Analytical Approach on Dynamic Tension of Free Hanging Configuration

Free hanging is the most simple and basic model of a flexible riser, and has advantages in terms of low cost and easy operation, installation and maintenance compared to other configurations, such as Lazy-S or Steep-S. However, the dynamic motion of the top end has direct impact on the riser behavior near the touch down zone (TDZ) and causes axial compression. Since flexible pipe is vulnerable to axial compression, we usually adopt buffer configuration like Lazy-S or Steep-S. Some technical means have been made to reduce the axial compression of free hanging, such as non-uniform weight distribution for Steel Catenary Riser (SCR) and heavy material wrapping near TDZ for flexible pipe. Despite the achievements of such research, it is difficult to say that there is a systematic and comprehensive explication on how the parameters of dynamic motion or the configuration of pipe effect the dynamic tension of a riser. Linearized analysis in frequency domain with quite small number of lumped-mass model is discussed in this paper. The simplified linearized analytic solution support or modify empirical knowledge which parameter (weight in air or water, an angle of the riser against still water plane, the motion of the top end or added mass) effect dynamic minimum tension of the riser. The aim of this paper is to provide starting point of analytical approach and show its possibility. Most of the key parameters which are empirically known to estimate dynamic tension of the riser is driven from analytical model in this paper. Comprehensive numerical calculation by Orcaflex is performed to verify the assumptions and support the results.