Abstract

`There are many floating (and fixed) oil platforms that have three or more large diameter legs close together, and many accounts of minor damage to equipment and secondary steelwork caused by ‘green’ water reaching much higher levels than anticipated even in relatively modest storms.

I shall make a link between the theoretical description of ‘near-trapped modes’ – close to standing wave patterns between the legs – and the practical issues highlighted above, taking as an example a simplified model of a large production semi-submersible rig with four closely spaced legs connected by submerged pontoons. The geometry is typical of floating rigs installed in the Gulf of Mexico, the North Sea and elsewhere.

1. Significant near trapping is predicted to occur below the deck of a realistic 4-leg rig in random waves, with water predicted to reach >2x the levels away from the rig.
2. This near-trapping can occur due to linear and nonlinear second order effects, but for realistic large structures in severe storms the frequency-doubling second order effects can be very large.
3. Whether the rig is restrained vertically (a TLP ) or freely floating (a semi-sub), so it can ride the waves, has a dramatic effect on the risk of water reaching the main deck level, even with the same hull geometry.’