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PIPILINE WALL THICKNESS CALCULATION REPORT

INTRODUCTION
1.1     General

The AFANG field is located in block-00 OML 419, approximately 45 km of the south-eastern coast of Wakanda in approximately 40 meters water depth. The field, initially brought into production in 1997 is owned by the Joint Venture APC/PDP and is operated by APPDPLPC Nigeria Limited (APPDPLPCNL). The OML 419 block is shown in the following figure.

Figure 11: Project Scope

1.2      Scope of Document

The purpose of this document is to present the minimum required wall thickness for the 12-inch APC-2 to PDP-1 Crude Export Line, and the associated spools.

The required wall thickness was calculated based on the following criteria:

  • Internal pressure containment based on paragraph A403.2.1, ASME B31.4-2016.
  • Collapse due to external pressure based on Section 4.3.2.1, API RP 1111, 4th Edition, Dec. 2015.
  • Propagating buckles due to external hydrostatic pressure based on Section 4.3.2.3, API RP 1111, 4th Edition, Dec. 2015.
  • Buckling due to combined bending and external pressure based on Section 4.3.2.2, API RP 1111, 4th Edition, Dec. 2015.
  • Welded and seamless wrought steel pipe, ASME B36.10M-2018.

The internal pressure containment criteria has been assessed for operating, hydrotest, and installation conditions whereas collapse, propagating buckles and buckling due to combined bending and external pressure criteria have been assessed for installation and operating conditions due to external hydrostatic pressure.

1.3     Abbreviations
Term Definition
API American Petroleum Institute
ASME American Society of Mechanical Engineers
CA Corrosion Allowance
CRA Corrosion Resistance Alloy
HP High Pressure
ID Internal Diameter
LAT Lowest Astronomical Tide
MLP Mechanically Lined Pipe
Mpa Mega Pascal
NA Not Applicable
OD Outside Diameter
Pcf Pound per Cubic Foot
Psf Pound per Square Foot
PSL Product Specification Level
WT Wall Thickness
RP Recommended Practice
SMYS Specified Minimum Yield Strength
   
1.4     References
S/No Document
1 Pipeline Design Basis
2 ASME B31.4-2016, Pipeline Transportation Systems for Liquids and Slurries.
3 API Specification 5L 46th Edition 2018.
4

5

6

7

API RP 1111, Design, Construction, Operation, and Maintenance offshore

Hydrocarbon Pipelines (Limit State Design), 4th Edition, December 2015.

Metocean Data

ASME B36.1-2018, Welded and Seamless Wrought Steel pipe.

   
Reference Drawings
8

9

10

LP-NG-HCD2023-DWG-002:-12-Inch APC-2 to PDP-1 Spool-1 G.A & Details Drawing

LP-NG-HCD2023-DWG-003:-12-Inch APC-2 to PDP-1 Spool-2 G.A & Details Drawing

LP-NG-HCD2023-DWG-004:-12-Inch APC-2 to PDP-1 Spool-3 G.A & Details Drawing

 

2.0        result summary and conclusions
2.1        Result Summary

The wall thickness calculation has been performed considering the internal pressure containment, propagating buckles, collapse due to external pressure, and buckling due to combined bending and external pressure criteria.

Based on the calculations, the required wall thicknesses for the pipeline and spool are presented in Table 2‑1 for the 12-inch APC-2 to PDP-1 Crude Export Line.

The governing criteria for the pipeline and spool wall thickness calculation is the pressure containment criteria.

 

Table 2‑1: Wall Thickness for 12-inch APC-2 to PDP-1 Crude Export Line

Pipeline Description CA

inch
(mm)

Min Req. Thickness Based on Pressure Containment (1)

Inch

(mm)

Calculated WT

+ CA

inch

(mm)

Selected WT

inch

(mm)

 

 

12-inch APC-2 to PDP-1 Crude Export Line 0.118

(3.00)

0.277

(7.04)

0.395   (10.033)  0.500   (12.70)  
12-inch APC-2 to PDP-1 Crude Export Line Spools 0.118

(3.00)

0.342

(8.69)

 0.460

(12.7)

  0.500

(12.7)

 

Notes:

  1. Calculated wall thickness inclusive of CA
  2. Wall thickness was selected based on ASME B36.1 Ref. 5.

 

Table 2‑2: Bend Thinning

Bend Radius Selected Pipe Wall Thickness;  inch (mm) Bend Thinning (%) Pipe Wall Thickness after bending; inch (mm) Min Req. Thickness

inch (mm)

Remark
12-inch APC-2 to PDP-1 Crude Export Line
5D 0.562

(14.27)

