Internal riser tensioning system

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

• an internal load shoulder located in an axial bore in the surface wellhead assembly;
• a tubular mandrel which has a lower end secured to a section of casing;
• a hanger having an external shoulder which lands on the internal load shoulder;
• a latch mounted to the hanger for latching to a mating groove in the bore to retain the hanger on the internal load shoulder, the hanger extending downward around the mandrel;
• a gripping mechanism between the hanger and the mandrel for allowing upward movement of the mandrel relative to the hanger but preventing downward movement of the mandrel relative to the hanger;
• a running tool which releasably engages an interior portion of the mandrel;
• running string having a lower end secured to the running tool for lowering the casing into engagement with the subsea wellhead;
• a launch adapter having a tubular outer body releasably secured to an upper end of the hanger by rotational movement, the launch adapter slidably and sealingly engaging the bore of the surface wellhead assembly and slidably and sealingly engaging the running string, the launch adapter having an internal engagement lug;
• whereby
• the blowout preventer may be closed around the running string and hydraulic pressure applied to the launch adapter to force the launch adapter and hanger downward relative to the mandrel until the shoulder of the hanger lands on the internal load shoulder;
• the running tool and mandrel may be pulled upward relative to the hanger after the lower end of the casing has been secured to the subsea wellhead to apply tension to the casing, the tension being held by the gripping mechanism;
• and
• the running tool may be released from the mandrel and brought upward into contact with the engagement lug by lifting the running string, wherein rotating the running string releases the launch adapter from the hanger for retrieval along with the running tool.

In an offshore well system having a subsea wellhead and a surface wellhead assembly which is located on a platform and which has an axial bore, the improvement comprising in combination:

• a load support mechanism which is radially movable from an extended position protruding into the bore of the surface wellhead assembly and a retracted position retracted from the bore of the surface wellhead assembly;
• a tubular mandrel which has a lower end secured to a section of casing;
• a hanger having an external shoulder which lands on the load support mechanism while in the extended position, the hanger having a lower portion extending around the mandrel;
• a gripping mechanism between the lower portion of the hanger and the mandrel for allowing upward movement of the mandrel relative to the hanger but preventing downward movement of the mandrel relative to the hanger;
• a running tool which is lowered on a conduit from the platform, the running tool releasably engaging an interior portion of the mandrel for securing the lower end of the casing to the subsea wellhead;
• wherein the running tool and mandrel may be pulled upward relative to the hanger after the lower end of the casing has been secured to the subsea wellhead to apply tension to the casing, the tension being held by the gripping mechanism to allow removal of the running tool;
• and
• wherein to retrieve the casing, the running tool may be again lowered into engagement with the mandrel and the load support mechanism moved to the retracted position to relieve the tension in the casing.

A method for connecting a casing string between a subsea wellhead and a surface wellhead assembly located on a platform, comprising:

• providing an internal load shoulder within an axial bore in the surface wellhead assembly;
• attaching a lower end of a mandrel to the casing string;
• mounting a running tool to a running string and releasably engaging the running tool with the mandrel;
• providing a hanger which has an external shoulder and providing the hanger with an internal gripping member which engages the mandrel to allow upward movement of the mandrel relative to the hanger but prevent downward movement of the mandrel relative to the hanger;
• releasably securing a launch adapter to an upper end of the hanger, the launch adapter slidably and sealingly engaging the bore of the surface wellhead assembly and slidably and sealingly engaging the running string;
• connecting a riser and a blowout preventer to the surface wellhead and lowering the casing string through the riser, blowout preventer and surface wellhead;
• securing a lower end of the casing string to the subsea wellhead while the external shoulder of the hanger is spaced above the load shoulder;
• then
• closing the blowout preventer around the running string and applying hydraulic pressure to the launch adapter, which forces the hanger and the lower extension downward relative to the mandrel until the external shoulder lands on the load shoulder;
• then
• pulling upward on the running string and the mandrel while maintaining the external shoulder of the hanger on the load shoulder to apply tension to the casing string, and once a desired amount of tension is reached, relaxing the pull, causing the gripping member to grip the mandrel to support the casing string in tension;
• then
• releasing the running tool from the mandrel and pulling it upward with the running string into engagement with the launch adapter;
• then
• releasing the launch adapter from the hanger and retrieving the running string, launch adapter and running tool.

A method for connecting and removing a casing string between a subsea wellhead and a surface wellhead assembly which has a bore and which is located on a platform, comprising:

• (a) providing a load support mechanism within the surface wellhead assembly which is movable from an extended position protruding into the bore and a retracted position retracted from the bore;
• (b) attaching a lower end of a mandrel to the casing string;
• (c) mounting a running tool to a running string and releasably engaging the running tool with the mandrel;
• (d) providing a hanger which has an external shoulder and providing the hanger with an internal gripping member which engages the mandrel to allow upward movement of the mandrel relative to the hanger but prevent downward movement of the mandrel relative to the hanger;
• (e) securing a lower end of the casing string to the subsea wellhead;
• (f) landing the external shoulder of the casing hanger on the load support mechanism while the load support mechanism is in the extended position;
• (g) pulling upward on the running string and the mandrel to apply tension to the casing string, and supporting the tension through the external shoulder of the casing and the load support mechanism;
• then
• (h) releasing the running tool from the mandrel and retrieving the running string;
• then, to retrieve the casing,
• (i) running the running tool back into engagement with the mandrel with the running string and pulling upward on the running string;
• then
• (j) retracting the load supporting mechanism and lowering the running string to remove the tension in the casing;
• then
• (k) releasing the casing from the subsea wellhead and retrieving the casing with the running string.