A simple model for heave-induced dynamic tension in catenary moorings

An empirical model for the dynamic tension due to vertical motions at the top of a catenary mooring is presented. The model is applicable for forcing at ocean wave frequencies (as opposed to slow drift frequencies). The model calculates the standard deviation of the tension as the sum of an inertial term proportional to heave acceleration and a drag term proportional to quadratic heave velocity. Using numerical simulations, the model is shown to capture coupling between inertia and damping effects. The proportionality parameters of the model are expressed as effective mass and drag constants times a linear function of the mean tension at the top of the mooring. Formulae are derived for calculating these constants in terms of the hydrodynamic and material properties of the mooring. Comparison of model results to measurements from an instrumented oceanographic mooring and simulations of a lazy wave riser under ranges of conditions yield maximum errors of 8 and 11% and root mean square errors of 2 and 3%, respectively. The greatest errors occur in situations where there is high mean tension and large dynamic forcing and when horizontal motion produces significant tension effects.

[1]  J. R. Morison,et al.  The Force Exerted by Surface Waves on Piles , 1950 .

[2]  G J Lyons,et al.  ADVANCES IN MOORING LINE DAMPING , 1995 .

[3]  J.I. Gobat,et al.  The dynamics of shallow water oceanographic moorings: experimental and numerical results , 1999, Oceans '99. MTS/IEEE. Riding the Crest into the 21st Century. Conference and Exhibition. Conference Proceedings (IEEE Cat. No.99CH37008).

[4]  William C. Webster.  Mooring-induced damping , 1995 .

[5]  H. M. Irvine,et al.  The linear theory of free vibrations of a suspended cable , 1974, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[6]  Jason I. Gobat,et al.  The dynamics of geometrically compliant mooring systems , 2000 .

[7]  Carl M. Larsen.  Flexible riser analysis — comparison of results from computer programs , 1992 .

[8]  Michael S. Triantafyllou,et al.  DYNAMIC ANALYSIS AS A TOOL FOR OPEN-SEA MOORING SYSTEM DESIGN , 1985 .

[9]  E. Huse,et al.  Mooring Line Damping Due to First- and Second-Order Vessel Motion , 1989 .

[10]  Masahiko Nakamura,et al.  Slow drift damping due to drag forces acting on mooring lines , 1991 .

[11]  Jason I. Gobat,et al.  Modeling the Dynamics of a Shallow-Water Oceanographic Surface Mooring Using Full-Scale Data , 2000 .

[12]  J. I. Gobat,et al.  WHOI Cable: time domain numerical modeling of moored and towed oceanographic systems , 1998 .

[13]  Mark A. Grosenbaugh.  Designing Oceanographic Surface Moorings to Withstand Fatigue , 1995 .

[14]  Mark A. Grosenbaugh.  On the dynamics of oceanographic surface moorings , 1996 .

[15]  M. S. Triantafyllou,et al.  Damping Of Moored Floating Structures , 1994 .

[16]  P. Hall.  On Symmetric Bootstrap Confidence Intervals , 1988 .