Lesson twenty-two Examples of offshore Structures By far the most common type of fixed offshore structure in existence today is the template, or jacket, structure illustrated in Fig. 1. This type of structure consists of a prefabricated steel substructure that extends from the seafloor to above the water surface and a prefabricated steel deck located atop the substructure. The deck is supported by pipe piles driven through the legs of the substructure into the seafloor. These piles not only provide support for the deck but also fix the structure in place against lateral loadings from wind, waves, and currents The construction and installation or a template structure plays a central role in its design. The substructure is usually prefabricated on its side at a waterside facility and then placed horizontally on a flattopped barge and towed to its offshore location. At the installation site, the substructure is then slid off the barge and uprighted with the help of a derrick barge and allowed to sink vertically to the seafloor. Once the substructure is in place, pipe piles are inserted through its legs and driven into the seafloor by means of a piles are driven to predetermined depths, they are cut off at the top of the substructure and the prefabricated deck stabbed into the piles and connected with field 解 Fig. 1 Artist's rendering of a modern template structure In its completed form, the deck weight is carried entirely by the piles, with the substructure providing bracing against their lateral movement. a typical oil drilling and production platform is shown in Fig. 2. This structure is located off Louisiana in about 300 ft of water in the Gulf of Mexico. The deck measures approximately 60*120 ft and, with operating equipment, weight about 2million pounds. The weight of the substructure is about 4million pounds. The eight pipe piles driven through the legs of the substructure have outside diameters of 4 ft and wall thicknesses of about 1 in in addition to
Lesson Twenty-two Examples of Offshore Structures Template Structures By far the most common type of fixed offshore structure in existence today is the template, or jacket, structure illustrated in Fig.1. This type of structure consists of a prefabricated steel substructure that extends from the seafloor to above the water surface and a prefabricated steel deck located atop the substructure. The deck is supported by pipe piles driven through the legs of the substructure into the seafloor. These piles not only provide support for the deck but also fix the structure in place against lateral loadings from wind, waves, and currents. The construction and installation or a template structure plays a central role in its design. The substructure is usually prefabricated on its side at a waterside facility and then placed horizontally on a flattopped barge and towed to its offshore location. At the installation site, the substructure is then slid off the barge and uprighted with the help of a derrick barge and allowed to sink vertically to the seafloor. Once the substructure is in place, pipe piles are inserted through its legs and driven into the seafloor by means of a piles are driven to predetermined depths, they are cut off at the top of the substructure and the prefabricated deck stabbed into the piles and connected with field Fig. 1 Artist’s rendering of a modern template structure In its completed form, the deck weight is carried entirely by the piles, with the substructure providing bracing against their lateral movement. A typical oil drilling and production platform is shown in Fig.2.This structure is located off Louisiana in about 300 ft of water in the Gulf of Mexico. The deck measures approximately 60*120 ft and, with operating equipment, weight about 2million pounds. The weight of the substructure is about 4million pounds. The eight pipe piles driven through the legs of the substructure have outside diameters of 4 ft and wall thicknesses of about 1 in. in addition to
8 main piles -48-in diameler wetted at we 300p# 4 skirt Fig. 2 Typical offshore template structure off Louisiana in the Gulf of Mexico (a)------installed(b)-----substructure illustrating skirt pi these, four skirt piles are placed around thethe base of structure. All piles are driven 200 to 300 ft into the seafloor. The structure is designed to withstand a result lateral force of about 3 million pounds from wind, waves, and currents during extreme hurricane conditions. Because the wave forces are greatest near the water surface, this resultant force acts near the top of the structure The structure is therefore also designed to withstand a base-overturning moment of the order of 700 million foot-pounds. These loads and moments are five to seven times those caused by extreme winds on a typical 25-story, 300-ft-tall building on land For structures designed for waters greater than about 350 ft, two variations of the basic eight-leg template design have been considered. The first has been to increase the number of legs of the structure so that, with skirt piles, the structure can carry additional deck loads and resist the increased lateral loading