Lightweight risers increase rigs’ water depth ratings

Alloy risers can increase the water depth capacity of a rig by as much as 50%.

As water depths increase, risers for drillships and semisubmersibles become some of the most critical components of the drilling operation. Risers must be strong enough to maintain integrity during deployment and operation; be light enough to be handled and supported by the floating drilling unit; and be constructed of materials that resist corrosion while exposed to seawater. Lightweight, alloy risers provide a means of increasing the water depth rating of floating rigs by as much as 50% without requiring expensive upgrades, Fig. 1. Because of their lightweight construction, alloy risers are easier to handle, require fewer buoyancy devices, and can enable floating rigs to increase their water depth capabilities, when compared to conventional steel risers. The first generation of lightweight deepwater (DW) risers has been used successfully for nearly a decade on two drillships and a semisubmersible rig in Brazil, while drilling in water depths as great as 7,200 ft.

Fig. 1. Extending water depth with lightweight risers.

Based on the experience of more than nine years of operation and more than 4,000 days of drilling with lightweight, alloy risers, a second-generation DW system has been developed and is now commercially available. In addition, an ultra-deepwater (UDW) riser system concept is available, using a higher-strength alloy, which is designed for drilling in water depths of up to 12,000 ft.

LIGHTWEIGHT RISERS

On floating rigs, the riser is the heaviest item used in drilling. Each 75-ft bare joint in a steel riser weighs, in air, approximately 25,000 lb. A string of 100 bare joints has a total weight of 2.5 million lb. In comparison, a lightweight, alloy riser joint of comparable strength weighs, in air, 16,850 lb, so a string of 100 bare joints has a total weight of 1.685 million pounds, or 32.6% less than the steel equivalent.

Risers are encased in buoyancy modules to reduce their effective weight in water, and steel risers require buoyancy modules on 95% of the joints. Because of their lighter weight, alloy risers require buoyancy modules on only 33% of joints, decreasing storage and weight capacity requirements on the rig. In addition, buoyancy modules on steel risers need to be of larger diameter (54-in. OD) than those used on alloy risers (46-in. OD), and only half of each alloy buoyant joint needs to be equipped with buoyant materials, Fig. 2. This combination—of lighter weight and reduced need for buoyancy modules—can increase the water depth capacity of a floating rig by as much as 50%, without extensive modifications to increase deck capacity and weight handling capability.

Fig. 2. Risers with buoyancy. The top image depicts an alloy buoyancy riser joint; the lower image depicts a steel buoyancy riser joint.

REDUCED RIG MODIFICATIONS

Rig operators, who elect to use steel risers to drill at water depths beyond a unit’s initial depth rating, will need to invest in significant modifications. As water depths increase, the rig’s structural support must be enhanced to accommodate the additional weight of a greater number of steel riser joints, plus the tensioner system required to stabilize the riser during drilling. Also, the rotary table and chain drive on the drawworks must be upgraded for the greater weight. Upgrades needed to increase a rig’s water depth capacity from 4,000 ft to 6,000 ft, and the additional riser joints needed to reach the new depth, require a total investment of approximately $49.5 million, including the revenue lost by taking the rig out of service for the upgrades. However, if the operator decides to reduce weight by using a lightweight riser system, the required investment would be approximately $26.7 million, a 46% reduction, Table 1.¹

HISTORY OF LIGHTWEIGHT RISERS

Development of a lightweight riser system began in 1996, when a major offshore drilling contractor commissioned an engineering study to determine the feasibility of using a riser manufactured from lightweight alloys. In 1999, an alloy was selected for the system, an initial design was completed, and prototype testing was conducted. Two full-scale joints were built and tested in 2001, and design improvements were made, resulting in the first-generation, lightweight riser system design.

During 2002, the first 100 joints were manufactured, at the Progress facility in Samara, Russia, which is the only plant in the world capable of extruding such high-quality, large-diameter aluminum tubulars (Alcoa acquired this facility in 2005). In 2003, the first string was placed offshore Brazil, on one of the contractor’s drillships, effectively extending its water depth rating from 4,000 ft to 6,000 ft (5,000-ft contractual depth). After the first deepwater riser deployment, the flange bolts, inserts and washers were replaced with an improved design to enhance reliability and ease maintenance. In 2004, the modified deepwater riser was returned to service and performed to expectations at a maximum water depth of 4,800 ft.2 Over the next two years, the drilling contractor outfitted another drillship and a semisubmersible with first-generation alloy riser systems, which have all performed to expectations. Experience gained while using these systems led to a few design modifications, resulting in a second-generation deepwater system.

Lightweight riser material and design. The alloy selected for the DW lightweight riser system is a high-strength, Al-Zn-Mg alloy, with a yield strength of 50,700 psi and an ultimate tensile strength of 58,000 psi. The riser system comprised of this material has a 1.5-million-lb tension capacity. The yield strength of the aluminum alloy is comparable to material used in steel riser systems, Table 2.

Riser joints are constructed of extruded alloy with no longitudinal seams, Fig. 3. Each joint body is 75 ft long, made of two 37-ft pipe sections, fusion-welded at the center, with flanges attached at each end; steel bolts are coated to prevent corrosion. Each riser joint also includes two choke and kill lines, a hydraulic line and a booster line, whose dimensions and pressure ratings are listed in Table 3. All auxiliary lines are extruded from the same alloy as the body. Service line pins and boxes are made of steel suitable for sour gas service.

