Corrosion behavior and electrochemical corrosion of a high manganese steel in simulated marine splash zone

The corrosion behavior of a high manganese steel in simulated marine splash zone environment was studied by dry-wet cyclic corrosion experiment and electrochemical experiment. Corrosion kinetics, composition, surface morphology, cross-section morphology, element distribution, valence state, polarization curve and electrochemical impedance spectroscopy were analyzed with the aim of characterizing the characteristics of corrosion product films. The results show that in chloride-containing environment, in the initial corrosion products, Mn oxides with porous structure lead to higher corrosion rate. As corrosion extends, the formation of alloy element oxides in corrosion products changes the corrosion properties of rust layers at different stages. Mo oxides form a stable passivation film, which reduces the influence of chloride ion on corrosion. Ni oxides in the inner rust layer facilitate the transformation of goethite, and Cr oxides in the outer rust layer increase the densification of the rust layer. The stability and compactness of Fe3O4, α-FeOOH and FeCr2O4 in the later corrosion products inhibit the corrosion action of manganese iron oxides and slow down the corrosion rate. With the corrosion durations, the corrosion current density of the sample with the corrosion product film first increases and then decreases, and the corrosion potential first moves negative and then shifts in a positive direction subsequently, indicating that the protective effect of the corrosion product film is gradually significant.

Corrosion Product Film of a Medium-Mn Steel Exposed to Simulated Marine Splash Zone Environment

The corrosion behavior of a medium-Mn steel in a simulated marine splash zone was studied by a dry–wet cyclic corrosion experiment and electrochemical experiment. The corrosion products were characterized by corrosion rate calculation, composition detection, morphology observation, element distribution detection, valence analysis, polarization curve, and electrochemical impedance test. The results show that the corrosion products of the sample mainly include γ-FeOOH, FexOy, MnxOy, and a small amount of (Fe,Mn)xOy, and the valence state of iron compounds and manganese compounds in different corrosion stages changed obviously. In the initial corrosion products, Mn is enriched significantly and facilitates the electrochemical reaction of corrosion process. The content of Ni in the inner rust layer is high. The semi-quantitative analysis of the corrosion product elements shows that the atomic concentrations of Cr and Mo increase significantly in later corrosion products, indicating that the dense isolation layer formed by alloy element compounds in the corroded layer is the main factor to improve the protection ability of the rust layer at the end corrosion stage of the sample. With the corrosion durations, the corrosion current density of the sample with the corrosion product film first increases and then decreases, and the corrosion potential first moves negative and then shifts in a positive direction subsequently, indicating that the protective effect of the corrosion product film is gradually significant.

Riser Robotic Inspection - Reducing Safety Risk While Improving Efficiency and Effectiveness

Integrity management is an ongoing lifecycle process for ensuring safe operation and fitness for service of offshore oil and gas production systems, including risers. Risers offer a means of transporting fluids between subsea wells and the host platform crossing the splash zone that is probably the most critical region for corrosion and exposure to external damages. Furthermore, with their proximity to the personnel on the platform and to the topside equipment, risers are considered safety critical, and are therefore, subject to planned inspections followed by an engineering assessment of the findings. This paper discusses the motivation and business driver for developing and implementing a new and cost effective risers’ inspection methodology in the splash zone based on innovative robotic platforms. The technical features and the capabilities of the robot are outlined. Traditionally, risers’ inspections are carried out by rope access technicians and divers or ROV below the water line using conventional technologies as spot ultrasonic thickness measurements, traditional radiography and visual assessment. This type of inspection is based on a first visual assessment followed by NDE testing only if some finding is spotted. Internal defects or defect under coating, e.g. splashtron, can be easily overlooked, compromising the entire assessment process. Additionally such activities are often limited by accessibility, weather, and Personnel On-Board (POB) accommodations, but primarily they involve risks to inspector's safety. Backbone of the presented methodology is the use of a robotic crawler that has the key advantage to inspect autonomously the risers, navigating over obstacles like clamps and supports. The robot can carry a variety of payloads for visual inspections, surface profiling, and NDE examinations with the ability to scan large surfaces with or without coating and detect internal and external defects. It can operate in the topside, splash zone and subsea sections of the riser. The inspection data are processed in real time for an immediate assessment of the integrity of the asset. Examples are presented and comparison is made between traditional inspection methodologies and robotic autonomous methodologies to demonstrate the improvement of inspection effectiveness and efficiency. The paper also discusses the potential areas of future development, which include Artificial Intelligence (AI) algorithms to further automatize the process and methodologies of risers’ inspection and data analysis.

Permeation Characteristics and Surface Accumulation of Chloride in Different Zones of Concrete along Altitude in Marine Environments

The accumulation characteristics of surface chloride in concrete in different zones are different in the marine environment. A series of laboratory experiments were conducted to investigate the surface chloride and permeation characteristics of concrete in a simulated marine environment. The experimental results indicated that the surface chloride and chloride profiles of concrete in different zones of marine environment decreased in the following order: tidal zone > splash zone > submerged zone > atmospheric zone. The width of the ascent zone of Cl− concentration at tidal and splash zones was far less than that of the influential depth of moisture transport (IDMT), and the range of convection zone was dependent on the IDMT. Cl− at splash and tidal zones penetrated into concrete as a bulk liquid by non-saturated permeation driven by a humidity gradient. The change of chloride profiles in concrete along the altitudinal gradient was consistent with that of the cyclic water absorption amount (CWAA). The transport rate of chloride was the highest at the highest point of the tide.

Experimental Wave Flume Tests in ROV-Wave Interaction Effects on the Line Tension for a Work Class ROV in Splash Zone

Integrity and stability of Remotely Operated Vehicle (ROV) when passing through the splash zone is one of the main concerns in the design of an ROV-umbilical system. Due to the lightweight nature of ROV in water, the umbilical experiences repetitive rapid transitions between slack and taut as the ROV travels through the splash zone. These rapid transitions induce tension spikes in the umbilical, namely snap forces, that can endanger the launch and recovery of an ROV. Therefore, it is important to ensure that the tension spikes do not exceed the safe working load of the umbilical. In this study, launch and recovery of a deep-water work class ROV are experimentally investigated using a 1:10 scaled ROV model through a series of wave flume tests. Different regular and irregular waves are generated in the flume while the ROV model is hung over the flume in four different positions. The tension time-history in the line is measured and recorded using a load cell at the top-end of the line. A simplified numerical model for launch and recovery of the ROV is developed and the numerical results are compared with the experimental ones. It is shown that the presented simplified model can be accurately used for analysis of launch and recovery of the ROV.