Hydrodynamic Analysis of Mooring Lines Based on Optical Tracking Experiments. (December 2007) Woo Seuk Yang, B.S.; M.S., Chung-Ang University Chair of Advisory Committee: Dr. Richard Mercier 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.

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Hydrodynamic analysis of mooring lines based on optical tracking experiments

Mooring line damping due to low-frequency superimposed with wave-frequency random line top end motion

17th International Conference on Offshore Mechanics and Arctic Engineering

To maximise effectiveness, towed underwater sonar bodies generally require perturbations from the steady state motion to be minimised. The instantaneous position of the towed body is influenced by the unsteady, wave-induced motion of the surface vessel, transmitted via the tow cable. This can render the trajectory of the underwater body beyond acceptable limits for sonar operations.
In an effort to decouple this motion, a two-part tow configuration has been employed. This thesis describes a three-dimensional dynamic computer model developed to investigate two-part tows by modelling the individual cables separately and coupling them dynamically. This approach also enables the modelling of series and parallel multiple tow configurations. The cable system, modelled using a three degree-of freedom finite difference approach, is then coupled to the six degree-of-freedom underwater towed bodies at the appropriate locations of the cable system. The modelling of the cable as a continuous medium and the derivation of the stress wave speeds are also presented, followed by the validity and.effects of representing it as a discretised model.
The solution to the dynamic equations describing the motion of the discretised tow configuration is carried out using an implicit multi-step numerical technique, subject to specific boundary conditions. This allows the values to be improved through an iterative procedure, until sufficient convergence is achieved.
An introduction into the use of such numerical techniques in engineering and an analysis of the numerical procedure used in the solution are also presented. The requirements for accuracy and numerical stability of the integration technique are investigated and a guide to the time interval for the time stepping algorithm is deduced.
The computer model is successfully validated using experimental results from scaled model tests in a circulating water channel. These results together with those obtained from full scale sonar trials utilising small coastal craft are used to further investigate the behaviour of the two-part tow configuration to varying parameters. The experimental results and the computer model enables the user to identify the optimum tow configuration for the prevailing conditions.
The thesis also presents a detailed review of the various methods available to investigate underwater cables and vehicles, together with prediction methods for their hydrodynamic coefficients. The latter coefficients for the scaled models used in the project are obtained via experimental procedures.
Although a number of investigations have been carried out dealing with aspects of underwater towing operations, the strength of this investigation lies in that it combines these aspects, i. e. mathematical modelling, computer simulation, prediction of the hydrodynamic coefficients, scaled model experiments, full scale trials, and the analysis of the numerical technique, into one study.

Computer simulation and investigation of underwater two-part and multi tow systems

At present the mooring technology of turret moored tankers is challenged as exploration activities for oil and gas move into deep and ultra-deep water and more hostile environments. The more hostile environment concerns the Atlantic Frontiers and the typhoon prone areas. For the field development in these water depths and under these weather conditions, the availability of a cost effective mooring system can be an important factor in determining economie success. The approach for developing fields in the mentioned areas has typically relied upon the adaption of mooring technology developed for shallower water depth applications. To improve the understanding of and the insight in the reliability and integrity in the application in the deep water mooring systems, numerical design tools can be used. As the ^fcerical design tooi a by model tests validated computer program based on the fully integrated dynamics of turret moored F(P)SO systems has been recently developed and applied to deep water. Further the computer program can be used to optimize the system and to enhance the safety of the design by applying numerous combinations of extreme weather conditions in order to discern the most severe weather condition. In this paper the program has been described. The results of the validation with model tests on a tanker moored with a conventional mooring system in 350 m water depth are shown. The results of the computations agree well with the results of the model tests. Finally by means of computations with the tanker moored with a conventional system with and without spring buoys were investigated in 400 and 1200 m water depth applications. By varying extreme metocean parameters the understanding of and the insight in the reliability and integrity in the mooring system can be improved.

Behaviour of Turret Moored Tankers in Combined Extreme Metocean Parameters

The computation of low frequency mooring forces on a tanker exposed to survival conditions requires knowledge of the hydrodynamic excitation and reaction forces. The low frequency excitation is caused by wave drift forces. The reaction forces on the tanker are a combination of added mass forces, wave drift damping forces, viscous forces and damping forces due to interaction with the mooring chains. In this paper a direct simulation method for a chain-turret moored tanker is applied for the correct instantaneous low and high frequency motions of the tanker, including the dynamic behaviour of the mooring chains. The latter is modelled by a lumped mass method. Comparison is made of the direct simulation method and an approximate method using the average chain damping. An evaluation of the chain damping contribution is made using model test results and computations.

R.H.M. Huijsmans

Papers

An Improved Method for Calculating the Contribution of Hydrodynamic Chain Damping on Low-Frequency Vessel Motions