Method and apparatus for air can vent systems

The invention is generally related to risers for floating offshore oil and gas production structures and more particularly to air-can tensioning devices for the risers. An air-can riser tensioning device for use in the production of oil and gas at offshore locations includes at least one air-can used to place a vertical force on the riser. The air-can includes an open port that extends into the water toward the sea floor to a depth sufficient to prevent water from entering or air from leaving the air-can during its upward and downward vertical movements while in use. A manufacturing method includes fabricating the air-cans out of one material and fabricating the open port out of another material, and in particular making the open port from a material that is more corrosion resistant than the material the air-can is fabricated from. The manufacturing method further includes fabricating a passage in one soft tank for fluid communication with a second soft tank, and connecting the second soft tank with the passage.