Pipeline Commissioning

Pipeline Drying Methods

Methods to remove moisture from pipelines after hydrostatic testing: air drying, nitrogen drying, vacuum drying, and methanol slug treatment.

Launch Calculator Drying methods

Target dewpoint

-20°F to -40°F

Transmission pipelines require -20°F minimum; arctic service -40°F or lower.

Air drying

5-20 days

Lowest cost method; requires desiccant dryer for -40°F dewpoint air supply.

Vacuum drying

1-3 days

Fastest method; water evaporates at low temperature under vacuum.

Contents

Use this guide to:

  • Select appropriate drying method
  • Estimate drying time and equipment
  • Specify and verify dewpoint requirements

1. Overview

Pipeline drying removes residual water after hydrostatic testing before introducing hydrocarbons. Proper drying prevents hydrate formation, internal corrosion, and off-spec gas delivery.

Pipeline drying cross-section showing dewatering pig pushing bulk water, residual water film on pipe walls, dry gas flow direction, and the three stages: before drying, drying zone with evaporation, and dried section
Pipeline drying process: Pigging removes 95-99% of bulk water, drying removes remaining film and trapped pockets.

Hydrate formation

Pipeline blockage

Water + gas at low temp/high pressure forms ice-like plugs.

Internal corrosion

Metal loss

Free water enables CO₂ and O₂ corrosion.

Product quality

Off-spec gas

Exceeds 7 lb/MMscf water content limit for sales gas.

Equipment damage

Slugging

Water slugs damage compressors and foul instruments.

Key Definitions

  • Dewpoint: Temperature at which water vapor condenses at given pressure
  • Water content: Mass of water per gas volume (lb/MMscf)
  • Hydrate formation temp: Temperature below which gas hydrates form (varies with pressure)
Example: A 10-mile, 12" pipeline holds ~7,400 bbl of water after hydrostatic testing. Pigging removes 95-99%, leaving ~75-370 bbl as film water and trapped pockets that must be removed by drying.

Method Comparison

Method Cost Time (10 mi) Dewpoint Best For
Air drying $10-50k 5-20 days -20 to -40°F Onshore, budget-sensitive
N₂ drying $50-200k 2-5 days -40 to -80°F Critical schedule, high purity
Vacuum $20-100k 1-3 days -60 to -100°F Offshore, remote locations
Methanol slug $5-30k 1-2 days N/A* Hydrate prevention, small lines

*Methanol absorbs water rather than achieving a specific dewpoint.

2. Drying Methods

Four primary methods for pipeline drying, each suited to different project requirements.

Air Drying

Air drying system process flow diagram showing air compressor (100-500 scfm), twin tower desiccant dryer with regeneration heater, pipeline under drying with wet and dry sections, and outlet dewpoint monitor displaying -35°F
Air drying system schematic: Process continues until outlet dewpoint matches inlet ±5°F for 24 hours.
Process: 1. Pig to remove bulk water (95-99%) 2. Introduce dry air at -40°F dewpoint or lower 3. Circulate continuously, monitor outlet dewpoint 4. Complete when outlet matches inlet ±5°F for 24 hrs Equipment: Compressor + desiccant dryer (100-500 scfm) Advantages: Low cost, simple, no permits Disadvantages: Slow (5-20 days), weather-dependent

Nitrogen Drying

Process: 1. Pig to remove bulk water 2. Circulate dry N₂ (-60°F to -100°F dewpoint) 3. Monitor outlet dewpoint 4. Can combine with pressure testing N₂ Consumption: Volume = Pipeline volume × 5-10 changes (for -40°F) × 10-20 changes (for -60°F) Cost Example: 10 mi × 12" = 41,500 ft³ 10 changes = 415,000 scf @ $0.15/scf = $62,000 Advantages: Fast, very low dewpoints, inert Disadvantages: High cost, asphyxiation hazard

Vacuum Drying

Vacuum drying system showing sealed pipeline with blind flanges, vacuum line, condenser capturing evaporated water, vacuum pump targeting 1-10 torr, and vacuum gauge reading 5 torr. At 1 torr water boils at 40°F, at 5 torr at 75°F
Vacuum drying system: Water evaporates at low temperature under vacuum. Complete when pressure rise < 1 torr/hr.
Principle: Water boils at low temp under vacuum At 1 torr (0.02 psia): Water boils at 40°F At 5 torr (0.1 psia): Water boils at 75°F Process: 1. Pig and seal pipeline (blind flanges) 2. Pull vacuum to 1-10 torr 3. Hold while water evaporates 4. Pressure rise test: <1 torr/hr = dry Advantages: Fastest (1-3 days), no consumables Disadvantages: Must be sealed, collapse risk

Methanol Slug Treatment

Principle: Methanol absorbs water (0.7 lb H₂O per lb MeOH) Process: 1. Calculate water to remove from GPSA charts 2. Size methanol slug (typically 20-40% of pipe volume) 3. Push slug through with dry gas or nitrogen 4. Collect methanol/water mixture at downstream end Best for: Gathering lines, hydrate prevention Caution: Toxic/flammable - full PPE required

Dewatering Pigs

Run before drying to remove 90-99% of bulk water:

Pig Type Removal Application
Medium-density foam 70-90% Initial pass
High-density foam 85-95% Second pass
Bi-directional + squeegee 95-99% Final dewatering
Best practice: Run 2-3 pigs in sequence before drying to minimize residual water.

