bonded Epoxy • Density of coating (D c ): 70.0 pcf • Coating thickness (T c ): 16 mils • Pipeline placement: Land Based in Trench with 4’-0 of Cover above Top of Pipe • Backfill material: Loose, Poorly Graded Silty Sand • Specific Gravity of Backfill Material: 1.44 • Density of backfill material (D b ) = 1.44 x 62.4 pcf = 89.9 pcf (use 90.0 pcf) • Span between anchor sets: Simple Span with Pin-Pin Ends • Maximum vertical displacement at Mid-Span between Anchor Sets = L d /360 • Minimum Factor of Safety (FS) for Mechanical Strength Of Hardware/Anchors = 2.0 • Minimum Factor of Safety (FS) for Anchor Soil Capacity = 2.0 • Soil data: As shown in Figure 8-23

3

7

Sand

5

28

Sand

7

21

Sand

10

30

Sand

12

21

Sand

13

60

Clay

15

60

Clay

20

380

Clay

25

500

Clay

30

250

Clay

35

460

Clay

40

1250

Clay

45

2000

Clay

50

1560

Clay

55

1250

Clay

60

2250

Clay

65

1320

Clay

70

750

Clay

75

750

Clay

BOREHOLE BH-1 SAMPLE DATA FIGURE 8-23

Required Ultimate Capacity (per Anchor Set)

Allowable Span Length

Net Buoyancy, W n , is a uniformly distributed load.

SOLUTION

NET BUOYANCY (W n ) Properties of pipe: •

Weight per linear foot (W p ):

SCHEMATIC DIAGRAM FIGURE 8-24

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