INTRODUCTION
1.1Â Â Â 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 1–1: Project Scope
1.2Â Â Â 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.
1.3Â Â Â 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 |
1.4Â Â Â 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 Â Â Â Â Â Â result summary and conclusions
2.1 Â Â Â Â Â Â 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 |
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 Â Â (10.033) | Â 0.500 Â Â (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:
- Calculated wall thickness inclusive of CA
- Wall thickness was selected based on ASME B36.1 Ref. 5.
Bend Radius | Selected Pipe Wall Thickness;Â 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 |
2.2Â Â Â Â Â Â Â Conclusion
- 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.
- 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.
- 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.
- 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.
3.0Â Â Â Â Â Â Â Design data
The following sub-section outline the design data used in performing the wall thickness calculation.
3.1Â Â Â Â Â Â Â 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) | Â Psi | 30022811.7 | 1 |
Steel Density | pcf (kg/m3) | 490 (7850) | 1 |
Corrosion Coating Density | pcf (kg/m3) | 57.434 (920) | 1 |
3.2Â Â Â Â 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 | Â 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 |
3.3Â Â Â Â Â 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 |
4Â Â Â Â Â Â Â Design Factors
The design factors applied in the calculations are presented in Table 3-5.
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 |
4.0Â Â Â Â Â methodology
4.1Â Â Â Â Â 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.
4.2Â Â Â Â Â 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);
F1Â Â Â Â Â Â Â Â Â Â Hoop stress design factor;
DÂ Â Â Â Â Â Â Â Â Â Â Outside diameter of pipeline/ riser.
4.3Â Â Â Â 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.
4.4Â Â Â Â 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:
|
Where:
|
||||||
|
||||||
|
Pc          Collapse pressure;
Collapse factor;
Pe          Elastic collapse pressure;
Py          Yield pressure at collapse;
SÂ Â Â Â Â Â Â Â Â Â Â Â Yield strength;
EÂ Â Â Â Â Â Â Â Â Â Â Modulus of elasticity;
ν            Poisson ratio.
4.5Â Â Â Â 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
4.6Â Â Â Â 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.
4.7Â Â Â Â Minimum Wall Thickness Determination
In accordance with reference [4], the minimum allowable wall thickness (tm) is defined as follows:
|
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
5.0Â Â Â results
5.1Â Â Â 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 |
5.2Â Â Â Â 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;Â 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 |
5.3Â Â Â Â Â Â Â 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 |
  Â
Â
 Notes
3.15mm step change is seen here, as the 12.75-inch spool and induction bend have a different wall thickness