MOTION REDUCTION FOR SPAR PLATFORM WITH HYDRO-PNEUMATIC TENSIONERS

The Spar platform is one of the options to support dry-trees from a floating unit, particularly in increasing water depth. The recent Spar developments in Gulf of Mexico (GOM) favor Spar Supported Hydro-pneumatic Tensioners to support Top Tensioned Vertical Risers (TTR). The trend has been made possible by, as well as promoted the long-stroke RAM type hydro-pneumatic tensioner market. Currently up to 35 ft stroke tensioners are technically available and 28 ft ones are installed and field tested on GOM spars. The primary advantage, compared with buoyancy cans, is its possible integrated installation. Aside from taking payload from the hull, hydro-pneumatic tensioners have stiffness and can effectively reduce the heave period closer to the dominant wave period. It's imperative to control the resonance motion, particularly of a classic spar which is generally low damped in heave. This study investigates the heave reduction by using an air-spring-water-column Vibration Absorber (VAB) mounted vertically inside the spar, the vibration absorbers consist of vertical caissons, open at the bottom to the sea and closed at the top. Each caisson consists of a water column providing mass (unsupported by the Spar) and a dual-chamber air spring, which interacts with the Spar. The motion of the water column produces pressure variations out-of-phase with the resonant motions of the Spar, resulting in reduced heave. Damping of the vibration absorber is provided by an orifice connecting the two air chambers, and is used to allow effective operation across varying frequencies. The detailed background of VAB and the application can be found in references [1] and [4]. This paper describes a classic spar platform with its heave motion reduced by using VAB to the point that market available hydro-pneumatic tensioners can be used. The theory of the Vibration Absorber and the procedure of configurating the system are introduced in this paper. The platform, mooring system and global motion analysis of the proposed facility are described for GOM applications with performance comparable to an existing truss Spar platform. Both frequency domain and time domain analysis of the coupled motions are included in this study.