Integrity assessment of welded structures designed with reduced safety

Predmet ove doktorske disertacije je istraživanje ponašanja zavarenih konstrukcija sa smanjenim stepenom sigurnosti prema krtom lomu i otpornost na prsline u cilju ocene njenog integriteta i veka. U tom slučaju posebna pažnja se posvećuje mestima koncentracije napona, jer se krti lomovi ovih konstrukcija po pravilu događaju upravo na takvim mestima. Radi detaljnog proučavanja nakstanka i rasta prsline, neophodna je primena principa mehanike loma, jer klasični pristup konstruisanju (projektovanju) preko faktora koncentracije napona i čvrstoće u tom slučaju nije dovoljan. Primenom parametra mehanike loma, kao što je faktor inteziteta napona, čiji opseg definiše brzinu rasta prline, a čija kritična veličina (žilavost loma) određuje kritičnu dužinu prsline. Time se obezbeđuje pouzdana procena integriteta konstrukcije (ako se dokaže da je faktor intenziteta napona manji od žilavosti loma), odnosno procena veka konstrukcije (primenom zakona mehanike loma na izračuvavanje krritičnih parametara za poznatu ili pretpostavljenu početnu dužinu prsline, njenu kritičnu vrednost, kao i izračunati opseg faktora intenzitata napona). Konzervativni pristup u svim koracima ove procedure obezbeđuje pouzdanu eksploataciju konstrukcije sa smanjenim stepenom sigurnosti.The subject of this doctoral dissertation is the research of the behavior of welded structures with a reduced degree of safety on brittle fracture and crack resistance in order to assess its integrity and age. In this case, special attention is paid to the stress concentration points, since the fractures of these structures tend to occur exactly in such places. For the detailed study of the crack and growth of the crack, it is necessary to apply the principle of fracture mechanics, since the classical approach to design (design) through the factor of stress concentration and strength is not sufficient in that case. Using the fracture mechanics parameter, such as the voltage intensity factor, whose range defines the rate of crack growth, and whose critical size (fracture toughness) determines the critical crack length. This ensures a reliable assessment of the integrity of the structure (if it is proven that the stress intensity factor is less than the tensile strength of the fracture), or the assessment of the century of construction (by applying laws of fracture mechanics to calculate critical parameters for known or assumed initial crack length, its critical value, as well as calculate the range voltage intensities factor). A conservative approach in all steps of this procedure ensures reliable exploitation of a structure with a reduced degree of safet

Analysis of the estimated remaining service life of gas rectification columns

Gas rectification columns are process apparatuses which are very important part of each onshore plant. Sudden failure of these types of columns causes huge daily losses, while fluid leakage can be very dangerous for the people and hazardous for the environment. For that reason, a regular and correct examination of the column is of great importance for the continual process of leading. Together with the detailed examination of the column, a risk-based inspection was applied to maximize savings, both material, time, and costs. The expected remaining service life of rectification columns was also analyzed, while the corrosion rates were calculated according to the various international standards. The minimum required column wall thickness was calculated according to the most commonly used international standard and the obtained difference was analyzed. Detailed analysis of the total risks of the column due to potential failure is presented

Structural Integrity Assessment of Welded Pipeline Designed with Reduced Safety

The main goal of this paper was to assess the integrity of welded joints in the main pipeline of the reversible hydropower plant "Bajina Bašta". Japanese steel Sumiten 80P (SM 80P) was used as the parent material. European recommendation for pipeline safety factor is equal to 1.7 and this value was used for calculations of the RHPP "Bajina Bašta", whereas the value recommended by Japanese standards is 2.1. A relatively small safety factor, which is different from the Japanese recommendation (since the material itself is Japanese), represented one of the main reasons for a detailed investigation of the pipeline structural integrity and safety, using the prototype. In the case of pressure vessels, the welded joint is a location of stress concentration, which can act in the same way as residual stresses. Assessment of prototype test results is possible to perform based on stress and strain calculations of vessels with ideal geometry. For this reason, the solution for thin-walled vessels is given, both in elastic and elastic-plastic areas. Numerous tests were performed in order to obtain a reliability assessment necessary for the construction of the pipeline, since the consequences of potential failure would be disastrous in this case. A numerical simulation, based on the experimentally determined mechanical properties of the material used, was also performed in order to obtain the stress/strain distribution. These results were then compared to the experimentally obtained ones, and it was concluded that there is a good level of compliance between numerical and experimental results

