Webster's unabridged, a copy of which weighs down a table in the ship's library, defines contingency in part as "something whose occurrence depends on chance or uncertain conditions; a possible, accidental, or chance event". A few days ago, on board JOIDES Resolution, being firmly determined that we would drill a deep hole into the ocean crust, we learned about contingency.
First of all, with any drilling, there are events which can foul you up. A lot of the time, this ship does piston coring, recovering miles (or, for scientists, kilometers) of soft mud. Last leg, on the west African margin, some 8 km of mud, most of it green and stinking, according to the survivors, came on board, this to understand the effects of some cold water entering the South Atlantic - actually, some important things like the Namib desert and rainfall in Europe. The mud was obtained by piston coring, which does a very nice job of recovering material with the consistency of silly putty without deforming it. The total of 8 km was ballyhoo'd as a record; four full containers of it left the docks at Cape Town bound for a refrigerated repository in Bremen, Germany. A fine patina of green dust is spread throughout the labs.
For drillers, piston coring is a lead-pipe cinch. The only wear and tear is on them, tripping pipe. There is almost nothing in a piston-cored hole which constitutes any threat to the coring bit, the stands of heavy drill collars above them called the bottom-hole assembly (BHA, another acronym), or the pipe itself. Problems may arise from the weather, from failure of equipment in the derrick itself, from accidents, or from medical emergencies. Usually nothing happens (the ship has an excellent safety program, and requires rigorous medical examinations of all who come on board). Piston-coring legs are almost always enormously successful.
Coring into the ocean crust is another matter altogether. The rocks are hard, not silly putty. Volcanic rocks are full of holes. They are riven with fractures, variously produced by contraction upon cooling, by hydraulic overpressure from circulating high-temperature fluids, and by faulting. In some places, these factors are so overwhelming that core bits last barely a dozen hours, and return to the deck nothing more than pock-marked hunks of steel. All their cutting equipment, including the four inward facing cones which actually do the cutting, their core guides, and their cone-mounts, can be abraded to nothing in very little time. Holes might literally grind to a halt after only twenty meters of coring. The action on the rig floor is a constant battle to make a hole despite what the drillers describe as torquing. Sometimes, the hole is not a hole, but what drillers call a "wallow". Often, the holes (or wallows) collapse, and the end of the drill string gets stuck, occasioning hours of hard labor to get the string unstuck. Failure means that an explosive charge has to be sent down the pipe, to sever the drill string above the point where it became stuck. This is no small matter, since just by themselves, hard-rock coring bits these days cost up to about $7,500, and BHA's a cool $100,000. A good part of the challenge of ocean crust drilling thus has been to discover just where it can be drilled.
Among many failures over the years, we have found a few "sweet spots". There are perhaps half-a-dozen holes where penetration into volcanic rock has exceeded 500 m. There is one truly exceptional hole, a legend in its own way, which is more than 2100 m deep, exactly 1837 m of which is in volcanic rock. This particular hole, which is in the eastern Pacific and numbered 504B, was visited by our ship, JOIDES Resolution, and its predecessor, Glomar Challenger, no fewer than seven times over a period of fifteen years, in order to accomplish this. It is the one place where drilling has penetrated through the basalt flows and far into the feeder system, called the "sheeted dike complex", which supplied those flows. The hole has been poked and prodded, and all sorts of geophysical experiments have been accomplished in and around it. It is, so far, the one place in the ocean crust where scientific ocean drilling has come closest to achieving some of the goals of the old "Mohole" project, which was abandoned after a promising start some 35 years ago. The idea at that time was to explore the entire ocean crust in a single drilled hole, and then to core substantially into the upper mantle beneath it. The target was a seismic discontinuity, one that essentially produces a kink in the travel paths of seismic waves, whether generated by earthquakes or explosions, that has been detected almost everywhere at about 7 kilometers depth beneath the ocean basins. The seismic discontinuity is named after the Croatian geophysicist who discovered it, Andrija Mohorovicic, but since this name does not trip lightly over the tongue, the discontinuity is called the Moho, for short. The Mohole project fizzled in a welter of cost overruns, bungled management, and politics, but it set the stage for the nearly thirty years of scientific ocean drilling, primarily in sediments and sedimentary rocks, that by itself has revolutionized much of the Earth sciences.
