Project Engineer: Matt Stahl |
Staff Liaison: Adam Klaus |
Scientific interest in the nature of gas hydrates in marine sediments resulted in a mandate to provide a coring tool capable of retrieving deep ocean cores at in situ pressures. The Pressure Core Sampler (PCS) was designed to obtain samples of gas hydrates at near in situ pressures.
The PCS was deployed 46 times during Leg 164, in each case using the new externally operated ball valve. Three deployments were for borehole water samples only. The PCS recovered 75% or more hydrostatic pressure on 70% of the deployments and exceeded 90% on 56% of the deployments. Average core recovery for the 43 coring runs was 0.30 m (30%). Recovery of soft sediments was clearly enhanced by the "push-in" cutting shoe, which was successfully deployed as deep as 501.8 mbsf (997B-15P, 93% hydrostatic pressure, 0.30 m core), well past the last APC core (166.9 mbsf).
Figure 1 and Figure 2 summarize the performance of the PCS during Leg 164. Pressure retention was generally successful, although not perfect. Core recovery varied considerably. Gas sampling was highly successful. One core yielded over 8 liters of gas, mostly methane. Many cores yielded gas in quantities several times the volume of the core recovered. Nearly all this gas emerged only after the pressure was reduced to roughly 500 PSI. For this reason, near-perfect pressure retention turned out to be desirable but unnecessary. Three times, the PCS was run specifically to recover water from the bottom of the borehole and each time very little gas, almost entirely air, came off the manifold. This supports the assertion that nearly all of the gas drawn into the manifold evolved from the core rather than from the water contained in the PCS. Coring runs on which we had zero recovery yielded similar results.
The "push-in" cutting shoe performed very well, especially in soft material, and was successfully deployed as deep as 501.8 mbsf in a hole where piston coring stopped at 166.9 mbsf.
Recovery with the auger cutting shoe was mixed. It was run several times near the surface without much success. At Site 997, the auger shoe was employed after recovery with the push-in shoe had deteriorated and the formation was so hard that it was decided not to push in any deeper. For Core 164-997B-21P (549.9 mbsf), the auger was run with a 6-in extension, 15-20K WOB, 60 rpm, and 40 spm (~200 GPM). This attempt was remarkably successful, producing a 0.9-m, nearly solid core. The next four attempts with the auger cutting shoe used similar parameters and produced no core whatsoever. One notable difference between success and failure was that the Core 21P run cut very quickly. That core was cut in less than 3½-min, whereas those that followed took substantially longer (6-11 min). This may provide a clue to the consistent, successful rotary coring obtained with the PCS.
Land testing placed the modified piloted cutting shoe in a distant third place for core recovery. For this reason, it was not run until the last PCS core in the last hole. This was run with a 6-in extension and similar parameters to the successful auger shoe run for Core 164-997B-21P. However, the attempt met with no success.
Generally speaking, the basket core catchers performed fairly well. In several cases, they came back partially or completely destroyed. Apparently, a firmer basket catcher is called for. Trying another variation on the basket catcher should be considered, this time using a thicker material than the original 0.007 in (probably 0.010 in).
The flapper core catcher ran a few times with disappointing results. On its first run, the PCS returned with core, and the flappers were closed; however, the flappers could be opened by a light push, revealing several inches of space above the flappers before the first piece of core. This suggested that core fell out of the barrel first and then the flappers closed. It appears that the flapper core catcher played no constructive role whatsoever.
Even before its first run, it was clear that the torsion springs used to push the flappers inward were inadequate despite having been optimized for the application. After the first run, a flapper catcher was modified by grinding material off the back side and epoxying rubber bumpers to the back side. This, used in conjunction with the existing torsion spring, was intended to push the flapper more forcefully into the path of the core before it could fall out. Because this fix limited rotation of the flapper to about 80 degrees, some material was ground off the front side to provide a nearly full opening for core to enter the barrel. The run that used these modifications recovered very little core because of jamming in the catcher.
In firm formations that tended to tear up the basket catchers, an empty catcher sub was run, relying on expansion to jam the core inside the barrel. Although successful on many occasions, this is a risky proposition, that should be limited to hard material that is likely to expand inside the barrel immediately. Table 1 summarizes the results obtained with each type of catcher.
Core Catcher Type | Runs | Average Recovery |
---|---|---|
Basket | 25 | 0.31 |
Flapper | 3 | 0.03* |
Empty | 13 | 0.33 |
Experimental | 2 | 0.39 |
*This includes one run as designed, one with rubber bumpers, and a run without springs.
Both types of inner barrels were run. Although the type of barrel used was noted for only 23 of the 43 runs, the uncoated barrels appear to have performed better than the teflon-coated barrels (average recovery = 0.42 m vs. 0.34 m).
The recovery of gas from the manifold raised questions about the volume of the PCS and the amount of air that can be trapped inside. The total volume of the PCS sample chamber is 4.4 L without a core. Approximately 100 mL of air is trapped in the sample chamber, compressed to hydrostatic pressure, and apparently driven into solution. A full core occupies approximately 1.4 L and would decrease the amount of water in the chamber from 4.4 to 3.0 L.
Leg 164 provided an excellent testing ground for a number of different designs (cutting shoes, core catchers, inner barrels, and the new style ball valve, which allows pressure testing right before the run). However, the "push-in" cutting shoe was by far the most significant departure from what has been run in the past. This shoe takes a core somewhat like the APC and was quite successful in sediments to 500 mbsf. That worked well until the formation got very stiff.
Unfortunately, there still is no good way in which to rotary core with the PCS. Neither of the rotary cutting shoes tested on Leg 164 worked any better than the original one, despite encouraging land test results. It will require a significant amount of engineering time (to build cutting shoes and test them at a good facility) to solve this puzzle.
Other possible modifications to the PCS have been proposed, although no further development effort is planned because of budgetary constraints.