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Jetting the 30" Foundation pile
Jetting vs. conventional drilling and cementing in deepwater e.g. due to unconsolidated
nature of sediments may present advantages e.g. Reduced time /costs, less tripping time and
no waiting on cement. Also Where deepwater mud-line is soft and unconsolidated, jetting
benefits vs. conventional drilling and cementing pipe, can result, especially if there is a
risk of hole collapse.
Additionally, from the perspective of structural design considerations, e.g. bending and
buckling, it is also important to determine the length of the foundation pile required to
support the pipe itself and conductor casing. In that, the length of the foundation pile
should be long enough to provide sufficient skin friction between the pipe and the
formation for support of the BOP and subsequent casing strings.
Another consideration is that jetting ROP will slow down and eventually stop. The
foundation pile should thus not also be designed too long or operational loss will result.
Unfortunately apart from some sea-bottom samples i.e. topsoil at seafloor no information is
often available about the soil condition in the first few hundred feet below the seafloor.
Insufficient data therefore is often not available to determine the minimum length of the
structural casing. Additional measures then being required to ensure successful jetting
operations, e.g.
Pre well planning:
· Perform a geo-technical analysis
· Perform shallow seismic survey techniques and studies.
Prior to drilling
· A pilot hole to 200 to 300 ft. below the mudline. Based on information gained, it
would be determined how many joints of foundation pile to be jetted. If the
sediments below the seafloor was too hard, to inhibit jetting, conventional
drilling and cementing would be used.
· A mud mat may also be utilized to provide additional support once a pile had
been jetted to its required depth.
· Jetting bit position in relation to the foundation pile shoe is critical and its exact
positioning is very formation dependent in order to optimise; WOB, less jetting
time, less damage to the soil and improved verticality of the jet string. |
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发表于 2009-5-4 15:03
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