Still, because scientific ocean drilling has been predominantly sediment- oriented since 1968, true crustal drilling legs, which have exploration of the ocean crust, pure and simple, as their primary objective, and which thus constitute the ultimate legacy of the Mohole project, come along but rarely. This is why, for example, it took so long to drill the 2 kilometers at Hole 504B, and why it has taken ten years to return to our own hole, 735B, in the Indian Ocean, and continue the coring which started so auspiciously when we were all somewhat younger. Ship time is precious, and there are lots of other things to do with it. A lot of easier things to do than crustal drilling. In any case, Hole 735B is another sweet spot. During Leg 118, ten years ago, the benchmark drilling depth of 500 m was reached in just 17 days of operations at the site. There was no torquing, no sticking at any time. Recovery, which represents the amount of rock actually brought up to the rig floor in each cored interval of 9.5 m length here averaged 87%. At grand daddy Hole 504B, recovery typically was only in the range of 10% to 20%. At the end of Leg 118, after some very successful downhole measurements and experiments, Hole 735B was left clean and open. The rocks cored were gabbros, the principal material thought to underlie basalt lavas and their feeder dikes, and to overlie the mantle itself. Here we could explore the deep ocean crust, getting away from all the fractured, brittle, and hard-to-drill rocks at the top of the ocean crust, and really make progress in understanding all its underpinnings. We thought that Hole 735B would be a magnet to draw the ship into the Indian Ocean. The magnet, however, proved to be weak. Other objectives and a circumnavigation intervened. It took ten years to get back.
So, more than a week after arriving, and things are going ... very well indeed. About as predicted. The coring is easy and the recovery is high. One of our scientists, a participant on several other crustal drilling legs which have typically low recovery, describes his reaction as "slack-jawed amazement" as core after core comes on deck with more than 8 m, more than 9 m, or even a full 9.7 m recovery of fresh long pieces of gabbro, the heart of the magma chamber that once resided beneath this portion of the Southwest Indian Ridge.
Interesting things are seen in the cores. They are very complex. Far from being monolithic, the gabbroic layer of the ocean crust now emerges as being once an extremely dynamic place, having been faulted, deformed, and contorted as magma passed through it en route to volcanoes erupting above it. The hole is pushed to 890 m, well past the 500-m benchmark, and we realize that the contorted rocks are the norm, that the entire crust at this location likely was involved in deformation while it still was forming. This is a grand scientific story in the making.
Four bit runs are completed without incident. The bits themselves return with their cutting studs well worn, but otherwise undamaged. Core recovery has steadily increased, with improving weather, easy drilling conditions, and growing confidence and experience of the drillers. Then, following a routine fifth re-entry, and while lowering the pipe from the reentry cone to the bottom of the hole, we learn about contingency.
Mike Storms, our Operations Manager, is revising his estimate of just how far we might be able to drill by the end of the leg. Mike is an old hand, having started with the Deep Sea Drilling Project in the 70s. He's been at his current job for several years, now, following a long stint with Engineering Development. Among his credits is the initial design for the piston-coring device which allowed the previous leg to recover 8 km of mud. Drilling Superintendent Wayne Malone, who was present for the drilling ten years ago, comes off the rig floor. He tells Mike that there is a bridge in the hole, and that the drill string is torquing like mad, and that it can't get past the bridge. The bridge is only 120 m down, and the drill string has been moved without incident past this point for bit replacement and reentries, both up and down, something like ten times during two legs. Mike continues with his revised estimate and waits.
An hour and a half later and there is still no progress. More torquing. Mike and Wayne confer with the co-chief scientist on duty, namely me, and then decide to pull the drill string up, and replace the coring bit with another designed solely for drilling. What could have happened? Here is where drilling devolves to an art form. All we know about what might be wrong at the end of nearly 1 kilometer of pipe is what gets recorded on the rig floor. This includes pipe length and fluctuations in something called weight-on-bit. Weight-on bit is just what it says. It represents the difference between the total weight of the drill string, and the weight actually measured at the rig floor. It's something like the difference between your own weight on a scale, and the weight you read if you cheat and hang on to the door knob, except that the weights involved are a whole lot more. Also, the driller has to operate within certain limits. The difference between actual and measured weight cannot exceed the weight of the drill collars constituting the bottom-hole assembly (again, the BHA). If that difference is exceeded, then some portion of the pipe above the BHA will be put into compression, risking breaking of the pipe. The pipe has to be kept under tension, or it will break. Weight on-bit, however, fluctuates. This is because the ship moves up and down with the waves. While drilling, there is a piece of equipment which is put into place at the top of the drill string that tries to take care of this. It is called the heave compensator. While lowering pipe, however, neither the heave compensator nor the top drive, which turns the pipe, are in place. Imagine, then, encountering a sudden unexpected obstruction without a heave compensator, and without the ability to rotate the pipe.
Something like this is what apparently happened. No hole, not even 735B, is utterly like a rifle bore. We have been describing fractures filled with vein material in the core for days. We have been drilling and pumping cuttings from the bottom of the hole past the wall of the hole, between it and the pipe, for days. All of this action apparently loosened some piece of rock, and it fell across the hole while we were changing the bit or while we were lowering the pipe. Mike and Wayne speculate on what might have happened. One possibility is that a narrow ledge, one that had been missed during all previous re-entries, this time had finally been hit by the end of the drill string. The weather, which had been calm for three days, on this morning was fairly rough. A substantial fluctuation in weight-on-bit, occasioned by a larger-than-usual heave of the vessel, evidently occurred just at the time of this encounter. This may have knocked the rock out. Alternatively, or in addition, slamming of the bit into an obstruction may have damaged the bit, making it impossible to core past the obstruction. Imagine jumping onto your bathroom scale from a distance of about five feet. Time to move in the big guns.