8 0.517

(13.13)

0.508

(12.9)

OK

 

2.2        Conclusion
  1. The selected wall thickness 0.5-inch for the 12.75-inch spools and Pipeline respectively, are found to satisfy all the design criteria of internal pressure containment; Collapse due to external pressure; Propagating buckles and buckling due to combined bending and external pressure.
  1. The selected wall thickness of 0.5 for the 12.75-inch spools, allows for induction bends thinning allowance of 8% for 5D bends while still meeting Pressure Containment criterion.
  1. The selected wall thickness 0.5-inch for the 12.75-inch spools and pipeline satisfies the requirements of piggability since there is no step change.
  1. The selected wall thickness 0.5-inch for the 12.75-inch spools and pipeline respectively, are also capable of withstanding hydrotest pressures 166.3 barg.
3.0        Design data

The following sub-section outline the design data used in performing the wall thickness calculation.

3.1        Pipeline Data

The pipeline data is presented in Table 3-1.

Table 3‑1: General Pipeline Data

Description Unit Value Ref.
Outside Diameter inch (mm) 12.75 (323.9) 1
Specification API 5L,Grade X-65, PSL-2 Seamless 3
Corrosion Allowance (Pipeline Section) inch (mm) 0.118

(3)

1
Corrosion Allowance (Spool Section) inch (mm) 0.118

(3)

1
Corrosion Coating Thickness inch (mm) 0.109

(2.7)

1
Poisson’s Ratio 0.3 1
SMYS (X65) psi (MPa) 64, 977

(448)

1
SMTS (X65) psi (MPa) 77, 015

(531)

1
Modulus of Elasticity (12.75-inch Pipeline)  Psi 30022811.7 1
Steel Density pcf (kg/m3) 490 (7850) 1
Corrosion Coating Density pcf (kg/m3) 57.434 (920) 1
3.2     Pressure and Temperature Data

The pressure and temperature data for design conditions are given in Table 3-2.

Table 3‑2: Pressure and Temperature Data

Pipeline  Design
Pressure (psig)
Hydrotest
Pressure (psig)
Design
Temperature

(oF)

Hydrotest
Temperature

(oF)

Ref.
12-inch APC-2 to PDP-1 Crude Export Line 2088.54 2411.35 176 84.2 1

Table 3‑3: Content Density

Pipeline Description Unit Value Ref.
12-inch APC-2 to PDP-1 Crude Export Line Density of Content Pcf (kg/m3) 11.86 (190) 1
3.3      Environmental Data

The environmental data used for the wall thickness calculation are given in Table 3‑4.

 

Table 3‑4: Maximum and Minimum Water Depth along Pipeline Routes

 

Pipeline

Water Depth(LAT) m(ft)
Minimum Maximum
12-inch APC-2 to PDP-1 Export Crude Line 40 40

 

4        Design Factors

The design factors applied in the calculations are presented in Table 3-5.

Table 3‑5: Design Factors

Description Symbol Value Ref.
Propagation Buckle Design Load Factor fp 0.8 4
Collapse Factor fo 0.7 4
Installation Bending plus External Pressure Factor f1 3.33 4
In-place Bending plus External Pressure Factor f2 2 4
Hoop Stress Design Factor Kh 0.72 (Pipeline)

0.9 (Hydrotest)

2
0.6 (Spool)

0.9 (Hydrotest)

2
Weld Joint Factor fe 1 2
Temperature Derating Factor ft 1 2
4.0      methodology
4.1      Acceptance Criteria

Pipeline/Spool wall thickness has been calculated in accordance with ASME B31.4-2016 and API RP 1111 2015. The criteria comprise of the following:

  • Internal pressure containment (ASME B31.4-2016, paragraph A403.2.1)
  • Buckle Propagation (API RP 1111 2015, section 4.3.2.3.2)
  • Collapse due to external pressure (API RP 1111 2015, section 4.3.2.2)
  • Buckling due to combined bending and external pressure (API RP 1111 2015, section 4.3.2.3)

The methodology used to calculate the required wall thickness for the above criteria are detailed in sections 4.2 to 4.5.

4.2      Internal Pressure Containment

The minimum wall thickness for the design pressure containment has been calculated as follows:

 

…………………………………………………. [4.1]

Where:

th            Wall thickness;

Pi            Design pressure;

Po           External hydrostatic pressure;

Sy           Specified minimum yield stress (SMYS);

F1           Hoop stress design factor;

D            Outside diameter of pipeline/ riser.

