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May 15, 2010

Advantages of offshore concrete structures

Although traditional shipbuilding uses steel for almost the whole structure, concrete has also been used in the past in a few vessels. But in the last years, the offshore industry has been discovering the great possibilities that concrete presents as compared to steel, and it is being used in more and more projects, such as floating LNG terminals and in the development of the MOB concept (Mobile Offshore Base), as shown on the picture.

Concrete present a series of advantages of a big importance when talking about a structure for permanent use in the middle of ocean. These advantages supported by examples in Ref. [1], are:

Lower maintenance costs.

  • This fact is supported by studies of floating concrete docks back in the 1970’s, showing dramatic savings, requiring less than 10% the maintenance of similar all-steel docks.
  • Sare and Yee report negligible repair and maintenance costs for the 19 pre-stressed concrete barges constructed in the Philippines during 1964-66 for Lusteveco. The average annual maintenance costs of the concrete barges are found to be about 1/3 compared to steel barges.

Lower fabrication costs.

  • The fabrication cost of Yee’s barges showed a saving of 16 percent compared to that of steel.

Downtime of the structure.

  • In Yee’s barges, in the period 1974 to 1975, the total downtime per floating barge per year for maintenance work was six days for the concrete structures. The similar steel barges had an average downtime of 24 days.

Longer life of the structure.

  • In a concrete structure, there is not a significant additional cost related to extension of design life from for instance 30 years to 50 years or 70 years. One reason is the fact that reinforced and pre-stressed concrete is not sensitive to fatigue.

Better motion behavior.

  • The motion characteristics of a concrete hull are typically better than for a steel floater designed for the same purpose. This conclusion can be drawn based on reports from ship captains (World War II ships and Yee’s barges), several studies and recently confirmed by both analyses and model testing for very large FPSO’s (BP Atlantic Frontier Stage 2 / Schiehallion, hull length 280 m). The generally larger mass and draught, result in improved motion characteristics.

In the table below summarized from a similar one in Ref. [2] for floating LNG terminals, we can see the already mentioned advantages (and even more) of concrete compared to the ones for steel:

ADVANTAGES FOR CONCRETE HULLS ADVANTAGES FOR STEEL HULLS
Reduced Down-Time due to Inspection Fabrication in Existing Shipyards
Reduced Maintenance Costs Potentially Lower First Cost for One Hull
Economies of Scale Traditional Engineering
Good Impact Resistance Traditional Construction
Low Center of Gravity/Good Station Keeping Behavior/Reduced Motions More Steel Fabricators are Available
Excellent Fatigue Life More Steel Designers are Available
High Mass Moment of Inertia
Prestressing Not Required
Slower Thermal Response/Better Insulation
Resistance to Fatigue and Crack Propagation
Resistance to Buckling

In conclusion, it is expected that concrete could be a good alternative to steel for using in the structures for colonizing the oceans.

References:

[1] Sandvik, Knut; Eie, Rolf; Advocaat, Jan-Diederik (of Aker Kvaerner Engineering & Technology AS); Godejord, Arnstein; O.Hæreid, Kåre; Høyland, Kolbjørn; Olsen, Tor Ole (of Dr.techn.Olav Olsen a.s). Offshore Structures – A new challenge. How can the experience from the marine concrete industry be utilized. Acapulco: XIV National Conference on Structural Engineering, 2004.

[2] Berner, Dale; Gerwick, Ben C. Large Floating Concrete LNG/LPG Offshore Platforms. U.C. Berkeley, 2000