ABSTRACT

Detailed analysis of the existence of highly non-linear local flow phenomena close to a platform column exited by a large and steep wave are presented. The objective is to improve the physical understanding of this phenomenon possible essential for accurate numerical treatment of the ringing load problem. Wave elevation close to the column circumference, particle velocities and pressure are investigated mainly in regular wave conditions. Experiments are used in combination with linear diffraction theory and nonlinear codes.

INTRODUCTION

The ringing response effect has shown to induce unexpected large extreme response in terms of tether tension on the Heidrun TLP. The importance of ringing effects on a TLP is related to the level of wave frequency response, the natural period of the platform/tether system in heave, roll and pitch, vertical centre of gravity position and steepness/amplitude of the incoming random waves. As a secondary effect, some ringing events can induce dangerous snap loads in tethers if the pretension is not sufficiently high. The importance of ringing will increase with water depth since the natural period in heave, roll and pitch will increase. This can to some extent be compensated by building the platform substructure in steel and consequently keep the vertical centre of gravity higher than the mean water level. Nevertheless, extreme non-linear ringing responses are expected to constitute up to 30–40 % of the extreme tether tensions on future deep water TLP's. The development of rational and sufficiently accurate prediction and simulation tools is therefore very important for use in pre-design and verification of the TLP concept. This is in particularly important for deep water where experimental verification of the TLP with complete tether mooring is not feasible due to laboratory limitations.

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