Summary
Structural casing jetting operations have become commonplace in deepwater
environments. Having originated in the Gulf of Mexico (GOM), structural casing
is now jetted in most deepwater basins in the world. Very little literature has
been published regarding jetting practices. The lack of published literature,
combined with the complex mechanics and hydraulics of the process and the lack
of detailed-soil data at most locations, results in an operation that is
heavily dependent on the experience and expertise of the rig-site team. This
paper reviews current deepwater structural casing jetting job design and
operational practices. Also included are several case histories of jetting
failures presented to share learnings with the industry and to stimulate the
sharing of learnings and practices.
Introduction
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.
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
evolve (Reimert 1975). 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 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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
2 October 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
27 March 2007
- Manuscript approved:
17 August 2007
- Version of record:
20 March 2008
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