Methodology
This analysis compares minimum wall thickness requirements across three widely-used
subsea pipeline design codes. Each code uses a different philosophy:
DNV-ST-F101 (2021)
Uses Load and Resistance Factor Design (LRFD) with partial safety factors
on both loads and resistance. The most granular approach with separate factors for safety class
(Low/Medium/High), material resistance (γm=1.15), and condition
(γSC varies by safety class). Performs four limit state checks: pressure containment,
collapse, propagation buckling, and combined loading.
API RP 1111 (2015)
Uses Working Stress Design (WSD) with single design factors applied to
material strength. The burst check uses a Barlow-based approach with fd=0.72.
Collapse uses an elastic-plastic transition formula. Generally the least conservative
of the three codes for typical offshore conditions.
PD 8010-2 (2015)
Also uses Working Stress Design (WSD) based on British Standards tradition.
Similar design factor approach (fd=0.72 for hoop stress) with an additional
Von Mises equivalent stress check (σe ≤ 0.9 × SMYS). Includes collapse and
propagation checks similar to API.
Common Parameters
- Grade: API 5L X65 (SMYS = 448 MPa, SMTS = 531 MPa)
- Water depth: 500 m (external pressure ≈ 5.02 MPa)
- Internal pressures: 10, 15, 20, 25 MPa
- Corrosion allowance: 1 mm
- Safety Class: Medium (DNV), standard factors (API, PD 8010-2)
Applicable Codes & Standards
- DNV-ST-F101 (2021) — Submarine Pipeline Systems
- API RP 1111 (2015) — Offshore Hydrocarbon Pipelines (Limit State Design)
- PD 8010-2 (2015) — Code of Practice for Pipelines – Part 2: Subsea Pipelines
- API 5L (2018) — Line Pipe (material grades)
Summary: Minimum Wall Thickness by Pipe Size and Code
At 20 MPa internal pressure, 500m water depth, X65 grade, Safety Class Medium
| Pipe Size |
OD (mm) |
Code |
Min WT (mm) |
Governing Check |
Status |
| 6" |
168.3 |
DNV-ST-F101 |
8.51 |
Propagation Buckling |
PASS |
| 6" |
168.3 |
API RP 1111 |
7.62 |
Propagation |
PASS |
| 6" |
168.3 |
PD 8010-2 |
8.57 |
Propagation |
PASS |
| 8" |
219.1 |
DNV-ST-F101 |
10.79 |
Propagation Buckling |
PASS |
| 8" |
219.1 |
API RP 1111 |
9.90 |
Propagation |
PASS |
| 8" |
219.1 |
PD 8010-2 |
10.90 |
Propagation |
PASS |
| 10" |
273.1 |
DNV-ST-F101 |
13.19 |
Propagation Buckling |
PASS |
| 10" |
273.1 |
API RP 1111 |
12.30 |
Propagation |
PASS |
| 10" |
273.1 |
PD 8010-2 |
13.30 |
Propagation |
PASS |
| 12" |
323.9 |
DNV-ST-F101 |
15.47 |
Propagation Buckling |
PASS |
| 12" |
323.9 |
API RP 1111 |
14.58 |
Propagation |
PASS |
| 12" |
323.9 |
PD 8010-2 |
15.58 |
Propagation |
PASS |
| 16" |
406.4 |
DNV-ST-F101 |
19.14 |
Propagation Buckling |
PASS |
| 16" |
406.4 |
API RP 1111 |
18.31 |
Propagation |
PASS |
| 16" |
406.4 |
PD 8010-2 |
19.31 |
Propagation |
PASS |
| 20" |
508.0 |
DNV-ST-F101 |
23.65 |
Propagation Buckling |
PASS |
| 20" |
508.0 |
API RP 1111 |
22.87 |
Propagation |
PASS |
| 20" |
508.0 |
PD 8010-2 |
23.87 |
Propagation |
PASS |
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Assumptions & Limitations
- All analyses use API 5L X65 grade (SMYS = 448 MPa, SMTS = 531 MPa)
- Water depth fixed at 500 m for all cases (P_ext ≈ 5.02 MPa)
- Corrosion allowance = 1.0 mm internal, 0.0 mm external (intact coating assumed)
- Fabrication tolerance = 12.5% (DNV default for seamless pipe)
- Ovality = 0.5% (Dmax-Dmin)/Dnom — conservative for good-quality pipe
- Seamless pipe fabrication (α_fab = 1.0 for DNV, no UOE derating)
- Temperature derating not applied (operating temperature < 50°C assumed)
- Safety Class Medium used for DNV (typical for production pipelines)
- Lifecycle phases use simplified bending moments and tensions
- No dynamic loading or fatigue effects considered
- Wall thicknesses from standard pipe schedules per ASME B36.10M
Chart 1: Lifecycle Utilisation by Design Code
Max utilisation across limit states for each lifecycle phase. Use dropdown to select pipe size.
Chart 2: Minimum Required Wall Thickness vs Pipe Size
At 20 MPa internal pressure, 500m water depth, X65 grade. Shaded band shows code penalty.
Chart 3: Utilisation Heatmap — Pipe Size × Internal Pressure
Each cell uses the minimum standard WT from the pipe catalog. Green = safe, Red = overstressed.
Chart 4: Steel Weight Penalty — Code Conservatism Cost
Difference in required WT between most and least conservative code, at 20 MPa.
Chart 5: Governing Limit State Distribution
Which limit state governs across all parametric cases, broken down by code.