Bir Jib Krenin Deneysel Gerilme Analizi

Abstract

Günümüzde gelişen sanayi ile birlikte kaldırma ve taşıma makinalarına olan ihtiyaç oldukça artmıştır. Artan talebi karşılamak için üreticilerin, geleneksel yöntemlere ek olarak yeni üretim ve kontrol mekanizmaları kullanmaları büyük önem kazanmıştır. Jib krenler, endüstriyel kaldırma ve taşıma işlemlerinde yaygın olarak kullanılan bir tür transport makinalarıdır. Bu tip krenlerin kullanıldığı özel alanlardan biri olan açık deniz petrol platformları kendine özgü koşulları gereği özel tasarımlara ihtiyaç duymaktadırlar. Bu tip platformların petrol veya gaz kuyuları içermeleri nedeniyle güvenlik önlemleri, kara ortamındakilere göre farklılık göstermektedir. Bununla beraber hava koşulları ve çalışma şartları gereği bu alanda özel standartlar uygulanması kaçınılmaz olmuştur. Açık deniz petrol platformlarında kullanılacak jib krenlerin imalatı dünyada yayınlanmış farklı standartlara göre tasarlanabilir. Bunlardan en önemlileri FEM ve API standartlarıdır. Bu tez çalışmasında API standartlarına uygun olarak tasarlanmış iki adet jib kren incelenmiştir. Sonlu elemanlar metodu günümüzde gelişen teknoloji ile birlikte karmaşık sistemlerde rahatlıkla uygulanabilmektedir. Gerçeğe yakın simulasyonlar yapılması ile tasarımcılar birçok deneyi bilgisayar ortamında gerçekleştirmektedirler. Böylece tasarım süreci önemli ölçüde azalmakta, dolayısıyla zaman ve maliyet tasarrufu sağlanmaktadır. Yöntemin farklı alanlarda, güvenilirliği ve uygulanabilirliği deneysel yöntemlerle test edilebilmektedir. Gelecekte deneysel yöntemlerin yerini, büyük oranda bilgisayar ortamında yapılan modellemelere bırakması beklenmektedir. Bu çalışmanın hedeflediği önemli noktalardan biri sonlu elemanlar metodu ile elde edilmiş olan veriler ile deneylerden elde edilmiş olanların karşılaştırılması ve hata oranlarının tesbit edilmesidir. Böylece, gelecek tasarımlar için bir kaynak oluşturulması amaçlanmıştır. İlk etapta, müşteri istekleri doğrultusunda yapılan analitik hesaplamalar ile tasarlanan jib krenler bilgisayar ortamında 3 boyutlu olarak tasarlanmıştır. Ardından tasarlanan modellerin, paket programlar kullanılarak sonlu elemanlar metodu ile statik yük analizleri yapılmıştır. Söz konusu statik yük analizleri daha sonra sahada gerçekleştirilecek olan deneysel analizler için bir temel teşkil etmektedir. Tasarımcı tarafından, bilgisayarda gerçekleştirilen bu uygulamaların ardından, belirlenen kritik noktaların incelenmesi için deney aşamasına geçilmiştir. Analizlerde tesbit edilen yüksek gerilmelerin oluştuğu noktalara strain gageler yapıştırılmış ve analog-dijital dönüştürücü özellikli veri toplama sistemi ile toplanan sinyaller dönüşüm denklemleri kullanılarak işlenmiştir. Dönüşüm denklemleri için gelecek uygulamalarda da kullanılabilecek kodlar hazırlanmıştır. Krenlerin çalışma ortamlarında gerçekleştirilen bu deneylerden elde edilen sonuçlar ile sonlu elemanlar analizlerinden elde edilenlerin karşılaştırılması ile literatüre katkıda bulunulmaya çalışılmıştır. Çalışma süresince açık deniz petrol platformu üzerinde deneysel çalışma yöntemleri, gerçek zamanlı veri toplama ve işleme gibi konularda bilgi ve tecrübeler kazanılmıştır.

