When Transocean's Express-class semisubmersibles were launched in 2001, the three ultradeepwater rigs were expected to deliver drilling efficiencies through their innovative Tri-Act derrick and other newly designed systems.

With over 30 wells of experience, the Cajun Express, Sedco Express and Sedco Energy are now demonstrating significantly faster well-construction times, compared with prior-generation rigs.
Time-savings plus safety performance and improved downtime have drawn significant attention from clients. After the Cajun Express constructed the Run for the Roses well for Murphy Exploration & Production Company in the US Gulf of Mexico, the client was convinced of the rig's savings capability.
"The Cajun performed very well," Steve Pulkowski, general manager of drilling for Murphy, said. "We did a post-mortem after the well and found that the Cajun achieved a 21% time advantage, compared with a prior-generation rig."

Pulkowski said the Cajun Express is well suited to "pedal-to-the-metal" power drilling. "We found that you save the most time in the first 20 to 30 days with efficiency gains in the big hole sections. The conditions were right to drill, drill, drill."

Because the Cajun Express can beat the drilling curve to target depths of wells, the rig has been rewarded through its incentive contracts. Since April 2002, the Cajun Express has achieved 50% of its performance bonus potential, beating drilling curves at double-digit rates up to 45%, or a combined 42 days ahead of planned total depth (TD) curves. The incentive-based performances have saved clients' time and money, increased the rig's effective dayrate and boosted morale among Transocean and vendor employees through bonuses in paychecks.

"The performance bonus is a good deal for everyone," Pulkowski said, adding that other bonus-related drivers include safe operations and compliance with US Minerals Management Service standards. "We did very well with safety on the Run for the Roses well with zero recordable incidents and we met all MMS requirements," Pulkowski said.

Saved

Time is saved in several areas on the Express-class rigs, including off-line tubular-handling and tool makeup. The 20-inch-diameter and smaller casing is racked back in the Tri-Act derrick in 120-foot (36-m) stands (three, 40-foot (12-m) joints) versus the standard single joints. Also racked back in the derrick are doubles of riser joints and triples of Range 3 drillpipe in 135-foot (41-m) stands (three, 45-foot (13-m) joints). Four bottom hole assemblies (BHAs) - already made up at two outboard mousehole stations and tested offline - can be racked back in the derrick.

Then there are faster penetration rates, supported by four 2,200-horsepower mud pumps coupled with a 7,500-psi fluid circulating system.

"Just being able to pump 2,000 gallons of mud a minute versus 1,200 gallons a minute in the top-hole section with a conventional rig makes a big difference," said Jimmy Nobles, Cajun Express rig manager.
Looking ahead, Pulkowski and others expect that the Cajun Express will be well suited to further drilling in the deepwater Eastern Gulf of Mexico, as that area becomes more mature. "Where that rig can really shine is on a development project," he said. "With known geological conditions and deepwater, you could keep improving, improving, improving on drilling performance."

Chip Keener, Transocean's director of deepwater marketing said, "The Express-class rigs are an excellent tool for both exploration and development programs and they are particularly well suited to the deepwater markets of the Gulf of Mexico, West Africa, Brazil and the Mediterranean. The crews have done well in optimizing rig performance under a challenging sequence of programs and clients, and we're still improving."

Incentive

After drilling in the Mediterranean off Egypt and making Apache's first deepwater discovery there, the Sedco Express is currently working in Brazil for Petrobras with an incentive-based contract. Under the agreement, the rig's dayrate increases significantly when it performs faster than the historical average drilling curve of a prior-generation drillship.

The Sedco Express has not only beaten the drilling curve established under the contract, it recently drilled the fastest well in the history of Petrobras in Brazil at just over six days. The rig surpassed the client's previous fastest well-construction by approximately two days, working in 4,910 ft (1,500 m) of water off Brazil's eastern coast.

