Under the Sea By Chris Greco

On March 25, 2012, filmmaker James Cameron descended to the deepest place in the ocean, the Mariana Trench’s Challenger Deep in the western Pacific Ocean. He accomplished this task in the Deepsea Challenger submarine, which has been equated to a “vertical torpedo.” Cameron spent a few hours exploring the sea floor while collecting samples and video footage. Cameron’s only disappointment was that he did not see any strange deepwater creatures. I will investigate some of the physics of this event. 

First, let’s look at the pressure associated with Challenger Deep, the deepest point of the Mariana Trench. Challenger Deep is at a depth of 35,756 feet (10843m). This is more than a mile deeper than Mount Everest is tall! At this depth, the density of sea water actually increases by 4.96% due to compression because of the great weight of the sea water above. The normal density of sea water is 1.025 x 10³ kg/m³. The density of sea water at Challenger Deep is (1.025 x 10³ kg/m³) x (1.0496) = 1.076 x 10³ kg/m³. 

We can determine the pressure at Challenger Deep using the equation:

        P = ρgh + pressure of the atmosphere

where P= pressure, ρ= density of sea water, g= 9.80m/s², h=10843m, and the pressure of the atmosphere= 1.013 x 10⁵ N/m² 

So P = (1.076 x 10³ kg/m³)(9.80m/s²)(10843m) + 1.013 x 10⁵ N/m² = 1.144 x 10⁸ N/m² 

To put this in perspective, this pressure is the equivalent of three SUV’s sitting on your big toe!

 

Clearly, Cameron’s Deepsea Challenger submarine had to be well outfitted to withstand these high pressure conditions. The Deepsea Challenger looked like this:

http://deepseachallenge.com/the-latest/page/4/

The submarine was 7.3m long and had a mass of 11.8 tons (10705kg). Cameron sat in a tiny compartment made out of thick steel. The remainder of the vertical column was made out of syntactic foam called Isofloat, a solid composed of hollow microspheres.  

For the sake of calculations, I approximated the submarine to be a cylinder with diameter of 2.0m (I couldn’t find a measurement for the diameter of the submarine, but by looking at scaled images of it, I concluded that its width was a little over ¼ of its length). The radius is 2.0m/2 = 1.0m.

I decided to determine the buoyant force on the submarine when it was submerged in the sea water at Challenger Deep. The buoyant force is the net force on the submarine exerted by fluid pressure and acts upward.  

 F₁

                         ↓
                   Cylinder

 ↑  

                        F₂

F_B = F₂-F₂ = ρ_FgA(h₂-h₁) = ρ_FgAΔh

The density of the fluid (compressed sea water) = 1.076 x 10³ kg/m³, g= 9.80m/s², A = πr² = π(1.0m)² = π m², Δh = 7.3m 

So:

F_B = (1.076 x 10³ kg/m³)(9.80m/s²)(π m²)(7.3m) = 2.4 x 10⁵ N 

Finally, despite its capabilities to withstand intense pressure, the Deepsea Challenger actually compressed and lost three inches (0.0762m) in length under the pressure at Challenger Deep. Normally, when an object is submerged in a fluid (which exerts a pressure on the object in all directions), the object is subjected to inward forces from all sides and its volume will decrease. I read in the textbook that when there is a volume change (as when an object is submerged in a fluid), we use the Bulk Modulus B. However, the only data I could find was that the length of the Deepsea Challenger changed, not its radius. Therefore, although the volume of the submarine changed due to the change in length, we can also just examine this problem through the Young’s Modulus E since the submarine was compressed (force acted inwardly on the object) with just a change in its length. This compression is due to the force of the pressure at Challenger Deep. We can use the Young’s Modulus E to approximate the magnitude of this force. The submarine’s volume was composed of 70% Isofloat and this would therefore be the most appropriate Young’s Modulus E value to use. However, I could not find a value for the Young’s Modulus E of Isofloat so I will instead use the second most abundant material composing the submarine, steel. For this calculation, we assume that the whole submarine is made of steel.

Young’s Modulus:

ΔL = (1/E)(F/A)L₀    or     F/A = E(ΔL/L₀)    or   E = (F/A) / (ΔL/L₀)
where L₀ is the original length of the object, A is the cross-sectional area, ΔL is the change in length due to the applied force F, and E is the Young’s Modulus.

So for the Deepsea Challenger:

L₀ = 7.3m
ΔL = 0.0762m
A= πr² = π(1.0m)² = π m²
Young’s Modulus E = 200 x 10⁹ N/m² (for steel)

So the applied force of the pressure that causes the change in length can be calculated using:

F=E(ΔL/L₀)A = (200 x 10⁹ N/m²)(0.0762m/7.3m)(π m²) = 6.6 x 10⁹ N (a very large force!).

Note: The Young’s Modulus E of Isofloat is likely smaller than the Young’s Modulus E of steel so the applied force of pressure is realistically smaller than this calculated force.

 

Calculating the approximate Bulk Modulus of the Submarine:

When an object is submerged in a fluid (which exerts a pressure on the object in all directions), the object is subjected to inward forces from all sides and its volume will decrease. I read in the textbook that when there is a volume change (as when an object is submerged in a fluid), we use the Bulk Modulus B (although we didn’t discuss it in class).

Since the length of the submarine was reduced by 0.0762m at Challenger deep, the volume (V= πr²h) of the submarine also changed:

Volume of Deepsea Challenger at Surface of Ocean= V₀ = π(1.0m)²(7.3m) = 22.9m³
Volume of Deepsea Challenger at Challenger Deep = V_f = π(1.0m)²(7.3m-0.0762m) = 22.7m³      
ΔV= V_f-V₀ = -0.239 m³

Pressure at Surface of Ocean = P₀ = ρgh + pressure of the atmosphere

               = 1.076 x 10³ kg/m³)(9.80m/s²)(0.00m) + 1.013 x 10⁵ N/m² = 1.013 x 10⁵ N/m²
Pressure at Challenger Deep = P_f = 1.144 x 10⁸ N/m²
ΔP = P_f – P₀  = 1.143 x 10⁸ N/m²

Bulk Modulus:

ΔV/V₀ = -(1/B)ΔP    or  B = -ΔP/(ΔV/V₀)
where ΔV is the change in volume, V_o is the original volume, ΔP is the change in pressure, and B is the bulk modulus. 
The minus sign means that the volume decreases with an increase in pressure.   

Using our knowledge of the situation, we can calculate the Bulk Modulus B of the submarine. Since the submarine was composed of mostly Isofloat (70% of the volume of the submarine was Isofloat), we can say that the Bulk Modulus we calculate is most closely related to the Bulk Modulus of Isofloat. 

B = -(1.143 x 10⁸ N/m²) / (-0.239 m³/22.9m³) = 1.1 x 10¹⁰ N/m²

This Bulk Modulus for Isofloat is large, but it is smaller than the Bulk Modulus of steel (140 x 10⁹ N/m²). This supports our above prediction that the Young’s Modulus of steel is larger than the Young’s Modulus of Isofloat and consequently caused our applied force of the pressure on the submarine to be slightly larger than it should realistically be. 

Referenced Article:

http://news.nationalgeographic.com/news/2012/03/120325-james-cameron-mariana-trench-challenger-deepest-returns-science-sub/

Video Clip:
http://www.youtube.com/watch?v=Y2tm40uMhDI