With developing industry, the need for lifting and transferring machines has considerably increased nowadays. In order to meet this demand manufacturers have to use new design and control technologies in addition to traditional methods. Furthermore, economic conditions and rivalry between companies enforce them to decrease the time used for the design and production of machines. Using computer technologies in design, manufacturing, and analysis process can take some advantages in this field to the companies. On the other hand, reliable classic calculations and conventional methods still are in use in a lot of companies. In order to have confidence, on computer based analysis, comparing these two ways is so important for reliability of the projects. There are a lot of kinds of transporting machines in the industry such as factories, shipyards, construction areas, and storages. Jib cranes are one of these kinds of machines used mostly. Design of jib cranes working in an off-shore petroleum and natural gas platform is a detailed and hard assignment because of their working conditions. About this, some standards and regulations are published by various foundations. FEM and API standards are the most well known standards in the world. In this thesis, two off-shore jib cranes were examined in terms of static structural analysis. All experimental studies were done according to SAE J987 referred in API Spec 2. According to the standard nominal load must be multiplied by 1.33 for the tests simply. In SAE J987, three conditions are stated for the tests. Firstly, Initial Reference Test Condition is defined as no-stress or zero stress condition of the crane structure before assembly process. Under this condition, the initial reference readings for each gage are obtained, N1. Second is Dead Load Stress Condition, the completely assembled crane structure on the test site and in the test position or attitude, ready to apply the specified live load at specified radius, N2. The last one is Working Load Stress Condition, the completely assembled crane structure on the test site and in the specified position, supporting the specified rated load, N3. Dead Load Stress (S1) is the stress computed by using the difference in the readings obtained (N2-N1) and Working Load Stress (S2) is the stress computed by using difference in the readings obtained (N3-N1). Resultant Stress (Sr) is the maximum stress induced in the structure as a result of S1 or S2, whichever is greater in absolute magnitude. During this work, all notations were translated in Turkish. For the success of the test, measurement results have to be in limits that are stated before. Finite elements methods can be used easily in analyzing of complex mechanical problems by a designer due to high technology. An engineer can design and analyze a construction and get results about his work with models look like their real design fairly. Computer based technologies lessen time that is spent identifying weak and critical points of a design at the same time. It can be said that computer analysis will be used in place of experimental studies in the future but, nowadays, current standards still require real observing results at the working conditions. So, comparison of virtual and real analysis is an actual area for researches. In this context, two jib cranes, created and analyzed by a designer at some commercial computer programs which are produced on 3D designs and analysis, are subject to this thesis. Moreover, it is aimed to generate a source for future workings about jib cranes static load test experiments with results and suggestions in the study. The first step of the project is to determine design parameters requested by the client. Secondly, a designer creates detailed 3D models of the cranes. At this point, some practical computer codes could be written so that a designer could use in the future easily by changing a few parameters. Both two cranes in the thesis have same design at all but they are on different platforms on the sea. The third step is the computer analysis using finite element methods at a commercial program. 3D model of the crane were transferred from a CAD program to a FEM program in convenient file format. Afterwards, required values and descriptions must be set up in the analysis program. Each crane must hoist the 16 tons load with 32 meter boom radius and 12 tons load with 37 meter boom radius in the normal conditions. As stated in the standard the nominal load must be multiplied by 1.33 during computer analysis. Approximately 22 tons test load was used for each crane briefly. As a result, four critical points were identified: one on boom, one on leg, and two on pedestal back and front structures. At these points, crane structures have maximum stress levels when loaded. But, in question areas’ stresses were substantially under the yield point of the St 52 steel. At first look although this confirms the design, these results are needed to be verifying by experiments before saying last words. So, critical points which are needed to be examined in the tests are found out in this easy way instead of complex calculations. Furthermore, this provides designers to go back in the process instantly and change some values in order to get optimum results. After the computer analysis, the next step is the application of strain gages on the structure. The main objective of this application is obtaining the strain values at points under high stress by using strain gages. Then strain - stress relationships can be used to compute stresses. The strains on the surface are measured in order to get the internal stress of the parts. Generally, a strain gage consists of three levels. Grid- shaped sensing element of thin metallic resistive foil (3 to 6 μm thick) is put on a base of thin plastic film (15 to 16 μm thick) and then, these two elements are covered with a thin film for protection. Strain gages must be tightly bonded to sensing elements in order to elongate according to strain on the measuring parts. Fundamentally, when there is a change in the shape of metals, most of them undergo a change in electric resistance. The principal that is used to get strain is to measure the resistance change in strain gages. Generally, the sensing element of the strain gage composes of a copper-nickel alloy foil. To protect stain gages from environmental and mechanical effects that can develop during transportation and assembly process, a protective coating named M-Coat F commercially, is used for covering them. The alloy foil of the gages has a rate of resistance change proportional to strain with a certain constant called gage factor. For copper-nickel alloy resistance, it is around 2. Basically, 10-3 strain value causes 0.2% resistance chance in a gage having 120Ω resistance. In this thesis, three types strain gages were used. Gage factor is 2.125 ± %0.5 for C2A-06-062LW-350, which is the type used mostly in this study and produced by Vishay Micro-Measurement. For other gages are 2.120 ± %0.5 and 2.095 ± %0.5 at 240C. Because of the difficulties on the measurement of such minute resistance change by a conventional ohmmeter accurately; they are measured with a dedicated strain amplifier using an electric circuit called as Wheatstone bridge. In experiments, to obtain data from strain gages and present it, Prosig P8048 device was used. Sixteen slots are available on it, but only four of them were enough to measure four strains simultaneously. This device also includes analog-digital converter to convert the signals coming from strain gages. DATS is a program used with Prosig, which can show the values of output voltage. To determine which Wheatstone bridge will be used during studies is also possible by users. On the other hand, some equations are written to calculate strain and stress results by using voltages. These equations are coded at MATLAB parametrically. Thus, they can be run by easy changes at next studies like this. In conclusion, static test load experiments were done for each crane according to API Spec 2. Three types of measurements were done. These were initial reference test, dead load test and, working load tests. After getting results from these tests, resultant stress was calculated for every point. According to the standard, resultant stress must be compared with the yield point of the material that is used with reliability factor. As a consequence, computer based analysis confirmed reasonably by experimental tests and all result were in the limits allowed. The strain gages at points on the legs of the each crane were damaged before test. It was assumed that some environmental or human effects could cause this situation. Owing to inappropriate circumstances, it was impossible to reapply the strain gages on this point before tests.