научная статья по теме О ВЛИЯНИИ КОРРОЗИОННОГО ВОЗДЕЙСТВИЯ НА РАЗВИТИЕ УСТАЛОСТНЫХ ТРЕЩИН НА МОРСКИХ НЕФТЕГАЗОВЫХ СООРУЖЕНИЯХ Науковедение

Текст научной статьи на тему «О ВЛИЯНИИ КОРРОЗИОННОГО ВОЗДЕЙСТВИЯ НА РАЗВИТИЕ УСТАЛОСТНЫХ ТРЕЩИН НА МОРСКИХ НЕФТЕГАЗОВЫХ СООРУЖЕНИЯХ»

Староконь И.В., кандидат технических наук, доцент Российского государственного университета нефти и газа им. И.М. Губкина Космачев К. А.

(Московский государственный университет им. М.В. Ломоносова)

О ВЛИЯНИИ КОРРОЗИОННОГО ВОЗДЕЙСТВИЯ НА РАЗВИТИЕ УСТАЛОСТНЫХ ТРЕЩИН НА МОРСКИХ НЕФТЕГАЗОВЫХ СООРУЖЕНИЯХ

В статье рассматривается особенности процессов коррозии для опорных блоков морских стационарных платформ. Предлагается коррозионные дефекты рассматривать как концентраторы напряжений, изменяющих напряженное состояние элементов опорного блока и характеризуемых величиной коэффициента концентрации напряжений (К). Предлагается метод численного расчета этого параметра. Обработка статистических данных отчетов о проведении диагностических обследований позволила установить, что наиболее интенсивно коррозионный износ идет в зоне переменного смачивания. Используя полученные данные отчетов, был проведен численный расчет величин коэффициентов концетрации напряжений при различных параметрах коррозионных дефектов.

Ключевые слова: усталостные трещины, морские нефтегазовые сооружения, коррозионное воздействие, переменные напряжения, коэффициент концентрации напряжений.

ABOUT IMPACT OF CORROSION ON THE PROGRESS OF FATIGUE CRACKS IN THE OFFSHORE OIL AND GAS INSTALLATIONS

Aggressive sea water environment, which is typical for operating conditions of offshore structures, is caused metal corrosion. In the presence of stresses, caused by different loads, corrosive effect leads to the formation and development of cracks on offshore structures that could lead to the destruction or leakage with subsequent flooding of the entire structure. In this article we will focus on corrosion fatigue of offshore structures and influence of this phenomenon on the redistribution of stresses in components of offshore constructions. Problem of fatigue failure offshore oil and gas structures was studied by both domestic and foreign authors. A detailed analysis is given at reports [1 , 2, 3 , 4, 5 , 6, 7 ]. Corrosion in sea deposits has its own specific characteristics and depends on various factors. With increasing of temperature increases the rate of galvanic corrosion that is caused by occurrence of difference thermogalvanic pairs due to the difference of temperature of individual sections of the same structural element MNGS [4]. Heated under the influence of solar radiation to a higher temperature, surface plot of constructive elements of offshore oil and gas structures transformed to anode is subjected to a more intense corrosive wear in contrast to underwater areas with small temperature differences in structural elements [4]. Significant effects on the rate of corrosion has a salt, which dissolved in the sea water, which converts seawater electrolyte with high conductivity [4]. According to Professor Borodavkin P.P., sea currents have a significant influence on corrosion processes of offshore oil and gas structures [1]. They constantly deliver new corrosive elements (e.g, saturated salt water), which has not yet reacted, to the already corroded areas of structural elements of offshore oil and gas constructions, thereby repeatedly increasing corrosion. It has to be noticed, that the corrosion damages on the offshore constructions are the result of violation of the work of the corrosion protection system [1]. Corrosion processes occur most intensively in areas of paint damage or other insulating material, as shown by analysis of the reports of inspection surveys offshore platforms. Poor adhesion refinishing or its complete absence led to serious corrosion damage even in the presence of existing ECP systems. Moreover, it should be taken into account that the corrosion rate is influenced by the state and chemical composition of the material structural elements of the offshore structures seasonality factor, air and water temperature, air humidity, the constructive form of welded joints and components, time, nature of the load cells and other factors [1,4,5, 6, 7]. To date, is developed various theories of stress corrosion. However, in the

