Suggestions:
1. Reference
Fink D.G., Beaty H.W., "Standard Handbook for Electrical Engineers," 14th Ed., McGraw-Hill, 2000,  Section 27 Lightning and Overvoltage Protection
(for background on lightning and references)
2. Industry Standards.
http://ewh.ieee.org/soc/pes/lpdl
3. Safety code. If you have all steel structure floating on the water, then the lightning system is not required. However, if the structure contains, non-metallic (it means less conducting) layers as electrical isolator, then you need the lightning system.
4. Essentially, to avoid a direct lightning strike to your gas vent, an additional and separate taller structure with a lightning system would have to be built.

On a ligh gas vent (onshore),  I had a small 1" steam line run up the vent with a valve at the vent's base. When a storm approached one of the operators would crack open the the valve. Solve our problem of having a vent fire with every thunder/electrical storm (rain and snow).