ABSTRACT

The contribution to the load on offshore structures from the design current is very significant. Research related to the determination of wave current interaction is therefore important.

This paper discusses

  • which combination of wave conditions and current should be used in the design

  • influence of current on wave spectra

  • incorporation of current in the Morison formula for calculation of forces on slender offshore structures

  • wave-current kinematic models for the wave crest

  • higher order nonlinear resonance effects caused by wave-current interaction

  • effect on the loading and response of structures by introducing current.

The discussion is exemplified with reference to calculation of response of slender jackets and jackups. Recommendations with respect to research needs are given in "Summary and Conclusions".

INTRODUCTION

In the design of slender offshore structures it is important to accurately compute the total water particle velocity of the wave-current flow field. Such structures are in the ultimate limit state condition (NPD, 1984) dominated by the drag loading which is proportional to the square of the wave and current velocity. Therefore it is important to determine the factors influencing the total wave-current velocity and the associated structural response (Karunakaran, 1990). These factors are

  • the wave field for the selected return period combined with the relevant current

  • the influence of the current on the wave field (Hedges, 1987)

  • wave kinematics model (Gudmestad, SpidsQe and Karunakaran, 1990) as well as wave plus current kinematics model for the wave crest. This is required for a proper time domain history analysis

  • selection of forcing equation and hydrodynamic coefficients whereby the wave plus current kinematics are transformed into forces on the structure

  • higher order resonance effects caused by nonlinear wave phenomena (free surface effects, use of nonlinear wave theory and application of the nonlinear drag forcing term), wave-current interaction effects and wave-current-structure resonant response (SpidsQe and Karunakaran, 1989).

For engineering application it is necessary to select

  • wave-current interaction model

  • methodology for calculation of wave forces

  • structural analysis models

and it is therefore required that a choice of data/methods/models be made for those factors mentioned above. We will take a practical approach and try to isolate the most important factors bringlng forward recommendations about future research needs.

IMPORTANT ASPECTS RELATING TO WAVE CURRENT INTERACTION
Wave Climate

The design environmental climate for offshore structures is determined by the selected probability of exceedance for the design event.

Traditionally, a 10−2 annual probability level of exceedance has been selected (NPD, 1984) for design of structures on the Norwegian Continental Shelf. The consistent treatment would be to investigate which combination of wave (significant height/peak period) and current would lead to a reasonable safety level In view of critical failure modes. The effects of a joint description are indicated by Haver and Winterstein, (1990). Due to lack of data it has, however, been very difficult to prepare the required combined statistics. NPD (1984, 1990) have selected a combination of a annual probability of exceedance for the wave and a 10-I annual probability of exceedance for the current. This is expected to represent a conservative approach.

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