3. Drying Calculations

Residual Water Estimation

After pigging (95-99% removed): Film method: M = π × D × L × t × ρ Where t = 0.001-0.005" film thickness Example - 10 mi × 12" pipe: M = π × 1.0 ft × 52,800 ft × 0.000167 ft × 62.4 lb/ft³ M = 1,730 lb ≈ 200 gallons residual water

Water Content from GPSA

Water content of natural gas vs temperature chart from GPSA showing logarithmic plot with curves for 100, 250, 500, 1000, and 2000 psia pressures, with sales gas spec line at 7 lb/MMscf and arctic/standard transmission spec reference lines
Water content of natural gas vs temperature at various pressures. Source: GPSA.
Temp (°F) 100 psia 500 psia 1000 psia
-402.00.60.35
-205.51.40.8
40347.84.3
801112614

Water content in lb/MMscf. Source: GPSA.

Methanol Requirements

MMeOH = (Wvapor + Wresidual) / (0.7 × P) Where: Wvapor = Vapor-phase water = (Winitial - Wtarget) × V Wresidual = Residual free water after pigging (typically 1-2% of pipe volume) 0.7 = Methanol absorption capacity (lb H₂O/lb MeOH) P = Methanol purity (fraction, e.g. 1.0 for 100%) Example: 10 mi × 12" at 500 psia, 80°F → -20°F dewpoint W_initial = 26 lb/MMscf, W_target = 1.4 lb/MMscf V = 0.041 MMscf Vapor water = (26 - 1.4) × 0.041 = 1.0 lb Residual water (1.5% of 310,000 gal × 8.34 lb/gal) ≈ 38,800 lb Total water = 1.0 + 38,800 ≈ 38,800 lb Methanol = 38,800 / 0.7 = 55,400 lb = 8,360 gal Note: Residual free water dominates methanol requirements. Vapor-phase water is negligible in comparison for post-hydrotest drying.

Drying Time Estimates

Method 5 mi pipe 10 mi pipe 20 mi pipe
Air drying3-10 days5-20 days10-30 days
N₂ drying1-3 days2-5 days4-10 days
Vacuum12-24 hr24-48 hr48-72 hr
Schedule contingency: Add 50-100% to calculated time. Weather, equipment issues, and higher-than-expected water extend actual duration.

4. Dewpoint Specifications

Dewpoint requirements based on hydrate formation temperature and operating conditions.

Hydrate Formation Temperature

Natural gas hydrate formation curve for 0.6 specific gravity gas showing hydrate zone below curve where ice-like plugs form, safe operating zone above, gathering systems and transmission pipelines operating ranges, and required dewpoint with 20-30°F safety margin
Natural gas hydrate formation curve. Conservative approximation: Th ≈ 32 + 8.9 × ln(P/14.7) °F for 0.6 SG gas.
Conservative approximation for natural gas (0.6 SG): Th ≈ 32 + 8.9 × ln(P/14.7) °F At 500 psia: Th ≈ 63°F At 1,000 psia: Th ≈ 70°F At 1,500 psia: Th ≈ 73°F Note: This simplified formula overpredicts hydrate temperature by 8-13°F vs the Katz gravity chart, erring on the safe side. Required dewpoint: Dewpoint < Tmin - Safety margin Standard margin: 20-30°F below min operating temp

Industry Standard Dewpoints

Service Dewpoint Spec
Gathering (<500 psia)-20°F @ operating P
Transmission (500-1000 psia)-20°F to -40°F @ 1000 psia
High-pressure (>1000 psia)-40°F @ operating P
Arctic service-60°F to -80°F
Sales gas (contract)7 lb/MMscf @ 14.7 psia

Pressure Effect on Dewpoint

Important: Dewpoint rises with pressure compression. Gas at -20°F dewpoint @ 500 psia → Compressed to 1,000 psia → New dewpoint ≈ -10°F (rises ~10°F per 2× pressure) Best practice: Specify dewpoint at MAOP "Dewpoint ≤ -20°F at 1,200 psig"
Dewpoint vs. water content: Dewpoint is intuitive for operations (relates directly to hydrate risk). Water content (lb/MMscf) is used in contracts. Always specify pressure basis.

5. Verification & Testing

Confirm dewpoint meets specification before commissioning.

Dewpoint Measurement

Method Accuracy Application
Chilled mirror±0.5-2°FLab standard, most accurate
Capacitance sensor±3-5°FPortable field use
Aluminum oxide±2-5°FOnline monitors

Monitoring Procedure

Typical pipeline air drying curve for 10-mile 12-inch pipeline showing outlet dewpoint temperature declining from +75°F to -40°F over 120 hours, with four phases: bulk water removal, film evaporation, deep drying, and verification hold
Typical pipeline air drying curve showing the four drying phases and acceptance criteria.
Sample points: 1. Inlet - verify dry gas supply 2. Outlet - primary measurement (slowest to dry) 3. Low points - water accumulates here Acceptance criteria: • Outlet dewpoint ≤ target for 24 hrs continuous • Multiple sample points within ±5°F • Dewpoint stable (not rising) Sample system: 1/4" SS tubing, fast-loop flow, coalescing filter, heat trace in cold weather

Troubleshooting

Problem Cause Solution
Dewpoint not decreasingLow flow, wet inlet airIncrease flow, check dryer
Dewpoint stallsCoating water releaseContinue drying, heat if possible
Dewpoint rises after meeting specLeak, water intrusionCheck seals, sample lines
Cannot reach targetInsufficient inlet dewpointAdd dryer stage

Documentation

Required records: • Drying procedure and equipment • Dewpoint logs (inlet/outlet readings) • Calibration certificates • Final acceptance readings (multiple points) • Sign-off by company and contractor Acceptance statement example: "Pipeline dried to -20°F dewpoint at 1,000 psig. Outlet readings averaged -23°F over 24-hr period."
Critical: Never commission based solely on calculated drying time. Always verify measured dewpoint at outlet meets spec for 24+ hours. Many hydrate failures occur from inadequate verification.

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