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The spar was fabricated using Schedule 40 6” inner diameter PVC pipes, which were used due to PVC’s durability, lightweight, adequate strength, and just as importantly, its excellent corrosion resistance. The ocean is a harsh environment and in order to maximize the functional life of the spar mooring, considerations of material properties were consistently made throughout the design process. PVC was also found to be a generally low-cost option relative to other industrial thermoplastics, especially in the size needed for the spar mooring. In order to create a spar buoy out of the pipes, closed-cell pour foam was used to fill half of every spar segment with the buoyant foam. Making use of Archimedes’ Principle, it was determined that the spar would need to displace a greater volume of fluid without adding too much mass to the system, in other words, decrease the spar’s density. The buoyant force of the spar with the foam was calculated and found to be significantly greater in magnitude than the gravitational force that would otherwise cause it to sink. A 32kg/m3 density foam was concluded to be sufficient in guaranteeing that the spar would float and naturally tend upward towards the surface.
The 10m tall spar mooring was composed of five 2m-long segments. The operational concept was that the segments would be prepared with their fastened ADVs and functional components before being loaded on the boat from which they would be deployed which would minimize the underwater work needed from the divers. The design was developed so that it would accommodate the deployment procedure that would consist of dropping a heavy railroad wheel anchor with the spar base, to which a spar segment would be connected, followed by stacking and inserting the remaining segments, and lastly, attaching the guy wires that stabilized the spar mooring structure.
Access to the ADV mounted inside the spar segment was made possible by the access port cavity that would be capped with a cutout door that mated with the face of the hole where it would be rigidly held in place with a set stainless steel hose clamps. The project sponsor gave insight on the practicality of hose clamps and their prominent use in oceanographic research deployments. As a result, their efficient fastening capabilities were found to be sufficient for many applications on the spar mooring and were used extensively throughout the design. Hose clamps were also utilized for mounting the ADV battery housing in the spar and securing the probe sensor to the holding fixture on the cutout door that would also be subsequently secured to the spar with hose clamps, as mentioned. The five spar segments all had these mounting fixtures and were arranged in a manner that ensured all the probes were in different orientations. This was done considering the turbulent region in the wake of the spar from an incoming flow could make for inaccurate measurements by the sensor probe in the wake. Thus, having the probes protruding out in various directions would ensure the majority of the sensors would always be recording accurate data.
The motivation for the base design was to construct a mechanism that allowed for the spar to be held down by an anchor wheel. The base design had to accommodate the anchoring methods used by the sponsor’s research team, which consisted of using a large railroad wheel that could weigh upwards of half a ton. This posed a challenge in that the anchoring wheel would be the first part thrown overboard during deployment and wherever the wheel was to land, was where the spar mooring would be assembled. The option of moving the wheel once it was on the seabed was not an option due to its immense weight which would be amplified by the water pressure at the seafloor. Furthermore, there was a high likelihood that the surface on which the anchor wheel landed would not be perfectly flat, requiring the spar base to have 2 degrees of freedom so that the spar mooring could be vertically aligned during deployment
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