and overturning moment A second modification has been based on the observation that, with taller structure and increased base widths, the interior piles become less effective in resisting overturning moments. As an alterative to the eight-pile structure, consideration has thus been given to the four exterior corners of the structure Template structures, as described earlier, are especially suited to soft-soil regions such as the Gulf of Mexico, where deeply driven piles are needed to fix the structure in place and carry the required deck loadings. In regions where hard soil conditions exist and pile driving is more difficult, an alternative structural form has been developed which, in place of piles, relies on its own weight to hold it in place against the large lateral loads from wind, waves, and current. These structures have large foundational elements which, when ballasted, contribute significantly to the required weight and which spread this weight over a sufficient area of the seafloor to prevent failure. Such structures are generally referred to as gravity structures In their more popular form, gravity structures. Are constracted with reinforced concrete and consist of a large cellular base surrounding several unbraced columns which extend upward from
Fig. 2 Typical offshore template structure off Louisiana in the Gulf of Mexico (a)------installed (b)------substructure illustrating skirt piles these, four skirt piles are placed around the the base of structure. All piles are driven 200 to 300 ft into the seafloor. The structure is designed to withstand a result lateral force of about 3 million pounds from wind, waves, and currents during extreme hurricane conditions. Because the wave forces are greatest near the water surface , this resultant force acts near the top of the structure. The structure is therefore also designed to withstand a base-overturning moment of the order of 700 million foot-pounds. These loads and moments are five to seven times those caused by extreme winds on a typical 25-story, 300-ft-tall building on land. For structures designed for waters greater than about 350 ft, two variations of the basic eight-leg template design have been considered. The first has been to increase the number of legs of the structure so that, with skirt piles, the structure can carry additional deck loads and resist the increased lateral loading and overturning moment. A second modification has been based on the observation that, with taller structure and increased base widths, the interior piles become less effective in resisting overturning moments. As an alterative to the eight-pile structure, consideration has thus been given to the four exterior corners of the structure. Gravity Structures Template structures, as described earlier, are especially suited to soft-soil regions such as the Gulf of Mexico, where deeply driven piles are needed to fix the structure in place and carry the required deck loadings. In regions where hard soil conditions exist and pile driving is more difficult, an alternative structural form has been developed which, in place of piles, relies on its own weight to hold it in place against the large lateral loads from wind, waves, and current. These structures have large foundational elements which, when ballasted, contribute significantly to the required weight and which spread this weight over a sufficient area of the seafloor to prevent failure. Such structures are generally referred to as gravity structures. In their more popular form, gravity structures. Are constracted with reinforced concrete and consist of a large cellular base surrounding several unbraced columns which extend upward from
the base to support a deck and equipment above the water surface. Structures of this kind were installed in the North Sea during the mid-1970s. figure 3 illustrates the main features of these structures.This particular structure is referred to as a CONDEEP(concrete deep-water)structure and was designed and constructed in Norway One advance was of the gravity structure over the template type is the reduced time needed for on-site installation. This is especially important in hostile areas such as the North Sea, where unpredictable weather conditions make it highly desirable to limit the construction time needed to 时国场 斜 一E. Fig 3 Illustration of a concrete gravity platform used in the North Sea fix the structure in place. Another advantage is the very decks weights that can be carried by the massive concrete columns Deep-water design forms For water depths greater than about 1000 ft, the weight and foundation requirements of traditional offshore structures make them less attractive than other design forms Two such forms re the guyed tower and tension-leg platform The guyed tower concept is illustrated in Fig 4. It consists of a uniform cross-sectional support structure held upright by several guy lines that run to clump weights on the ocean floor. From the clump weights, the lines then run to conventional anchors to form a dual stiffness mooring system Under normal operating loads, the clump weights remain on the seafloor and lateral motion of the
