🚢 UTI/MMC – Complete Tank Gauging Procedure for Oil Tankers
Merchant Navy Life – Advanced Technical Series
1️⃣ Introduction
Accurate cargo measurement is one of the most critical duties onboard oil tankers. The primary tool used for this purpose is the UTI/MMC (Ullage–Temperature–Interface / Multi Measurement Cable). When used correctly, it provides:
- Ullage (distance from reference height to cargo surface)
- Cargo temperature in °C or °F
- Oil–water interface (water detection)
- Tank bottom depth / BS&W
- Cross-checking of radar/fixed gauges
- Evidence for B/L, ROB, OBQ & cargo disputes
All operations are performed gas-tight through a vapor lock valve, ensuring zero vapor release and full compliance with ISGOTT & OCIMF.
2️⃣ Safety Requirements – MUST FOLLOW
✔ Closed Gauging Only
Modern tankers operate fully closed:
- No ullage caps opened
- No hatch opening
- No vapor escape
The UTI probe passes through a 2-inch vapor lock valve, always maintaining tank pressure.
✔ Intrinsically Safe Equipment
UTI/MMC must have:
- Explosion proof rating (Ex ia or Ex ib)
- Intrinsic safety certificates
- Valid calibration date
- IECEE/ATEX approvals
A non-certified device cannot be used inside vapor-rich atmospheres.
✔ Bonding / Grounding Cable
Before lowering the probe:
👉 Connect the bonding clip to ship’s structure
This eliminates static electricity buildup — a critical ISGOTT requirement.
3️⃣ How UTI/MMC Detects Oil & Water 💧🛢
UTI probe uses conductivity difference:
🔹 Oil = Non-conductive
→ No alarm, no signal.
🔹 Water = Conductive
→ Conducts electric signal →
Buzzer + LED light activates.
This system allows extremely accurate water detection even for thin water layers, and is essential during:
- Crude ROB measurement
- Clean/dirty tank changeover
- Seawater contamination investigation
- Pre-loading inspection
- Bunker operations
- BS&W analysis
▶️ Video 1 – Recognizing Oil–Water Interface (UTI)
4️⃣ How to Take an Ullage / Interface Reading 📏
🔸 Step 1 — Safety Precautions
Per ISGOTT 6th Edition:
✔ Verify intrinsic safety certification
✔ Connect bonding cable
✔ Wear anti-static gloves
✔ Ensure IG pressure safe to open valve
✔ Confirm correct gauging point and correct tank
✔ Check wind direction
✔ Use proper PPE
✔ Confirm vapor lock valve fully functional
🔸 Step 2 — Perform Zero Check
Before lowering, ensure:
- Tape is at TRUE zero
- Probe tip is clean & dry
- Temperature reading stabilized for at least 10–15 seconds
- Check digital display brightness, battery voltage
- Check valve’s “zero ullage reference height” marking
🔸 Step 3 — Lower the Tape Gently
Lower slowly to avoid:
- Wetting the tape
- Disturbing oil surface
- Splash in volatile products
First alarm = ullage point
Record:
📌 Ullage (m/cm)
📌 Temperature (°C)
📌 Time of measurement
Take 3-point temperature for cargo documents:
- Top
- Middle
- Bottom
This is mandatory for terminals and surveyors.
🔸 Step 4 — Detect Water Interface
Lower further until:
🔔 Second alarm = water level
Record interface depth accurately.
This is crucial for BS&W calculation.
5️⃣ Tank Bottom Measurement (BS&W)
If tank contains crude or dirty product:
- After interface detection, lower probe to reach bottom
- Read total depth
- Calculate:
Water Cut = Interface Depth – Ullage
BS&W (Bottom Sediment & Water) is essential for:
- Loading acceptance
- Cargo custody transfer
- Discharge loss quantity
- Cargo claims
- Pre-wash/inspection
6️⃣ Cross-Checking Radar / Fixed Gauges 📊
Every tanker must compare:
- Radar reading vs MMC reading
- Difference must be within ±3 to ±6 mm (company dependent)
If mismatch:
⚠ Make remark in logbook
⚠ Inform CCR
⚠ Cross-check again with 2nd officer
⚠ Investigate radar calibration issue
⚠ Report to office if persistent
7️⃣ Reference Height (R.H.)
R.H. is a shipyard-defined fixed measurement from:
Deck datum → Tank bottom or ullage reference point
Correct calculations depend on correct ullage + R.H.
8️⃣ List & Trim Corrections
If vessel is not even keel/upright:
✔ Apply List Correction Table
List to port = negative correction
List to starboard = positive correction
✔ Apply Trim Correction Table
Aft trim = alternate correction
Forward trim = opposite correction
Corrected ullage is used for tank capacity table.
9️⃣ Finding Cargo Quantity from Tank Tables
Steps:
- Use corrected ullage
- Enter Tank Capacity Table
- Read corresponding Volume (m³)
- Apply VCF (Temperature Correction)
- Apply Density correction
- Final cargo quantity =
Volume × VCF × Density @15°C
(This full calculation will be in next article.)
🔟 Common Problems & How to Fix 🛠
❌ No alarm at oil
→ Probe contaminated, clean with lint-free cloth.
❌ No water alarm
→ Probe corrosion / cable broken.
❌ Temperature fluctuating
→ RTD sensor damaged → recalibration needed.
❌ Wrong ullage
→ Tape stretched or slipping.
→ Check zero alignment.
❌ Moisture inside probe
→ Equipment must be removed from service.
❌ Vapor lock valve leaking
→ Stop gauging immediately.
1️⃣1️⃣ Best Professional Practices ✔️
- Always wipe probe and tape before/after use
- Never allow tape to touch tank walls
- Keep lowering speed steady
- Reconfirm readings three times
- Verify with another officer (SIRE requirement)
- Record measurements in CCR + cargo logbook
- Cross-check with radar/deck display
- Keep spare batteries & O-rings in CCR locker
- Clean vapor lock valve seals regularly
1️⃣2️⃣ Regulations & References
- ISGOTT 6th Edition
- OCIMF SIRE 2.0
- SOLAS Chapter II-2
- MARPOL Annex I
- OCIMF Ship Inspection Guidelines
- Company SMS
- Terminal Measurement Guidelines
▶️ Video 2 – Full UTI/MMC Measurement Guide
📌 Conclusion
UTI/MMC is a precision instrument and a core competency for all deck officers.
When operated correctly, it ensures:
✔ Accurate ullage
✔ Correct temperature profile
✔ Reliable oil–water interface detection
✔ Safe, gas-tight measurement
✔ Accurate ship/shore quantity comparison
✔ Compliance with inspections and cargo auditors
Mastering UTI/MMC operation means mastering professional tanker cargo handling.
📘 Note
This article is for educational and training purposes only.
Always follow your Company SMS, ISGOTT, OCIMF and manufacturer instructions.
🔜 Coming Soon
“Full Cargo Calculation Guide – Step-by-Step for Tanker Officers”
🔗 Additional Reading & External Sources