Long Buoyant Flexible Pile Frames for Support of Deepwater Offshore Platforms

ABSTRACT This paper is about Long Buoyant Flexible Pile Frames for support of deep-water offshore platforms. These Long Buoyant Flexible Pile Frame structures combine elements of bottom-founded and floating systems. They consist of one or more piles (legs) rigidly connected to bending members (trusses) at the sea floor, and also near their upper ends below or above the water surface. The piles are large-diameter steel cylinders. There is a watertight bulkhead in each pile with the pile open to water below the bulkhead and open to the atmosphere above the bulkhead. The buoyancy of the piles supports the structure plus the payload. Overturning moment caused by wind, wave and current is resisted by the buoyant force offset horizontally from the anchorage and from bending induced at the top and bottom of each pile by the bending members. INTRODUCTION Long Buoyant Flexible Pile Frames are very long cylindrical steel piles are anchored at the sea floor and are rigidly connected to bending members near the water surface. Figure 1 shows a typical four-leg design. The bending members will restrain joint rotation and thus keep the tops of the piles very close to vertical alignment. The piles could be anchored by being driven into the sea floor; however, because they are most likely to be used in very deep water, the more practical anchorage would be with skirt piles. Bottom trusses connect the skirt piles to the Buoyant Piles and also connect Buoyant Piles to one another. The piles have watertight bulkheads with the piles open to the atmosphere above the bulkheads and open to the water below the bulkheads. The top bending members are rigidly connected to each Buoyant Pile. If the bending members connect two or more piles, they form a simple rigid frame. Figure 2 shows a single-leg design. The top bending members of this design are rigidly connected to the Buoyant Pile with their other ends (outboard ends) connected to steel tendons which keep the bending member very close to horizontal alignment and keep the platform level. Figure 3a shows a four-leg rigid frame design with depth and Buoyant-Pile diameters noted. Figure 4a shows a similarly noted single-leg design. This paper is based on design and analysis in compliance with API RP 2A using assumed operating requirements and environmental forces typically associated with the Gulf of Mexico. MAIN PRINCIPAL Long Buoyant Flexible Pile Frames have two important components that cause them to be stable and to resist the environmental forces of wind, waves and current (WWC forces). The first component is the rigid-bending connection at the anchorage and the top of the piles. As the WWC overturning forces cause the tops of the buoyant piles to move laterally, as in Figures 3b and 4b, bending moment is induced at the anchorage and at the pile tops. The induced bending moments are righting moments.