The invention relates generally to remedial systems and methods for subsea structures. More particularly, the invention relates to systems and methods for pulling subsea structures such as primary conductors that have been bent from vertical.
In offshore drilling operations, subsea wells are built up by installing a primary conductor in the seabed and then securing a wellhead to the upper end of the primary conductor at the sea floor. A blowout preventer (BOP) is then installed on the wellhead, and a lower marine riser package (LMRP) mounted to the BOP. The primary conductor is typically installed in a vertical orientation to facilitate and simplify the installation of the BOP and LMRP onto the wellhead, which is coaxially aligned with the primary conductor. A lower end of a drilling riser is coupled to a flex joint on the top of the LMRP and extends to a drilling vessel or rig at the sea surface. A drill string is then suspended from the rig through the drilling riser, LMRP, BOP, wellhead, and primary conductor to drill a borehole while successively installing concentric casing strings that line the borehole. The casing strings are typically cemented at their lower ends and sealed with mechanical seals at their upper ends.
During drilling operations, drilling fluid, or mud, is delivered through the drill string, and returned up an annulus between the drill string and casing that lines the borehole. In the event of a rapid influx of formation fluid into the annulus, commonly known as a “kick,” the BOP and/or LMRP may actuate to seal the annulus and control the well. In particular, BOPs and LMRPs comprise closure members capable of sealing and closing the well in order to prevent the release of high-pressure gas or liquids from the well. Thus, the BOP and LMRP are used as safety devices that close, isolate, and seal the wellbore. Heavier drilling mud may be delivered through the drill string, forcing fluid from the annulus through the choke line or kill line to protect the well equipment disposed above the BOP and LMRP from the high pressures associated with the formation fluid. Assuming the structural integrity of the well has not been compromised, drilling operations may resume. However, if drilling operations cannot be resumed, cement or heavier drilling mud is delivered into the well bore to kill the well.
In the event that the BOP and LMRP fail to actuate or insufficiently actuate in response to a surge of formation fluid pressure in the annulus, a blowout may occur. The blowout may damage subsea well equipment and hardware such as the BOP, LMRP, or drilling riser. For example, falling debris (e.g., a severed riser) resulting from a blowout may bend the primary conductor from the “as installed” vertical orientation. Bending of the primary conductor can also arise if the surface vessel drifts too far and exerts sufficiently large lateral loads on the LMRP and BOP via excessive tension applied to the riser extending from the surface vessel to the LMRP. In general, if the bending loads and associated stresses do not exceed the yield strength of the material forming the primary conductor, the primary conductor will not plastically deform and should rebound to its vertical orientation when the bending loads decrease sufficiently. However, if the bending loads and associated stresses exceed the yield strength of the material forming the primary conductor, the primary conductor will plastically deform and become permanently bent (i.e., the primary conductor will not rebound to its vertical orientation when the bending loads decrease).