Fatigue Performance of High Strength Riser Materials Subjected to Sour Environments

07121-DW1403

Goal
The goal of this project is to assess the fatigue performance of new high strength materials required for ultra-deepwater high pressure, high temperature (HPHT) risers, under representative conditions. These materials include high strength steel and titanium linepipe, high strength forgings, and nickel-base alloys.

Performers
Southwest Research Institute, San Antonio, TX 78238
BP, Inc. (cost share contributor)

Background
The quest for offshore oil and gas is leading the industry into deeper waters and more hostile environments compared to those encountered in previous exploration and development projects. Increasing water depth results in production from reservoirs with increased pressures and in some cases reservoirs can exhibit high temperature and/or “sour” (hydrogen sulfide or H₂S) environments. These conditions limit the use of conventional materials, particularly for production risers, which must be capable of withstanding extremes of pressure and temperature. Safe access to these reservoirs requires the development of enabling riser technologies in the form of new materials with higher strength (lower weight), as well as increased resistance to sour production brine and seawater environments. These demands represent a significant technical challenge since while higher strength materials are available; they often exhibit increased sensitivity to aggressive environments. Several suppliers of tubular materials have developed higher strength products that would enable the design of light-weight risers needed for deepwater application. However, information is lacking on the fatigue performance of these new materials under environments representative of those anticipated in ultra-deepwater development areas. The objective of this project is to generate corrosion fatigue data on these new materials to fill this information and data gap.

The work carried out under this project will quantify the fatigue properties of high strength line pipe material (steel and titanium). An assessment of the fatigue properties of high strength forgings and nickel base alloys could occur in a subsequent project phase. Fatigue life (S-N) and fatigue crack growth rate (FCGR) behavior will be evaluated under various environmental conditions. The aim is to explore several different materials and systems and determine those which exhibit the best properties.

Potential Impacts
Assessing the fatigue performance of new high strength materials representative of those encountered in deep, sour service will assist operators in the design of new light-weight HPHT risers. Such designs will increase the technical feasibility of deepwater development projects, while at the same time decreasing development time and reducing technical risks. This will accelerate the speed with which ultra-deepwater development can safely proceed, and in some cases, will enable production in areas where it would not otherwise have been possible.

Accomplishments
Work on this project began on December 15, 2008. To date, three of the test materials have been procured and test specimens have been machined. Initial frequency-scan crack growth rate testing is underway at constant mechanical driving force (ΔK) in order to determine the optimum test frequency to be employed in subsequent FCGR and S-N testing. Several initial frequency-scan tests have been completed in seawater with cathodic protection and in sour brine environments.

A draft of the Technology Transfer Plan has been prepared. A draft Project Management Plan with work breakdown structure that concisely addresses the objectives and approach for each task with all major milestones and decision points has also been prepared. This plan is presently being updated since the Project Working Committee has identified additional sour brine testing that would be desirable and SwRI is working with RPSEA to fit these tests into the scope and schedule in view of the presently available resources. The Project Working Committee was established to ensure that the project results meet industry requirements and to facilitate technology transfer to industry. Specifically, in order to offset a portion of the costs of these additional tests into the current project, the planned baseline testing in laboratory air at SwRI will instead be performed by NETL at their Albany, OR lab.

Two additional test machines and chambers were set-up for frequency scan testing (in sour brine environment). All test machines and chambers for S-N environmental testing to be performed at SwRI were prepared and the machining of all S-N specimens for C110, C125S, and Q125 steels were completed. The team then developed a detailed test matrix for S-N fatigue testing in air, which included target stress ranges and expected fatigue lives.

Crack growth frequency scan tests at K-range of 20 MPa-m1/2 and R=0.5 on C110 and C125SS steels in both seawater and sour brine environments were completed. From there, the lab air fatigue crack growth rate (FCGR) testing in the 151 ksi steel was completed. These data have been analyzed and compared with the four steels previously tested; the overall trend is for the FCGRs, at a given ΔK, to increase systematically with increasing strength level: the largest increase is about 50% for increasing yield strength from 114 ksi to 151 ksi.

Current Status
The project has been completed. The final report is available below under "Additional Information".

Project Start: December 15, 2008
Project End: March 14, 2010

DOE Contribution: $ 800,000
Performer Contribution: $ 200,000 (contributed by BP)
In addition, most, if not all, pipe materials to be tested will be donated to the program by material suppliers.

Contact Information:
RPSEA – Jim Chitwood (jchitwood@rpsea.org or 713-372-2820)
NETL - Jay Jikich (Sinisha.Jikich@netl.doe.gov or 304-285-4320)
Performer Company – Steve Hudak (shudak@swri.org or 210-522-2330)

Additional Information

Final Project Report [PDF-6.87MB]