Flexible pipes are used extensively for production and transport of oil and gas in the offshore industry. It is assumed that about 60% of the Norwegian production of hydrocarbons is flowing through flexible pipes on its way from the reservoir to the consumer. The pipes have a multi-layered structure of steel armour wires and polymer layers that are prone to corrosion and subsequent failures if the armour wires are exposed to water containing corrosive gases like CO2, H2S and/or O2. Water and corrosive gases diffuse from the bore into the annulus or enter the annulus through damages in the outer polymer sheath that protects the pipe from direct exposure to sea water. Several incidents of critical corrosion of the armour wires have been reported over the last 20 years. Some of the corrosion incidents are related to exposure to CO2 and oxygen when the outer sheath has been breached. However, incidents with brittle fractures and ruptures typically associated with hydrogen embrittlement and/or H2S corrosion have also been observed. With increased souring of many reservoirs, including North Sea fields, there is a growing concern for environmental cracking. The objective of the Kjeller Flexible Cracking I (KFC-I) project was to establish comprehensive understanding of the annulus chemistry and its effect on hydrogen uptake and environmental fracture of the armour wires. The research partners in the project were IFE and NTNU. The project was financially and technically supported by several operators, suppliers, and service companies. Two PhD students at NTNU were working full time in the project.
Six different armour steels were studied in the project. Corrosion experiments were performed to determine the corrosion rates and how the concentration of dissolved corrosion products in the annulus developed with time for various combinations of variables like temperature, CO2 and H2S partial pressure, and type of steel. The new understanding has been implemented in the test procedures used for fatigue testing and stress corrosion cracking testing by several vendors and operators. It is also used by IFE and NTNU in spin-off projects addressing fatigue and cracking of armour wires in flexibles.
The relationship between H2S partial pressure, concentration of dissolved corrosion products and consumption of H2S was studied. It was shown experimentally that there is significant consumption of H2S when the steel is exposed in a gas phase with a relative humidity of 60 – 100%. Reliable data for H2S consumption is regarded as very important for safety assessments and for cracking and lifetime predictions of flexibles. The result from the JIP is gaining increased interest amongst the participants and it has been suggested further work on H2S consumption in the gas phase and a common approach for controlling the H2S concentration during testing.
Absorbed hydrogen will reduce the mechanical properties of a metal. Combined diffusion and corrosion tests were run to determine how hydrogen is absorbed and diffuses into the different steels at different CO2 and H2S concentrations. TDS (Thermal Desorption Spectroscopy) was used to determine the amount of hydrogen and how the hydrogen was trapped in the steel. Stress/strain curves were established, and the fracture surface was characterized. Wires with both smooth surfaces and surfaces with sharp notches were exposed in the tensile experiments and the experiments were performed on micro- and nano level. A simulation model combining experimental data and geometry data was established. This model can be used to simulate different crack geometries (2-D) and to see how absorbed hydrogen affect initiation of crack growth (critical stress).
