Extending the life of steel catenary risers

John MacDonald and Lee Tran of 2H Offshore discuss a methodology to help operators extend the life of steel catenary risers in order to keep pace with new subsea tiebacks projects.

Intelligent PIGS  can be used to assess internal corrosion.

A number of production and export steel catenary riser (SCR) systems in the Gulf of Mexico (GOM) are approaching the end of their design life – typically 20-25 years – which is the anticipated time of use for the SCR that designers will use in their calculations. Operators are challenged to extend the life of existing SCRs to keep pace with subsea tieback wells. The answer to this challenge is to conduct a life extension assessment.

An overlay of a bathtub curve showing the component failure rate versus the number of years different GOM risers have been in operation shows two distinct trends. First, GOM riser component failures, to date, show recorded failures have been early in life due to unexpected circumstances. Second, the earliest installed SCRs are approaching the end of their design life, which means an increased likelihood of aging failures.

Premature failure of an SCR would most likely occur due to the presence of a variety of different anomalies that were not expected in the original design. In addition, the risers approaching the end of the design life were originally anticipated to be pulled out of service. However, a number of SCR systems are likely to be used beyond their original design life and will need a life extension assessment.

Life extension assessment

Life extension assessment activities.

Engineers need to reassess the situation based on the new projected design life. For example, consider an SCR that has been designed for 20 years’ service, which after the 18th year carries produced fluids at a much lower flow rate (typical at the end of life of a deepwater field). A new well is drilled nearby to the existing riser and is expected to produce for seven years. Rather than install a new riser, savings can be found by demonstrating that the existing riser can meet the combined 25 year service life. To accomplish this, a life extension assessment is carried out in three stages:

1. System design and operational information is gathered.
2. The current SCR condition is evaluated against “fitness for purpose” requirements.
3. The condition at the end of the new service life is predicted based on a combination of the existing conditions and remaining predicted design loads.

Determining whether life extension of an SCR system is possible requires understanding and evaluation of the potential failure mechanisms. These could be overstress, fatigue, internal or external corrosion, erosion, elastomer failure in the flexjoint, blockage or something that gets dragged across the SCR such as a mooring line.

Evaluation of each failure mechanism requires past operational data, direct assessment measurements, future expected operational data and system modeling. While the process is a straightforward one, in reality the information needed to complete each step is often interspersed with gaps and inconsistencies. For instance in the GOM, most of the infield flowlines and risers are not designed for in-line inspection, and where this is possible, the inspection technology often has inadequate resolution/accuracy to identify small but critical cracks or flaws in the system. Finally, where direct assessments such as a cathodic protection surveys or ultrasonic tests are completed, the results can sometimes be inconclusive.

Left side: Documented system failures. *Current age of first riser type installed.  Images from 2H Offshore.

Information gathering

If the possibility of a life extension exists, operators can take small steps during an SCR’s operating life to increase confidence in a life extension assessment meeting long-term needs. Below are suggested evaluation methods that can be used to gather information for life extension assessments. Both direct assessment and engineering analysis are discussed.

• Fatigue: strain monitoring or motion loggers (for vortex induced vibration) can be used to assess fatigue. Riser analysis software can be calibrated to accurately estimate fatigue life based on actual measured environments. For older SCRs, the analytical methods employed during the original design can be refined with newer software. If good fabrication records are available, then the actual tolerances and material properties can be used. Lab tests can also be conducted using the actual crude oil properties to generate reservoir-specific material crack growth Paris Curves for the fitness for service assessments.
• Internal corrosion: in-line inspection PIGS can be used for direct assessment from inside the pipe. Alternatively, newer tools such as ultrasonic inspection or computer-aided tomography can be used externally but only cover a small area. Corrosion probes and chemicals analysis can be used to evaluate general expected amounts of wall loss. Corrosion analysis models can identify the corrosion hotspots to calibrate general corrosion rates from coupons to more specific predicted wall loss values along the length.
• External corrosion: in-line inspection PIGS can be used for direct assessment for the outside diameter of the pipe as well. In addition, ROV visual inspection and a cathodic protection survey can be used to assess external corrosion. Advanced imaging technology allows corroded sections to be scanned and mapped for FEA analysis.
• FlexJoint elastomer degradation: Similar to the above, a combination of external survey, 3D mapping and predicted degradation rates based on pressure/temperature and rotation data can be carried out.

Information needed for a life extension assessment includes a combination of operational data and direct evaluation of the component, which is then input into an engineering analysis model for evaluation. The problem is where this data cannot be gathered, an engineer must rely more heavily on the underlying analysis assumptions, such as predictions, instead of actual measured data, which increases uncertainty.

