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" to 0.040").

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.
All 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:
Length 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 deck.

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 P-5