When one thinks of “retro,” a number of things may come to mind – old clothes, vintage furniture, maybe even a classic car – but in the world of subsea well operations, there are three words that best fit: Riserless drilling systems.
Riserless drilling systems are based on schemes that have been around for the better part of 50 years. They’ve always been on the backburners of companies looking to increase the efficiency of drilling operations and reduce the heavy costs that come with them. But the technology was not there to make those schemes a reality. In the last decade, however, as technology has caught up to theory, riserless drilling has become an attractive alternative to standard methods.
Out with the old…
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| Reelwell's dual drillstring |
For decades, risers have been a standard in drilling operations, but as companies wade further offshore, traditional drilling methods will pose greater financial and technical difficulties.
Risers are more cumbersome to use in deeper waters; the money and labor needed to build a riser capable of handling the stresses drilling beyond 5000- 6000ft, as well as the platform required to hold it, are extensive. Large risers also increase the risk something could go wrong with equipment. A malfunctioning riser in a deepwater well could significantly impact the environment.
Watkins patented the first riserless system in 1969, in part to balance subsea well pressures and make it easier for the drill pipe to re-enter the drilling hole. At the time most offshore rigs drilled relatively shallow waters, and although companies continued exploring deeper water, any problems could be solved by increasing the size of the marine riser and the subsea wellhead.
Today, it’s a different story. The US National Oceanic and Atmospheric Administration (NOAA) says about 63% of the 1.6 million b/d of oil produced in the Gulf of Mexico come from depths lower than 1000m (3280ft).
Riserless systems are not foolproof. Several studies have pointed out flaws, including difficulities in cleaning a top hole, potential tubular failures, and increased torque and drag. There’s the potential for oil or natural gas to seep through the tophole and into seawater, causing environmental problems and potentially endangering the rigs or drill ships. Gas bubbles building underneath a rig could trigger an explosion, or in the case of a drillship, cause the density to drop enough to make the ship sink.
In a 2011 study examining drill casings in riserless topholes, Robello Samuel of Halliburton and John Gradishar of Shell pointed out buckling failures with some riserless systems, in which poor hold conditions led the casing to compress above the mud line. In that same study, they proposed a “new modeling approach” to help deal with the torque and drag issues.
But regardless of the potential pitfalls, the technology is there, and companies are out there with systems that are becoming the new standard in the industry.
The RDM-Riserless
One system getting the most attention right now is the RDM-Riserless from Norwaybased Reelwell. Developed in 2004, this system hinges on a dual drill string, a 6.625-in. pipe that comes with a 3.5-in. inner pipe – in other words, a mini-drill in a mini-riser. Drilling fluid flows down through the annular channels of the well and comes back up through the inner pipe. Rock cuttings are transported to the surface through the inner pipe along with drilling fluid, which leaves them out of the well annulus and helps keep the hole clean.
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| Illustration of AGR's riserless recovery systems |
As the return flow of drilling fluid comes through a closedloop high-pressure system, the Reelwell system helps mitigate some of the possible pitfalls of conventional drilling. With its ability to switch between managed pressure and underbalanced drilling without any pressurized equipment on the surface, it can help make deepwater drills safer. And it’s lighter: a 21-in. riser puts out around 1000kg (2200lbs) of liquid per meter, but the Reelwell drill cuts that volume down to 50kg (110lbs) per meter.
“It’s a major weight difference,” Reelwell CEO Jostein Aleksandsersen said. “The main capacity of these drilling weights is to handle the fluid volume. It reduces the need for a higher capacity [because] you don’t need to fill the riser.”
The end result of all this is a system that can support the increased pressures of a deepwater and public recognition. Reelwell received a Spotlight on New Technology Award from the Offshore Technology Conference in 2013, one of 15 developments to be so honored.
Reelwell has marketed its riserless system as a means for companies to get more use out of their older rigs. Instead of building new rigs to operate heavier risers, they can retrofit its drill and save on construction costs.
In that sense, Aleksandersen calls it a “rig of opportunity.”
“That’s the major reason of going into the market,” Aleksandersen said. “It’s an opportunity. If you look at the situation of deepwater drilling today, there are not enough sixth-generation rigs out there. By introducing [riserless drilling systems], you can go into the marketplace with lower-capacity rigs. You don’t need newer rigs.”
The system was tested onshore using the Nabors Rig 97 at various depths between 600m (1968.5ft) and 4250m (13,943.57ft) with a 6.625-in. dual drill string on an 8.75-in. portion of Shell Canada’s Groundbirch field, in eastern British Columbia. The test compared the RDM-riserless system to a conventional riser-based drilling system. Reelwell says that test showed its system has “a competitive rate of penetration” to a riser. The RDM-Riserless was not used offshore until last year when BG Brazil chose Reelwell to develop a deepwater drilling system to be used at 2300m.
AMG’S RMR
Another system that’s been widely accepted for much of the past decade is the riserless mud recovery (RMR) system from Norweigan company AGR.
RMR has been available for shallow-water rigs since 2003, but in recent years this technology has been tested in deep waters. It is a system centered on a subsea pump module that lets operators retrieve cuttings and used drilling fluid through a mud return line, allowing for better volume control when drilling a tophole. It can be used for closed-loop riserless drilling, and also with dual gradient systems.
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| Right-RMR system has been used on over 500 wells |
AGR participated in a 2008 study off the South China Sea along with BP America, Shell, PETRONAS, and the Norweigan Research DEMO 2000 program to test RMR in a deepwater setting. In the study, engineers ran a drill from a floating rig through 4657ft of water, eventually reaching a depth of 7404ft. They found that systems operated without a problem. The 7404ft depth was 169ft short of the original goal, but the discrepancy was not great enough to cause much concern.
The RMR system is now used in drilling operations the world over. As of last year, RMR and CTS, its sister system, have been used on more than 500 wells. AGR claims that several drilling operations have used RMR to help drill top holds in tough downhole conditions, and it says that its system has allowed operaters to use smaller casings for their risers, which in turn allows them to build lighter, longer risers that can be used for deepwater drilling. The example they point to most often is one where an operator went from a 2-in. casing to a 13.6.25-in. casing, allowing them to drill down to 7710ft.
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