Hydraulic pressure assisted casing tensioning system

In an offshore well system having a subsea wellhead and a surface wellhead which is located on a platform, the system having a removable riser with a blowout preventer extending upward from the surface wellhead, the improvement comprising in combination:

• an internal load shoulder located in the surface wellhead;
• a tubular mandrel having a lower end which is secured to a section of tieback casing;
• a casing hanger having an external shoulder;
• a tubular extension pipe secured to the casing hanger and extending downward around the mandrel;
• gripping means between the extension pipe and the mandrel for allowing upward movement of the mandrel relative to the extension pipe but preventing downward movement of the mandrel relative to the extension pipe;
• a conduit which extends upward from the mandrel through the extension pipe and the surface wellhead;
• and
• means for sealing between the extension pipe and the surface wellhead and sealing between the casing hanger and the conduit in a manner which allows downward movement of the casing hanger relative to the mandrel in response to hydraulic pressure;
• whereby
• the blowout preventer may be closed around the conduit to provide a sealed annulus in the riser below the blowout preventer around the conduit, so that hydraulic pressure may be applied to the annulus to force the casing hanger and the extension pipe downward relative to the mandrel onto the load shoulder;
• and wherein,
• the conduit and mandrel may be pulled upward relative to the casing hanger and the extension pipe after securing the lower end of the tieback casing to the subsea wellhead to apply tension to the tieback casing and the extension pipe, so that the gripping means can grip the mandrel to support the tieback casing and the extension pipe in tension once a desired amount of tension is reached.

In an offshore well system having a subsea wellhead and a surface wellhead which is located on a platform, a removable riser string and a blowout preventer extending upward from the surface wellhead, the improvement comprising in combination:

• an internal load shoulder located in the surface wellhead;
• a tubular mandrel having a lower end which is secured to an upper end of a section of tieback casing, the mandrel having a plurality of circumferentially extending external grooves;
• a casing hanger having an external shoulder and an extension pipe which extends downward from the casing hanger and surrounds at least an upper portion of the grooves of the mandrel;
• a ratchet ring carried by the extension pipe in engagement with the grooves on the mandrel;
• a conduit which extends upward from the mandrel through the extension pipe and the surface wellhead;
• an inner seal located between the casing hanger and the conduit;
• an outer seal located between the extension pipe and the surface wellhead;
• the casing hanger and the extension pipe having an extended position relative to the mandrel while the string is lowered through the riser and blowout preventer and secured to the subsea wellhead, the extended position locating the external shoulder of the casing hanger above the load shoulder while the lower end of the string is securing to the subsea wellhead;
• means for applying hydraulic pressure to an annulus in the riser around the conduit above the inner and outer seals and below the blowout preventer while closed to force the casing hanger and the extension pipe from the extended position downward relative to the mandrel onto the load shoulder;
• and
• latch means for holding the external shoulder of the casing hanger on the load shoulder, allowing the conduit and mandrel to be pulled upward to apply tension to the extension pipe and the tieback casing, so that the gripping means can grip the mandrel to support the extension pipe and the tieback casing in tension once a desired amount of tension is reached.