Floating equipment is generally used in the lower section of the well to help:
Reduce strain on the derrick while guiding casing past ledges and slough zones in the hole;
Provide a landing point for bottom and top cementing plugs (pumped ahead of and behind the slurry as part of cementing operations);
Provide a backpressure valve to prevent cement from flowing back into the inner diameter of the casing after the cement has turned the corner into the annulus and the top plug has been bumped
Basic floating equipment includes the float collar and either the guide shoe or float shoe:
The guide shoe runs on the first joint of casing to be run into the hole to help maneuver the casing past annular irregularities. The guide shoe includes side ports and an open end to enable fluid circulation for mud conditioning, hole cleaning, and cement placement.
The float shoe contains a backpressure valve that prevents fluids from entering the casing while the pipe is lowered into the hole and prevents cement from flowing back into the casing after placement, while enabling circulation down through the casing.
Float collars are placed one to three joints above the guide shoe or float shoe. They provide a seat for the cement plugs, the bottom plug pumped ahead of the cement and the top plug behind the full volume of slurry. Once seated, the top plug shuts off fluid flow and prevents over-displacement of the cement. The space between the float shoe and the float collar provides a containment area to entrap the likely-contaminated fluids from the wiping action of the top cementing plug, securing the contaminated fluid away from the shoe where a strong cement bond is of primary importance. Float collars include a backpressure valve and serve basically the same function as the float shoe.