No abstract

Flexible risers are critical equipment for the production of oil and gas in offshore fields around the world. The annulus region may condense acidic water containing H 2 S exposing the tensile armour steel wires to a sour environment, which may promote sulphide stress cracking (SSC) and/or hydrogen-induced cracking (HIC). This work aims to evaluate through electrochemical and constant displacement tests the effectiveness of thermal spray aluminium (TSA) coating to mitigate SSC and HIC in high strength steel used to manufacture the tensile armour of a flexible riser. Electrochemical tests confirmed the anodic behaviour of the coating against the steel and estimated a service life of 2.5 years. The constant displacement test indicated that the non-coated samples failed by SSC although some cracks were also found in the samples mid-section due to HIC. For Al-coated samples, no signs of SSC and HIC were observed. The TSA coating showed a dual barrier effect, hindering either corrosion of steel or hydrogen up-taking, and neither SSC nor HIC was observed. Samples with a coating defect were cathodically protected and no HIC was observed.

In general, tensile armour wires of flexible pipes that are designed for sour applications have their strength limited to 850 MPa due to the possibility of embrittlement phenomena to occur. A Thermally Sprayed Aluminum (TSA) coating 250 μm thick was applied to high strength steels with UTS of 1470 MPa and YS of 1280 MPa. Three specimens conditions were evaluated: full coating, no coating and coating with a designed defect. The load was applied using a four point bending fixture, maintaining a constant stress of 90% of material’s yield strength. All tests were performed in accordance with recommendations of NACE TM 0177 method B. The test solution was distilled water with NaCl 5.0% saturated with a gas mixture of 10,000 ppm of H2S in balance with CO2 during 720 hours. It was observed that samples without coating were more susceptible to the effect of the environment presenting higher degradation and failure. The fractures presented typical characteristic of the Sulfide Stress Corrosion Cracking (SSCC). Furthermore, it was detected parallel cracks to the surface of the wires indicating the embrittlement phenomenon of Hydrogen-Induced Cracking (HIC). On the other hand, coated samples with and without defects did not fail during the 720 hours of testing. A posterior non-destructive testing and a metallographic analysis did not identify the presence of cracks. These results were attributed to the physical barrier of the aluminum coating and the cathodic protection generated by the preferential aluminum corrosion. This preliminary study shows that TSA coatings can be a good alternative to increase the corrosion resistance of armour wires in sour environments allowing the application of higher strength steels.

Subsea umbilicals are a vital component for offshore operations, being responsible to provide hydraulics, fluid injection, power and/or communication services to and from subsea facilities. They can be designed for both static and dynamic applications and can be deployed in harsh environments where potentially damaging factors such as corrosion, tension, pressure, clashing, entanglement, among others are commonly present. For armored umbilicals, the tensile armor wires of these structures are responsible for the tensile strength and its fatigue life is a key design consideration for subsea umbilicals subjected to dynamic loading. Despite its considerable importance, there is no standardized methodology for conducting fatigue tests of tensile armor wires of umbilical cables. Some general guidelines are given in technical publications such as ISO 13628-5 standard and DNV’s recommended practice F401, but there is still need for more detailed information regarding fatigue testing of these components. This lack of methodology for fatigue testing of circular wires that comprise the tensile armor of these structures lead to the motivation for this study. The main advantage of using specific data from an adequate testing methodology of tensile armor wires — instead of using standard SN curves — is to reduce conservatism, mainly considering damage calculations for the overall structure. Additionally, with the increasing operating water depths, an excessively conservative design can lead to a considerably heavier structure which will induce higher topside stresses. Therefore, it is of great importance to have a well defined methodology to obtain SN curves that can be used in the design phase of umbilical cables which can lead to an optimized structure and also keep an adequate degree of conservatism. In this work, it will be presented several attempts to achieve an adequate methodology to obtain SN curves for circular tensile armor wire of umbilicals and the final methodology adopted will be described in detail. Tensile armor wires in a wide range of diameters and strength were tested in order to validate the proposed methodology. Additionally, the normalized fatigue results from several tests are also presented and compared to SN curves commonly used during the design phase of umbilical cables.