1,720,998 research outputs found
Low-cycle fatigue assessment of mooring chains based on the critical plane approach
No full textThe integrity of mooring chains is important to the safety of a range of offshore platforms for the exploitation of natural resources, including oil, gas, wind, and wave energy sources. However, mooring line failures have been occurring earlier than they are designed for with a high number of these failures occurring due to fatigue. The failures occur in the first three years, when operating in harsh sea environments, indicating that the failure mechanism could be attributed to low-cycle fatigue, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stressstrain responses of the material, leading to cyclic plastic damage accumulation. However, the traditional SN approach, suggested by mooring standards, does not consider these effects, and it is proposed that the lack of this consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. Therefore, this thesis aims to develop a fatigue approach, based on a critical plane multiaxial fatigue criterion. This method is capable of accounting for the mean load and cyclic plasticity effects on the fatigue damage, which governs the low-cycle fatigue of mooring chains. This allows for an investigation into how fatigue damage prediction changes with a more accurate representation of the low-cycle fatigue regime under stochastic tension loading in service for intact mooring chains. A finite element model with explicit solver is developed and shown to be robust for modelling representative stresses in mooring chains when compared with other finite element models available in the literature. Using the determined finite element model, the critical plane approach is developed based on the multiaxial stress-strain conditions at the hotspot and this is correlated with the fatigue life derived from tests taken from the mooring standards. Subsequently, the developed critical plane approach is compared with the traditional SN approach using a mooring line simulation of an FPSO as a case study. The result found that the critical plane approach predicts significantly higher proportions of low-cycle fatigue than the traditional SN approach in the mooring line loading. This is because the mean load effect is detrimental when it is above 20% of the minimum breaking load, shown to make up a large proportion of the loads in high sea-states. However, the effect of the cyclic plasticity is limited as the mechanisms are only applied when the tension load exceeds 70% of the minimum breaking load due to the strain hardening from the proof load application during the first cycle. The traditional SN approach is shown to be conservative when the mean load is below 20% of the minimum breaking load, but after this, it is non-conservative, which is observed in harsh environments where the majority of mooring chain failures were witnessed. The low-cycle fatigue analysis in this study explains why current fatigue assessment in mooring chain applications is non-conservative and demonstrates that the critical plane approach should be accounted for in mooring line design
Feasibility study of selected riser concepts in deep water and harsh environment
No full textOne of the well-known riser systems, the Steel Catenary Riser (SCR), has been an attractive choice for the riser system in deep water. However, the main challenge of the SCR is large motions from the host platforms due to the harsh environment. The large motion of host platforms may induce excessive buckling and fatigue at the touchdown point. By screening the downward velocities at the hang-off point in the time history graph, the time at which the critical responses (i.e. buckling utilization, bending moment and compression) peak is identified. This study investigates the feasibility of the SCR configuration in terms of the capability to cope with the vessel motion.Several types of the SCR configurations are proposed in this study. The selected configurations of SCR in this study are conventional SCR, Weight Distributed SCR (WDSCR), and Steel Lazy Wave Riser (SLWR). The feasibility of the three riser configurations was analyzed in terms of strength and fatigue performance to understand the limitation of one over the other. The “lazy wave” configuration efficiently absorbs the vessel heave motions. Thereby the SLWR configuration is proven to be the most robust configuration to cope with large motion of the host platform. This study proves that although the SCR feasibility is limited due to vessel heave motion, innovative solutions can be established to extend its feasibility in order to cope with the vessel heave motion in harsh environment
Low-cycle fatigue assessment of offshore mooring chains under service loading
No full textThe integrity of mooring chains is essential to the safety of a range of offshore platforms. However, mooring line failures are occurring earlier than their design lives, with a high number of these failures occurring due to fatigue. Early in the fatigue life of the component fatigue initiation processes occur, where the fatigue hotspot is sensitive to the mean load and there is plastic strain accumulation from the multiaxial stress-strain responses of the material, leading to cyclic plastic damage accumulation. The traditional SN approach suggested by mooringstandards does not consider these effects, and it is proposed that this lack of consideration under low-cycle fatigue conditions is the reason for the current non-conservative fatigue assessments of mooring chains. This paper aims to develop a fatigue approach based on a critical plane multiaxial fatigue criterion for mooring chains that can consider the damage-induced by the cyclic plasticity and the mean load effect, to investigate the importance of incorporating low-cycle fatigue into the mooring chain life prediction. To develop the criticalplane approach, the multiaxial stress-strain states are extracted for the critical plane at the fatigue hotspot from a finite element model of a mooring chain. This is then correlated with a fatigue life prediction provided by conventional fatigue design data. It uses a simulation of an FPSO as a case study to demonstrate the importance of low cycle fatigue, which shows that the mean load effect is significant in reducing the fatigue life for mooring chain applications, while the effect of fatigue damage-induced cyclic plasticity is limited. The fatigue damageaccumulation predicted by the critical plane approach is significantly higher than that of the traditional SN approach and should be accounted for in mooring line design