The big gun in this case is a tri-cone drilling bit. Once the drill string is retrieved, Mike shows me both the bit which had encountered the obstruction - it proves to be undamaged, and the tri-cone bit. This is clearly a much more rugged contraption. Its bearings and cutting cones are larger, and their teeth bigger. A brute. There is no passage for a core. This will simply cut a hole. In his office Mike now describes to me what he thinks will happen.
Some piece of rock has evidently slipped into the hole at an angle. The angle probably is quite steep, in accordance with the angle of many of the fractures described in the cores. The reason for the torquing is that the core bit wedged in between this angled rock, and the vertical wall of the hole. One can see that the paint on the sides of the bit is abraded, but that the paint has not been removed from the cutting surfaces of the cones facing the flat bottom of the bit. They never had a chance to cut into the rock. So this time, with the tri-cone bit, although there will be some torquing, the strategy will be to use low weight on bit, allowing it to kerf into the angled rock, and then begin to cut it. With nothing more to discuss, I go to awaken our other co- chief scientist, Henry Dick, with the news. Henry's reaction is difficult to convey in words.
At a meeting of anxious scientists that afternoon, Mike explains all of this, and offers the prediction that this should work. Our logging scientist, Gerry Itturino, brings a printout of a borehole televiewer log from the previous leg. It shows a decidedly messy zone at about the level where we encountered our obstruction. This is not especially reassuring. Mike says that a day will be lost to clean the obstruction and the rest of the hole with the tri-cone bit, then retrieve this bit and replace it with a coring bit. The choice unanimously (vote of Mike and Wayne) goes to the unused bit which first encountered the obstruction. I go to bed, leaving the next several hours of wait-and-see to Henry. We don't say much to each other, but know the other's thoughts. There are what-ifs to consider. Especially, what if it doesn't work? We'll have to start a new hole. Well, what kind of position are we in to do that? Originally, we staged a logging run to provide structural information which, with contingency now staring us straight in the face, could guide us to a new target. The particular logging tool of interest is called the formation micro- scanner (FMS, to continue the alphabet soup). It is quite good at showing up fractures and other structure in the walls of a hole. The tool didn't even exist ten years ago. Naturally, however, this time, at the start of the leg, it chose not to work. Now, above the obstruction, there are only about 120 m of hole that can be logged. Not much scope for detecting structure.
How about survey information? We have our near-bottom video survey from the drilling of ten years ago, but that's it. A repeatedly requested near bottom mapping expedition of the surrounding platform, jointly sponsored by U.S., Canadian, and British scientific funding agencies, finally got past all the funding agencies, and was scheduled this past summer. It will take place in March, after our drilling cruise. Good timing, that. We can stay within our video survey, or carry out another short one some distance away. But in which direction? With such ruminations, we contemplate the future.
Here's the dénouement. Of course, as in all good fairy tales, everything worked out. The hole has been cleaned, the obstruction cleared, in a sort-of Heimlich maneuver for a drill hole, and we are now back to coring. If the mantle is within reach, we can still try for it, with our poor-man's Mohole. The drama, however, may not be over. We understand now that the hole is fragile. The more we work it, the deeper we go, the more the sum of the fragility will increase. There is risk. It is obviously more substantial than we realized, and it will increase. We are now at 960 m, having already drilled more than 90% of the distance drilled during Leg 118, at something like 80% recovery. There is no reason we shouldn't get at least to 1 kilometer. Then this hole will have to count as one of the premiere achievements of scientific ocean drilling. The cores, meter after spectacular meter of them, will be studied for years to come.
We can imagine what it will take to do a genuinely deep hole into the ocean crust, several kilometers worth, at some time in the future, even a Mohole. Plans are afoot to do so. However, an experience like this suggests that we will really have to bow our necks to do substantially better than a 735B or a 504B. The problems will occur. This one was solved fairly easily by an off-the-shelf tri-cone bit and some savvy and very skilled drillers. There was one other last-gasp arrow in our quiver, a massive flat-bottomed bit-like device called a "junk mill", because it is usually used to grind up metallic junk such as cones broken from coring bits and left in a hole. There are other arrows out there, like downhole casing, but they are much more expensive and take a lot of time to install. The general strategy for deep crustal drilling is emerging. First, find the sweet spots and stick with them. Second, stock your quiver. Third, do more of this type of drilling, first of all to find the sweet spots, but also because the only way genuinely to reduce the risk is with experienced drilling crews. Finally, be patient.
In the meantime, at least for us, it's still "Core on deck!".