Element’s pipeline testing facilities perform fatigue testing on steel catenary risers in our laboratory and on-site at the point of fabrication. Our testing containers can quickly be mobilized to various locations, making Element both flexible and faster than any other testing provider on the market.
Offshore oil companies commonly employ Steel Catenary Risers (SCRs) as a means of connecting surface installations with subsea pipelines. Steel catenary risers are subject to harsh environmental conditions that include cyclic loading as a result of wind and wave motion, tidal flow, vortex-induced vibration (VIV), and operational changes to temperature and pressure. These conditions make welds, and other stress concentrating features, sensitive to fatigue cracking when in service. To mitigate against premature failure while in service, fabricators are often required to test the fatigue resistance of any proposed weld procedures before their use in SCR installations.
Weld fatigue testing
Two approaches are used to assess the fatigue sensitivity of welds; small-scale testing on extracted coupons, and full-scale testing on entire welds:
- Small-scale testing, while cheaper, has several limitations. The residual stress profile of the weld is altered by coupon extraction, the applied stress profile is not representative of operation, and it can miss stress-sensitive features present in the original weld.
- Full-scale testing, conversely, allows the entire weld to be tested without alteration to its stress profile, using rotating bending stress that more accurately simulates the stress conditions seen in service. This stress can be applied by using large equipment to mechanical bend the welded pipe string, or by vibrating the test string near its natural resonant frequency (near resonance method). The advantage of this is that it can produce high-stress ranges in samples using relatively small, low powered vibratory motors. It tests the full 360 degrees of girth welds, and it can provide higher cycle frequencies (generally: 25-30 Hz), which speeds up testing.
Element’s mobile fatigue testing solutions
Element’s resonant fatigue testing rigs are fully integrated into modified shipping containers, making the rigs self-contained, modular, and mobile. Each container is equipped with its internal lifting system, and all equipment necessary to load, test and unload prepared test strings. The containerized design has several benefits over conventional lab-based models:
- It allows each rig to be mobile, allowing for quick and easy mobilization between Element and Client sites.
- It creates an intrinsic barrier between the test equipment and personnel by limiting access and improving security, as well as increasing safety for operators and site staff.
- It enables tests to be remotely monitored through the use of external workstations and CCTV cameras.
- It removes the need for overhead lifting during sample loading due to the presence of integrated dual hoist systems.
- It allows the rigs to be located in uncovered areas outside of site buildings, freeing up valuable floor space.
To find out how our experts can advise on weld fatigue testing on catenary risers, or to request a quote, contact us today.
Standards we test to
Although there is no standard covering fatigue testing on steel catenary risers, Client requirements are usually dictated by expected service conditions and design criteria.
The design assumptions are generally expressed as the requirement to meet a particular design curve given in standards and recommended practices covering resistance to fatigue in steel structures, such as DNVGL-RP-C203, BS7608, API RP2A-WSD or UK DoE Offshore installations: guidance on design, construction, and certification.
Element has experience with testing for conformance to all of these documents. Although SCR full-scale fatigue tests are conducted ‘in-air,’ ‘knock-down’ factors can be applied to testing requirements to account for environmental conditions. We can assist with the determination of these factors through environmental small-scale fatigue testing at one of our corrosion testing facilities.
Video: Fatigue Testing on Steel Catenary Risers
Steel catenary risers are subject to harsh environmental conditions and operational changes to temperature and pressure. Our video explains how Element performs fatigue resistance testing of components before their use in SCR installations.
This paper seeks to provide an overview of the approach to the validation of the fatigue performance of pipeline girth welds. In particular, it addresses the variables that need to be considered when planning a test program to qualify such welds' fatigue performance.
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