ABSTRACT

Wave kinematics and wave profile data collected in extreme wave conditions during the WADIC experiment in the central North Sea have been analyzed. It is confirmed that wave profiles observed by buoys in steep waves are more symmetric than derived from fixed instruments. The steep waves do not necessarily occur in the highest sea state, however. There are signs that suggest that the WAVERIDER does not follow the surface during the most extreme waves. For sub-surface current meters directional linear wave theory fits the observations reasonably well. Further, measurements made in the splash zone compared favourably with linear theory using conditional simulation and prediction techniques.

INTRODUCTION

The Wave Direction Measurement Calibration Project (WADIC) had the primary objective to evaluate operational, commercially available directional wave measurement systems under severe open ocean wave conditions.

There are two principal methods for in situ measurements of ocean waves, both of which were employed during WADIC; those made on a fixed platform using wavestaffs, pressure transducers, current meters, lasers etc., and those made from moored buoys.

Wave induced orbital velocities and associated wave profiles are of considerable interest in the design analysis of offshore installations and the operability and stability of ships, in particular under high wave conditions. The WADIC field experiment resulted in some unique measurements of wave properties measured using various wave measurement systems in sea states up to nearly 11 m (36 ft) significant and single waves as high as 20 m (65 ft.)

In this paper we focus on two aspects; first, an analysis of parameters defining the asymmetry of extreme and steep waves as seen from fixed and moored sensors and, secondly, comparisons of measured storm wave orbital velocities with predictions and simulations based on directional linear wave theory.

FIELD EXPERIMENT

The WADIC field experiment took place over a four month period during winter 1985-86 in the vicinity of the Edda platform on the Ekofisk field in the central North Sea (Fig. 1).

Rapid response vector measuring current meters together with pressure transducers were mounted on a purpose built tower attached to the north west corner of the platform (Fig. 1). Further a pentagon array (? 7 m) of downlooking Thorn EMI lasers were located above and to the side of the tower. The highest current meter was a Marsh McBirney whilst the remaining seven were Simrad Ultrasonic Current Meters (UCM-10). The current meters in the splash zone were not as high as had originally been conceived due to sea floor subsidence in the Ekofisk area. Seven directional wave buoys in addition to a Waverider buoy were moored within 1 km of the platform.

Signals from the various sensors mounted on the platform and some of the buoys which telemetered to the platform were logged on a central computer. Data was recorded at 2 Hz sampling rate over 40. minutes normally once every 3 hours but almost continuous recording was initiated during storms. Other of the buoy systems were self-logging.

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