Wave drift forces

by

Hi there,

A few things to chew on for those who like to think along:

 

Wave drift forces, are forces due to absorption or reflection of waves. Waves carry momentum, and stopping or reversing that, will cause a reaction force. Structures with a small waterplane area, like spars or semi-subs, do not reflect a significant amount of waves; they leave the overall wavefront more or less undisturbed. This is however not the case for large ‘floating island’ type structures, or breakwaters. The wave-drift forces on an (approximately) infinite wall can be calculated from the formula found here. So substituting some numbers, for a breakwater or island, placed in clubstead ocean conditions with a significant waveheight of up to 8 meters (4m aplitude), this translates into almost ten tons of force per meter of structure exposed to the waves.

In shallow waters, these type of forces might be combatted successfully by means of plentiful mooring, although even that is stretching todays technology (link). In deep waters, mooring against these forces does not seem plausible at all (link).

A single ton of thrust requires about 60kW of engine power, which means designing for absolute positioning against these forces by means of powered thrust is not very plausible. Even if tugboats were working at full engine power neck to neck along the entire length of the structure, they still wouldnt be capable of even holding the structure in place.

So absolute positioning does not seem likely, but what about lazy, time averaged station keeping? The dependance of wave-drift forces on wave height is quadratic, so under more typical calm weather conditions, these forces would not be nearly as bad, nor would their time-averaged effect be. Yet given that these large structures would act as a wavebreak, there will be no significant waves on the leeward side. The island will move with unstoppable force, but the potential neighboring seastead island it is moving towards, will experience no such impetus. Hence, having mutiple such large islands drifting in relative proximity would be complicated, to say the least.

 

Your thoughts? Is this a dealbreaker for any wavebreaking type of structure? Or could these problems be dealt with somehow?

8 comments

  1. Joep 2:08 am

    Hi Eelco,

    There’s way too much math in your link for me :) What exactly is "drift force"? Is it the force excerted by a wave only when it’s pushing or the average force excerted after a complete crest / trough cycle is completed?

    Could we use the "aikido" breakwater design (by Patri I guess) on the bottom of this page http://www.seasteading.org/seastead.org/book_beta/On%20The%20Water.html by adding "flaps" that go up and down, letting waves move up and break freely but when they go down stand up to pull the structure towards the direction the waves come from?

     

     

  2. Eelco 7:46 am

     Exactly; drift forces are the net time-averaged horizontal force exerted on the platform by the waves. The particulars of the math in that link are not very relevant; the simplified conclusions at the end are most useful.

    There is essentially a direct relationship between stopping (breaking or reflecting) the waves and drift forces. The travelleing waves are transmitting momentum, and deflecting that will require force, analogously to the way deflecting a moving ball requires force.

    It seems as if islands of this type, or breakwaters, would have to account for experiencing a substantial amount of drift during storms.

  3. Melllvar 12:08 am

    Certainly sounds sobering, here’s some possible work arounds (I make no claim that they will be successful though)…

    1) Build the seastead asymmetrically so that a good portion of the wave energy will be converted to rotational motion of the seastead rather than translational… I highly doubt this will work though, for two reasons: 1) it would require a pretty weird design, and 2) more importantly, it would eventually rotate to find a point where the wave forces on both sides of the seastead would be equal, and full translational motion would resume… it might be possible to "reset" the rotation though (possibly by changing the C of M by switching water between ballast takes at different ends of a seastead).

    2) Don’t put large seasteads close together, or at least keep them far enough a part and sparse enough that tug boats could pull them perpendicular to the direction of the waves long before they would collide.

    3) Put barriers between them to make any collision non-catastrophic (millions of recycled tires possibly?)… but considering the masses and forces involved, I’m not sure you’d find anything strong enough to use as a barrier that wouldn’t be ground away during a prolonged collision.

    4) Connect them so that they all move together… probably unlikely too since that would require a heck of a strong connection between them.

    5) Build it so that one side is made to cut through the waves while the other side is made to resist motion by having a high area pushing through the water (example, the side facing the waves is pointy while the other side drops off to a large depth and acts like a heave plate)… the structure is then rotated to keep up with the direction of the major waves. One problem is that, with waves coming from different directions, it would be impossible to design it to keep up with both 90 degree and 180 degree wave direction differences.

    I realize these aren’t great solutions, buts its all I could think of.

    Edit: Also, its real nice to see more real technical work being done on the engineering aspects. Feel free to keep us updated, this kind of stuff keeps me awake at night (in a good way).

  4. Eelco 7:44 am

    Those are infact very sensible comments; keep em coming. Same here, ive been puzzling with this stuff most of my waking hours over the past days. And the good thing is; instead of getting in the way of my responsibilities as it used to, it IS my responsibiliy. Yay!

     

    Ive considered 2, but im not yet at the point where I can quantify whether or not it would work out though (how long can storms last, how fast and far would that push the structure, and would that all add up to a sensible scenario? hard to tell)

    3 would be possible from a material pov: fancier alternatives to tires exist that could do the job. Yet if they are meant to operate as seperate islands, they would have docks and boats and other fragile structures at their perimiter, so shoring up just for the occasion without crushing lots of stuff might not be possible.

