Sea pen Engineering prototype ideas
Possible Materials
Then pen used to house Keiko was made of foam-filled polyethylene pipes, which were designed to float on the surface with the currents and also resist the pressure of any ice that might form. A company called Spectra-Net made the netting used to set the perimeter of the pen. The material they chose was graphite and Kevlar for high strength. The issue here is that this is still netting, which can lead to a risk of entanglement. To reduce the risk of entanglement, a closed wall pen would be the best solution. This would be different than Keiko’s pen, which was designed to move. A closed wall pen will certainly need to be anchored to keep it in place, which would drive the cost up as well. The proposed areas where this sea pen would be constructed are salt water. This means that any material used in construction would have to be corrosion resistant. Possible materials for the walls would be stainless steel or plastic. Stainless steel is more expensive and actually can corrode if the saltwater strips the thin passive layer off of the surface, so plastic would be more ideal for this application. Three types of plastics that would hold up in this environment are Polyvinyl Chloride (PVC), Polyethylene Terephthalate (PET), or Isoplast. All three offer resistance to seawater, with Isoplast being the strongest of the three. Isoplast is also more expensive, so a cost/benefit analysis would need to be done to determine if the extra cost would be worth the added longevity of the walls.
Water Filtration
Another design consideration would be filtration in the pen (will also be discussed below). A suggestion to increase the water flow in the pen would be to drill or cut holes into the walls to allow water to flow through. This offers the functionality of a net without the problem of entrapment, since plastics are much more rigid. Hole size would have to be worked out. A lot of little holes could create a sort of permeable membrane, where fresh water can flow through the pen with the current but the holes won’t be big enough for fish to get stuck in. This, or some sort of filtration, must be included so the water does not stagnate.
Potential Issues
A possible issue that must be considered is the embrittlement of the material at lower temperatures. Plastics are more sensitive to changes in temperature than metals such as aluminum. Steels also exhibit a similar sensitivity to temperature. Using Neah Bay as a reference point, the temperature in this area is very cold. The range for the year can go from 45 °F to 53 °F, with a yearly average of 50 °F. More tests on the chosen material would need to be performed at these temperatures to ensure that the strength requirements will not be compromised by the low temperatures.
How would it be anchored?
The construction of the pen would ultimately determine the anchoring necessary. The advantage of a pen with netting is that it would not necessarily need to be anchored. Assuming a net-walled pen is ruled out, new ideas have to be researched for securing the pen to the ocean floor. As suggested above, to avoid entrapment, some sort of solid material possibly with little holes drilled in it will be necessary. ~more information necessary~
