OPERATIONS - Further Notes
|Selection and availability of coretubes and liners
A client must decide if they want to use a steel coretube + liner or
a bare aluminum coretube. This decision is to be made in part as a function
of what is locally available. For overseas work, due to the fragility
and bulkiness of liners, one needs to see if liners can be found locally,
off-the-shelf, or can be locally extruded to custom specs. Air shipment
of a few adequate steel tubes and corenoses to fit the liners are not
an expensive proposition. We stress that the diameter clearance between
the coretube's ID and the liner's OD should be in the order of 1-2mm (0.020"
|Length of coretubes vs. length of sample
||If a client wants a 15ft. (4.5m) core sample, the coretube
needs to be 16ft. (4.8m). This is because some 6" are lost when inserting
the coretube into the vibrohead and another 6" are lost with the attachment
of the corenose and retainer.
|Vessel operations and drawworks
The size of the vessel does not have as much relevance as does its maneuverability,
although is must be large enough to support an A-frame of adequate size
along with working deck space.
Taking a core with our vibrocorers is a relatively fast, but not an
instantaneous operation, therefore the vessel must be able to maintain
its position in the core site and remain on position while the vibrocorer
is deployed and coring. The vibrocorer is deployed from the vessel with
the winch line and the vibrocorer's electrical cable. If the vessel
drifts away from the vibrocorer operating on the sea floor, the tension
on the winch line can pull the vibrocorer over or the electrical cable
may not have sufficient length and may snap. This will damage the connectors
and could cause an electrical short or damage the vibrocorer's motors.
Also, if the vessel drifts or swings on its anchor chain, the vessel
will not be over the vibrocorer during the extraction of the coretube
from the sediment resulting in the winch cable's vibrocorer-to-ship angle
not being vertical. This can make the recovery process very difficult.
Bent coretubes, and/or loss of coretubes and samples can be expected.
this means, the vessel must have either the ability to deploy several
anchors to maintain position or in the case of deep water coring, a good
real time maneuverability.
Nighttime operation: If a client wishes to work during the evening hours
the working area on the deck must be well lit and with lights on top of
the A-frame to cover the work area behind the stern.
A-frame size and load capacity: To determine the necessary height
needed for a vibrocoring operation, please use this following calculation:
of coretube + 4ft. (1.2m) for the vibrohead, lifting bridle, shackle
and lifting eye in the end of the winch line (most eyes are made with
3 cable clamps, making them approximately 10-12 inches long that will
not pass through the sheave under load.)
The measurement is made below the sheave hanging from the A-frame. Example:
To get a 15ft. core sample, use a 16ft. coretube +4ft, thus a total of
20ft. working height required. Please note that if a pivoting A-frame
is used that working height is measured not when the A-frame is vertical
over the deck, but rather when it is tilting over the stern clearing the
Two types of A-frames or crane: A pivoting A-frame is preferred. If a
fixed A-frame is used, the vessel must provide a second winch to pull
the vibrocorer aboard the vessel. If a sea crane is to be used, it must
be able to work at sea with the roll of the vessel not affecting the boom's
position or length and it must have its own winch, not a winch at some
other location on the deck.
Both the drawworks and the winch and wire line must be able to handle
a minimum working load of 2 tons. If sand is expected, a 3 ton system
should be used.
|Penetration capabilities of the P-5 Vibrocorer
||Penetration depths and recovery rates depend on many factors
such as the water content of the sediment, particle size and shapes, compaction
/ density, and even calcification. There is no core site that is exactly
the same, thus predicting correct penetration depths can not be done. However,
the following examples will attempt to define the P-5's capabilities. All
cases used a 4"OD steel coretube 20ft in length with a 0.083" or 0.120"
wall thickness and liner during various operations from 1990-95.
|Pure coral sands and reef debris
||i.e. Red Sea, South pacific atolls and reefs, West coast of
Australia. Water depths 60-400ft. The penetration depths ranged between
6-12ft. As the percentage of shell fragments increase and the calcite sand
percentage drops the depth decreases. This may be due to larger angular
fragments that will not rearrange themselves allowing passage into the corenose.
Also, the more abundant calcium could be cementing the fragments. Be aware
these core sites require the most force to remove the coretube from the
sediment compared to any other. Small vessels under 50ft. will have difficulty
extracting the coretube from the sea floor. There is always a chance that
the coretube will be lost when coring in this sediment type.
|Arctic till and glacier debris:
||Arctic ocean, Bering Sea, Gulf of Alaska, Hudson River, The
Great Lakes of N. America. Water depths 50-500ft. The penetration depths
will range between 3-15ft. As the debris increase into cobble size the penetration
depth decreases. Retention of the sample will vary greatly. A large cobble
logged in the corecatcher can allow the fine particles to be lost, however
it can also completely seal the catcher yielding 100% recovery.
|Shallow Continental shelf sand & silts
||West and East coast of N. America, Mediterranean Sea, Strait
of Gibraltar. Water depths 20-800ft. In these sediments the percentage of
sand vs. silt will determine the penetration depth. A pure silica sand,
large homogeneous grain size, will compact during vibration. Sample penetration
can range between 8-20ft. with the longer core being achieved in deeper
water depths (lower energy levels during deposition.) As the percentage
of silt increases the coring penetration will also increase. Heterogeneous
sediments will core the best. In many cases using a 20ft. coretube the final
penetration depth will stop after the coretube has encountered a clay horizon.
The P-5 can usually recover a 2-5ft. terminal plug of a stiff dry clay.
Stiff dry friable clay is defined by us as "pushing a screw driver into
the sample is very difficult". A clay with higher water content will allow
a longer plug.
|Deep Ocean Sediments
Gulf of Mexico, China Sea, North Atlantic, East coast of Africa, Indonesia.
Water depths 600-1200ft. The P-5 vibrocorer has for many years been used
as a backup tool for the piston corer and dart corer during hydrocarbon
surveys. When the dart corer encounters sand deposits, 1000-3000ft. water
depths, penetration and recovery are usually zero. The coretube of the
dart corer becomes bent upon impact with the sea floor. When this happens
the P-5 is deployed. Penetration and recovery rates in these water depths
depend more upon the ability of the vessel remaining exactly on location,
not pulling the vibrocorer over, and also the water currents dragging
on the winch and electrical power cable. The P-5 has the coring power
for a 20ft. core in this condition, but there is still a chance of a washout
of the sample from the corecatcher during the long travel back to the
surface. Water swirling around the corenose can wash the coretube clean
even with a perfect vacuum seal on the top of the coretube. A provision
or modification to the corecatcher and or weightstand to prevent washout
should be considered. Many 10ft. and 20ft. samples have been recovered
in this depth of water.
Penetration capabilities of the P-3 Vibrocorer: 10%-30% less than the