The fatigue driven criticality of steel catenary riser welds requires attention to all aspects of their production and inspection. The miss match of pipe ends to be welded is one of he main fatigue sensitive sites for failure. Weld profiles are also another fatigue driven failure issue. Allowable defect size requires extraordinary attention to inspection issues such as wall thickness variations, and ultrasonic technology issues including material sound velocity changes, ultrasonic beam profiles and pipe surface condition. Each of these critical welds requires a "pedigree" that clearly defines constraints while documenting the essential properties of the weld. This paper discusses some of the critical issues and approaches to ensure the consistent production and inspection of steel catenary riser welds.
Recovery of oil and gas from deepwater deposits requires the use of some type of floating production facility such as a Tension Leg Platform (TLP), FPSO, or other facility that is not rigidly secured to the sea floor. These facilities are designed to allow a certain amount of structure motion due to the environment. The steel catenary riser (SCR) pipelines that provide transport of product from the field to the facility or from the facility to onshore processing plants must be designed and constructed to maintain integrity even with long term exposure to fatigue damage from constant motion. All aspects of the design, construction, and installation of these suspended pipelines must address the potential fatigue related failure modes. Issues that need not be considered for export pipelines resting on the seas floor or for pipeline risers attached to the fixed legs of stationary platforms now become critical design and construction issues for these suspended pipelines. Misalignment of the pipe ends prior to welding, which can represent an area of localized stress, is normally not a design factor for standard pipelines, and, depending on the applicable codes, can approach values of 2 - 3 mm. For SCR, his misalignment must be limited to 0.5 - 0.7 mm. This requirement imposes limits on pipe end ovality and wall thickness variations.
Critical defect size for these SCR welds is much smaller than normally encountered for pipeline welds. With critical defect heights in the range of 0.5 -1.0 mm, the inspection requirements are much more stringent than those used for standard pipeline welds. To ensure accurate detection and defect sizing, specialized ultrasonic inspection is typically utilized as the inspection method. To provide sizing accuracy and resolution, many inspection variables must be closely monitored and controlled, including weld bevel tolerances, pipe wall thickness variations, ultrasonic beam profiles, and ultrasonic scanner stability as a function of position on the pipe.
There are many variables that must be addressed and controlled to ensure that SCR welds will maintain integrity during many years of exposure to cyclic loading. The variables must be addressed during all phases of fabrication and construction including material specifications, weld procedure development, pipe end sorting and matching, production welding and inspection, and installation. Most of the above variables are closely linked.
