Extreme-torque Connection Drives A Step Change In Drillstring Design

The dramatic increase in horizontal wells from roughly 2005 to the present has been a driving catalyst in the development of new drilling technologies. This trend began in the offshore sector and then moved to land applications as economics continued to shift and drilling market share became predominantly land-based. Drillpipe was noted as a critical piece of drilling equipment that needed considerable improvement if it was to fulfill the industry’s needs. Horizontal drilling in U.S. land operations has become the new norm, and internationally the transition from vertical to horizontal drilling is ongoing at various speeds in different countries. Some countries show significant growth, like Russia, and others are barely launching their horizontal programs. To address the needs of these new programs, a new development in drillstring design is sometimes necessary.

Changes in string design, connection performance
The most common drillstring design features a combined string of 5 in. on the top and 3½ in. on the bottom. This design has been the dominant choice for vertical wells across the world for decades. Despite its long-term success in such applications, this string design is not suited when the horizontal section extends beyond several thousand meters or feet, as the change in drilling direction challenges the string’s torque capacity and brings hydraulics limitations. The choice is then between using proprietary premium drillpipe connections to overcome torque and hydraulic limitations or keep pushing American Petroleum Institute (API) rotary-shouldered connections (RSC) but risking major issues with fatigue life and washouts. Based on the need to surmount these difficulties, drillpipe technologies were focused on performance at the expense of ease of use and maintenance costs. The cost of such premium drillpipe technology was acceptable in the context of large-budget offshore projects, as the technology was deemed as enabling. The need to reduce costs became more apparent as the industry trudged through the last downturn, and changing the drillpipe connection was a potential solution.

New connection for new drillstring design
National Oilwell Varco (NOV) recognized that in the new drilling paradigm a fourth-generation connection was necessary and developed Delta. Table 1 details the performance metrics of drillpipe connection technology through four generations, with Delta showing both excellent technical performance and lowest total cost of ownership.

TABLE 1. A comparison of drillpipe connection technologies through four generations shows consistent improvement in performance. (Source: NOV)

When developing Delta, NOV took customer feedback as the primary design driver. The torque capacities provided by existing connections were enough in 95% of cases, especially on land, because higher makeup torque requires more powerful torque equipment. The most significant concern was the cost of maintaining the drillpipe connection through inspection and repair. The new design uses the experience gained from the second- and third-generation connections, and its unique thread profile promotes deeper stabbing, faster makeup and improved ruggedness, which means that each connection can take more wear before needing repair. When repair is necessary, there is 50% more room for a reface, which consumes 1/32 in. of tong space, before the connection must be recut, which also consumes 33% less material than for the second-generation connection.

Case histories
On a project in the Middle East, a conventional tapered string design using 5 7⁄8-in. and 5-in. drillpipe was specified for a large offshore project. After several years of operation, it was clear that a change in design would be necessary to drill longer wells. The solution was to replace a tapered drillstring with a single-sized 5½-in. string. The selected drillpipe incorporated several unique features:
• A high-torque capacity, which had never been available for a tool joint with 6½-in. outer diameter (OD) and 3¾-in. inner diameter (ID);
• 6½-in. tool joint OD limit allowed fishing in an 8½-in. hole with a full-strength overshot; and
• 3¾-in. tool joint ID needed to minimize hydraulic pressure losses while drilling the 12¼-in. section.

Historically, significant attention had been given to reducing repair rates. The operator wanted to keep the same rate of repair or improve it and wanted to reduce expenses as well. This led to the introduction of a superslim and extreme torque capacity drillstring design with Delta connections—a new design for the area. Though the 5 7⁄8-in. pipe body provided the highest tensile and torque rating, the 5½-in. option allowed the customer to use one size for the entire string and carry the required drilling torque (Figure 1). In this instance, the drillpipe with the highest tensile/torque capacity was not the right choice.

Results from inspection data confirmed better total cost of ownership, as the new string design had a recut rate of only 1.2% versus the industry standard of 10% to 12%, and sometimes 15% for challenging projects like this one. Note that as initial inspection numbers are generally the lowest, the recut rate will most likely fall in the 3% to 4% range as the sample size grows. The operator also saved two days on project time due to the reduction in stick/slip occurrences, saw an increase in ROP, improved imaging data for geosteering and saved an additional 4.5 days by using a single-size string instead of the previous two-size one. This eliminated pickup and laydown pipe operations during drilling with the single-size string and drillpipe-associated operations, such as BOP testing and top drive saver subs installation, were reduced. The operator spent significantly less on handling gear and backup pipe due to the standardization of the drillpipe size.

On a project in the U.S. in early 2018, a major operator introduced well design changes for Permian Basin horizontal wells, which increased the lateral sections of their wells from a 6 1⁄8-in. to 6¾-in. hole size. Previously, 4-in. drillpipe was used to drill the 6 1⁄8-in. interval. The operator was initially looking to switch out the 4-in. lateral pipe for 4½-in. pipe, believing that the change would improve hydraulics and pipe stiffness. After examining a third-generation connection with a 5 3⁄8-in. tool joint OD, the operator chose the fourth-generation Delta connection with a 5¼-in. OD, as a smaller OD would mean a decrease in equivalent circulating density related issues. After further consideration, the operator decided to use the 4½-in. string for the entire well. Though this meant drilling the tophole section more slowly with the 4½-in. string rather than the 5-in. string, the economics were in favor of the single size string as it removed the need for laydown and pickup and the need to carry more accessories (Figure 2).

As with the Middle East project, though the larger drillpipe size had a higher tensile rating, giving it more benefits when drilling the surface and intermediate sections, the more balanced 4½-in. size with its extreme torque capacity saved time on tripping operations, reduced the number of accessories necessary and eliminated costs associated with pipe size switching. Of note was that the amount of recuts required using the new string design was only 4%, less than half of the recut percentages for the previous design. As recut is the most expensive part of the repair, every percentage point in this category is of critical importance.

Conclusion
Drillpipe must not only be looked at from a drilling performance point of view, and the recent market downturn has helped the industry to reconsider this point, especially on land operations. High-performance drillpipe will never be the least expensive option, but it can save on costs through its life cycle. Companies that want to be effective on a long-term basis must consider a combination of three aspects before making a purchasing decision: drilling performance (i.e., torque, hydraulics and fatigue life), serviceability and total cost of ownership. The fourth-generation Delta connection incorporates all three considerations.