Production of oil and gas in the Brazilian pre-salt will face several technical challenges. One of them that is a major concern is the presence of CO2 in high concentrations. Indeed, since the local regulatory agency requires increasingly stricter recommendations, it seems unlikely the possibility of simply release CO2 in the atmosphere. Besides that, as it happens since the 1970’s in USA, enhanced oil recovery (EOR) using CO2 might be a great opportunity and has to be considered. If that is the case, as soon as CO2 is separated from the oil in the top side, it has to be pressurized and transported through pipelines into the reservoir. The material of choice for that pipeline would be API 5L X65 since it is widely used and most available. The working pressure can reach up to 500 bar or more and it is an important issue to consider the fact that a suddenly depressurization due to a crack in the pipeline or a failure in a fastener or flange can promote a local abrupt decrease in temperature down to −60°C or even lower. That concern in addition to the fact that cathodic protection has to be used in the pipeline for corrosion control has the potential to produce embrittlement due to a combination of low temperature and hydrogen charging and ultimately lead to a catastrophic failure. The use of nickel to improve steel toughness has been used extensively and some grades or classes of nickel containing steels have been created for special applications. The aim of this work is to evaluate whether a nickel containing steel would be a more suitable material to manufacture high pressure CO2 transporting pipelines, taking in account a possible leak and the corresponding effect of a local and abrupt drop in temperature on the fracture toughness of the material. The effect of hydrogen due to cathodic protection on fracture toughness is also evaluated. The results indicated that under the experimental conditions and materials examined here the 9%wt nickel steel is not sensitive to low temperature and hydrogen, practically maintaining its original fracture toughness.
The aim of this paper is to establish a relationship between strain and acceleration, obtained at the center of the specimen, during resonant fatigue test of a full-scale X-65 seamless rigid pipe. The pipe was submitted to five different strain levels. Strain and acceleration on the outer surface of the specimen were measured and recorded using strain gages and accelerometers located at the center of the pipe, which is the point of maximum stress along the pipe structure during the resonant fatigue test. Thus, strain and acceleration values acquired from experimental tests were processed and compared with FEM modeling results. The results obtained by comparison between experimental and FEM modeling data were satisfactory, with a difference of approximately 0.05%. With these results, this work produced a reliable tool to obtain test parameters, considering different pipe geometries, in order to perform further resonant bending tests becoming the acceleration as an alternative to control test frequency instead of strain values.
Exploration and production of oil and gas in ultra-deep waters will face several technical challenges. One major concern of the static failure of flexible pipes is the occurrence of damage on external sheath and high resistance bandages that in some cases can generate radial instability of those structures. The new ultra-deep water fields will require a better understanding of those failure mechanisms and relationship between compressive loads and defect sizes to guarantee a safe operation, otherwise the expected service lifetime will not be achieved. Besides that, historical data of in-service failures of this type of equipment shows that several flexible pipes have to be early replaced due to missing of proper data to evaluate damaged structures. Therefore, even worse results are expected on ultra-deep water field application. Flexible risers comprise multiple structural layers, which combine leads to characteristics of resistance, tightness and desired flexibility, both for its installation and operation. Regarding the mechanical strength, flexible riser structure must withstand several load modes acting together or isolated. Within this context, axial compression acting individually or combined is the responsible for radial instability of flexible pipes. Radial instability occurs mainly when the flexible pipe suffers damage on the outer layers, which are responsible for radial strain resistance. This damage on the external sheath and bandages occurs due to the launching procedure, project or material failures, wearing, excessive loading, abandonment procedures or possible falling. Damages on the external bandage layer, together with axial compressive load may lead to catastrophic failures due to radial strains, as well known as birdcaging or lateral bucking, thereby leading to a complex local analysis in the search for solutions capable to predict riser behavior. Therefore, this study intends to build a relationship between the size of the defects and compressive loads for flexible risers that leads to birdcage formation, which consequently reduces the pipe expected life. The measurements were performed in full scale mechanical tests of two sizes of flexible risers. After that, finite element method models calibrated and validated with mechanical tests data, were used to extrapolate the results for other possible defect scenarios. The case studies for an analysis of the relationship between compressive loads and sizes of defects which lead to radial instability and consequently to pipe stiffness decreasing, were two flexible pipes of different sizes, widely used on offshore applications, with produced defects. Besides that, thirty-two conditions were analyzed through the model developed with variations in the size of defects, according to riser geometric limits in length and width. The results indicated that the radial instability in flexible pipes with defect on high resistance bandages does not reach the failure criterion for axial stiffness if compressive loads are limited to a threshold. Also, the defect size on bandage of flexible pipes subjected to compressive loads influences the radial instability, reducing the stiffness up to five times according to obtained results, especially depending on its length and without significant dependence of the width. One of the simulated conditions presented a change on the deformation distribution located near the manufactured defect, indicating another type of instability known as lateral buckling.
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