Analysis of the estimated remaining service life of gas rectification columns

Gas rectification columns are process apparatuses which are very important part of each onshore plant. Sudden failure of these types of columns causes huge daily losses, while fluid leakage can be very dangerous for the people and hazardous for the environment. For that reason, a regular and correct examination of the column is of great importance for the continual process of leading. Together with the detailed examination of the column, a risk-based inspection was applied to maximize savings, both material, time, and costs. The expected remaining service life of rectification columns was also analyzed, while the corrosion rates were calculated according to the various international standards. The minimum required column wall thickness was calculated according to the most commonly used international standard and the obtained difference was analyzed. Detailed analysis of the total risks of the column due to potential failure is presented

Repair welding of pressure equipment with unacceptable defects

The paper describes non-destructive testing and the repair welding technology for eliminating defects in the pressure equipment of RHE BB (Bajina Basta) (located in Serbia). The obtained results had justified the proposed technology

Repair welding of pressure equipment with unacceptable defects

The paper describes non-destructive testing and the repair welding technology for eliminating defects in the pressure equipment of RHE BB (Bajina Basta) (located in Serbia). The obtained results had justified the proposed technology

Assessment of integrity of pressure vessels for compressed air

Fracture mechanics approach for structural integrity assessment is described in the case of pressure vessels for compressed air. The results of non-destructive testing indicated unacceptable defects on the vessels and set a base for application of failure analysis diagrams (FAD) and risk-based analysis. Risk matrix are applied with the aim of assessment and analysis of the integrity of the vessels

Assessment of integrity of pressure vessels for compressed air

Fracture mechanics approach for structural integrity assessment is described in the case of pressure vessels for compressed air. The results of non-destructive testing indicated unacceptable defects on the vessels and set a base for application of failure analysis diagrams (FAD) and risk-based analysis. Risk matrix are applied with the aim of assessment and analysis of the integrity of the vessels

Integrity assessment of ammonia storage tank by non-destructive testing

Ammonia is a very dangerous fluid, and it is so important that its storage tanks are safe. Fusion welded joints (in further text: welded joints) are the 'weakest' location in some structures, in this case - tanks, and the detection of defects in welded joints during manufacture is also very important, since defects may cause failure with catastrophic events. NDT techniques that have been performed on welded joints show insight into the quality of the performed welding technique, and results imply whether tanks are safe to be put into exploitation. Three different methods are used for testing: visual, penetrant, and ultrasonic (in this case a new method is used). The first two methods show no irregularities, but third NDT method shows the necessity for tank repair. All of these tests are performed in accordance with relevant standards that are discussed, as well as the used NDT methods in general, with their ability and restrictions

Manufacturing and integrity of ammonia storage tanks

Manufacturing ammonia storage tanks represents a very demanding and responsible task. The paper presents the methodology for manufacturing two ammonia storage tanks in Russia, along with all the necessary steps. The base material used for tank manufacture is steel P355NL2. Instructions and requirements in EN 14620 standard need to be followed. This standard dictates all previously mentioned steps, hence the standard itself is shown here. The welding technology of two steel tank shells is also shown. Integrity assessment of the tanks is necessary to carry out immediately before the tanks are put into service. NDT results for welded joints and results of adequate tests performed on the tanks themselves provided insight into the quality of the performed work, and have shown that the selected welding technologies have met all requirements in terms of strength, thus confirming that the tank integrity is satisfactory