Articulated tower platform is one of the compliant structures which is economically attractive especially as loading and mooring terminal to deep waters. These platforms are comparatively light compared to the conventional fixed platforms.
An articulated tower is a linear structure , flexibly connected to the sea bed through a cardon joint and held vertically by the buoyancy force acting on it. The part of the tower emerging form the water supports the super structure designed to suit the particular application eg. A tanker to be loaded etc., As the connection to the sea bed is through the articulation the structure is free to oscillate in any direction and does not transfer any bending moment to the base.
The extension of the concept of the single leg articulated tower has led to the development of a new type of platform with several columns which are parallel to one another. The columns are connected by universal joints both to the deck and to the foundations. The use of universal joint ensures that the columns always remain parallel to one another and the deck remains in horizontal position. This type of platform is called multi leg articulated tower which may have three or more columns. The articulated tower which can be used at very large water depth may also have one or more number of joints in the intermediate level. Such towers having joints in the intermediate level are called multi hinged articulated towers.
As the articulated tower is compliant in nature, it moves with the waves and thus the wave force and bending moment along the tower will be less compared to a fixed structure. Usually the natural period of the towers is of the order of 40 to 90 seconds and hence dynamic amplification factor is small.
Examples of Articulated Towers
An articulated is a light structure, hence it is easy for fabrication and installation.
The deepest articulated tower single point mooring in the North Sea is the Stattford tower designed by Equipment Mecaniques at Hydraulics (EMH) for 145 m water depth.
Two articulated single point mooring systems designed by Chicago Bridge and Iron Co (CBI) have been installed in offshore Brazil in 122 m water depth.
The single anchor leg moorings (SALMS) with multi hinged articulated tower in the Thistle field in the north sea was installed in 1977 at 174 m of water depth.
The most recent SALM is in the Hondo field in the Sanata Barbara channel and was designed by IMODCO for Exxon. This tower is in 150 m of water and permanently moored to a 50000 DWT tanker. The articulated tower used in the Fulmar field as a loading and unloading system started operating in 1982.
The articulated loading columns in the Maurean field located in a water depth of 93 m installed in 1982.
CG Dorris have developed a deepwater gravity concrete tower as production platform for a water depth of 490 m of water .
Several investigations have been carried out by Kokkinowrachos and Butt (1978) and kokkinowrachos et.al. (1985) have considered the motion for tower with two degrees to of freedom and parametric studies have been carried out. Kirk & Jain (1977) and Chakrabarti and Cotter (1979) have studied the response of articulated tower and motion analysis. The nonlinear drag term was linearised and the oscillations were obtained. Dutta and Jain (1990) have investigated the response of an articulated tower to random wind, wave and current forces.