Offshore Oil & Gas Process Skid Piping

Scenario

Project Type: Compact process skid for gas dehydration and glycol injection on a North Sea platform. Location Context: Harsh marine environment (salt-laden air, ambient -10°C to 40°C), space-constrained deck area, high vibration, and strict weight limits. Requires compliance with ASME B31.4 (liquid hydrocarbons) and DNV-OS-F101 (subsea piping). Constraints: Must withstand 1,200 psi design pressure at 150°C (302°F) while minimizing corrosion allowance and structural load. Carbon steel is acceptable but requires 3 mm corrosion allowance per ISO 15156-2 due to CO₂/H₂S presence.

Given Data

• Internal Pressure: 1,200 psi
• Nominal Pipe Size (NPS): 4 inches
• Material: Carbon Steel (ASTM A106 Gr. B)
• Operating Temperature: 302°F (150°C)

Calculation

Using Barlow’s formula with corrosion and mill tolerance allowances:

First, determine base wall thickness:

• $P = 1200$ psi
• $D = 4.500$ in (OD of NPS 4 pipe)
• $S = 17,000$ psi (allowable stress for A106 Gr. B at 302°F, per ASME B31.4 Table A-1)
• $E = 0.85$ (weld joint factor for non-radiographed butt welds)
• $Y = 0.4$ (for ferritic steel)

$$t_{\text{calc}} = \frac{1200 \cdot 4.500}{2 \cdot 17{,}000 \cdot 0.85 \cdot 0.4 + 1.2 \cdot 1200} = \frac{5400}{11{,}560 + 1440} = \frac{5400}{13{,}000} = 0.4154\ \text{in}$$

Add corrosion allowance (3 mm ≈ 0.118 in) and mill tolerance (12.5% per ASME B36.10M): $$t_{\text{min}} = \frac{0.4154 + 0.118}{0.875} = \frac{0.5334}{0.875} = 0.610\ \text{in}$$

Standard schedules: Schedule 80 = 0.237 in (too thin); Schedule 120 = 0.337 in; Schedule 160 = 0.438 in; XXS = 0.636 in → meets 0.610 in requirement.

Result and Decision

Recommended Pipe Schedule: Schedule 160 (equivalent to XXS for NPS 4) Wall Thickness: 0.438 inches (nominal), but actual specified wall = 0.636 in (per ASME B36.10M XXS) — selected to satisfy corrosion + mill tolerance margin. The procurement specification mandated ASTM A106 Gr. B Schedule 160 pipe with full-body UT and PWHT. All flanges were upgraded to Class 600 RF to match the schedule’s pressure rating.

Lesson

In corrosive offshore environments, the ‘minimum required’ wall thickness from Barlow’s formula must be augmented before selecting a schedule — not after. Failure to pre-apply corrosion allowance and mill tolerance leads to underspecified piping that cannot pass third-party verification (e.g., DNV classification). Always calculate t_min including all allowances, then round up to the next available schedule.