Uniform designs win out in ultra-deepwater activity

Once again, deepwater rigs are finding their way into the US Gulf of Mexico (GoM). With a renewed focus on safe, efficient execution, operators are seeking out agile, purpose-built rigs to venture back into the region.

The age gap typically is used to describe the situation of most crews; however, this phrase aptly describes rig fleets as well. Providing uniform design with an ample supply of modern equipment is an important first step for most drilling contractors seeking work charters in the world's deepwater basins.

The company’s 8500 Series of semisubmersibles feature a uniform design, which simplifies training from rig to rig and eases recertification for working in the US GoM. Pictured is ENSCO 8505 prior to undertaking sea trials in the GoM in April. (Images courtesy of Ensco)

A younger breed

Now the second largest drilling contractor in the world, Ensco has moved swiftly in the past five years to become a major player in the world's ultra-deepwater drilling market. Aside from the company's fleet of ultra-deepwater DS Series, its fleet of ultra-deepwater semisubmersibles, referred to as the ENSCO 8500 Series, is major news this month.

The ENSCO 8500, the first of seven semisubmersibles, commenced operations in 2009 and has built a history of performance drilling a major discovery in the US GoM's Lucius field that year. In 2011, the ENSCO 8503 drilled another significant discovery for Tullow offshore French Guiana. The ENSCO 8500 Series is currently one of the strongest workhorse fleets in the industry. Proven to be efficient and cost-effective drilling machines, the fleet accumulated a record of 97% utilization in 2011.

The company is now completing sea trials on its sixth semisubmersible in the series, ENSCO 8505. As part of a rig-sharing agreement, the vessel will be drilling for Anadarko, Noble Energy, and Apache later this year.

Uniform design

The story of the company's 8500 series begins with the design. According to Mark Burns, senior vice president, Western Hemisphere at Ensco, "Our strategy was to design a rig that addressed the greatest need as opposed to designing rigs to a 'wish list.'"

The idea was to target what Burns referred to as "the heart of the deepwater market." According to IHS ODS-Petrodata, the largest number of deepwater wells since 2005 has been drilled in water depths ranging from 914 m to 2,286 m (3,000 ft to 7,500 ft). The 8500 series rigs are capable of drilling in of up to 2,591 m (8,500 ft) water depth.

Burns added, "We also wanted to have a cost-effective, proven design, so we leveraged our experience with the ENSCO 7500 that had been operating successfully for five years."

The end result is a series of rigs that are durable, fully equipped, and suitable for both exploration drilling and development projects.

The company has leveraged its relationship with the Keppel Fels shipyard in Singapore to deliver each of the 8500 series with both standard and specialized equipment based on the central needs of operating in ultra-deepwater environments.

The company's strategy of designing the series to be identical is paying off. "Given the demand for qualified crew members, the benefits of not having to retrain workers who transfer from one rig to another are magnified in the current environment," Burns said. In addition, the identical nature of the 8500 series has proven invaluable in the post-Macondo recertification process. "Once one rig was recertified, the others quickly followed."

A driller's rig

According to David Hensel, vice president, North and South America (excluding Brazil) at Ensco, "The ENSCO 8500 Series truly represents a 'driller's rig.'"

Each of the series is outfitted with heave compensation packages that provide constant control of the drillstring despite average surface conditions experienced in most ultra-deepwater environments.

The substructure features an open height of 14 m (46 ft), which allows more room for tree handling. With a regular staff of approximately 70 personnel, the design is equipped for 150 persons-on-board to accommodate regular crews and added specialists during specific, crucial operations. In addition, the rig also features deck space with an open design that provides great flexibility for installing operator-provided equipment. The vessel's low profile reduces weight and overall height, which provides a better environmental profile.

The drilling package has the ability to work offline both on the drill floor and in the moon pool. The mud system is designed as a split system capable of handling both drilling and completion fluids simultaneously.

The ENSCO 8500 Series features the latest generation of BOP equipment. The six-ram BOP comprises a functional profile configuration with the sixth ram being a test ram. The blind and casing shear ram contain 22-in. operators at 5,000 psi, and these are capable of shearing all operator-specified drillpipe. After the new regulations were passed, the ENSCO 8500 and 8501 were the first rigs to be recertified for operations in the US GoM.

"To date, our deepest water operating depth in drilling mode is 2,195 m (7,200 ft), but we've been approved to operate as the rigs are currently configured in up to 2,804 m (9,200 ft) of water for a completion," Hensel said.

The rig has performed workover operations in 2,438 m (8,000 ft) of water. As a testament to the series performance, ENSCO's 8500 Series has joined the "six-mile" club drilling a well to 10,302 m (33,800 ft) measured depth. "We have current approval for a well planned to 11,430 m (37,500 ft)," Hensel said. The rigs have run 13 5/ 8 -in. casingstrings weighing up to 1.8 MMlbs, which is considered typical for an average subsalt well in the US GoM.

Strong currents are just one of the challenges confronted by drillers in ultra-deepwater environments. The dynamically positioned 8500-series rigs are classified as DP-2. Operating in French Guiana, the ENSCO 8503 ran in currents up to 5.8 knots. To put this in perspective, typical operating currents in the US GoM range around 2 knots.

The latest addition

As of mid-April, ENSCO 8505 was mobilized to the GoM (Corpus Christi, Texas) from Singapore, which took 39 days. According to the company, its 8503 improved its deepwater acceptance testing to 49 days, which was 10 days sooner over the 8502. "We expect to test the 8505 in 49 days or less," Hensel said.

As its latest addition to the series heads back into the deepwater GoM, the company is poised for future growth. The company's strategy behind its latest generation of semisubmersibles is long-term. "I've worked in the contract drilling business my entire adult life, and I can tell you that it is the best example I can think of where a company has had a well thought out strategy, enacted a nearly flawless execution, and cultivated great success with customers and excellent financial results," Burns said.