December 17, 2010

Costs of Floating Breakwaters

As introduced in the book by Patri and Wayne, Breakwaters are really important for ocean colonization: they present significant cost benefits which are essential to make an ocean community of a certain size, not just a single fancy hotel or business park. It is also a subject being discussed in the engineering forums.

The advantage of using a breakwater is that it eliminates all the problems caused by waves. Structures become much cheaper, safer, and easier to expand, seaplanes can land, and cargo is easier to offload. But to do this, you must dissipate the tremendous energy found in ocean waves and do it continuously, for years on end, even during severe storms. If the breakwater fails, suddenly your structures must face waves they were not designed for, which may be disastrous. In principle, the floating breakwaters are the most suitable ones, as the artificial ones resting over the sea bottom are limited to a few meters in depth.

The problem as introduced in a previous blog post is the cost. How much would it cost to establish a floating breakwater in the ocean? Here we show some numbers.

According to FDN Engineering, the floating breakwaters can be categorized into 3 types:

T-Block, used only at locations with water depths up to 6 meters or wave heights up to 1.1 meters.
U-Block, recommended for more severe conditions with water depths up to 12 meters or wave heights up to 2.5 meters.
Heavy Duty U-Block, for even more extreme conditions.

They are shown in the graph below:

The table below gives an overview of the design possibilities of these types of floating breakwaters. These numbers are only indications, as the exact numbers depend on the local conditions and the clients’ wishes. The cost estimate is an estimation of the direct costs of the build of a breakwater, including anchorage and installations, excluding VAT. 

Heavy Duty U-Block
up to 20 m
up to 30 m
50+ m
3 – 4 m
4 – 7 m
7 – 18 m
Height (total)
3 – 4 m
4 – 7 m
7 – 18 m
Water depths
up to 6 m
6 – 12 m
> 12 m
Wave heights
up to 1.1 m
1.1 – 2.5 m
> 2.5 m
Cost estimate
3.000 € / meter
€ 5.000 € / meter
10.000 € / meter

In any case, floating breakwaters are still an unsolved the problem for seasteading purposes because the biggest Heavy Duty U-Block was the semi-floating jetty installed in Monaco hinged to the coast, set in water 55 m deep and with a design maximum wave height of 4.9 m. Also, as already mentioned, according this thesis, for water depths over 60 m the cost of the floating breakwater is unknown. We reproduce here the graph from page 15 of that thesis. The line of cost/m finishes precisely in 55 m, the water depth reached in Monaco, with a cost per meter around 75.000 €.

It still seems a sizable engineering challenge to establish a floating breakwater in the middle of the ocean outside of the EEZ. But on the other hand, and as it was suggested in the previous blog post and in the engineering forum, installing them in a seamount in benign waters might be a possibility.