Yang, Woo Seuk (2007-12). Hydrodynamic analysis of mooring lines based on optical tracking experiments. Doctoral Dissertation. Thesis uri icon

abstract

  • Due to the complexity of body-shape, the investigation of hydrodynamic forces on mooring lines, especially those comprised of chain segments, has not been conducted to a sufficient degree to properly characterize the hydrodynamic damping effect of mooring lines on the global motions of a moored offshore platform. In the present study, an experimental investigation of the hydrodynamic characteristics of various mooring elements is implemented through free and forced oscillation tests. Since no direct measurement capability for distributed hydrodynamic forces acting on mooring line segments such as chain and wire rope is available yet, an indirect measurement technique is introduced. The technique is based on the fact that hydrodynamic forces acting on a body oscillating in still water and on a stationary body in an oscillatory flow are equivalent except for the additional inertia force, the so-called Froude-Krylov force, present in the latter condition. The time-dependent displacement of a slender body moving in calm water is acquired through optical tracking with a high speed camera. The distributed hydrodynamic measurements are then used to obtain the force by solving the equation of motion with the boundary condition provided from tension measurements. Morison's equation is employed along with Fourier analysis to separate the inertia and drag components out of the total fluid force. Given the experimentally-derived information on hydrodynamic behavior, the resistance provided by a mooring line to a floating structure is briefly studied in terms of damping and restoring force in a coupled dynamic system.
  • Due to the complexity of body-shape, the investigation of hydrodynamic forces on
    mooring lines, especially those comprised of chain segments, has not been conducted to
    a sufficient degree to properly characterize the hydrodynamic damping effect of mooring
    lines on the global motions of a moored offshore platform. In the present study, an
    experimental investigation of the hydrodynamic characteristics of various mooring
    elements is implemented through free and forced oscillation tests. Since no direct
    measurement capability for distributed hydrodynamic forces acting on mooring line
    segments such as chain and wire rope is available yet, an indirect measurement
    technique is introduced. The technique is based on the fact that hydrodynamic forces
    acting on a body oscillating in still water and on a stationary body in an oscillatory flow
    are equivalent except for the additional inertia force, the so-called Froude-Krylov force,
    present in the latter condition. The time-dependent displacement of a slender body
    moving in calm water is acquired through optical tracking with a high speed camera. The
    distributed hydrodynamic measurements are then used to obtain the force by solving the
    equation of motion with the boundary condition provided from tension measurements. Morison's equation is employed along with Fourier analysis to separate the inertia and
    drag components out of the total fluid force. Given the experimentally-derived
    information on hydrodynamic behavior, the resistance provided by a mooring line to a
    floating structure is briefly studied in terms of damping and restoring force in a coupled
    dynamic system.

publication date

  • December 2007