Executive in Action: Jean-Francois Segrétain, Deputy Technical Director, Bureau Veritas

If you stand in a long corridor on a large container ship you can see it moving in a snake-like motion in front of you. “It really is something you can’t ignore,” says Jean-François Segrétain, Deputy Technical Director of the Marine Division for Bureau Veritas. “These effects need careful analysis as they can have a major impact on hull fatigue.”

“Whipping” & “Springing”

Bureau Veritas has classed the largest container ship in the world so far, the 16,000-TEU CMA CGM Marco Polo, built by Daewoo Shipbuilding & Marine Engineering in South Korea for French operator CMA-CGM. In the new generation of mega-container ships, whipping and springing that cause this snake-like movement can account for a massive 50 percent of the total vibration of the ship’s hull. “Whipping” is the shock on the forebody caused by the motion of the vessel through waves, and “springing” is an oscillation of the whole vessel.

For very large container ships, around 400m long, the main energy in ocean waves is the same order of magnitude as the ship’s natural frequency, so they excite the vibration of the ship. For smaller vessels, such as container ships of around 10,000 TEUs, and also for very large tankers, the effect is much smaller, accounting for perhaps 10 percent of the vibration.

While longitudinal phenomena such as hogging and sagging are well known and accounted for, it should be mandatory to analyze springing and whipping in large container ships, says Segrétain. “Both these phenomena can be predicted by computation nowadays,” he explains. “It is a relatively complex calculation, but it must be done because these ships are much more flexible than ships before.” As it is a fatigue problem, it doesn’t mean global scantlings need to be increased, or the thickness of the deck. Rather, reinforcing is required on the connections of the longitudinal bars to the structure of the ship.

Size Effects

There are other safety concerns with these very large container ships. To ensure the safe maneuverability of such large vessels, the rudder must be sized appropriately and must take into account the effects of sea and wind and the height of containers on deck. This is often studied during tank tests and doesn’t present a technical challenge, says Segrétain: “Due to the sheer size of the ship, you have to be careful. This being said, the reaction of the ship is not essentially different from any type of very large ship, such as oil tankers and bulk carriers.”

Other size effects need to be considered, including bridge visibility. Marco Polo has a second castle in a more forward position in order to meet IMO regulations. Operationally, draught is also very important. At around 16m in large container ships, it is very close to the maximum permissible depth of many ports, so dredging and operational depth must be considered.

However, there is one aspect in which mega-container ships offer a safety advantage because of their size. The acceleration experienced by containers on deck is slightly less than on smaller vessels, so the risk of losing containers overboard is reduced.

Managing the Risk

Segrétain is satisfied that the risks associated with Marco Polo are well managed: “The risk is managed, certainly. In particular, the structural risk has been studied intensively by our society. If you consider risk as a product of the probability of damage, this probability has been lowered to a satisfactory level.” However, he cautions yards and owners to perform similar studies to those undertaken by Bureau Veritas, particularly with respect to springing and whipping. He believes these studies should be mandatory for large container ships. – MarEx

Wendy Laursen can be reached at [email protected].