Method of Determining the Sizes of Corrosion Defects of Elements of Marine Oil and Gas Industrial Constructions on the Basis of Data on Temperature Contrasts

Offshore oil and gas platforms located on offshore fields are exposed to various stresses, the most intense of which is corrosive. Corrosion caverns arising in different areas of offshore oil and gas structures cause local stress concentration, and can cause overstresses of structural elements with its subsequent destruction. The most dangerous are corrosion defects “V”-shaped, capable of increasing the value of operating stresses more than 2 times. The article presents the results of experiments and a method of estimating the size of corrosion defects on the basis of data on the difference between defect-free and corrosion-affected areas. The use of this technique will allow to determine the sizes of corrosion damages with high efficiency and to estimate their influence on the safety of operation of offshore oil and gas platforms.


Introduction
Having established the thermal conditions on the outer and inner walls of the structural elements of offshore oil and gas facilities, let us turn to the consideration of an equally important phenomenon, namely, the appearance of a temperature contrast in areas with defects. On the basis of this phenomenon, a new scientific direction has been developed-thermography, which studies the differences in temperature fields between defective and defect-free sections of the material of various elements, and on the basis of an analysis of the data obtained, they conclude that the size of the defects.
Various authors predicted the appearance of this method for complex diagnostics of offshore oil and gas facilities and showed the advantages of this method in comparison with other methods of nondestructive testing. However, specific results and formulas allowing to determine the size and assess the danger of the detected defects in the marine oil and gas field structures have not been developed.

Analysis of existing researches
According to the authors of [1, 2] thermography with the method of sufficient accuracy, it is possible to identify corrosion defects, namely, in the planar form and the maximum depth projection, according to the formula, determine: (1) where: ∆Ldepth of corrosion defect, L-wall thickness, Т d и T nd -temperature in the defective and defect-free zones.
This formula is given in [1]. According to this formula, the ratio of temperatures in the defect and defect-free zones, i.e. The temperature contrast is always a constant.
However, the experiments carried out by the author showed some deviation from the results obtained in [2]. In figure 1 shows graphs of the temperature variation in the region simulating a single corrosion defect, and in the defect-free region that have been subjected to the heat flux. In the author's opinion, the temperature contrast of the defective and defect-free zones will depend on the power of the heat flux, i.e. the ratio of the density of the heat flux to the duration of its effect on the sites under study.
This conclusion is well confirmed by experiments conducted by the author. An experimental model was developed simulating corrosion lesions with various depths and surface dimensions. To satisfy the similarity condition of the experiment, these defects were filled with iron oxide. The photograph of the experimental setup is shown in figures 2 and 3. A heat flux of different density was directed to the defects. In all cases, there was a significant discrepancy between the rate of temperature growth in the defective and defect-free zones figure 5. Especially sharply there was a drop in temperature in the defective zone after the heating ceased. Figure 1. Graphs of temperature variation in defective (SP 1) and defect-free (SP 2) regions.

Aims and objectives of the research
The author poses the following questions:  Determine whether the value of the temperature contrast is a constant value that does not depend on the duration of the exposure and the density of the heat flux.  Determine whether it is possible to conduct thermal control of offshore oil and gas facilities due to the action of only solar radiation, or it will be necessary to apply the forced heating method (active thermal control method).  Establish a relationship between the temperature contrast, the dynamics of heat flux and the depth of corrosion defects.

Justification of conditions of the experimental research
An experiment was carried out to solve these problems. Consider the effect of heat flow on a single defect.    A thermal constant flow was sent to the defect, which revealed the following. Because the defect is modeled by a "V" or "U" shaped groove filled with oxide, then the maximum temperatures were reached at its apex middle figure 6. If the defect is removed from the center to its boundaries, the temperature tends to align with the defect-free surface. In addition, the ratio of the dynamics of temperature growth when the defect and defect-free zone is heated is not a constant value. In addition, the following feature was noted. After the heat flux ceases, a sharp drop in temperature occurs in the defect zone. Even taking into account that the temperature in the defect zone was much higher than the surface temperature of the defect-free zone, the rate of its fall after stopping the heating of the decrease was significantly higher. And after a certain period of time, the temperatures were equalized, and then the temperature in the defect zone decreased below the values of the defect-free zone. When cooling the sample, small values of the temperature difference, close in their values to the noise, were noted. Considering the peculiarities of the application of methods of forced and free heating, it can be said that the most obvious defects were found during forced heating, which is explained by a higher temperature contrast. To obtain a practically applicable formula for estimating the depth of corrosion defects, let us carry out the following experiment. In the wall of a pipe with a diameter of 133 mm and a wall thickness of 8 mm (with a uniform corrosion layer), drill holes of various diameters simulating corrosion damage in depth that were filled with Fe 2 O 3 . The relationship between the following parameters is studied:  The ratio of the depth of corrosion damage to the wall thickness.  Dynamics of the influence of heat flow (ΔQ·Δt).  Temperature ratio of defect-free and corrosion-damaging sections (temperature contrast).
Calculation of the density of heat flow taking into account the duration of its effect is carried out in accordance with the methodology given in [1]. The density of the heat flux was calculated with respect to the defect-free zone of the metal. The experiment was carried out 6 times with respect to each point. The results of the experiment are shown in figure 9.   We will process the experimental data. We calculate the density and dynamics of heat fluxes, as well as temperature contrasts for each of the samples simulating corrosion defects.

Results of the experimental research
As a result of the pilot study, the following is established:  The magnitude of the temperature contrast is greater the deeper the defect and the greater its geometric dimensions in the projection.  The magnitude of the temperature contrast is affected not only by the depth of the defect, but also by its dimensions in the projection to the area. For example, the defect 8x1.5 and 4x4.5 have the same temperature contrast 0.27. Therefore, the author proposes to introduce the notion of an "equivalent volume" of a defect.
We shall calculate the formula from the experimental values obtained, which will describe the relationship between the equivalent volume of the corrosion defect and the dynamics of the heat flux and the temperature contrast: (2) where: V деф -equivalent defect volume; D -dynamics of heat flow; С -temperature contrast; and constants k 1 =0,00463937; k 2 =-8,70561; k 3 =132,525; k 4 =1432,5; a=3748,46.