Compliant Tower Platform
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A compliant tower (CT) is a fixed rig structure normally used for the offshore production of oil or gas. The rig consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are designed to sustain significant lateral deflections and forces and are typically used in water depths ranging from 1,500 to 3,000 ft. (450–900 m). At present the deepest is the Chevron Petronius tower in waters 623 m deep (https://en.wikipedia.org/wiki/Compliant Tower).
Basic Characteristics of Compliant Tower Platform
The compliant tower platform is suitable for seawaters between 60 and 200 m. Also, shafts can be series connected, which could allow larger water depth. In 1977, the single anchor leg mooring (SALM) with multi-hinged articulated tower in the Thistle field in the North Sea was installed at the water depth of 174 m. The compliant tower platform can be used for single point mooring (SPM), loading terminals, control towers, and early or full production facilities. In some marginal fields where reserve size does not warrant large facilities, it may also be used as a production platform.
History of Compliant Tower Platform
In the year 1963, due to the needs of industries, the first compliant tower platform was designed. In 1968, the first full-scale compliant tower platform to be used for experimental purposes was built. This platform worked at a depth of 110 m. This platform existed for 3 years at the site, during which researchers carried out many tests demonstrating that the compliant tower platform can be used in offshore engineering. In 1977, Burns and O’Amorim proposed the design and construction of two compliant tower platforms (Burns and O’Amorim 1977). The two compliant tower platforms worked at a depth of 140 m. The design took into account the dynamic response and the strength of the base under the wind, wave, and current loads. The main structures of these two compliant tower platforms include base, universal joint, ballast chambers, buoyancy chambers, and the shaft. In the year 1990, the world’s largest SPM compliant tower platform was built in 1989 in the Timor Sea in the northwestern of Australia. This compliant tower platform can withstand waves up to 9 m, wind up to 47 m/s, and current up to 2 m/s.
Main Characteristics of Compliant Tower Platforms
The tower shaft is the main structural component and is hinged on the seabed. The tower shaft can be a tubular cylinder or a steel lattice column. There is normally buoyancy chamber near the water surface. At the bottom of the shaft, there is always a ballast chamber. The oil production system is set up on the deck. The buoyancy chamber near the water surface provides buoyancy, and the buoyancy forces ensure the compliant tower platform is in an upright position. The larger the buoyancy, the smaller the tower shaft’s motion caused by the wave force.
Besides, the damping effects of compliant tower are of great importance for the dynamic response of the whole platform. The influence of frictional damping, structural damping, and viscous damping on the vibration response amplitude is different. Frictional damping has the greatest influence on the amplitude attenuation of free vibration response without wave force, followed by structural damping, while viscous damping of fluid is the least (Hao 2008). In other words, the control of frictional damping can well control the response of the platform and enable the platform to quickly return to the equilibrium position after being stressed.
In addition, the difference between the compliant tower platform and the traditional steel structure platform is that there is no pile inserted into the seabed in the compliant tower platform, while the pile of the traditional steel structure needs to insert into the seabed to resist loads of various forces at sea. Therefore, it is more convenient to construct and install a compliant tower platform and cause little hazard to the seabed environment. The compliant tower platform relies on its own huge moment of buoyancy to adapt itself to the waves without affecting drilling and production operations. In the case of not using, the ballast water in the base can be drained, and the base drifts up a little. So the tower platform can be easily transported by tugboats.
Furthermore, because the compliant tower platform connects to the seabed, the capability to resist the seismic load deserves great attentions. Some researchers (Han et al. 2006) analyzed the natural vibration characteristics and the seismic response characteristics of the platform by establishing the nonlinear governing equations of the compliant tower platform under earthquake excitation. The influence of different depths and the friction coefficient of the bottom universal joint on the platform response were also discussed. It is found that the seismic load acts on the platform can cause vibration of the compliant tower platform; when only the average water depth is changed, while keeping other parameters unchanged, the seismic response decreases with the increase of water depth; and when the friction coefficient of the bottom universal joint is increased, the amplitude of the seismic response decreases, and increasing the friction coefficient properly can effectively reduce the seismic response of the compliant tower platform.
The compliant tower platform has its own advantages and disadvantages. Compared with the traditional steel structure platforms, this type of platform can save a lot of steel. Because of the small size of this type of platform, small wind and wave force, simple structure, and low cost, the compliant tower platform is an ideal platform for deep water. The natural frequency of this type of platform is much lower than the wave frequencies. A salient feature of this type of platform is that the buoyancy chamber is near the sea surface, so the wave load is principally caused by inertia.
Although compliant tower platform has quite a lot of advantages, there are also some disadvantages. The disadvantage of this platform is that due to practical and economic reasons, the buoyancy of the compliant tower platform is not enough, and the platform may swing too large in some cases. Sometimes the base may slide along the seabed. Besides, in severe sea state, the motion of the compliant tower platform can be too large to work normally.
- Burns GE, O’Amorim GC (1977) Buoyant towers for phase 1 development of Garoupa field. In: Offshore technology conference, Houston, TexasGoogle Scholar
- Han L, Tang Y, Yi C (2006) Seismic response analysis of articulated tower platform in the still water. China Offshore Platf 21(3):36–39Google Scholar
- Hao J (2008) Dynamic response analysis of articulated tower platform. Harbin Engineering University, Harbin. (in Chinese)Google Scholar
- Young RA (1983) Coupled analysis of compliant offshore platforms. Oregon State University, CorvallisGoogle Scholar