researching process of corrosive effect on the stress state of the offshore construction author is invited to refer to the classical theory of fracture mechanics. It is quite obvious that a significant impact on the change of stress state have various forms of surface damage of structural elements of offshore constructions. If we consider a solid surface corrosion, it leads to a uniform wall thickness thinned to a uniform reduction of the total load capacity. Far more dangerous are ulcerated or point corrosion damage extending from the surface into the base metal with small radius of curvature at the end of the defect. Such lesions dramatically alter the shape of the surface of the component of offshore construction, leading to changes in the overall picture of the state of stress and potentially become overvoltage elements known in classical fracture mechanics as stress risers. These elements are characterized by coefficients of stress concentrations - K - that determine the true state of stress in the area with a CD by multiplying the value of K at nominal voltages. The value of stress concentrations is the greater as the sharper form of corrosion defect. Corrosion defect in the form of the cavity is a local depression, characterized by the parameters of length, width, depth and radius of curvature at the end of the cavity. In order to establish the history of changes in the stress state of offshore construction it should be set the speed with which corrosion defect was developed, that mean to set the rate of corrosion. Employees of JSC «Institute «SHELF» have been made the experimental studies, the result of which was found that the corrosion rate of specimens fully submerged in the water is from 60 to 130 mm per year, the corrosion rate in the variable wetting can reach up to 1.3 mm per year. However, these studies were conducted without the influence of electrochemical systems of corrosion protection, insulation coating, cathodic protection, and other ways of dealing with corrosion wear, i.e the so-called free corrosion. In fact offshore structures protected from corrosion effect by various methods. In order to establish the real value of corrosive wear, turn to the data of materials diagnosing of marine constructions. On the SMP-4 was produced ultrasonic thickness wall structural elements aimed in a direction perpendicular to the tube axis, which allows to determine the corrosion rate of vertical speed parameter or increase the depth of corrosion defect. Let's have a look the SMP-4, which is set up on the Golytsenskoe field. The total height of the support block is 60 m, the unit has four supporting legs made of pipes 1420*15,7 mm. Adjustment of the support block design outbid 5 horizontal diaphragms 478 * 11 mm. The vertical struts are made of tubes 530 * 12 mm. Results of the research was processed by author and summarized in the table 1. From the results, presented in table 1, it follows, that the most corrosive wear of structural elements SMPs, equal to about 12% of the initial wall thickness is achieved in the variable wetting. In the underwater area corrosive wear and lower ranges 5-8,8% (depending on the undercurrents). In the area of atmospheric corrosion wear an average of 7%. The analysis of a large amount of statistical material of diagnostics SMPs, located in different fields of the Black Sea, showed that medium corrosive wear of the upper truss chords platforms such as SMP-4 being in operation for more than 30 years, is 10.8% lower chords - 14.6%, 10% and bracing struts 9%. On the offshore platform SMP-5, which is in operation for over 20 years and installed in the same area, the corrosive wear of topsides is higher, than that of the platform, mentioned before, and suggests that the intensity of corrosion damage depends on many factors. The average corrosion rate elements topsides is within 0,04-0,13 mm / year. Assessing the degree of damage to the production of metal bearing units, it should be noted that all the elements of platforms are affected by corrosion, but the intensity of the corrosion process depends on the positions of the elements of their design and workmanship. However, for all SMPs can be noted a very important trend: the actual state of the surface of metal parts manufacturing units in the variable wetting is characterized by the highest corrosion wear. Normal wear and tear items in this zone range from 25 to 40%. Maximum deterioration of the individual elements reaches 75-85%. Let's consider the theoretical basis of cracking corrosion attack at offshore constructions. The actual process of corrosive cavities characterized by constant change in their geometric parameters, and most importantly, the change in the radius at the crack front. Briefly the development of corrosion defect can be described as follows [2]: exposure of the metal and the emergence of local indentations (Fig. 1 a); repassivation to form an oxide film that prevents the de-

velopment of corrosion defect (Fig. 1b), the destruction of oxide films and promotions defect (Fig. 1c). Defect does not develop if the generalized force development defect (G) less generalized resistance force (F), which corresponds to the incubation stage, or stage stop due to lower stress levels, meeting the front CD with more durable material with oxide film, etc. Small CD grow steadily provided equality generalized force development defect and generalized resistance force [2]. Spasmodic same growth occurs CD provided it exceeds the generalized force on defect development value generalized resistance force. Most typically spasmodic development of corrosion defect with sequential shift relations between the values of G and F [2].

a) formation of the local b) repassivation c). removing of the defect

deepening

Pic. 1. Corrosion defect progress

Pic. 2. Corrosion defect of structural ele- Pic.3. Scheme of 2 corrosion defects of struc

Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.

Показать целиком