| With the rapid development of the national economy, both onshore and shallowwater oil resources are depleting, the range of offshore oil exploration isexpending. Thus, deep water oil development has become an important frontier of theoil industry. While deep water engineering is so expensive, with high maintenancecosts and high operational risk, therefore, it is particularly important to control themarine corrosion of steel which is widely used in engineering structures in this newenvironment. As a common method of anti-corrosion, cathodic protection is stilleffective for corrosion controlling of steel structures in deep water. Currently,cathodic protection standards for shallow water have been very mature, but thedesign of cathodic protection for deep water environment is still unclear. Therefore, itis necessary to study the process of cathodic protection in deep water, and optimizethe cathodic protection design, in order to meet the requirements of maximum safetyfactor and minimum weight while taking into account the economy.(1) In this paper, single factor experiments were used to study the galvanostaticcathodic protection process of Q235carbon steel samples under various temperatures,dissolved oxygen concentrations (DO), flow rates and hydrostatic pressuresin seawater. The electrochemical properties of the samples after cathodic protectionwere analysed by electrochemical impedance spectroscopy (EIS) and linearpolarization resistance (LPR). The concept of protection factor was put forward toquantify the calcareous deposits protective properties. Furthermore, various surfaceanalysis techniques were used to study the calcareous deposits morphologicalcharacteristics, elemental analysis and crystal structure identification, includingscanning electron microscopy (SEM), energy dispersive X-ray diffractionspectroscopy (EDX) and X-ray diffraction (XRD).The temperature experiments showed that the protective properties of calcareous deposits increased exponentially with temperature. The critical temperature ofcalcium carbonate crystal form transition ranged from15to20℃. Calcite formedbelow15℃, while aragonite precipitated at above20℃. Calcium content in thedeposits increased with increasing temperature. The DO experiments showed that thedeposition rate of calcareous deposits increased with dissolved oxygen concentrationdecreasing. Under lower dissolved oxygen concentration, cathodic reactiontransformed quickly from oxygen into hydrogen evolution reaction in the initialpolarization, and as a result, the protective properties of the deposits decreased. Theflow rate experiments showed that the protection potential declined in two processes,corresponding to the two deposition processes of crystal. Protective properties ofcalcareous deposits decreased exponentially with flow rate. Due to the larger crystalgrain size than magnesium hydroxide, calcium carbonate deposited more diffcultlyon the surface under high flow rate. Therefore, calcium content decreased in thedeposits with increasing flow rate. Hydrostatic pressure experiments showed that theprotection potential changes was less affected by pressure, but the protectiveproperties of calcareous deposits increased and then decreased with increasingpressure where had best protective properties under pressure of2MPa. As thepressure increased, calcium content decreased in the deposits. When pressure washigher than5MPa, magnesium ions involved in the deposition of calcium carbonateand calcium occupies a portion of the lattice position, resulting in the formation of(Ca,Mg)CO3.(2) In the development of deep water oil, pipelines would pass through thethermocline environment while were laid across the ocean from shallow water todeep water. The cathodic protection potential and current distributions on the wholepipelines would be influenced because of the inhomogeneous medium. In order tooptimize the design of cathodic protection systems for pipelines passing through thethermocline environment in deep water, numerical method was adopted， andcorresponding thermocline physical and finite element mathematical models （FEM）were built. Supplemental boundary conditions were applied on the interfaces of two different mediums in the thermocline model. To avoid the simulation errorsintroduced by the variation of cathode polarization curve with CP service time inseawater, a dynamic boundary condition based on Ohm’s law was put forward on thecathode surface. The solution of the boundary condition was analyzed based on thelaboratory experiments. A very good correlation has been obtained betweenexperimental and calculated potential distributions. Therefore, it can be concludedthat the numerical model is suitable to reproduce the potential distributions obtainedfrom experimental tests.The cathodic protection potential distributions on pipeline were studied by FEMcalculation with different anode numbers and locations. The calculation resultsshowed that it supplied the most efficient protection when the single anode waslocated at the thermocline zone (76.1cm), and the potential could reach theprotection potential value of-0.8V(vs. Ag/AgCl) in the early60h. The case of twoanodes calculation results showed that it provided the most efficient protection whenthe anodes were located at the thermocline and deep zone. The pipeline potentials ofthree anodes case after60h protection have been more negative than that of twoanodes case after120h. |
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