Calculate Tidal Force on Underwater Artwork Sculpture in Philippines

[James8626]

Can anyone assist. I'm workingg on a project for the installation of an underwater artwork sculpture. How do I calculate the tidal force on the structure to stop it moving on the seafloor. It's not anchored to the seafloor only sitting on it. Also what safety factor should be built into it for typhoon strength waves. The structure is going to be installed in the Philippines as a diving attraction

 

[anorlunda]

That is a very difficult question. I don't know how to do engineering calculations on something like that.

The forces of Typhoon waves depend on many factors, including the local shape of the sea bottom including sand bars. Sometimes, very heavy rocks get thrown on to shore, most times not.

I know that other projects built scale models of the sea, rather than calculating.

We have many knowledgeable engineers here on PF. Maybe one of them can give you a better answer than this.

 

[fresh_42]

I think the crucial factor is: how deep? Wind induced waves are to my best knowledge restricted to the surface, in contrast to e.g. tsunamis. This doesn't mean you couldn't have big amplitudes as in typhoons, but then again depth is the quantity that matters. IMO it's a more a question of meteorology than it is an engineering question.

 

[256bits]

diving attraction

You didn't say snorkeling, so maybe you mean scuba diving, which means a greater depth and farther out from shore.
Any indication of the underwater water currents during tides and storms at that particular depth?
Maybe you can estimate the worst, ie the velocity of the water and subsequent force on your art, if no data available, for toppling or twisting.
That part is probably doable to some extent by calculation for your member cross section and joint weld strength.
But as anorlunda eluded to, transfer of say sand or mud undercutting the base of the art could make it unstable after a storm.
In which case, a safety factor for that situation would be greatly unknown.

 

[Nik_2213]

These may help...
https://en.wikipedia.org/wiki/Wave_base
"At depths greater than half the wavelength, the water motion is less than 4% of its value at the water surface[2] and may be neglected."
https://en.wikipedia.org/wiki/Wind_wave
quote from latter:
wind 92 km/h (57 mph) fetch 2,627 km (1,632 mi) for 69 hr wave height 14.8 m (49 ft) wave length 212.2 m (696 ft)

 

[CWatters]

Perhaps you can make a model and test it in a water tank? Perhaps a university would be interested in taking it on as a project?

 

[JBA]

It has been a number of years since I was required to analyze the hydrodynamic drag forces no an exposed subsea pipeline but below is a summary of the steps used in that process.

The first step is to determine the area and the depth where want to install your structure and then probable storm for you want your sculptures to survive i.e. 10 yr, 50 yr or 100 yr. With that information there are references that can give you the predicted maximum predicted wave heights for those conditions. From there you can determine the wave circulation velocities and direction at your selected location; as well as any prevalent strong cross currents at that location. The next item is to determine the orientation of your sculpture to the predicted storm wave's and cross current's direction. With all of that established there are equations for predicting the hydrodynamic drag force on your sculpture(s). Depending upon what you find, there are two choices, either relocate your structure to a deeper area as given in post #5; or, to design a sufficiently large and heavy structure base or a piling support that is sufficient to resist the anticipated hydrodynamic drag loadings.

 

[anorlunda]

I just saw a report that says that Hurricane Irma sandblasted algae off the coral reef at depths of 45-65 feet (14-20 m) in the Florida Keys.

 

[berkeman]

Welcome to the PF, James.

How do I calculate the tidal force on the structure to stop it moving on the seafloor. It's not anchored to the seafloor only sitting on it.

That would seem to be a very problematic design constraint, and maybe insurmountable. Is there a way to apply for a permit to allow you to sink an anchorage point into the sea floor there? What is the composition of the sea floor there? Sand, rock, coral, etc.?

 

[anorlunda]

Welcome to the PF, James.

That would seem to be a very problematic design constraint, and maybe insurmountable. Is there a way to apply for a permit to allow you to sink an anchorage point into the sea floor there? What is the composition of the sea floor there? Sand, rock, coral, etc.?

One could easily make the anchor stronger than the art work; which makes it pointless. Also, anchoring does nothing to protect against "sandblasting."
If the art is at all fragile, the best remedy is to remove it from the water before the storm gets there. Indeed, a floating art work that uses an anchor to pull it down to the bottom, could be easily retrieved from shore in advance of a storm.

 

[CWatters]

If it's a diving attraction there won't be strong currents all the time, just the occasional storm to worry about.

 

[johnbbahm]

Perhaps a more empirical approach might be better than the analytical one.
This has been done before, and there are even videos describing the efforts.

Most artists would likely talk to you about what the complications are, the worst they could say is,
"no" they won't talk to you.

 

[anorlunda]

Very relevant video @johnbbahm. Some of those sculptures weigh 60 tons. I suspect many of them are located in sheltered lagoons.

Unfortunately, the OP has not been back here since posting this thread, so we probably will not learn more about the art or the location.