Remote Oil & Gas Wellhead Pressure Monitoring

Scenario

A brownfield offshore oil platform in the North Sea requires retrofitting of legacy pressure transmitters with modern 4–20 mA smart transmitters. The new transmitters must operate reliably over existing 22 AWG twisted-pair copper cable running 500 m (one-way) from the wellhead junction box to the central control room. Environmental constraints include ambient temperatures ranging from −25°C to +60°C, salt-laden air, and limited space for hardware upgrades. Power is supplied from a centralized 24 V DC system; however, voltage drop across long cable runs is a known issue on aging infrastructure.

Given Data

• Supply Voltage: 24 V
• Minimum Operating Voltage of Transmitter: 12 V
• Maximum Current in Loop: 0.02 A (20 mA)
• Load Resistance (PLC input card): 250 Ω
• Wire Gauge: 22 AWG
• Cable Length (one-way): 500 m

Calculation

The tool computes cable resistance using standard resistivity data for copper:

• Resistance per km for 22 AWG ≈ 53.3 Ω/km (at 20°C)
• One-way resistance = 53.3 Ω/km × 0.5 km = 26.65 Ω
• Round-trip cable resistance = 2 × 26.65 Ω = 53.30 Ω
• Total loop resistance = Load resistance + Cable resistance = 250 Ω + 53.30 Ω = 303.30 Ω
• Voltage drop at 20 mA = I × R = 0.02 A × 303.30 Ω = 6.07 V
• Residual voltage at transmitter terminals = 24 V − 6.07 V = 17.93 V → above minimum 12 V requirement ✅

Result and Decision

The calculated loop resistance (303.30 Ω) and resulting transmitter terminal voltage (17.93 V) are within specification. No cable upgrade or power supply modification was required. All 12 wellhead transmitters were deployed using the existing 22 AWG infrastructure, saving ~€85,000 in cable replacement and trenching costs.

Lesson

Always validate actual terminal voltage—not just loop resistance—against the transmitter’s datasheet minimum operating voltage, especially in retrofits where cable aging and temperature derating may increase resistance beyond nominal tables.