Double Hull Rolling

Large double hull tankers have much poorer resistance to roll than single hull ships. In the inelegant words of one senior Chevron mooring master, "Double hulls roll like pigs". In the loaded condition, this is the result of the high VCG of the cargo and the resultant reduced righting moment. In the ballast condition, the VCG is better than that for a single hull tankers but this is overwhelmed by the large increase in roll radius of gyration. These poor roll characteristics impose fuel-speed penalties and more importantly generate a significant operational problem in offshore lightering. The additional roll not only generates safety problems for the crew but impose large strains on the mooring lines and fenders. It's only a matter of time before we have a lightering casualty involving snapped mooring lines and a possible spill.

The purpose of this project is to

  1. model and quantify this problem,
  2. and propose and study possible alternatives for ameliorating the problem. These include operational changes (lighter into the sea rather than down swell), anti-roll tanks, larger bilgekeels, changes in beam etc.
This project will make use of the ability of CTX_MATE to compute the roll radius of gyration for any given loading pattern. But this has to be combined with a good ship motions program, which CTX needs for a variety of reasons. So the big job here will be to implement such a program at CTX. We will also have to disagregate the lightweight spikes and for the first time use the y and z coordinates.

Probably this is going to point to more beam which in turn points to twin screw.

Email about this project should be sent to roll@c4tx.org.