SAFETY

Don’t wait for a disaster to check stability

On most ships, the chief officer calculates stability without any secondary validation, until a casualty occurs. When stability failures and groundings happen, salvors and investigators pore over each detail of the ship’s loading. Naval architects may evaluate stability for the first time since design or modification.

Stability and loading calculations are not normally verified during inspections, surveys, or audits. They are also not a required drill or training activity onboard. Despite the crew’s best intentions and interest, the calculations are often inaccurate when the ship goes to sea.

When calculating the loading on a ship, the math sometimes doesn’t add up. The total weight of the ship (displacement) can be calculated and observed using the draft marks. But what if the displacement doesn’t match the drafts? The chief officer uses a “constant” weight to force the total weight of the ship to match the apparent draft.

Due to the size of ships, this constant weight can be huge. During recent containership groundings, salvors discovered the following constants: · 500 metric tons on a 12,000 TEU ship · 1,600 metric tons on a 20,000 TEU ship · -100 metric tons on a 1,300 TEU ship

In each of these three cases, the magnitude of the constant weight is about 0.5% of the displacement of the vessel. This is within the 2% tolerance allowed by the International Association of Classification Societies (IACS) for “Computer Software for Onboard Stability Calculations.” However, magnitude of the weight is only one clue used to judge if the constant weight is reasonable.

How much constant weight is too much? Where should it be applied? These questions aren’t answered. A mariner’s license training typically includes neither draft surveys nor loading computer usage. Rules for ships do not limit the magnitude or center of gravity of constant weights.

Drafts are not checked against calculations at all in some cases. The Paris and Tokyo Memorandum of Understanding on Port State Control conducted a Concentrated Inspection Campaign (CIC) in 2021. This CIC found that 5.4% of the 4,212 ships inspected had no evidence that calculated displacement and trim corresponded with observed drafts.

Lack of time in port compounds the problem of stability calculation by ship crews. Efficiency of the supply chain requires fast turnaround time in port. Containerships and Car Carriers are designed for fast loading and unloading. These ships carry thousands of individualized pieces of cargo, (containers or vehicles). The cargo often has different weights and sizes. Stowage plans provide the ability to load this cargo almost anywhere onboard. The chief officer is responsible for loading and securing the cargo, adding and removing ballast, and calculating vessel stability all before (quickly) departing port.

The UK’s Marine Accident Investigation Branch (MAIB) and the U.S.’s National Transportation Safety Board (NTSB) recently investigated separate Pure Car and Truck Carrier stability failures that revealed strikingly similar accident timelines.

Car Carrier 1:

• 0621: Loading vehicles in port • 1857: Loading complete • 2006: Departed port • 2109: Listed to 40° and grounded

Car Carrier 2:

• 1736: Arrive in port, being partial unload and loading • 2330: Discharged 265 cars, loaded 359 cars • 0053: Departed port • 0137: Listed to 60° and grounded

According to investigators, the chief officer made errors calculating the amount of ballast in both cases. Estimated weights for vehicles were used during planning. Both vessels departed before finalized weights could be entered into the loading computer. In one case, the chief officer intended to finalize stability calculations after departure. In the other, the chief officer performed calculations but the final load plan was emailed to the ship two hours after it capsized.

Ships must comply with stability regulations and stability should be calculated after loading and prior to departure. However, there isn’t a rule requiring a time when stability must be calculated. The car carrier incidents discussed are not isolated. In fact, the 2021 CIC revealed that 2% of inspected ships (79 total) did not have accurate or correct data for a departure stability check.

Port captains, operations managers, auditors, surveyors, and port state control officers can help chief officers ensure their vessels are stable. The chief officer must have accurate loading information. Stability calculation results must match the actual draft of the ship (with a reasonable constant). Don’t wait for a disaster to check stability calculations and procedures.

ANDREW LAWRENCE Principal Naval Architect U.S. Coast Guard Salvage Engineering Response Team