ME168: MECHANICS OF OFF-SHORE SYSTEMS

What are State-of-the-art Challenges of Offshore Systems?

Oceanic Waves and Offshore Structures

One of the major challenges of offshore structure design and analysis is the survivability against high momentum of storm waves. Storm waves are strong oceanic surface waves whose restoring force is the gravity pulling. If the restoring force is surface tension waves are called capillary waves (usually very short waves). If both gravity and surface tension play a leading order role, waves are called capillary-gravity waves. Capillary forces are in charge of generating small ripples and spray as in seen in the following photos.

A large offshore structure encountering waves (credit)                                                                                                                                               (credit)

An offshore Structure in North Sea (credit)                                                               Powerful Waves vs Lighthouse (Photo: Jean Guichard)

Off-shore Structures in Arctic

Working in the arctic waters poses new challenges to offshore engineering. Ice is a very complicated form of matter. The ice texture (and as a result mechanical properties) can be very much different from location to location and at each location from time to time. Native arctic people reportedly have about 180 words for different form of snow and ice. The mechanical property added to the variability of thickness, roughness and if the ice is connected or broken makes the problem even more complicated.

Offshore Structure in Arctic Waters (credit)

The Coast Guard Cutter Healy approaches the Russian-flagged tanker Renda while breaking ice around the vessel 250 miles south of Nome Jan. 6, 2012. The two vessels departed Dutch Harbor for Nome on Jan. 3, 2012. U.S. Coast Guard photo by Petty Officer 1st Class Sara Francis. (credit)

Off-shore Renewable Energy

Wind and ocean wave energy (wave energy, tide energy and ocean thermal energy) resides offshore. The major issue making these efficient renewable energy  resources expensive is survivability in the harsh ocean environment.

Artist rendering of an offshore wind tower in a mild storm

Gigantic Pelamis (first commercially deployed ocean wave energy converter) in waves (credit)

Mooring

There are a variety of offshore system designs based on the water depth at which these structures operating as well as other functionality requirements for these structures. If the offshore structure is not rigidly connected to the bottom, then mooring is necessary (taut or slack). Mooring lines are very long (in some cases kilometers) and have their own dynamics.

Different Types of Offshore Structures based on Operating Water Depth (credit)

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And finally, If you do not take ME168:

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