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Corrosion Resistant Aluminum Components for Improved Cost and Performance of Ultra-Deepwater Offshore Oil Production
Project Number
FWP-072971
Last Reviewed Dated
Goal

The goal of this project is to develop critical technologies that will support the industry’s development of aluminum risers for ultra-deepwater drilling. The primary technical objective to support this project is the development of high strength, corrosion resistant weldments that connect 7XXX series aluminum riser flanges and pipes. A secondary technical objective with this project is the development of technologies that will mitigate the corrosion of 7XXX series alloys. Theses technical objectives will be accomplished by: 1) Development of a friction stir welding process to join forged 7XXX aluminum flanges with extruded 7XXX pipes. 2) Establishing a post weld heat treatment schedule for 7XXX aluminum joints to improve corrosion resistance and weld strength. 3) Exploring cold spray applications as a corrosion mitigation strategy. 

Performer

Pacific Northwest National Laboratory (PNNL) – Richland, WA 99354

Background

Commercial oil production from conventional deepwater (<7000 feet) resources has been successfully demonstrated on three separate aluminum riser systems operating in Brazilian waters. More than 12 years of continuous service at depths up to 7200 feet have shown that low strength aluminum risers are viable in seawater environments.  Moving aluminum riser technology to ultradeep water (>7000 ft) requires the use of higher strength aluminum, such as 7XXX alloys, which present several critical challenges. For example, 7XXX aluminum risers were used to achieve water depths of 9,900 feet in the Perdido Oil Field, but corrosion issues encountered during the project prevented long term use of the aluminum riser string. One of the largest challenges is riser strength. The relatively low strength of aluminum alloys currently deployed in deepwater applications is insufficient for ultra-deepwater. In order to use existing deepwater riser designs (low strength aluminum) for ultra-deepwater applications, the flange and pipe wall thicknesses would have to be significantly increased to support the higher tension load (due to increased depth) and increased fatigue loading inherent to longer riser strings. This added mass would entirely negate all of the weight saving advantages of aluminum. Changing to 7XXX series aluminum requires development of the welding process that joins a forged flange to an extruded pipe, and post-weld heat treatment schedules to improve corrosion resistance and strength of the weld. 

Another challenge is seawater corrosion. Deepwater aluminum risers rely solely on sacrificial anodes to mitigate corrosion. Industry views this approach as too risky for ultra-deepwater applications due to the increased surface area of the longer riser string and greater current fluctuations across extreme water depths. To mitigate this risk, a defense-in-depth approach is proposed where a corrosion protection strategy will employ both sacrificial anodes and a corrosion coating system applied to the fabricated riser.

Detailed financial analysis has shown that replacing steel production risers with aluminum is a promising approach to dramatically improve the economics of oil production from ultra-deepwater resources. For example, extending the offshore depth from 4000 feet to 9000 feet would cost an estimated $33M using aluminum risers compared to $200-300M with conventional steel risers. This is because the use of steel risers requires that rigs be significantly modified to increase deck load capacity; an extremely expensive proposition. 
 

Impact

The replacement of steel with aluminum for construction of risers will greatly improve the economic feasibility of oil production from ultra-deepwater resources. Successful completion of this project will strengthen the upstream sector by enhancing the ability of the oil and gas industry to target resources that are currently beyond economic reach.

Specific benefits of aluminum risers include 34% weight reduction compared to steel, higher strength to weight ratio, lower string tensioning force, reduced deck load per foot of riser, and reduced transport costs which all lead to deeper and more cost-effective drilling capability. Despite this largely untapped global, and particularly US, opportunity for ultra-deepwater oil production, there remain significant materials, joining, and corrosion challenges that are currently preventing the deployment aluminum risers for ultra-deepwater applications. This project aims to address these technical barriers, thereby accelerating the transition from steel to aluminum production risers by industry.
 

Accomplishments (most recent listed first)
  • Completed baseline saltwater corrosion testing per ASTM D1141 and ASTM G31 at West Moreland Testing and Research Inc. on of 1” thick friction stir welded AA7175. 
  • Placed contract with Penn State Applied Research Laboratory for depositing cold spray Ni, Al-Zn, and Al-A2O3Zr2O3 corrosion barriers. 
  •  Achieved FSW joints with tensile yield strength having 65% of base metal strength in 1” thick AA7175 (end of project target = 80%). 
  •  Demonstrated ability to precisely control FSW tool temperature at 425C, 450C, and 475C during joining. 
  • Xymat Engineering currently fabricating a sub-scale demonstration riser for display at PNNL.
     
Current Status

PNNL is currently on track with all milestones and deliverables detailed in the statement of work.

Project Start
Project End
DOE Contribution

$1,500,000.00

Performer Contribution

$0.00

Contact Information

NETL: David Cercone (david.cercone@netl.doe.gov or 412-386-6571)
PNNL: Glenn Grant (glenn.grant@pnnl.gov or 509-375-6890)
 

Additional Information