There are more than 3500 dynamic unbonded flexible risers in operation worldwide. In addition, there is a considerable number and kilometres of flexible flowlines and jumpers on the seabed. The average riser age is more than 10 years and a number of risers are soon reaching their original design service life of 15-25 years. There is a need to learn more about the time driven degradation processes in flexible risers in order to ensure continued safe operation, and to meet the increasing demand for service life extension. By investigating a large range of damaged and intact unbonded flexible risers from West Africa, UK, The Netherlands and Norway, 4Subsea is continuously improving the understanding on how flexible risers and flowlines in operation degrade over time. As the variability of the degradation and failure development is significant, a high number of observations is needed to establish consistent trends and basis for reliable analysis and assessments. Trends of emerging challenges are observed through these investigations, however most of all it is recognized that the complexity of the annulus environment, corrosion processes and polymer degradation is high and deserves continuous attention. 4Subsea has investigated some 60 000 m of flexible risers, flowlines, jumpers, cables and umbilicals from 13 fields and 6 operators. Examination of used flexible pipes by 4Subsea and others have to a large degree validated the design assumptions and conclusions, however there are exceptions. Variability in degradation mechanisms and their development is found to be significant. Some weaknesses are identified and improvements are implemented in the recent updates of API specifications and recommended practices. The referenced investigations are performed for several operators, and it is seen that sharing information will give the operators considerable benefits. Cooperation initiatives overcoming confidentiality issues are progressing and will give improvements to reliability and safety for flexible pipes. Combined, significant improvements in basis for safety-, reliability- and life time analysis for the operated assets are experienced. In broad terms the experiences show that polymer ageing issues, and in particular challenges related to high temperature operation, need to be prioritized when investigating the possibility for life extension. Current methods for degradation modelling are uncertain. Tensile armour wire fatigue analysis needs to take into account dynamics from calibrated models, response measurements and advanced stress predictions as well as correct annulus environment to be representative for the real exploitable life of the structures. Pitting corrosion and loss of material due to corrosion are key factors to take into account in fatigue modelling. Serious corrosion issues are correlated with oxygen access as typically experienced in connection with large external sheath damages near the water surface. A successful life extension for a particular flexible pipe requires a thorough process starting with an assessment of current status of the flexible pipe with basis in original design data, operational data records, inspection, test and monitoring data. When the assessment of the current status concludes that the complete system is well suited for further safe operation, a prognostic assessment is performed based on best-available information on future service conditions. As the lifetime of a flexible riser or pipeline may be several decades, the premise for assessing safe operation may have to be re-established, incorporating new methods, knowledge and differing industry experiences. Experiences from investigations of more than 75 used risers and umbilicals provide a unique basis for focusing on life time assessment issues, as well as target areas of inspection, monitoring and research. This also involves various compensating measures such as external sheath repair, improvement of annulus vent flow, online condition monitoring or special inspection campaigns.
According to published statistics for flexible pipes, penetrating holes in outside covers of flexible pipes is one of the most frequent damage mechanisms. The corrosion and fatigue performances of tensile and pressure armour wires are directly influenced by the fluids in the pipe wall annulus. There are several incidents where cover damages have led to serious pipe failures. In this perspective the best strategy is to avoid cover damage, but for those cases where damage occurs it is essential to have systems in place for early detection, as well as capabilities for repair. Holes in the outside cover can create a range of different conditions in an annulus depending on location of the hole, configuration of the pipe and service conditions. CO2 driven corrosion in a confined water filled annulus has been investigated extensively and reported by several authors to give very low corrosion rates. However, the environments that armour wires are exposed to in certain parts of an annulus may differ significantly from confined water with CO2. One obvious example is the region around a penetrating hole in the outer cover where there may be repeated ingress of oxygenated seawater or air that mix with CO2 in the annulus. Such environments could cause high corrosion rates that may explain some observed failures. In many cases it is difficult to quantify the annulus environments precisely and suitable corrosion models have not been established. The consequences are large uncertainties in the prediction of corrosion type and rate, giving challenges for integrity assessment. This paper will identify and discuss unresolved corrosion issues related to outer cover damage linking it to field experience. Needs for developing further knowledge and models will be addressed. Efficient and reliable methods for repair of outer cover damage that can be mobilized soon are essential for restoring the integrity of pipes with damages to outer covers.
During Statoil’s recent riser replacement project more than 30 used risers have been dissected onshore [1]. The main objective has been to reveal the root cause of carcass axial tearing failures. However, this also gave the opportunity to investigate details of other carcass damages, armor corrosion, annulus environment, external sheath breaches and polymer ageing as described in [5], [6] and [7]. This paper discusses some of the new knowledge obtained from the dissections. New tools and methods have been developed for carcass pitch measurements in order to determine axial movements, maximum load level seen by the carcass and determine the utilization against tearing. Offshore free-volume annulus testing and gas sampling is compared to liquid sampling and actual liquid found when dissecting, with significant deviations observed. Expected annulus environment is assessed in light of actual observed corrosion, with generally less corrosion than expected, however at some few selected areas significant corrosion attacks are found. Residual stresses and condition of polymer layers is quantified, with generally more degeneration and larger changes than anticipated before riser recovery.
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