and dangerous - things happen. For example, as drill bits bore down beneath two miles of seawater and two more miles of rock, the gas bubbles they encounter pack the explosive power of a bomb.
Oil field veterans point out that a gas bubble the size of a ping pong ball at the bottom of one of these wells expands to the size of a small car by the time it reaches the surface. Suddenly freed from its million-year, high-pressure imprisonment, the gas decompresses violently in the underwater well's casing and riser, the protective shell of the well in rock and ocean, respectively. At the top of the well, high above the drilling ship's "moon pool" (or work area), a steel fabrication weighing tens of thousands of pounds takes the shock of what oilmen call a "kick."
Designing and building that equipment, known as a riser diverter, and its accompanying blowout preventer (BOP), are the core of the offshore drilling technology business at Stewart & Stevenson Services Inc. (S&S), Houston, Texas. Much of the rest of its business is designing and fabricating the riser (so called because it rises to the surface from the bottom of the ocean) and the riser's complex connections to the drilling ship. S&S sales for 1998 topped $1.2 billion.
These days, drilling ships work in ocean depths up to 10,000 ft. and, soon, 12,000 ft. And that's just halfway: from the seabed, drillers probe for oil and gas another 10,000 ft. into the rock of the earth's crust.
This is not only technology-intensive, but also capital-intensive. A new drilling ship costs upwards of $400 million. Outfitting a ship for a given drilling project can cost $25 million or more. Other than the shipyard itself, S&S is the biggest supplier of equipment for these vessels. Global Marine Corp. is its biggest customer.
The physics of this drilling runs into big numbers and big risks. The deeper the well, of course, the higher the pressures. These reach close to 30,000 pounds per square inch (PSI) in deep undersea wells. Moreover, the kicks generated by the high pressures increase exponentially, not linearly. This is a result of the effects of blasting upward through several thousand tons of drilling fluids, or "mud," en route to the surface. This makes the engineering of the riser diverter and BOP even more critical.
Because of the great distance they travel to the surface, kicks telegraph their arrival, giving the drilling engineer and crew time to prepare. All wells now have pump stroke counters on the mud pumps and flow meters. Drilling crews know how much is going down the well and how much is coming back up. When the upward flow climbs past the downward flow, a kick is coming. The bigger the difference in the two rates, the bigger the jolt, oil field experts observe, with some kicks reaching close to explosive force.
All this equipment is fairly standard in offshore drilling. "But at these working depths and pressures, every undersea well has a set of unique challenges," said Michael Giles, S&S design engineer. With so much at stake, virtually any new engineering design or modification has to be analyzed. At S&S this work is done with DesignSpace verification and optimization software from ANSYS, Inc.
Giles and fellow engineer Brian Sneed use DesignSpace V4.1.1 in both its stand-alone mode as well as embedded within Release 21 of Pro/ENGINEER from Parametric Technology Corp., Waltham, Massachusetts. Some work is also done in Release 14 of AutoCAD from Autodesk Inc., San Rafael, California, and the beta version of AutoCAD 2000.
"Because of Global Marine's unique requirements, the diverters and the BOPs were designed integrally," said Richard Olson, manager of engineering research and development for S&S. "To combine these two pieces of equipment is rare in the offshore industry and a first for us." A diverter always sits on top of the riser. Operating as a manifold, it channels the gas, oil, seawater, and drilling mud coming up the riser into holding tanks and processing machinery.
The BOP, in this case with a 43 in. diameter piston and dome, sits on top of the diverter. When there is a kick, the BOP's piston lets off the initial pressure jolt then clamps the well shut.
To add to the engineering challenges Giles addressed with DesignSpace, "this particular diverter design is one of a kind, designed oversized, and, in contrast to the usual industry practice, in one piece rather than three," Olson said. The diverter housing, which connects and fits the design to the Global Marine's drilling rig, is an additional component. "Even without an integral BOP, these are one-of-a-kind designs," he said. Early in the development process, Global Marine told S&S it would soon be working in seawater up to 12,000 ft. deep.
The diverter and its housing are roughly 6 ft. in diameter and 10 ft. high. The diverter body itself weighs 36,000 lbs. and its housing an additional 18,000. It has six outlets, three with 16 in. inside diameter (I.D.) openings and 750 lb. flanges meeting standards of the American Petroleum Institute (API). The flanges are bolted to the drilling ship's framework outlets.
The diverter, Giles noted, " is so big it takes 3,000 lbs. of weld metal and 16 2.5 in. diameter bolts just to hold it together." These 8-pitch bolts are critical. "They are installed under makeup pressure, the pull of the weight of the riser and torqued to 4,800 ft.-lbs. Installing the bolts under pressure like this more evenly distributes the stresses on the threads." Otherwise, he added, they might not hold.
The BOP piston was designed so it could take indirect shear forces and even a little bending. "The bending is due to a combination of required clearances and tolerances and a 50 percent safety factor," Giles reported.
"Working with DesignSpace, we tested and analyzed the constrained cylindrical forces on the upper part of the piston to
pinpoint
the high-stress areas," Giles said. "We had to be sure that when kick loads
hit, we would be able to avoid the bending. To simplify the analysis, we used
DesignSpace to isolate the cylinder from the parts of the piston that were not
going to be impacted," he explained. "And in our beta tests, we found that the
assembly-modeling capabilities in V. 5.0 let us do this even more
precisely."The critical link in all this equipment is the tensioner, which connects the BOP and riser diverter to the top flexible joint, the uppermost end of the riser. Hanging from the tensioner is up to 2 million lbs. - 2,000 tons - of riser. Because it's beneath the drilling ship and under water, the tensioner is made of a high-strength, corrosion-resistant AISI 4130 chrome-molybdenum steel - 62,000 lbs. Of it.
