Mooring- and Riser-Induced Damping in Fatigue Seastates

Abstract

Viscous damping due to drag on mooring lines and risers is seastate dependent and significantly affects the motion of a floating platform in deep water, particularly in everyday seastates. This in turn impacts design of the risers, which are typically controlled by fatigue. The dynamic interaction between the platform, mooring and risers cannot be evaluated using conventional uncoupled analysis tools, where each is analyzed separately. Rather, coupled analysis is required to provide a consistent way to model the drag-induced damping from mooring lines and risers. We describe a coupled, frequency domain approach (RAMS – Rational Approach to Marine Systems) for calculating the dynamic response of vessel, mooring and risers. In coupled analysis, the risers and mooring lines are included in the model along with the floater. In this way, damping of the floater motion due to drag on the mooring lines and risers is incorporated directly. It is also valuable to estimate the linear damping factors from the full, coupled analysis results. These damping factors may then, for example, be used in an equivalent linear model of the floating system in which the stiffness and damping effects of the mooring and risers are represented as additions to the floater stiffness and damping matrices. Such a model could be used to efficiently design a subsystem (e.g.; an export riser). We describe a technique to determine the equivalent linear damping factors from the coupled analysis results. This paper also illustrates the use of these methods for a West Africa FPSO. The need for coupled analysis is shown by comparing results from the fully coupled model with those obtained using an uncoupled method in which the mooring line damping is approximated.

Coupled dynamics of deepwater structures – issues and challenges

“…For ship-like floaters, the dominant source of roll damping arises from the drag on (spread moored) line structures including SCRs which can be captured only by a coupled model Garrett et al (2002b). Colby et al (2000) have noted that the results from a coupled model show a larger damping.…”

“…Most of them adopt numerical modelling and there have been a few studies comparing experimental and numerical modelling results (see, for example, Ormberg and Larsen 1998;Colby et al 2000;Ma et al 2000;Astrup et al 2001;Heuirtier et al 2001;Garrett et al 2002a;Garrett et al 2002b;Bhattacharyya et al 2003;Joseph et al 2004;Chen et al 2006;Low and Langley 2006;Yang et al 2012;Zhang et al 2008). Scaled model tests for deepwater systems inevitably have to be done in wave basins where water depth obviously cannot be scaled down.…”

Toward a Probabilistic Approach to Determine Nominal Values of Tank Sloshing Loads in Structural Design of Liquefied Natural Gas FPSOs

“…The first is the multibody hydrodynamic interaction between the FLNG and the LNGC [27][28][29]. The sec ond is the effect of tank sloshing on the FLNG response [30][31][32][33][34], The third is the hydrodynamic predictions for the coupled FLNG and the mooring system [36][37][38], The fourth is the horizontal plane stabilities of the single-point mooring of FLNG [39^12]. The hydrodynamic characteristics of FLNG are of great impor tance to the efficient exploitation of gas fields.…”

A reduced integration method for the coupled analysis of floating production systems

“…That is, one should not disregard the coupling between the hydrodynamic behavior of the hull and the structural-hydrodynamic behavior of the mooring lines and risers represented by Finite Element -FE models (Astrup et al, 2001;Chaudhury, 2001;Correa et al, 2002;Correa, 2003;Finn et al, 2000;Heurtier et al, 2001;Kim et al, 2001Kim et al, , 2005Ormberg and Larsen, 1998;Senra et al, 2002;Wichers and Devlin, 2001). Therefore special attention has been dedicated to the development and implementation of formulations for the coupling of the equations of motion that represents the hull and the lines (see for instance Garrett et al, 2002aGarrett et al, , 2002bGarrett, 2005;Kim et al, 2005;Rodrigues et al, 2007;Tahar and Kim, 2008;Low andLangley, 2006, 2008;Low, 2008).…”