Fatigue approaches for mooring chains subjected to wear degradation
No full textThere are currently 365 FPSOs in service around the world. These vessels all use mooring lines to maintain position and provide stability, keeping the vessel and cargo safe. However, more than 21 failures have occurred between 2001 and 2011 and approximately 50% of the reported failures occurred in the first 3 years of 20-year design life. Each mooring line failure represents the potential for serious environmental and economic consequences. Based on industry surveys, the most common failure mode is fatigue failure. In the current offshore standards, the surface degradation due to wear and corrosion is modelled as a diameter loss at a standards rate. To assess whether the uniform reduction in chain diameter suggested in the offshore standards is able to explain the early chain failures seen in service, this paper incorporates two wear rates into a fatigue life calculation; one wear rate is taken from DNV-OS-E301 and is compared against one taken from NORSOK M-001. Three fatigue life estimation approaches: tension, nominal stress and hotspot, are used to compare the differences in fatigue method. The stress in the chain is calculated using an analytical model, which is verified against an FE model. The effect of wear degradation on the ultimate strength of the chain is calculated based on the minimum breaking load. The results show that the diameter loss rates suggested in the offshore standards are not able to explain the early mooring chain failures seen in the past and that the reduction of diameter cannot solely explain the early failures seen in service. The hotspot approach, not often used in mooring line predictions, is best able to predict these shorter lives, as it offers more accurate fatigue predictions by considering high peak stresses compared to standard methods such as tension and nominal stress approaches
Selection of appropriate numerical models for modelling the stresses in mooring chains
No full textMooring chains are key components for floating platforms. The failure of these components can be catastrophic in terms of the economic and environmental impacts, especially when dealing with the potential failure of FPSOs. However, mooring failures have been regularly occurring much earlier in their service lives than expected, with almost 50% of the reported failures happening in the first 3 years of 20-year design lives. Although the operating stresses play a major role in determining the failure mechanisms of mooring chains, the methods of predicting the operating stresses in mooring chains vary in the openly available literature, and the accuracy of these different numerical methods for predicting types of mooring failures is unknown. There is currently little evidence provided for when one model is appropriate for a particular scenario. Therefore, this paper benchmarks the different available methods for modelling mooring chains under tension, including FE models found in the literature. These models are calibrated and verified against previous studies and compared with experiments and a developed FE explicit model. There is a significant difference in the way that the numerical models behave, which are discussed in terms of their applicability and limitations in modelling mooring chains. The results of this study show that the explicit modelling approach should be utilised for accurate assessment of mooring lines, as it provides the most realistic response, with a substantial reduction in the computational cost and without any convergence problems
PENGEMBANGAN BAHAN AJAR KIMIA ANORGANIK BERBASIS E-LEARNING
No full textABSTRAK Asadullah, Gilang Muhammad. 2013. Pengembangan Bahan Ajar Kimia Anorganik Berbasis E-Learning. Skripsi, Jurusan Kimia, Fakultas Matematika dan Ilmu Pengetahuan Alam, Universitas Negeri Malang. Pembimbing: (I) Dr. Munzil S.Pd., M.Si, (II) Drs. I Wayan Dasna, M.Si, M.Ed, P.hD Kata Kunci: bahan ajar, e-leaning, kimia anorganik. Kimia anorganik yang disampaikan dalam jenjang perguruan tinggi merupakan salah satu cabang dari ilmu kimia yang luas dan kompleks. Kimia anorganik mempelajari keberadaan senyawa anorganik di alam, sifat-sifat senyawa anorganik, serta reaksi-reaksi pentingnya. Salah satu bahan ajar yang digunakan mahasiswa jurusan kimia Universitas Negeri Malang sebagai sumber belajar materi kimia anorganik adalah handout yang hanya berisi teks, sehingga diperlukan media dalam bentuk visual seperti gambar, video, dan animasi. Bahan ajar berbasis e-learning adalah salah satu media yang dapat menyampaikan materi ajar dalam bentuk gambar, video, atau animasi serta memiliki askes yang luas sehingga dapat memperoleh update informasi yang cepat. Tujuan penelitian pengembangan ini adalah menghasilkan produk pengembangan dan mengetahui kelayakan bahan ajar kimia anorganik berbasis e-learning. Penelitian ini menggunakan rancangan penelitian pengembangan model 4-D (four-D models) yang dikembangkan oleh Thiagarajan. Penelitian pengembangan ini terdiri dari tiga tahap, yaitu tahap define (pendefinisian), tahap design (desain), dan tahap develop (pengembangan). Tahap selanjutnya yaitu disseminate (penyebaran) tidak dilakukan karena penelitian ini hanya melakukan uji validitas produk pengembangan. Uji validitas produk dilakukan melalui pengumpulan data dari validator ahli menggunakan instrumen uji kelayakan desain tampilan dan isi media. Pengolahan data hasil validasi dilakukan dengan cara analisis deskriptif dengan rata-rata skor jawaban pada setiap item dalam instrumen validasi. Produk bahan ajar yang telah dikembangkan berbentuk media website yang dapat diakses menggunakan perangkat komputer dengan jaringan internet melalui alamat http://anorganik.kimia.or.id. Kelebihan bahan ajar yang telah dikembangkan, yaitu memiliki gambar, video, dan animasi yang dapat membantu visualisasi materi kimia anorganik serta link-link yang mengarah ke website atau media lain yang membantu proses pembelajaran. Hasil analisis validasi isi, produk diyatakan layak digunakan dan validasi terhadap tampilan produk, diyatakan sangat layak dengan hasil analisi masing-masing skor adalah 3,18 dan 3,87 dari skala interval 1-4. Produk yang telah dikembangkan disertai dengan petunjuk dan back up bahan ajar yang dikemas dalam Compact Disk (CD)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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