The weight of riser extending two or more miles to the seabed means that the engineers must work their designs to a tensile strength capacity of 3.5 million lbs. The dead weight of this steel itself, when submerged, is enough to cause the riser to pull apart, especially under storm-related stresses. So S&S also supplies buoyancy modules that neutralize up to 98 percent of this weight. In the air, buoyancy modules weigh up to 73,000 lbs. It is for these kinds of loads that Mike Giles uses DesignSpace.
The tensioner is designed for frequent unclamping and reclamping as drilling operations progress. "We used DesignSpace to read out the stresses on the drilling rig whatever the boundary conditions and when loads are attached and lifting," Giles recalled. "In any kind of wind and sea, the rig has to be able to lift out and lay down the diverter and the BOP (which, combined, weigh over 75,000 lbs.) plus several more tons of flexible riser tubing."
DesignSpace lets Giles and Sneed rotate their models and slice them in half to see where the highest stresses are. "This is a great design help," Giles said. "And the ANSYS calculations reassured us that we can exceed ratings considerably, for example, a 103,000 PSI load on something rated for 75,000 PSI - provided that the loading is brief, infrequent, and isolated from most of the rest of the structure."
To make sure the tensioner and riser diverter stayed firmly connected, Giles designed eight lockdown actuators and lugs. The lugs, also known as setting dogs, were analyzed with DesignSpace in the linear plastic mode under both tension and compression. "The weight of the steel is the compression load, and the movement of the ship is the tension load," Giles pointed out.
For this analysis, Giles used ten-node tetrahedron gap elements with beam elements. "We used six and four degrees of freedom and, because they were not in contact, frictionless solvers," Giles reported. "Even though we used a thermal-mechanical model, we were able to do 50 percent less complex calculations.
"Results," he said, "were calculated for equivalent stresses with safety factors and equivalent stress with deformed shape. We also used the DesignSpace SHAPE command to edit the geometry in the model to conform to pre-calculated stresses. This is a very handy capability," Giles said.
"This was very easy to set up with DesignSpace," he continued. "Based on our beta tests, it will be even easier with DesignSpace 5.0. The most recent version will allow our models to account for different parts, different materials, and even different meshes. Without these capabilities," Giles added, "the solution could have taken 100 to 120 hours rather than the two we actually needed." The modeling and analysis were done on a PC with dual Intel Corp. Pentium 300 MHz. class CPUs.
Another vital DesignSpace analysis involved the clamping of the tensioner to the bottom of the diverter housing. Taking all the load of the riser weight, plus the lateral stresses of waves and current, is a grooved shoulder just a half-inch wide in a steel flange ring 1.375 in. thick. In comparison to the huge opening in the bottom of the tensioner - a 78 in. diameter - this is relatively small. "We used DesignSpace analyses to make sure the connecting components could withstand 55,000 lbs. Of upward pressure from the riser," Giles reported.
The riser diverter and housing got a lot of attention from DesignSpace. The primary function of the diverter is to send drilling mud and whatever else comes up to the mud pits and shell shakers for cleaning and reuse. It also keeps drilling mud in the riser from going overboard and contaminating the ocean.
But in the everyday reality of engineering, this is all about force containment. Each diverter housing is customized to the particular rig. The two that S&S is building for Global Marine are the largest ever built - and are unique in many other ways. These two have a 78-in. through-bore while most offshore rigs are 49.5 in. and many are still 39.5 in. One reason for such a large bore, in addition to the ship's heave and roll, is that ocean currents can pull the riser as much as ten degrees away from the vertical.
To accommodate ship movement, DesignSpace was used to redesign the extended 16 in. API flanges. "We wanted to make sure they could always be unbolted," Giles said, "and that they would not jam or bind together due to pressures from below or the ship's motion. Otherwise, they would have to be cut apart." To link the diverter to the drilling ship's structure, the flanges point five degrees below horizontal. "To make sure we got this right, I questioned everyone and everything, even what DesignSpace told me," Giles said. "Then I dove right in."
Sneed used DesignSpace in his work on the riser's connections to the drilling ship. These included the tensioner ring, a piece of equipment which keeps the riser under tension and as straight as possible. Despite the best anchoring methods yet devised, the drill ship is still a floating vessel. Despite sophisticated technology to dampen the effects of the sea, it still rolls, yaws, pitches, and heaves. Without tensioners, flexible joints, and telescoping joints, the ship's ceaseless movement would make deep-sea drilling far more costly if not impossible. To further compensate for the motion of the drill ship, a tensioner ring, essentially a gimbal bearing underneath the ship, is anchored to the seafloor.
Summing up, Giles observed that, "DesignSpace is for the R&D and production guy. (Full) ANSYS is for the analyst. The key to DesignSpace is that the design engineer can use it as he goes along, just drag and drop. The savings is that we don't have to stop, create another geometry file, e-mail it to the analyst, and then wait while he or she comes up with the answers. Best case, that can take a day or more.
"Perhaps more important, the inevitable delay disrupts the designer's thinking process," he continued. "And there is always the chance the geometry sent to the analysts will have to be modified while it is in their hands. This means the results may not match the revised geometry, and that can render the analysis effort useless."