Senior project · Preliminary design · Cal Poly
A tidal turbine platform sits about 10 meters below the surface. Our team developed a surface-operated buoyancy concept to bring it up for maintenance and lower it back to its operating depth.
My contributions: concept selection, preliminary buoyancy calculations, and work on the concept prototype.
Sponsored by Ian Bohannon of IM Bohannon Mechanical Design. This page covers the preliminary design and concept prototype; full-scale performance remains to be verified.
The design challenge
The submerged platform is held near its operating depth by catenary mooring chains. To service the turbine, the system must raise the platform, then return it to depth without requiring divers or a remotely operated vehicle.
Selected concept
A technician would connect a compressor to a hose at the surface. Air flows through a manifold into four submerged bladders. As the bladders expand, they displace more water and lift the platform along with additional mooring chain. Venting air lets the platform settle back toward its operating depth.
Protective cages and isolated air branches are part of the preliminary design. Valve selection, bladder materials, and controlled deflation still need analysis and testing.
Concept selection
We compared winch systems, rigid ballast tanks, and inflatable bladder arrangements against criteria including line management, reliability, cost, operation, and exposure to marine growth. The selected inflatable system scored 797.5 in the weighted decision matrix, the highest of eight concepts. It removes submerged winch drums and cable-spooling mechanisms, while allowing separate bladder branches for potential fault isolation.
Preliminary engineering analysis
The platform's upward buoyancy is balanced by suspended chain at its operating depth. Raising it brings more chain off the seabed, increasing the load the bladders must support. The team's first calculation estimates about 72 kN of additional lift and at least 7.15 m³ of total submerged air volume. That estimate excludes bladder and cage weight and design margin.
Near 10 m depth, air is under roughly twice the surface pressure. As the platform rises, the air expands and buoyant force changes. The next analysis will compare that expansion with the changing chain load throughout ascent.
Four independent bladders could limit air loss if one branch fails. The team still needs to show that the remaining three can lift the platform without unacceptable tilt or uneven mooring loads.
Concept prototype
In the small concept model, air transferred between connected balloons and they inflated unevenly. That observation led the team to consider individual pilot-operated check valves to isolate the bladders and a controlled way to vent them during descent. The model demonstrated an issue to solve; it did not validate full-scale lifting performance.
Next verification: test balanced inflation, controlled venting, valve reset, and operation with one bladder disabled in a scaled tank prototype.
Team project with Garrett Caltabellotta, Jonah Hulvershorn, Nick Glenn, and Kai Lambert. Sponsor: Ian Bohannon, IM Bohannon Mechanical Design. Images and technical figures are from the team's preliminary design review materials.