Year: 2015 Language: English Author: T. Koole Genre: Other Publisher: Delft University of Technology Format: PDF Quality: eBook Pages count: 116 Description: Jack-Ups have been used since 1954, and have become the most widely used Mobile Offshore Drilling Unit (MODU) for offshore exploration and development purposes that are carried out by oil companies such as Shell. They have been a subject of much research during the end of the 1980’s, but have seen limited publications since the change of the millennium. In the meantime, the industry has seen the introduction of a new breed of Jack-Up designs, capable of operating in water depths up to 170 meters in harsh environments such as The North Sea. In order to ensure safe operation of these newer Jack-Ups, a study is carried out to update Shell’s in-house knowledge. This allows the company to gain more insight into the various designs available and also helps to identify the governing design parameters within Jack-Up assessments. Furthermore, the future of Jack-Up designs is investigated as it will help Shell shape prospective offshore developments. A market analysis identifies the trends within Jack-Up designs which can be used to generate the models used in the main body of this research. Quasi-static environmental loads are analysed to investigate the effects of using Jack-Ups in greater water depths. The dynamic response is then added as Jack-Ups are known to show significant dynamic behaviour. Timedomain FEM is used as it allows for the incorporation of the non-linearities that are present in the system. The final chapters of this thesis focus on the future of Jack-Up design. Firstly, the limits of the current design philosophy are identified. Secondly, four possible solutions to overcome these boundaries are proposed and briefly analysed to evaluate their potential. This thesis has shown that modern Jack-Ups are characterised by large triangular hulls, holding three spacious truss legs. Rack-chocks form a very stiff leg-hull connection which has allowed designers to reduce brace diameters and increase chord spacing. This has lead to an increase in problems associated with Rack Phase Difference. Wind loads increase significantly with these modern rig designs but wave loads remain fairly unchanged due to increased leg spacing. Modern Jack-Ups therefore become more wind-dominated when compared to older designs. This stresses the need for accurate calculation of wind-loads during JackUp assessment. The dynamic response analysis and its non-linearities remain to play an important role in the assessment of the structure. Significant dynamic amplification of the oscillating wave loads is found. The future of the current Jack-Up design philosophy is limited to approximately 200 meters. As hull weights and water depths increase, a strong increase in leg stiffness is required to prevent resonance from occurring. Fabrication and operational limits prevent the stiffness from increasing further. Solutions to overcome these boundaries show the difficulties of designing hybrid structures like Jack-Ups; improvements made in one area will often lead to compromises being made in another.
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Modern Jack-Ups and their dynamic behavior
Language: English
Author: T. Koole
Genre: Other
Publisher: Delft University of Technology
Format: PDF
Quality: eBook
Pages count: 116
Description: Jack-Ups have been used since 1954, and have become the most widely used Mobile Offshore Drilling Unit (MODU) for offshore exploration and development purposes that are carried out by oil companies such as Shell. They have been a subject of much research during the end of the 1980’s, but have seen limited publications since the change of the millennium. In the meantime, the industry has seen the introduction of a new breed of Jack-Up designs, capable of operating in water depths up to 170 meters in harsh environments such as The North Sea. In order to ensure safe operation of these newer Jack-Ups, a study is carried out to update Shell’s in-house knowledge. This allows the company to gain more insight into the various designs available and also helps to identify the governing design parameters within Jack-Up assessments. Furthermore, the future of Jack-Up designs is investigated as it will help Shell shape prospective offshore developments.
A market analysis identifies the trends within Jack-Up designs which can be used to generate the models used in the main body of this research. Quasi-static environmental loads are analysed to investigate the effects of using Jack-Ups in greater water depths. The dynamic response is then added as Jack-Ups are known to show significant dynamic behaviour. Timedomain FEM is used as it allows for the incorporation of the non-linearities that are present in the system. The final chapters of this thesis focus on the future of Jack-Up design. Firstly, the limits of the current design philosophy are identified. Secondly, four possible solutions to overcome these boundaries are proposed and briefly analysed to evaluate their potential.
This thesis has shown that modern Jack-Ups are characterised by large triangular hulls, holding three spacious truss legs. Rack-chocks form a very stiff leg-hull connection which has allowed designers to reduce brace diameters and increase chord spacing. This has lead to an increase in problems associated with Rack Phase Difference. Wind loads increase significantly with these modern rig designs but wave loads remain fairly unchanged due to increased leg spacing. Modern Jack-Ups therefore become more wind-dominated when compared to older designs. This stresses the need for accurate calculation of wind-loads during JackUp assessment. The dynamic response analysis and its non-linearities remain to play an important role in the assessment of the structure. Significant dynamic amplification of the oscillating wave loads is found. The future of the current Jack-Up design philosophy is limited to approximately 200 meters. As hull weights and water depths increase, a strong increase in leg stiffness is required to prevent resonance from occurring. Fabrication and operational limits prevent the stiffness from increasing further. Solutions to overcome these boundaries show the difficulties of designing hybrid structures like Jack-Ups; improvements made in one area will often lead to compromises being made in another.
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Modern Jack-Ups and their dynamic behavior
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