Frank Hartley • Houston
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Rotary steerable systems, bit designs
Bit design for rotary steerable systems is a topic of major concern to both manufacturers and users, and it came under intense discussion by four manufacturers recently. Representatives of Smith, ReedHycalog, Hughes Christensen, and Security DBS traded views on the subject at a recent meeting of the International Association of Directional Drillers in Houston.
Wyatt Pritchard with Security DBS said that understanding the interaction of a bit’s cutting structure with the formation will lead to optimal design characteristics that determine the efficiency, directional control, and vibration mitigation of the bit.
“One of the main pieces of our design is to see how the cutters and the rock interact,” Prichard said. Once we do that, we need to understand how the forces interact with the arms.”
According to Pritchard, their roller cone series are designed so that when the teeth interact at a normal angle with the formation, they remove a great amount of rock with steerability.
“We are always concerned with lateral aggressiveness,” he said, “with what it’s going to take from the required side force to get the bit deviant. We want to get it going in a certain direction.”
Drilling simulator
To understand how the bit interacts at the cutter to rock interface, designers want to know how the cutter engages, he said. Once the designers model in the drilling simulator and determine the actual engagement area of the cutter, then they have a better understanding of what it will take to increase penetration rate, Pritchard said.
One of the key components is the requirements of long gauge and short gauge bits, he stressed.
“To understand how many cutters to put on and the difference between active gauge and passive gauge, we have to look at the application, the BHA design, formation strength, and the cutter engagement area,” Pritchard said.
There is no doubt that rotary steerable technology has improved performance, said Steve Barton of ReedHycalog. “You have to remember that to optimize performance you have to eliminate certain aspects such as hole tortuosity, torque instability, low penetration rates, durability, and the inability to steer effectively,” Barton said.
Many of these factors, combined with the bit characteristics, affect penetration, he said. “One solution is the stable cutting structure in which many models were prepared to look at the lateral stability of the drill bit to maximize efficiency. You’re getting your actual stability from the circumferential gage coverage, so you still keep your short gage and your short bit but you’re not compromising the steerability.”
“Another aspect which will help in the system-matched design is steerability modeling. Here we can understand what tool are required in a certain projection and then optimize actual drilling proficiency,” Barton says.
It’s not enough to drill a fast quick hole, said Mark Morris of Hughes Christensen. The driller also needs to maintain hole quality.
“Instability is always an issue,” Morris said. “One thing we really need to emphasize is torque control, which is one of the important failure mechanisms.”
Bit evolution
Over the past few years, the industry has seen an evolution of the rotary steerable bit, Morris said. When the system first came out it was focused on just steerability with side cutting ability to make the system work. As the industry has learned more about the system, it has become more comfortable with the technology, due to a little more balanced approach at the design.
Looking for performance drilling, hole quality issues, managing torque control and steerability aspect, the bits went from very poor gages to a standard that maximizes the performance and provides steerability, Morris said.
“If you want a rotary-specific bit, you need to know which system you are running on. Each system operates somewhat differently. The more we can learn about the system, the more we can build the software to model the system,” Morris said.
“The one thing we want to do is match the bit to the formation,” Morris said, “make one long run, and make the interval without sacrificing the performance of the bit.”
Drilling companies may specify various kinds of bits depending on the region in which they’re operating, said John Williams, diamond bit product manager for Smith Bits.
Little consistency
“We see very little consistency across the world as related to the different demands we encounter,” Williams said.
“One of the key things you need to remember is that performance of the bit will be different in different rock types. It’s not always the bit’s fault,” Williams said.
There has been a major shift of new technology development over the past several years from operators to service providers, making it more important than ever for the industry to have a forum where the development and implementation of these new technologies can be openly discussed, said IADD executive director Alan Tannehill.
“IADD facilitates these open discussions between operators and service providers so that each can better understand the needs and concerns of the other as we try to promote the development and acceptance of new technology that will keep the directional drilling industry innovated and strong for future generations.”•
New jackup order
Noble Corp. has received a letter of intent from Shell E and P Offshore Services B.V. to for two new F&G JU-2000E enhanced premium jackup drilling rigs to be constructed by China Shipbuilding & Offshore International Co., Ltd./Dalian New Shipbuilding Heavy Industry Co., Ltd. in Dalian, China.
The Shell commitment includes a primary term of two years for each rig, with Shell having five one-year contract extension options for each rig at rates to be mutually agreed. Noble estimates that the aggregate gross revenues for each rig commitment over the initial two-year primary term will range between $74-79 million (not including reimbursement of costs of mobilization and demobilization, which will be borne by Shell).
The F&G JU-2000E is a high specification Friede & Goldman designed jackup with a 75-ft cantilever reach, rated water depth of 400 ft, and rated drilling depth of 30,000 ft. The F&G JU-2000E is also a harsh environment design rig capable of operating in the North Sea.
An indirect subsidiary of Noblr, Noble Drilling Holding LLC, has entered into contracts with DNS for the construction of the two F&G JU-2000E jackups. Based on its contracts with DNS, Noble expects delivery of the first rig in the third quarter of 2007 at an estimated total delivered price of approximately $153.4 million and delivery of the second rig in the first quarter of 2008 at an estimated total delivered price of approximately $156.4 million.
