Structural casing (also known by some operators as conductor casing, surface casing, or surface conductor) is defined as the first string of casing installed in the well construction process. By definition, it provides structure and support for all the casing strings, the subsea Christmas tree, and blowout preventer (BOP) stack. Structural casing must be able to resist bending moment loads imposed on it by the mobile offshore drilling unit (MODU) and by future production workover operations. In certain areas with shallower water, the casing may also need to resist loads imposed by trawler net entanglement. The casing must be installed reasonably straight/vertical, usually with less than 1-degree angle, to avoid drillstring wear on wellhead and BOP components.
Jetting of structural casing has become the preferred method of installation in most deepwater environments where seafloor sediments allow the technique to be used. Operators have found the technique to be faster than the historical method of drilling a rathole and cementing the casing in place. Some deepwater basins, however, have harder seafloor sediments, boulders, or rubble zones that prevent jetting from being an effective technique.
The history of jetting can be traced to the first floating rigs used in the U.S. GOM in the 1960's. Minton (1967) describes the installation process used to install structural casing from the first floating rigs developed in the early 1960's. One hundred feet of 29½ in. × 1.0 in. wall thickness casing was set using a combined drive-jet process. The casing was connected to the drive-jet bottomhole assembly (BHA) by means of a J-slot tool with a 3-ft stroke to allow driving action with the BHA. The BHA consisted of 5½-in. drillpipe with two 22-in. leaded drill collars, weighing a combined 60,000 lbm, to impart impact and add additional penetrative weight into the sediments. Fig. 1 illustrates this assembly. Jetting occurred through a jet sub (no bit or motor) and returns were taken outside of the structural casing. Fig. 2 depicts the process and shows both fluid and jetted solids were forced to the mudline along the outside of the casing. Minton alludes that, even from the early days of floating drilling, settling of structural pipe has been a concern.
The jetting technique has spread to other geologic basins/geographical areas around the world. Salies et al. (1999) state that jetting of 30 in. structural casing began in the Campos basin of Brazil in 1993. The use of jetting to set structural pipe in the deepwater of West Africa countries such as Angola, Nigeria, and Congo emerged in the middle-to-late 1990's. Operators in deepwater basins off Trinidad, Canada, Australia, and southeast Asia have all adopted structural-casing jetting as the preferred method of installation.
Beck, R.D., Jackson, C.W., and Hamilton, T.K. 1991. Reliable Deepwater Structural CasingInstallation Using Controlled Jetting. Paper SPE 22542 presented at the SPEAnnual Technical Conference and Exhibition, Dallas, 6-9 October. doi:10.2118/22542.