Question 1

During a crane slewing operation with a 40-tonne load, a generator trips and the DP system goes to degraded mode. The crane is mid-slew. What should the DPO and crane operator do?

Accepted Answer

DPO immediately alerts the crane operator to stop slewing and hold position; DPO assesses new DP capability with reduced power and compares against current environmental conditions using the ASOG; crane operator should arrest the slew and hold the load stationary to prevent additional pendulation forces until the DP situation is stabilised A generator trip during a crane lift with a heavy load is a compounded risk: the vessel has less power for DP while simultaneously carrying a load that increases the demand on thrusters. Stopping the crane slew prevents additional pendulation and removes the dynamic moment from the equation while the DPO assesses whether the remaining power is sufficient to hold position safely with the suspended load.

Question 2

How does a heavy crane lift affect the DP system's position-keeping model and why does this matter?

Accepted Answer

Lifting a heavy load shifts the vessel's centre of gravity upward and athwartships (depending on crane position), altering the vessel's roll and pitch characteristics and therefore the environmental force response — a DP system not updated with the correct crane position and load may miscalculate the forces needed to maintain position, reducing accuracy Modern DP systems allow the DPO to input crane position and estimated load so the DP model can compensate for the changed stability and force balance. On vessels without automatic crane/load input, the DPO should manually update the model. A heavy load on an extended jib creates significant moment that the DP system must account for to maintain heading and position accurately.

Question 3

What is load pendulation and why does it create a risk for DP position-keeping?

Accepted Answer

Load pendulation is the swinging of a suspended load in an arc below the crane hook, which creates a dynamic, oscillating side force transmitted through the crane structure to the vessel hull — the DP system must counter this cyclic force, and the constant changing direction of the force can cause oscillating thruster demands that stress both the DP system and the power management system A pendulating load creates a lateral force that swings across the beam of the vessel, requiring the DP system to provide corrective thrust in alternating directions. This cyclical demand can cause undesirable roll motions that further increase pendulation (amplifying the problem), and on vessels with limited power, can contribute to demand spikes that stress the PMS or trip generators.