Question 1

A DPO notices the vessel starting to drift off position during a 50-tonne crane lift with the load 30 metres off the deck over the vessel's side. What is the priority order for actions?

Accepted Answer

Immediately alert the crane operator to stop slewing or jib movement (to prevent pendulation worsening); assess whether the drift is controlled (DP working hard) or uncontrolled (excursion); maintain or increase thrust to arrest drift before alerting client/structure; only initiate ASOG decision if drift reaches amber limit — do not drop the load without direct emergency cause A load 30m off the deck and over the side during a DP drift is a multi-hazard scenario: dropping the load is a last resort that creates its own hazards (personnel, ROVs, subsea assets below). The first action is to stabilise the crane (prevent additional pendulation forces) while addressing the DP issue. Uncontrolled load dropping should never be the first response to a DP problem.

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.