Hart's E&P was invited to experience Transocean Sedco Forex's unique dual-activity derrick (DAD) drilling rig firsthand. We visited Discoverer Spirit, an Enterprise Class drillship on location in the Deepwater Gulf of Mexico.

"Right from the beginning, you experience the efficiency (of the DAD system)," said Mac Polhamus, offshore installation manager aboard the dynamically positioned drillship Discoverer Spirit. Polhamus was referring to the ability to run riser and land the blowout preventer stack with the forward rotary (FR) and hoisting system while making up drill pipe or casing using the aft rotary (AR). Not a moment is wasted when the Spirit is in dual-activity mode; in fact, the control room is a beehive of activity with two drillers and two assistant drillers working as teams on each rotary. Traditionally, the drillers handle the rotary controls, drawworks and pumps, while the assistant drillers operate the pipe-racking systems (PRS) and iron roughnecks. But many of the controls are fully redundant, so both team members have access to the same data and can work in tandem if required. Hardware too is redundant, so if one PRS is being maintained, for example, the other can be positioned to work either rotary.
Discoverer Spirit is the second of three Enterprise Class ships. Discoverer Enterprise and the brand-new Discoverer Deep Seas round out the fleet. The rigs are contracted to Unocal, BP and Chevron, respectively. Other high-efficiency rigs in the company's fleet include three Express Class semisubmersibles.
Bob Scott, vice president, operations support, revealed how the dual-activity process is paying off. "Efficiency varies with the type of operation," Scott said. "Once we've populated the derrick with pipe, efficiency takes a major leap." One of the design criteria for both the Enterprise Class drillships and the Express Class semisubmersibles is the ability to move with pipe in the derrick. Although the rigs are noticeably different, the ability to perform other necessary tasks while the main rotary is engaged in drilling is the key to their success. "In the past, as you progressed to the bottom of the well, a typical time line had so-called flat spots where time was expended with no corresponding downward movement," Scott said. "Our objective was to eliminate those flat spots."
While the hardware is impressive, a great deal of the efficiency improvement comes from the massive amounts of data available to drilling crew members. At any time, each person in the decision chain - from driller to rig manager - knows what's going on. This allows them to anticipate problems and work proactively to avoid them. It also permits an unprecedented level of communication. Monitors throughout the rig provide instant access to live color video from a dozen strategically positioned cameras, many of which can be remotely controlled, zoomed in or out, or swiveled to get a view of just about any area on the rig. Also available on the closed-circuit TV system are data screens that provide summaries of key drilling parameters in real time.
It is no coincidence the workplace is partitioned according to the degree of human intervention required - with safety as the overriding concern. "For the main rotary, where efficiency is key, we like to call it the Gobi Desert approach - totally devoid of human life," smiled Scott. "For the second rotary, we use the Oasis approach - where people gather to assemble and inspect tubulars as well as test and configure the next phase of the operation. Even so, we assist the workers with lots of mechanical equipment to make their jobs easier and more efficient." Positive knuckle-joint cranes perform like giant robots, eliminating the need for slings and chains. These can grab tubulars at their precise balance points. Human intervention for attaching or releasing slings from a swinging load or climbing to a stabbing board is not required. "Our goal was to take people out of potentially dangerous positions," Scott said.
Not only does the DAD give drillers the opportunity to do two things at once, it gives them great flexibility in coordinating complementary activities. As a result, critical path time is being reduced by up to 40%. On the Express Class semisubmersibles, a unique TriAct derrick exhibits an equally unusual profile. "The TriAct derrick differs from the DAD in that it has only one rotary," said Scott. "But it has two so-called porches that allow the marine riser to be preassembled in double stands." The rig can be running pipe from one side of the derrick while breaking down pipe on the other. "In concept, the Express Class is designed to deliver the same order of efficiency, but with less capacity," Scott said. "In either case, a great deal of thought and hard work went into streamlining repetitive tasks."
Purpose-built, the DAD system has the capacity to rack casing and drill pipe in 135-ft (41-m) stands. The extra height yields significant improvement in pipehandling efficiency because it reduces the number of connections required per trip by one-third. Two automatic fingerboards contain 258 slots each and can rack back more than 80,000 ft (24,400 m) of tubular goods. Each fingerboard can be accessed by either PRS. In effect, continuous inspection and maintenance of tubulars can be performed off line while the main rotary is busy drilling.