Fig. 3. The lightweight riser joints are constructed of extruded alloy with no longitudinal seams.

To limit corrosion, each lightweight riser joint is equipped with cathodic protection, consisting of bonded sacrificial anodes to deplete, in preference to the alloy riser and its components. Anodes on the deepwater system have provided protection against corrosion during nine years of operation on three rigs.

Before being deployed, full-scale riser joints successfully underwent a comprehensive testing protocol, which included tensile, hydrostatic fatigue and impact testing. In May 2003, the riser system received certification from American Bureau of Shipping, verifying that it was designed, constructed and maintained in accordance with ABS, ASME and API guidelines, and was approved for use on all drillships and semisubmersibles in the contractor’s fleet.

Operating considerations. While lightweight risers have significant advantages, the alloy risers must be handled per their specifications, which differ from those of steel. Rig crews must be trained on proper deployment, lifting, running and storage procedures. Anodes for cathodic protection must be kept in good working order, as defined in the specification.

As with steel riser systems, regular maintenance is required to keep the lightweight riser system operating reliably. However, lightweight alloy risers have been shown to require less deck maintenance during operation than steel risers. Approximately every two years, each riser joint should be disassembled, inspected and repaired. During these periodic maintenance cycles, alloy risers require less labor than steel riser systems. On the first-generation deepwater lightweight risers, most reliability issues were found in the connections of the auxiliary lines. These issues have been addressed with re-engineered connections in the second-generation deepwater system.

Experience with lightweight risers. After the first deepwater, lightweight riser system was deployed on a drillship in May 2004, similar systems were deployed on another drillship and a semisubmersible by the end of 2005. All three rigs have been operating in Brazilian waters. During nine years of operation, the deepwater risers accumulated more than 4,000 days in service while operating in water depths from 525 ft to 7,200 ft.

The deepwater, lightweight riser system has performed, as expected, and has effectively increased the water depth ratings of the three rigs without requiring expensive upgrades. As mentioned, some problems were encountered early in the development of the riser system on the auxiliary lines, including stab pin corrosion, and stress corrosion cracking failures in booster and choke-and-kill line couplings. Improved manufacturing procedures, and a new design for auxiliary lines, have been implemented to address these issues on the systems in operation and in the second-generation DW system.

Development of an ultra-deepwater riser system. Soon after deploying the first lightweight riser system in 2004, the offshore drilling contractor embarked on a program to develop an ultra-deepwater (UDW) riser system, using a higher-strength, Al-Zn-Mg -Cu alloy, which would be capable of drilling in water depths to 12,000 ft, with a tensile strength of 2.26 million lb. This riser system featured tapered-body joints to strengthen ends near flange connections, and was manufactured using a new friction stir welding technique, because fusion welding could not be performed on the stronger alloy, Table 4. After the friction stir welding process on the alloy was perfected, and after full-scale riser joints passed tension tests, a string of the high-strength riser joints was deployed near the end of 2007, to a semisubmersible drilling in the Gulf of Mexico in 9,000 ft of water. Another string of 150 joints was sent to Brazil for deployment on another semisubmersible.

During its first year of operation, the UDW riser system on the first rig experienced pitting corrosion on the main tube. This problem was traced to cathodic protection anodes that were not made of the optimal material. Auxiliary tube issues also occurred, and the higher-strength alloy risers were withdrawn from service.

Laboratory tests of two high-strength alloys and three anode materials, conducted by Alcoa at Batelle’s Florida Materials Research facility, exposed the alloys to sea water for up to one year in 2012 and 2013. The tests demonstrated that both alloys performed well in simulated deepwater applications, when the proper anode was used and cathodic protection was maintained.³

Further development of the UDW system continues, with design improvements expected to enable this system to exceed the water depth performance achieved with the three deepwater, lightweight riser systems.

In addition to the deepwater riser systems in service, another semi is gaining some of the benefit of using lightweight alloys by operating with a hybrid riser system that includes a steel main tube and alloy auxiliary lines. The hybrid riser weighs less than risers constructed entirely of steel and enables an improvement in the rig’s water depth rating.

ESTABLISHED CAPABILITY

The deepwater, lightweight riser system, incorporating improvements based on experience gained during nearly a decade of operation, is established and available to help extend the depth range of floating rigs to 8,000 ft. Nearly 60 floating rigs in operation today could drill, economically, in deeper water by using lightweight riser systems. The issues encountered during the introduction of the first-generation, deepwater riser system have been resolved and retrofitted into the three deepwater systems in operation. These commercial systems are drilling successfully in water depths down to 7,200 ft.

In addition, the improved UDW system has the potential to extend depth ratings to 12,000 ft, and offers an option for greatly enhancing the versatility of the world’s floating rig fleet. 

REFERENCES

1. 2H Offshore Subsea Riser and Engineering feasibility study, “Alcoa oil & gas aluminum drilling riser development,” February 2008.
2. Deul, H., “Aluminum alloy riser allows for deeper drilling on existing rigs,” SPE/IADC paper 92559, SPE/IADC Drilling Conference, Feb. 23-25, 2005.
3. Moran, J., and H. Pate, “Corrosion performance of aluminum alloys for use in deepwater riser systems,” NACE paper 4454, Corrosion 2014 Conference, March 9-13, 2014