DIAGNOSTICS OF COMPLICATED THERMAL REGIMES OF AN OIL PIPELINE

ABSTRACT

The regimes under consideration can arise both due to a violation of thermal insulation in the above-ground sections of the pipeline and during the cyclic operation of the pipeline. Studies of complicated modes during the cyclic operation of the pipeline made it possible to obtain the ratio of the cycle parameters that ensure long-term, safe operation of the pipeline.

Keywords: oil pipeline, well, thermally insulated, hydraulic regimes.

The issues of ensuring the reliability of the transportation of solidified oil products through aboveground oil pipelines without track heating are of particular importance. Therefore, when designing and operating oil pipelines, it is necessary to take in account the possibility of the appearance of complicated modes. It should be noted that for giving parameters (), the solution of the problem described above in a stationary formulation does not present any difficulties. Moreover, a non-stationary problem can be solved using a computer.

For thermally insulated oil pipelines with optimal values of thermal insulation thickness k2 ~ 1 W / C), a  ~ 50 - 400 W / ( * ° C). Then from expression (6) it follows that the appearance of a layer of solidified oil is possible only at  ~ 50, i.e., when the oil temperature differs from the pour point by 1–2 ° C. During normal operation of the oil pipeline, the indicated difference is much larger. Therefore, with optimal thermal insulation, taking in account the process of oil solidification on the pipe wall is of no practical value. Therefore, the issues considered are important only in the case of insufficiently good thermal insulation, as well as in violation of insulation in a certain section of the pipeline. There can be many reasons for these situations. These include removal of thermal insulation during repair or its replacement, aging of insulation, violation of the surface waterproofing layer of thermal insulation, etc. Most of these reasons are not controllable. Therefore, it is impossible to predict the moment of change in the properties of thermal insulation and the value of the indicated changes. In this regard, the question of the exact solution of the problem of the formation of a layer of frozen oil loses its meaning due to the uncertainty of the coefficients of the model.

In this situation, an important question is the assessment of the possibility of the appearance of a complicated regime, as well as the question of diagnosing the moment of occurrence and the degree of complication.

To assess the possibility of the appearance of complicated modes during normal operation, the most unfavorable conditions can be selected as model parameters. These conditions include the case of complete destruction of thermal insulation, for example, as result of freezing of moisture that has got under the surface layer of waterproofing. The condition for the appearance of a layer of frozen oil on the pipe surface is determined by the inequality f (l)<0. From expression (6) we obtain

                                                  (1)

The condition for the impossibility of long-term operation of the oil pipeline is found from the condition f (l /  -l)  0, i.e.

,                                    (2)

where  is the oil temperature at which long-term operation of the oil pipeline is not probable.

 Let us estimate the conditions for the fulfillment of inequalities (1) and (2). At  = 20, = 60,= 20 ° С,  = -40 ° С, we have the condition for the appearance of a layer of frozen oil  < 20 ° С; the condition of impossibility of long-term operation   < 2.7 ° C.

It can be seen from the above estimates that the situation of impossibility of long-term operation of the oil pipeline is carried out only at temperatures close to the pour point. However, a layer of solidified oil on the pipe surface can appear even if the oil temperature is significantly higher than the pour point. Note that a violation of thermal insulation can occur anywhere in the pipeline. Therefore, when using conditions (16) and (17), it is necessary to substitute the final oil temperature. Hence, we find that if the final oil temperature is higher than 0 ° C, then the appearance of a section with damaged thermal insulation is safe during continuous operation of the oil pipeline.

The value of  decreases significantly with decreasing diameter, also because of the explicit dependence of on the diameter. Therefore, for pipelines of small diameter () can be quite large. For example, at k(x) = 50 W/(m2 ° С),  D = 0.1m,  = 16, 7. Then  ~ 10 ° C.

During the operation of oil pipelines, periodic operating modes are often encountered. In conditions of complicated thermal conditions, the issues of the admissibility of the periodic operation of oil pipelines are especially important. When studying thermal conditions during periodic operation, it is necessary to solve a non-stationary thermal problem. Suppose that periodic operation is determined by periodic stops of pumping, that is, during the time the oil pipeline is stopped, and during the time it works.

To simplify calculations, consider a short section that satisfies condition (1). Then, during the pumping period, we use model (5) - (6) with the flow temperature  =  = const. During the shutdown period, the thermal regime is determined by the system of equations:

Let us consider the periodic mode of operation of a short oil pipeline with parameters:   = 18, = 2,   = 18,  = -9, = 1.

As seen from Fig. 1. in the first three cases, long-term operation of the oil pipeline in a periodic mode is impossible.

Figure: 1. Typical dependences   (t)

Operation of an oil pipeline in a periodic mode is possible only if the following inequalities are met: , where  is some function, determined numerically. When the ratio of times  and  falls into the zone above curve , (Fig.2), operation in a periodic mode is possible.

Figure: 2. Dependence

The appearance of a section with damaged thermal insulation affects the thermal and hydraulic regimes of the oil pipeline. Therefore, by changing these modes, one can judge the appearance of such a site. However, this approach is only applicable if the leg length is large enough so that integral changes in temperature or differential pressure can be measured. In case of violation of thermal insulation in a short section, the characteristic changes in the thermal regime are negligible. An increase in the pressure drop due to a decrease in the cross-section will be measurable only when the radius of the solidified layer is close to or equal to the critical one, it will be practically impossible to influence the solidification process. Therefore, control over the thermal insulation of the oil pipeline is of great importance. Moreover, the control should be especially careful if inequality (1) is satisfied.

References:

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2. Geill F., Russel R.J. Pumpaoility of Residual Fuel Oils. "Industrial and Engi­neering Chemistry", 1954, vol. 46, №6, pp. 1264-1278.
3. Hagasi N., Tnouge K. Transient heat transfer through a thin circular pipe due to unsteady flow in the pipe. II J. of heat transfer, Vol. 87, Series C, № 4, 1965. p. 101-110.
4. Yukin A.F., Hasanov M.R. Modeling of thermal processes during heating of pipelines with solidified oil product. // Oil and gas business,2003.-№1.-С. 213-221.