Polhamus enumerated several activities that lend themselves to dual-activity:
• making up and testing bottomhole assemblies while running casing;
• making up and testing logging-while-drilling strings or wireline logging tool strings off-line;
• casing make-up and setback while drilling;
• running the Christmas tree in on drill pipe from the AR while setting the production casing using the FR;
• running a horizontal tree using the AR and testing it at depth while drilling using the FR;
• picking up and laying down landing strings; and
• picking up and laying down drillstrings for inspection.
"With a sequential process, there is little flexibility," Polhamus said. "However, a dual-activity derrick has at least four different ways to perform any sequence of tasks. The result has been a step change in the way we plan and do our jobs."
A case in point
To better understand the inherent efficiency of the DAD system, imagine the rig is in drilling mode using the FR. The marine riser and subsea stack is in place. The following sequence illustrates the simultaneous use of the FR and the AR in a typical scenario:
FR: After drilling to total depth and logging, the operator decides to set pipe and complete the well.
AR: The production string is picked up off the pipe rack, made up, and is standing in the derrick.
FR: The drill pipe is removed from the well and is stacked in the derrick. Offline, the Christmas tree has been prepared and tested.
FR: The production string is run, hung in the wellhead and cemented.
AR: Meanwhile, the tree is run to the seabed on drill pipe.
FR: As soon as the cement is tested, the stack and riser are unlatched from the wellhead and hoisted out of the way (by just a joint or two). The ROV is used to facilitate landing the tree on the wellhead.
FR: The riser string is relatched. Then the entire system is tested. The well is ready to perforate.
AR: Meanwhile, the workstring used to set the tree is pulled and stacked.
These activities must be conducted sequentially on a conventional rig.
The sizzle of dual-activity drilling is the ability to batch-set development wells while drilling ahead through riser on a central well. J-laying of pipelines and jumpers as well as coiled tubing operations can be conducted concurrently with other activities. A significant savings is realized by the ability to test subsea equipment at depth. If no problems are found, installation can proceed immediately, reducing several days of critical path time. Even if a problem is discovered, remedial work usually can be completed off the critical path.
Identifying and capitalizing on efficiencies is a continual process with the goal being maximum use of the rig's equipment to safely minimize the operations' critical path. As in any drilling operation, this takes planning. Transocean Sedco Forex assigns an experienced drilling engineer to observe workflow and procedures and make suggestions for improvement. The idea has paid off. Many procedures have been implemented as a result of such observations or those of other drilling team members.
Besides these innovations, the company reports significant shortening of the learning curve as each new DAD rig is launched. According to one crew member, "The name Spirit is no coincidence. Working on this rig creates an atmosphere of spirit and high morale that filters down to each employee. The pride, teamwork and can-do attitudes are obvious. Together, they contribute to our outstanding safety record."
New-build crews receive valuable on-the-job training and experience on the sister ships so they are ready to go to work when their turn comes. In addition, a drilling simulator configured with the DAD and TriAct derrick programs is being relocated to Houston, Texas, from Pau, France. With Enterprise Class rigs, there are two rotary tables with virtually identical controls. One rotary is on line performing actual work on the well, so there is a certain amount of pressure on the operator. The other rotary is performing offline tasks, such as pipe makeup. Drillers and floor hands can train on the offline rotary out of the critical path with no pressure, building their skills until they are ready to take over the online system. Training crews in a nonstressful environment realize a big benefit in terms of safety.
This style of training has another benefit. It is human nature to resist change, and drilling crews are no exception. However, the combination of simulator training on the basics followed by on-the-job training in a nonstressful environment goes a long way toward allowing crews the opportunity to "sell themselves" on the new equipment and new ways of doing things. The company has seen that crews quickly take ownership of the new systems.
The way of the future
With emphasis focused on seabed installations, the DAD system can play a major role in field development. It can perform as an offshore crane. It can lay subsea pipelines and flowlines using either J-lay or back-lay techniques. "One of the advantages of bringing the simulator to Houston is to be able to sit with our operators and develop and test new techniques without risk," Scott said. "Complex tasks can be simulated and everyone trained before the idea is taken offshore. This will ensure that everyone is conversant with the objectives of the task and the techniques for achieving them.
"We are learning how to use software-controlled equipment, robotics if you will, from the aerospace and automobile industries who have been using it successfully for years." Scott added. "By taking proven concepts from these industries and adapting them to our own, we can go forward fairly confidently and quickly. As always, learning from others' experience helps reduce costs, which is an ever-present desire, both for our customers and ourselves."