Field moves between well locations provide more scope for saving time. Between two and four days are saved on each move, mostly by not having to run the blowout preventer (BOP) , which is suspended to at least 330 ft (100 m) above the seabed from a dedicated hang-off system fitted to Express-class rigs. At the new location, the top hole is drilled and the conductor casing is run with the riser and BOP hung-off. On long rig moves, the BOP and riser would be pulled and run at a new location. Even in these cases, the Express-class rigs have achieved time-savings compared with conventional riser operations.
Distinctive

The most distinctive feature of the Express-class rigs is their Tri Act derricks. The top drive, well center and two iron roughnecks are located in the derrick's center section, flanked by identical wing derricks on the port and starboard sides, each with two iron roughnecks. These wing derricks, or outboard stations, make up risers and tubulars in advance of - or parallel with - critical path activity at the well center. Two pipe-racking systems service the entire derrick.

The Sedco Energy used these and other advanced systems to save 33 days of operational time during 2002 for four clients in Nigeria, not including drill stem tests and some unanticipated well-control issues. The difference between planned and actual days of well-construction included 11 days saved for Conoco; 16 days for Statoil, three days for ChevronTexaco and three days for Canadian Natural Resources (CNR).
"Clients' written appraisals spoke of crews' strong work ethic and commitment to safe operations, as well as the rig's circulation and solids-control system, well-monitoring improvements and deck-management," said Ian Paterson, one of two Sedco Energy rig managers in Nigeria. "Their feedback also included areas for improvement that we are addressing in order to enhance safety and to further improve efficiency."
Enhancements

Safety, health and environmental-protection enhancements already in place include 30,000 sq ft (9,150 sq m) of unobstructed main deck area to reduce personnel tripping hazards and simplified arrangements for well-site equipment. Offshore life quality is improved by preventing rig-structure vibrations from affecting accommodations quarters, which are fully suspended on shock absorbers in a separate structure.

Double-skinned pontoons and columns provide a second barrier to prevent accidental environmental spillage. Low-emission engines are used and dump valves in the mud pits have been removed to prevent human error.

Lessons

Having close coordination between Transocean, clients and vendors on well planning and logistics far in advance of actual requirements is a key lesson learned. When materials are supplied to the Express-class rigs earlier than with prior-generation rigs, and all casing strings are prepared and racked back offline before running the casing, performance is significantly enhanced.

Casing strings have even been picked up for future wells while drilling, reflecting strong cooperation by operators who drill wells sequentially. This collaboration signifies a step-change in planning on their part. They all recognize the time-savings that can be achieved by utilizing offline capabilities. Co-operation also has helped reduce the Express-class rigs' downtime by more than half to 2.5% through early July 2003 versus 2002.

"Bringing together personnel from all the disciplines involved in constructing the well beforehand is one of the keys to achieving excellence in operations, as well as safety," said Sedco Express rig manager Svein Teksum in Brazil. "Petrobras has been very open to expanding the planning process. That's a key reason why our crews have performed efficiently while achieving a zero total recordable incident rate from January through early July 2003."

Closer collaboration also occurs when drilling. For example, communications are enhanced as function-related personnel work in one room, compared with different rooms on conventional rigs. The technically advanced integrated well construction center houses the driller, directional driller, wireline operator and remotely operated vehicle operator. The mud engineer, mud logger and cementer work in the fluids center. In addition, data-sharing occurs real-time. For example, the earth model can be updated as the well is drilled, improving decision-making and thereby reducing well cost.

Taken together, the Express-class rigs' new designs are delivering value through enhanced safety, well-construction time-savings and reduced downtime. However, there is always room for improvement.
Perhaps the biggest future efficiencies will result from current studies on adding off-line tree-handling. Engineering and operations personnel are exploring how to adapt the "multi-activity" functions used on the Deepwater Nautilus and Deepwater Horizon. With some modifications, Express-class rigs could hang off the riser and BOP beneath the starboard porch station while drilling the top-hole section at well center and simultaneously running top-hole conductor casing beneath the porch station and hanging it off in the transporter. The end result could be hours of additional drilling time saved. It will be interesting to see how these fifth-generation rigs can drive more innovations and efficiencies. After all, offshore drilling is a business in which, more than ever, safety and time-savings drive value - both for operators and contractors.