• Journal of the Korean Society of Marine Environment & Safety
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• v.26 no.1
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• pp.103-113
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• 2020

Jack-up drilling rigs are widely used in the offshore oil and gas exploration industry. Although originally designed for use in shallow waters, trends in the energy industry have led to a growing demand for their use in deep sea and harsh environmental conditions. To extend the operating range of jack-up units, their design must be based on reliable analysis while eliminating excessive conservatism. In current industrial practice, jack-up drilling rigs are designed using the working(or allowable) stress design (WSD) method. Recently, classifications have been developed for specific regulations based on the load and resistance factor design (LRFD) method, which emphasises the reliability of the methods. This statistical method utilises the concept of limit state design and uses factored loads and resistance factors to account for uncertainly in the loads and computed strength of the leg components in a jack-up drilling rig. The key differences between the LRFD method and the WSD method must be identified to enable appropriate use of the LRFD method for designing jack-up rigs. Therefore, the aim of this study is to compare and quantitatively investigate the differences between actual jack-up lattice leg structures, which are designed by the WSD and LRFD methods, and subject to different environmental load-to-dead-load ratios, thereby delineating the load-to-capacity ratios of rigs designed using theses methods under these different enviromental conditions. The comparative results are significantly advantageous in the leg design of jack-up rigs, and determine that the jack-up rigs designed using the WSD and LRFD methods with UC values differ by approximately 31 % with respect to the API-RP code basis. It can be observed that the LRFD design method is more advantageous to structure optimization compared to the WSD method.

• Journal of the Korean GEO-environmental Society
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• v.20 no.11
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• pp.11-22
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• 2019

Limit state design (LSD) and allowable stress design (ASD) are two main types of soil nailing design methodologies. In the LSD method, stability is determined by applying individual coefficients to ground strength, working load and etc. The ASD method calculates the safety factor and compares it with the minimum safety factor to determine the stability. The global design trend of soil nailing system is changing from the ASD method to the LSD method. The design method in Korea still adopts the ASD philosophy while others mostly do the limit state design. In this study, four soil nail design methods, 'FHWA GEC 7' in U.S. (2015), 'Clouterre' in France (1991), 'Soil nailing - best practice guidance' in U.K. (2005), 'Geoguide 7' in Hongkong (2008), and 'Design guide for slope in construction work' in Korea (2016) were applied to the evaluation of the stability and the results were analyzed comparatively in brief. It is revealed that the design method of 'the overall stability of soil nail walls' in Korea is the most conservative and next those by FHWA, Clouterre and CIRIA become more conservative in order. However, the difference of results obtained from FHWA and Clouterre is negligible. Also, this study found out that efforts to improve domestic design criterion are needed.

• Journal of the Korean Society of Marine Environment & Safety
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• v.29 no.7
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• pp.950-957
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• 2023

For safe mooring and towing between the ship and port, the equipment must be designed in accordance with the relevant international regulations. However, some small shipyards and engineering companies often do not fully comprehend the core contents. Therefore, the international regulations regarding towing and mooring equipment are reviewed and the bollard and chock are newly developed based on the Mooring Equipment Guideline 4 (MEG4) standards. A bollard is a mooring equipment used to fix a mooring rope to the hull. It has two columns and is mostly used in a figure eight pattern knots under the mooring condition. The chock, which is used to change the mooring rope direction coming into the ship from outside, is manufactured using a casting with curvature. The two mooring equipment are widely used in the stern, bow, and mid-side. Owing to the increase in the size of container vessels and LNG ships, the mooring rope load has increased and the safe working load of the mooring equipment must be revised. This study summarizes and examines the results of the allowable stress method obtained using finite element analysis modelling. To consider the mesh size effect, a reasonable criteria was suggested by referring the existing class guidance. Additionally, the safe working load was verified through nonlinear collapse analysis, and the elastic region against load increments was confirmed. Furthermore, the proposed evaluation method can be used to develop similar equipment in the near future.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.03a
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• pp.379-389
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• 2010

It is very important to determine a target probability of failure in reliability based design such as an allowable factor of safety in working stress design because they are indices to judge the stability of structures. We have carried out reliability analyses of nationwide gravity type quay walls and found that sliding and foundation failures of quay walls were dominant failure modes for every case of loads. And a target probability of failure for bearing capacity of foundation of quay wall was also determined in this study. Of several approaches which have been suggested until now, a couple of reasonable approaches were used. Firstly, in order to consider the safety margin of structures which have been executed so far, the reliability levels of existing structures were assessed. And then a mean probability of failure for the quay walls was estimated. In addition, life cycle cost(LCC) analyses for representative structures were performed. Probabilities of failure for several quay walls were calculated with changing the width of each quay wall section. LCC of quay wall which is requiring case by case during the service life was evaluated, and also the optimum probability of failure of quay wall which minimizes LCC was found. Finally, reasonable target probabilities of failure were suggested by comparing with mean probability of failure of existing structures.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.6 no.4
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• pp.61-68
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• 1986

This paper presents a method for developing a probability-based working stress design code for reinforced concrete. Reliability of reinforced concrete structural members is evaluated by using an advanced second moment reliability method, and then, a practical method for code calibration is shown in this paper. The target reliability indices for various structural elements are determined by considering the results of the numerical studies on the safety of the structures designed by the current code, and by reflecting the construction practice. A set of allowable stresses and safety factors for reinforced concrete is proposed as a possible substitute for the current safety provisions, based on the rational target reliability indices.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.34 no.5
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• pp.263-270
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• 2021

In this study, a reliability-based design optimization of a 130-m class fixed-type offshore platform, to be installed in the North Sea, was carried out, while considering environmental, material, and manufacturing uncertainties to enhance its structural safety and economic aspects. For the reliability analysis, and reliability-based design optimization of the structural integrity, unity check values (defined as the ratio between working and allowable stress, for axial, bending, and shear stresses), of the members of the offshore platform were considered as constraints. Weight of the supporting jacket structure was minimized to reduce the manufacturing cost of the offshore platform. Statistical characteristics of uncertainties were defined based on observed and measured data references. Reliability analysis and reliability-based design optimization of a jacket-type offshore structure were computationally burdensome due to the large number of members; therefore, we suggested a method for variable screening, based on the importance of their output responses, to reduce the dimension of the problem. Furthermore, a deterministic design optimization was carried out prior to the reliability-based design optimization, to improve overall computational efficiency. Finally, the optimal design obtained was compared with the conventional rule-based offshore platform design in terms of safety and cost.