A Circulating Swage or Side Entry Sub is an adapter that enables a temporary circulating line to be rigged to the top of the casing string. This allows circulation of fluids to help properly locate the casing string.
As shown in these photos, the typical Circulating Swage is a Crossover with a third connection, mid-point on one side. The two end connections are usually premium threads, to fit into the string. The side connection is usually a WECO (hammer union) connection. Note that the Subs in these photos are manufactured from one solid piece of steel.
Stewart Tubular Products will manufacture Circulating Swages to any specified length - inch or metric. Premium or API threaded end connections for any size, weight or grade may be provided.