Design and analysis of typical buoyancy tank riser tensioner systems

Abstract

The method for design and analysis of a buoyancy tank riser tensioner system (BTRTS) was put forward in this paper, taking the free standing hybrid riser’s top buoyancy tank as an example. The design procedure was discussed and was also illustrated in a flowchart, after a short description of the global arrangement, structure configuration, and the function of different types of buoyancy tanks (BT). The objective of this paper is to describe a way of developing a BT with minimal hydro force, maximal net lift, and no redundancy of compartments. The method of determining the main dimensions of the BT, namely the length and the outer diameter, was outlined. A series of investigations was conducted for a West Africa FSHR BT design, and the effect of the ratio of the length to the outer diameter (L/D) on the hydrodynamics and the weight of the BT was discussed. The methodology of designing the internal structure of the BT was presented. The effects of the number of compartments and the dimension of the inner stem on the BT weight and strength were compared. The relationship between inner structure and the number one index of the BT as well as the riser’s top tension factor (TTF) were illustrated for normal operating conditions and conditions with one or more compartments (or inner stem) damaged. A design instance was given in this paper, when L/D is 4–6, the BT weight and the drag force are compromised. When the BT is divided into 10 compartments, the riser TTF will reach the maximum value, and the ratio of the stem OD to shell OD is about 0.3. A global strength analysis method of the BT and the main load case matrix was also included in the paper, together with the local strength analysis of the buoyancy tank’s pad-eye assembly.