Heavy metals are major pollutants that pose threats to wetland environments. In the present study, surface sediments from wetlands vegetated by invasive species Spartina alterniflora in the Yellow River Delta were collected and determined for the mass fractions of Co, Ni, As, Cd and Pb. Results showed mass fractions of Co, Ni, As, Cd and Pb in the sediments of the S. alterniflora communities ranged from 8.5 to 16.0, 13.9–27.9, 3.2–13.8, 0.08–0.24, and 17.6–37.5 mg/kg dw, respectively, generally presenting an order of Pb > Ni > Co > As > Cd. The levels of heavy metals in sediments in the S. alterniflora communities were higher than those in the wetland vegetated by the native plant species Suaeda heteroptera. Correlations among metal elements were highly significant, suggesting that they might have the same sources. Clay and TOC were important factors affecting the spatial distribution of metals. The I_geo values of the investigated elements in the sediments were frequently lower than 0, revealing the slight pollution status of these metals. Relatively slight values of E_rⁱ and RI suggested that the potential ecological risks caused by the 5 metals were low. Our findings could provide a better understanding of the correlation between metal pollution and bio-invasion in wetland ecosystems.

In order to ensure deep-water flowline safety, this paper combined the axial temperature distribution model of the submarine pipeline and the distributed parameter circuit model of the skin effect electric heat tracing system; such work is conducive to proving that the heating effect of the skin effect electric heat tracing system depends on the distributed circuit parameters and power frequency of the system. Due to the complexity of the power supply device, the frequency cannot be increased indefinitely. Therefore, for the case that the input of the skin electric heat tracing system is constrained, a generalized predictive control algorithm introducing the input softening factor is proposed, and the constrained generalized predictive control strategy is applied to the electric heating temperature control system of the submarine oil pipeline. Simulation results demonstrated that the control quantity of the skin effect electric heat tracing system is effectively controlled within a constraint range, and also the values of heating power and power frequency are obtained by theoretical calculations rather than empirical estimations. Moreover, compared with the conventional control algorithm, the proposed constrained generalized predictive algorithm unfolds more significant dynamic response and better adaptive adjustment ability, which verifies the feasibility of the proposed control strategy.

In order to ensure the safety of fluid flow in deep-water submarine pipelines, a safe and energy-saving built-in skin effect electric heat tracing technology was adopted as the thermal management strategy. By analyzing the multi-physics conditions of the electromagnetic field-temperature field in the heating state of pipelines, the corresponding control equations were derived, and the physical model was established using the transient electromagnetic-temperature coupling field of Ansys Workbench. Simultaneously, we calculated the equivalent parameters of the oil pipeline and heating cable as the load when the system is heated to different temperatures. Thus, the main circuit parameters of the power supply for the built-in skin effect electric tracing system were designed, thus providing the theoretical guidance for the key performance optimization. For regulating the output power of the built-in skin effect electric heat tracing system, the nonlinear active disturbance rejection control strategy was newly adopted to achieve closed-loop power regulation based on the Buck chopper, and the system has the advantages of fast dynamic response and strong anti-interference ability. Finally, the feasibility of the proposed control system was verified by simulation.

In actual operation, the skin effect electric heat tracing system is powered by the inverter circuit, and the higher harmonic components are inevitably introduced. There is not much research on the influence of harmonic current on the electric heat tracing load loss. In this paper, the harmonic distribution under PWM driving mode is analyzed, and the load loss of the electric heat tracing body under harmonic current is derived. The co-simulation method of PWM inverter circuit and skin electric heat tracing body coupling is proposed. The simulation results of current and loss of co-simulation method and sine wave driving method are compared. The results show that the parameters of PWM driving mode increase to different degrees, which proves the correctness of the joint simulation method. At the same time, the influence of the output characteristics and modulation parameters of the PWM inverter circuit on the load loss of the electric heat tracing body is analyzed, which provides a theoretical basis for the determination of the parameters of the inverter circuit. In addition, combined with multi-physics coupling analysis, this paper obtains more accurate temperature distribution results of the electric heat tracing system without scaling the size of the electric heat tracing ontology. Not only can we intuitively understand the material selection and hot spot configuration of the electric heat tracing components, but also further understand the influence of current harmonics on the load loss in actual operation, so as to better realize the efficient heat tracing of the skin electric heat tracing system.