Abstract :

The dynamic response of jack-ups, tension leg platforms and compliant towers is significantly influenced by the amount of damping. Considerable uncertainty exists about the damping arising from the interaction of the structural members of the structure with the surrounding fluid. Alter a concise introduction to the hydro-elastic problem, the paper describes some carefully designed experiments to determine the in-line forces on an oscillating cylinder of 0.4 m diameter (β= 2.9 104 - 1.5 105) for KC in the range from 1 10 14. The results are presented in terms of drag and inertia coefficients. The drag coefficients are significantly lower than reported before.

Introduction

Two trends can be seen in the worldwide application of offshore structures. One is the extended application of existing infrastructure in water depths less than 150 m. This leads to an increased requirement for jack-ups for drilling, well testing, early production and satellite development. The second trend is the advance, particularly in the Gull of Mexico, into much deeper waters, which has initiated the development of tension leg platforms and compliant towers. Jack-ups, TLP's and compliant towers are distinguished from more conventional structures by the large dynamic responses that they may display to unsteady forces caused by wind, waves and current. These responses and hence the fatigue life of the structure are significantly influenced by damping which may stem from internal friction, interaction with the soil or the surrounding fluid. It is hardly possible to generalize the relative importance of these contributions, each situation has to be assessed separately with the appropriate physical models. At slightly larger amplitudes of oscillation the laminar solution becomes unstable leading to transition to turbulence and/or separation of boundary layers. Unfortunately, at small amplitudes the so-called Morison equation lacks physical justification, thereby complicating present design practices that are based on it.

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