Motion, Fatigue, and the Reliability Characteristics of a Vertically Moored Platform

As the search for hydrocarbons continues, offshore platforms are being installed in deeper and deeper water. platforms are being installed in deeper and deeper water. These platforms primarily have been of the fixed type, for water depths ranging from 10 to 1,000 ft (3 to 300 m). In deeper waters, the fabrication and installation of these platforms can become difficult and extremely costly. In addition, as the water depths increase, the first natural periods of vibration of these platforms increase from less than 1 second to 4 or 5 seconds and higher. Wave forces have significant energy at this level (Fig. 1); dynamic amplification also becomes significant. Thus, such structures may be subjected to a large number of cycles at a significant stress level, so that structural fatigue becomes a major design parameter. The vertically moored platform (VMP) concept Fig. 2) is a drilling and production platform specifically designed for deep water of 700 to 3,000 ft (200 to 900 m) or deeper. This platform can be fabricated and installed at a reasonable cost. A detailed engineering description of this concept has been presented.The VMP is a buoyant structure moored by vertical tubular members maintained in tension. The well casings are integral to the mooring risers, and the wellheads are on the deck. The basic motion is similar to that of an inverted pendulum; the structure is flexible horizontally and rigid vertically. While the VMP and semisubmersibles have similar horizontal motions, the latter have substantial vertical motions, making it difficult to tie in wells for production operations. VMP vertical motions are restrained. permitting conventional tie-in of wells at deck level.First, we describe VMP motions. Next, a computer model of VMP dynamic behavior is discussed, followed by a comparison of calculations with experimental data. We stress the selection of design environmental conditions for a VMP. An overview of the riser fatigue and reliability studies demonstrates the adequacy of the mooring system.

VMP dynamic characteristics are inherently different from those of fixed offshore structures (Fig. 1). The motions of the VMP jacket (within 6 degrees of freedom) have differing characteristics, depending on the corresponding stiffness of the mooring system. The flexibilities are high for surge, sway, and yaw motions, with natural periods in the 60-second range, where wave energy is negligible. Surge, sway, and yaw motions were critically damped during model tests and computer simulations. No resonance can be expected for these motions. The mooring system resists the mean environmental forces (e.g.. averaged more than 30). while jacket inertia resists the shorter period environmental forces. Under the most severe environmental conditions, in water 1,000 ft (305 m) deep, a maximum horizontal placement or surge of 150 ft (46 m) is predicted. placement or surge of 150 ft (46 m) is predicted. JPT

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