Fitness for Service of Flexible Pipes After Long-Term Subsea Storage Full of Seawater

Multi-layered flexible pipes have long been important components of PETROBRAS’ subsea production systems. Approximately 14,700 km of these pipes are in service throughout PETROBRAS’ assets in 2024, and more is planned to be laid down in the coming years. Flexible pipe design aims at complying with many functional requirements under safe conditions and is mostly regulated by API 17J Standard [1], along with other engineering rules endorsed by independent verification agents. The innermost metallic layer named "interlocked carcass", made of steels, is designed to resist collapse under external pressure. Therefore, any noticeable damage thereof is believed to cause some strength reduction. In general, stainless steels in carcasses are designed to resist most of the fluids conveyed by the pipe. If present in significant concentrations, however, chlorides and oxygen may trigger pitting and crevice corrosion, whose kinetics depends on many factors such as their concentration and renewal, the duration of exposure, the temperature, the pH and hydrodynamic conditions. This susceptibility limits the long-term exposure of interlocked carcass to seawater, which stands for a restriction with side effects on the overall logistics of flexible pipes. In 2000s, the manufacturers carried out studies on the corrosion of materials used in carcass and the manuals accompanying the recent pipe deliveries mention strict usage conditions. For example, carcasses made of AISI 304L stainless steel should never be left full of seawater for more than 30 days in shallow waters according to their advice. The requirements are coherent with the principle that, if the engineering fails to quantify the residual strength of a corroded component with confidence, no noticeable corrosion is considered acceptable.