the base to support a deck and equipment above the water surface. Structures of this kind were installed in the North Sea during the mid-1970s. Figure 3 illustrates the main features of these structures. This particular structure is referred to as a CONDEEP (concrete deep-water) structure and was designed and constructed in Norway. One advance was of the gravity structure over the template type is the reduced time needed for on-site installation. This is especially important in hostile areas such as the North Sea, where unpredictable weather conditions make it highly desirable to limit the construction time needed to Fig. 3 Illustration of a concrete gravity platform used in the North Sea fix the structure in place. Another advantage is the very decks weights that can be carried by the massive concrete columns. Deep-water design forms For water depths greater than about 1000 ft, the weight and foundation requirements of traditional offshore structures make them less attractive than other design forms. Two such forms are the guyed tower and tension-leg platform. The guyed tower concept is illustrated in Fig.4. It consists of a uniform cross-sectional support structure held upright by several guy lines that run to clump weights on the ocean floor. From the clump weights, the lines then run to conventional anchors to form a dual stiffness mooring system. Under normal operating loads, the clump weights remain on the seafloor and lateral motion of the
structure is restrained, However, during a severe storm, the clump weights are lifted off the seafloor by loads transferred from the structure to the clump weights through the guy lines. This action permits the tower to absorb the environmental loadings on it by swaying back and forth without overloading the guy lines. The guyed-tower concept is presently considered to be applicable to water depths of about 2000 ft Figure 5 illustrates the tension-leg concept. In this design, vertical members are used to anchor the platform to the seafloor. This upper part of the structure is designed with a large amount of excessive buoyancy so as to keep the vertical members in tension. Because of this ? Fig 4 Guyed tower concept for deep water tension, the platform remains virtually horizontal under wave action. Lateral excursions are also limited by vertical members, since such movements necessarily cause them to develop a restoring force. A major advantage of the tension-leg concept is its relative cost insensitivity to
structure is restrained, However, during a severe storm, the clump weights are lifted off the seafloor by loads transferred from the structure to the clump weights through the guy lines. This action permits the tower to absorb the environmental loadings on it by swaying back and forth without overloading the guy lines. The guyed-tower concept is presently considered to be applicable to water depths of about 2000 ft. Figure 5 illustrates the tension-leg concept. In this design, vertical members are used to anchor the platform to the seafloor. This upper part of the structure is designed with a large amount of excessive buoyancy so as to keep the vertical members in tension. Because of this Fig. 4 Guyed tower concept for deep water tension, the platform remains virtually horizontal under wave action. Lateral excursions are also limited by vertical members, since such movements necessarily cause them to develop a restoring force. A major advantage of the tension-leg concept is its relative cost insensitivity to
Fig 5 Tension leg concept for deep water increased water depths. At present time, it appears that the main limitation on the tension-leg platform arises from dynamic inertia forces associated with the lateral oscillations of the platform in waves. These become significant at water depths of about 3000 ft From"Offshore Structural Engineering,, by Thomas H. Dawson, 1983 Technical Terms 1. template structure导管架平台(结构)9. current潮流 2. fixed offshore structure固定式近海平10. flat topped barge平顶驳船 台(结构) 1l. derrick barge起重船 3. jacket structure导管架平台(结构) 12. pile driver打桩船 4. prefabricated steel substructure预制钢13. bracing撑杆,拉紧连杆 质导管架(底座) 14. oil drilling and production platform石油 5. seafloor海底 钻井和生产平台 6.deck甲板(平台) 15. skirt piles裙桩 7.(pipe)pile(圆管)桩 16. hurricane飓风 8.leg桩腿,支线 17. base overturning moment对基倾覆力矩
Fig. 5 Tension leg concept for deep water increased water depths. At present time, it appears that the main limitation on the tension-leg platform arises from dynamic inertia forces associated with the lateral oscillations of the platform in waves. These become significant at water depths of about 3000 ft. (From “ Offshore Structural Engineering”, by Thomas H. Dawson, 1983 Technical Terms 1. template structure 导管架平台（结构） 2. fixed offshore structure 固定式近海平 台（结构） 3. jacket structure 导管架平台（结构） 4. prefabricated steel substructure 预制钢 质导管架（底座） 5. seafloor 海底 6. deck 甲板（平台） 7. (pipe)pile （圆管）桩 8. leg 桩腿，支线 9. current 潮流 10. flat topped barge 平顶驳船 11. derrick barge 起重船 12. pile driver 打桩船 13. bracing 撑杆，拉紧连杆 14. oil drilling and production platform 石油 钻井和生产平台 15. skirt piles 裙桩 16. hurricane 飓风 17. base overturning moment 对基倾覆力矩