All subsea casing hangers are mandrel type, as shown in Fig. 5. The casing hanger provides a metal-to-metal sealing area for a seal assembly to seal off the annulus between the casing hanger and the wellhead. The casing weight is transferred into the wellhead by means of the casing hanger/wellhead landing shoulder. Each casing hanger stacks on top of another, and all casing loads are transferred through each hanger to the landing shoulder at the bottom of the subsea wellhead. Each casing hanger incorporates flow-by slots to facilitate the passage of fluid while running through the drilling riser and BOP stack, and during the cementing operation.