Coiled tubing is also used in completion programs to convey downhole hardware, fluids and materials. Frequently, wells drilled in unconsolidated sands require the wire mesh screen of a gravel pack (GP) to prevent sand production. Common GP installations involve a washdown procedure. First, the CT string is run to the GP depth. Gravel is then pumped through the coiled tubing. The CT string is retrieved to the surface, and a GP screen assembly is attached. As the cylindrical screen is run to the top of the gravel, fluid is pumped through the CT to agitate the gravel and settle the screen into place across from the perforations (at bottom right). The CT string is then retrieved to the surface. The GP keeps the sand in place while allowing formation fluids to flow through it. Should sanding begin later in the life of a well that does not have a GP, coiled tubing offers a means of installing a through-tubing GP completion, in which GP screens are installed through the existing production tubing without removing the original completion hardware.