Yesterday’s design vs. today’s design

Motion loggers installed on a riser help assess fatigue.

One of the main challenges with refining assumptions is accommodating today’s design requirements. As more failures occur in the industry, design regulations naturally respond with more stringent requirements, and life extension assessments are no exception to these updated requirements. One example can be seen through changes to governing codes and standards.

Hurricanes Ivan, Katrina, and Rita, which took place during the 2004 and 2005 US Atlantic hurricane seasons, caused significant structural damage to fixed and floating production platforms, semisubmersible and jackup drilling rigs. Among the destroyed were 123 fixed platforms and one floating platform. The US Mineral Management Service (MMS) was concerned about the platforms that suffered significant structural damage, as well as the potential for future damage to key energy infrastructure in the region. In response, the MMS issued new guidelines called the Assessment of Existing OCS Platforms and Related Structures for Hurricane Conditions, which updated the metocean criteria for use in assessing existing OCS platforms and related structures. Metocean changes such as this affect the expected loading on the riser system and must also be accounted for in life extension assessments.

Further examples of changes in design criteria include unexpected souring of the well or increased pressures and temperatures due to a newly drilled tieback, and changes in analytical methods used.

The bigger picture

Proving that riser life can be extended involves more than demonstrating the fatigue life of the pipe through analysis. Cathodic protection systems, coating systems and flexjoints need verification that they are operating as designed. The life extension of the mounting structure (which is part of the hull, and therefore subject to corrosion, overstress and fatigue) also requires verification. If the analysis predicts that a life extension assessment is not feasible, then other approaches can be taken.

Monitoring of riser motions can be used to determine the actual riser response to a particular environment. The finite element analysis is typically conservative, and having actual measured data allows better calibration of the analysis model. However, the most important item of any life extension assessment is the availability of past design data, knowing the current condition of the component and operational data. This information is key to more confident assumptions and reduced conservatism in the engineering models.

Integrity management (IM) as a process

One of the many advantages provided by a long-term IM program is the ready availability of condition assessment data for an SCR. An IM process by definition requires the periodic condition assessment of the equipment with a few examples described below:

• A design, fabrication, installation and operations dossier (DFIO) is a gathering of the reports, drawings, and assessments completed on a SCR through the life. The DFIO discusses each component, along with an exhaustive listing of documentation related to the component. Adding a hyperlink for each document in the list allows users to quickly find design information such as the baseline fatigue design report as well as documented anomalies.
• Regular inspections are used in IM programs to periodically assess the condition of a component. Inspections may include external visual inspections or nondestructive examinations to verify remaining wall thickness. Anomaly reports are typically used to document inspection or operational observations that are outside of the design intent, or could degrade the component’s function. The anomaly reports allow focused assessments and/or repairs through life that maintain in service SCRs in their optimal operating condition. In the context of a life extension assessment, inspection results and anomaly reports increase confidence that the SCR is operating as expected and not subject to premature degradation as opposed to life extension.
• Key performance indicators round out a regular inspection program providing context on how a system is performing as compared with design. For example, the measured wave spectrum can be compared to the spectrum used in the design analysis to evaluate if fatigue accumulation is expected to be less than that predicted. Or more directly, SCR motions can be directly monitored and used to demonstrate that actual motions are less than predicted in the analysis for a given sea state. Similarly, tracking corrosion inhibitor availability and corrosion coupons can give confidence (or otherwise) that corrosion rates are low.

A long-term SCR integrity program completes the first two steps of a life extension, information gathering and condition assessment, on a periodic basis. Significant time and effort is saved when compared to the challenge of going back and gathering details after 8-10 years in service.

Conclusion

As deepwater exploration grows, existing assets continue to age. Extending the life of these assets is economically favorable in comparison to bringing new ones online. Life extension includes evaluation of current conditions and updated predictions about future operations. The evaluation includes both directly measured data and engineering models to predict the remaining service life. With direct assessment being costly and at times inconclusive, the emphasis shifts to analytical assumptions. Nevertheless, with a good understanding of how SCRs operate and degrade, decisions can confidently be made about the service life of the system.

John MacDonald is a project manager at 2H Offshore in Houston. He has extensive experience in the integrity management of riser and subsea systems. He holds a BS in ocean engineering from Texas A&M University.

Lee Tran is a senior engineer at 2H Offshore in Houston. He holds a BS in ocean engineering from Texas A&M University and has five years of project experience in finite element analysis and fitness for service assessments of steel catenary risers.