Fuel changeover with a stuck viscosity controller on the approach
At 0450 the 6S60MC-C main engine is on 380 cSt HFO at 8 knots, 6 nm from the pilot station, with the bridge expecting manoeuvring speed at 0520. Company procedure requires changeover to MDO before manoeuvring. You start the changeover at 0455; the viscosity controller is set to 12 cSt and the steam tracing valve is in automatic. Ten minutes later the viscotherm at the engine inlet reads 24 cSt and falling, but the fuel temperature controller output is stuck at 100 % and the fuel oil unit inlet pressure has dropped from 0.8 to 0.55 MPa. The bridge calls to confirm the engine is ready for manoeuvring. The MDO service tank is at 32 °C. The second engineer is on deck inspecting the purifier room and the junior engineer is on his first contract. You are the EOOW alone in the control room.
1. State your immediate actions in the next five minutes, in priority order, and explain the risk each action addresses.
Read the worked answer
- Answer the bridge at once: the engine is not ready for manoeuvring; recommend manoeuvring speed and minimum telegraph movements, and give a realistic readiness time. This prevents the pilot expecting immediate full astern while the fuel system is unstable.
- Take the fuel temperature controller to manual and cut the steam to the tracing. A controller stuck at 100 % will keep heating the distillate and drive viscosity below the safe minimum; low viscosity increases plunger/barrel leakage, wrecks atomisation and can seize a fuel pump.
- Freeze or slow the changeover valve until temperature and viscosity are under control. The risk is a step change at the fuel pumps, not the changeover itself.
- Investigate the 0.55 MPa fuel pressure: booster pump suction strainer, discharge filter, changeover valve line-up, MDO tank level and any air ingress. Low pressure at manoeuvring can cause a fuel cut-out and blackout.
- Call the second engineer to the ECR; do not leave the console unattended.
Reasoning: the two hazards are too-low viscosity (seizure/leakage) and unstable fuel pressure (loss of propulsion in a confined channel). Both must be controlled before any 'ready' answer.
2. What viscosity, temperature and pressure limits govern this changeover, and how do you prevent thermal shock and pump seizure?
Read the worked answer
Use the engine builder’s instruction book, not a guessed figure. For a large two-stroke on HFO the usual inlet target is 10–15 cSt with a low-viscosity alarm near 9 cSt; for distillate the target is much lower, so the 12 cSt HFO setpoint must be changed or the viscotherm bypassed and the heater left on minimum. The controlling limit is the minimum viscosity at the fuel pump inlet, not the HFO number.
Thermal shock: limit the rate of change at the fuel pump inlet to about 2 °C per minute (builder’s figure). A fast rise or fall can crack pump barrels and distort clearances. Monitor inlet temperature, not just the viscotherm reading.
Pressure: keep fuel oil unit inlet pressure at the normal 0.7–0.8 MPa; 0.55 MPa is abnormal. Check booster pump, filters, changeover valve and tank temperature.
Prevention: steady engine load, slow valve movement, manual steam control, and cross-check viscosity against temperature and pressure. If viscosity falls below the alarm limit, stop heating and, if necessary, slow or reverse the changeover slightly to bring it back into band.
3. Draft the communication you give the bridge and the records you make. Justify why you would or would not declare the engine ready.
Read the worked answer
Bridge call: “Engine not ready for manoeuvring. Fuel changeover in progress; temperature controller defective and fuel pressure low. Recommend manoeuvring speed now and minimum telegraph movements. I expect to confirm readiness in 15–20 minutes. I will call you.” Then keep the bridge informed of any slip in that time.
Records: engine log (times, viscosity, inlet temperature, fuel pressure, controller status), bell book/movement book, fuel changeover record, defect report for the temperature controller, and notification to the chief engineer. If the ship’s SMS has a fuel-changeover checklist, annotate the deviation.
Justification for not declaring ready: viscosity is still falling with the controller stuck at 100 %, fuel pressure is 25 % below normal, and there is one engineer on the console with an inexperienced junior. Manoeuvring demands instant response to telegraph orders; a fuel pump seizure or low-pressure trip at that moment is a loss-of-propulsion risk. Declaring ready would transfer the risk to the bridge without the means to control it. Ready means stable viscosity, stable pressure, all alarms normal and a competent engineer available - none of these is yet true.