Jetting of Structural Casing in Deepwater Environments: Job Design and Operational Practices

Abstract

Structural casing jetting operations have become commonplace in deepwater environments. Having originated in the Gulf of Mexico, structural casing is now jetted in most deepwater basins in the world. Very little literature has been published regarding jetting practices. Indeed, jetting operations remain as much an art as a science to a large degree. This paper reviews current deepwater structural casing jetting job design and operational practices. Also included are several case histories of jetting failures presented in order to share learnings with the industry and to stimulate the sharing of learnings and practices. Introduction Structural casing 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 BOP stack. Structural casing must be able to resist bending moment loads imposed upon it by the mobile offshore drilling unit (MODU) and by future production workover operations. The casing must be installed reasonably straight/vertical, usually with less than 1° angle, to avoid drillstring wear on wellhead and blowout preventer (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 utilized. 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, do have harder seafloor sediments, boulders, or rubble zones that prevent jetting from being an effective technique. The history of jetting can be traced back to the first floating rigs used in the U. S. Gulf of Mexico in the 1960's. Minton1 describes the installation process used to install structural casing from the first floating rigs developed by Shell Oil Co. in the early 1960's. One hundred feet of 29–1/2-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) via a J-slot tool with a 3-ft stroke to allow driving action with the BHA. The BHA consisted of 5–1/2-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. Figure 1 illustrates this assembly. Jetting occurred through a jet sub (no bit or motor) and returns were taken outside of the structural casing. Figure 2 depicts the process and shows both fluid and jetted solids were forced to the mudline along the outside of the casing. Minton alludes to the fact that, even from the early days of floating drilling, settling of structural pipe has been a concern. In the 1970's, development of tools such as the positive displacement mud motor and the wellhead housing running tool allowed the jetting technique to evolve2. Ports in the wellhead housing running tool allowed returns to be taken inside the casing rather than outside the casing resulting in less soil disturbance. Positive displacement mud motors allowed the rotation of bits in the jetting string and more efficient break-up and fluidization of the sediments. The jetting technique has spread to other geologic basins/geographical areas around the world. Salies, Nogueira and Evandro3 state that Petrobras began jetting of 30-in structural casing in the Campos basin in 1993. ExxonMobil Affiliates introduced the jetting concept to the west African countries of Angola, Nigeria, and Congo 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.

Finite element analysis of deepwater conductor bearing capacity to analyze the subsea wellhead stability with consideration of contact interface models between pile and soil

“…Quiros and Little (2003) have done some research on deepwater soil properties and their impact on the geotechnical program. Akers (2006) has discussed the job design and operational practices in deepwater environments during jetting of structural casing. Guan et al (2009) casing for deepwater drilling and obtained the deflection differential equation of conductor under the combination of top vertical force and bending moment in theory.…”

Numerical Simulation of Jet Excavation in Conductor Jetting Operations in Submarine Soil: Mesh Analysis

“…The problem of this study consists in considering the first few seconds of initial drilling on seabed by conductor jetting, since this operation for deepwater environments is quite common and even a preferred installation method because the seafloor sediments allows this technique to be used (Akers, 2006;Wang & Song, 2018).…”

“…In this model, we considered a 17 1/2" drill bit and nozzles with 20° inclination that are jetting seawater at a velocity of 20 m/s (approximately 400 gpm), which it is compatible with the volumetric flow rate pointed out by Akers (2006) considering that the drill bit has 4 nozzles. For this case, we also assumed that there is no relative displacement between the drill and the conductor casing: bit stick-out = 0 cm.…”

“…The technique consists of using a drill bit to circulate fluids with the goal of removing sediments and allowing the conductor casing to penetrate the soil. This technique is usually faster if compared to historical method of drilling a borehole and, then, cementing the casing in place (Akers, 2006;Kan, 2018;Malouf, 2013;Wang & Li, 2014).…”

Tubular Bells: A Case Study of Large-Scale Batch Sets

“…Each well was designed first to jet-in and set 36-in. conductor to approximately 305 ft BML to provide structural support to the blowout preventer (BOP) stack, subsea tree, and production casing and tubing (Akers 2006). After ensuring sufficient skin friction had developed during the jetting process, the jet-in assembly would be unlatched from the 36-in.…”