    Ive considered 4, but it has drawbacks. If a seastead community would all need to grow rigidly around the same kernel, that would make rearranging very difficult. 

    Some version of 5 is my favored solution. An elongated island weathervaning into the waves will have far more real estate to justify the cost of station keeping for a given span of structure exposed to the waves. And the shape of the nose facing the waves does indeed matter; as its pointyness goes to infinity, the waves are only minimally deflected, and the wave drift force goes to zero. 

     

     

    One other complication ive run into recenty: its not just big islands that reflect a lot of waves: spar forests do too! Even though an individual spar does not reflect a lot of waves, the force on a cluster of spars is far greater than the sum of its parts. In mathematical-physical terms: the pattern of vertical columns provides a resonant cavity for waves of many wavelengths; these waves will be only partially transmitted, and partially reflected or absorbed. There isnt much literature on the subject, but as is my understanding for now, a sizable cluster of pillars would experience up to half the wave drift forces a solid reflecting structure would. That sucks, as their position is harder to control than that of a simpler single rigid structure to begin with.

    I am more and more starting to lean towards concrete pontoons/caissons as ive been weighing all relevant considerations. A system of modularly connectable concrete caissons arranged into elongated islands/barges seems like the most attractive solution on many fronts. If weathervaning into the waves, roll motion could be kept to a minimum, and by virtue of being made sufficiently long, it would have minimal pitch and heave motions. The big question here is how much concrete would be needed to make this strong enough w.r.t. sagging and hogging forces out in the open ocean, and how much would that cost? It hasnt actually been done before, so its hard to tell, but extrapolating from some other concrete projects, it might be one of the more cost effective options too.
     
    Depending on how the modularity works out, it could start out as a fairly small scale project in protected waters, and when it had grown into an island long enough to be comfortable in international waters could it consider moving there. If modularity proves technically too challenging, then we are essentially talking about a long concrete barge. Doing a good job of being stable in open ocean storms would require a 300-400m length caisson or so, so in that case wed be talking about a sizable upfront investment. Although a squarish concrete caisson would still be cheaper than a steel hull of comparable size, from what I can tell.
     
    Anyway, I digress.

     

  5. Wayne Gramlich 7:05 am

    Does this mean that TSI is finally done investigating breakwaters, or is thie merely wishful thinking on my part?

  6. wesley_Bruce 7:58 am




    Wave powered boats exit. If these are configured to pull up the wave train in the opposite direction to wave drift the two cancel out. Wave powered boats are http://pesn.com/2005/09/21/9600170_Wave_Propulsion/

    and http://www.popsci.com.au/gear-gadgets/article/2008-02/wave-runner

    Most don’t know that wave breaking technology does not need to be continuous a layered defence of buoys will work.

    See: http://www.freepatentsonline.com/4712944.pdf

    and

    http://docs.google.com/viewer?a=v&q=cache:MgyKPZ0lHvsJ:fluid.ippt.gov.pl/ictam04/CD_ICTAM04/FSM4/11758/FSM4_11758_new.pdf+wave+break+array&hl=en&gl=au&sig=AHIEtbQauD4vhtb7dMuEEzpqFnOY3lpiIQ

     

    This creates nodes of high and low wave hight/ energy. The sea steads would loiter in the low nodes and the wave power units in the high nodes.


     

     

    For everyone’s information I was involved with the original Oceania Project in a small way. I’m also in several space organisations. And I have a Degree in sustainable Development, sustainable agriculture and renewable energy,water and sewerage.

  7. wesley_Bruce 8:03 am




    Rexresearch has a round up of weird boats including a

    "Air Bubbles Employed To Form Breakwater"

    Part way down the page at about 15% down.

    See: http://www.rexresearch.com/boats/1boat.htm#wave

     

    We need to test this.  I have seen it mentioned elsewhere. I gather it works best if you time the bubble release to match the frequency of the wave. It undermines the peaks and smooths them.


     

     

    For everyone’s information I was involved with the original Oceania Project in a small way. I’m also in several space organisations. And I have a Degree in sustainable Development, sustainable agriculture and renewable energy, water and sewerage.

  8. ellmer 12:50 am

    I doubt a bit that the tremendous horizontal forces introduced by waves mentioned here do exist in practice…

    When i look around i see no anchor rig designed for such forces.

    Just look at the SIZE of those anchors compared to the size of the ship – do the designers expect them to hold the ship weight? – i doubt it. But they seem to work just fine in practice (they are only using the right anchor in this case – it is just working fine).

    The tremendous forces that lift the ship up – deliver it on the same spot in a circular movement.

    Drifting structures like icebergs follow ocean currents and wind directions – what indicates that those are the dominating forces. This is congruent with practical ship anchor design – anchors hold against wind and current – not wave forces.

    (see also comments on floating breakwater feasibility thread)

     

    Wil

    concretesubmarine.com

    European Submarine Structures AB

The comments are closed.