The LFV presents an obstacle for the logging operation, as a
closed LFV could snag a tool string as it is withdrawn from the
open hole into pipe. The LFV is engineered to pass tools through
a diameter of 3.625 in (9.2 cm), but to lock open or release
with the passage of a 3.75 in (9.5 cm) tool. For this reason
a go-devil (an attachment at the end of the tool string) is deployed
to open the valve as the tool passes through downward. As the tool
is withdrawn to the surface, the go-devil again closes the LFV.
Certain tools can work as a go-devil because
their diameter is very close to 3.75 in (9.5 cm). When the
RCB BHA is used, the LFV is not an issue. In RCB logging operations
the bit is released either in the hole or on the seafloor, or is
removed at the surface so logging tools pass through pipe with
no other obstructions.
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| Go-devil on the floor of the core tech shop of the JOIDES Resolution. |
There are two methods for running the go-devil:
1. The go-devil is attached to the bottom of each tool string, and the LFV
is opened and closed for each logging run. The disadvantage of
this is that it sometimes takes a few attempts to get the go-devil
through the LFV.
2. The go-devil is pumped down on its own before logging, and
the LFV stays open for all logging runs until by a tool with a diameter of 3.75 in (9.5 cm).
The disadvantage of pumping a go-devil here is that the hole cannot be deepened, because
the go-devil will be sitting at the bottom of the hole.