4.3     Buckle Propagation

A buckle in the submarine pipeline will propagate along the pipeline when the external over pressure (Po-Pi) exceeds the factored propagation fpPp as follows:

………………………………………………………….. [4.2]

 

………………………………………………………….. [4.3]

Where:

Pp           Buckling propagation pressure;

Propagation buckle design factor.

4.4     Collapse Due to External Pressure

Pipelines and Spool may be subjected to collapse when the external over pressure (Po-Pi) exceeds a certain critical value foPc as follows:

……………………………………………………………………[4.4]

 

 

Where:

.………………………………………………… [4.5]
………………………………………………………………[4.6]
…………………………………………………………  [4.7]

 

 

 

 

 

 

 

 

Pc           Collapse pressure;

Collapse factor;

Pe           Elastic collapse pressure;

Py           Yield pressure at collapse;

S             Yield strength;

E            Modulus of elasticity;

ν             Poisson ratio.

4.5     Buckling due to Combined Bending and External Pressure

Combined bending strain and external pressure load should satisfy the following:

……………………………………………………………………  [4.8]

To avoid buckling, bending strains should be limited as follows:

…………………………………………………………………….   [4.9]

…………………………………………………………………….   [4.10]

Where:

Collapse factor for use with combined pressure and bending loads = fo/ g(δ)

g(δ)        Collapse reduction factor = (1 + 20δ)-1

δ            Ovality

ε             Bending strain in the pipe

εb           Buckling strain under pure bending (t/2D)

ε1           Maximum installation bending strain

ε2           Maximum in-place bending strain

Bending safety factor for installation bending plus external pressure

Bending safety factor for in-place bending plus external pressure

4.6     Bend Thinning Consideration for Induction Bend Wall Thickness

The minimum wall thickness at any point on the pipe bend shall not be less than the greater of the minimum wall thickness required by the system design code incorporating the bend, or the minimum wall thickness of the installed adjoining pipe.

The requirement for 5D bends is about 8% thinning allowance. As such, the selected wall thickness (after bend thinning) was found to be greater than the calculated wall thickness (internal burst design wall thickness). This will be considered in selecting the required wall thickness for the risers only.

4.7     Minimum Wall Thickness Determination

In accordance with reference [4], the minimum allowable wall thickness (tm) is defined as follows:

…………………………………………… [4.11]

tm = (th + ta)

Where:

ta            =            Internal corrosion/erosion wall thickness allowance

th                   =            Calculated wall thickness

A nominal wall thickness (t) is selected to meet the criteria given below:

t ≥ tm

5.0    results
5.1    Pipeline Wall Thickness Calculation Results

The results of the 12-inch pipeline and spools wall thickness calculations are presented in table 5‑1, while that of the  bend thinning requirement is presented in table 5-2.

Table 5‑1: 12-inch APC-2 to PDP-1 Crude Export Line and Spool Wall Thickness Calculation Result

SPOOL AND PIPELINE WALL THICKNESS CALCULATION (INCH)
Water Depth (m) Internal Pressure Design Collapse Pressure Design Buckling Propagation Design
40 Spool (in) Pipeline (in) Spool (in) Pipeline (in) Spool (in) Pipeline (in)
HYD. T OPE HYD.T OPE INST INST INST INST
0.228 0.342 0.228 0.277 0.137 0.140 0.2 0.210

 

5.2     Bend Thinning Requirement

A bend thinning allowance of 8% for 5D induction bends has been evaluated based on the selected wall thickness. The result is presented in Table 5-5 and shows that the wall thickness selected is adequate.

Table 5‑2: Bend Thinning Requirement

Bend Radius Selected Pipe Wall Thickness;  inch (mm) Bend Thinning (%) Pipe Wall Thickness after bending; inch (mm) Min Req. Thickness Based on Pressure Containment;

inch (mm)

Remark
12-inch APC-2 to PDP-1 Crude Export Line
5D 0.5 (12.7) 8 0.46 (11.68) 0.342 (8.69) OK

5.3        Piggability and Internal Diameter Deviation

The pipeline and spool wall thicknesses satisfied the requirements of Piggability, based on the criterion of difference in internal diameter required by pigging tool manufacturers. See table 5-3

Table 5‑3: Piggability and ID Step Change

Pipeline Description Step Change from Spool to Induction Bend
in(mm)
Allowable

(mm)

 

Remark
Tool Rosen
12.75-inch APC-2 to PDP-1 Crude Export Line 0.124(3.15) 14 OK

   

 

 Notes

3.15mm step change is seen here, as the 12.75-inch spool and induction bend have a different wall thickness