Wind, wave and meteorological data have been measured by Shell UK Exploration and Production (a Shell/Esso joint operating company) in the Northern North Sea since 1975, initially at the Brent Bravo Production Platform and latterly, since September 1983, at the North Cormorant production platform located in the Northern North Sea north-east of the Shetland Isles.

The objectives of the data collection programme have been to

  • obtain reliable data for the prediction of more reliable design criteria for new structures in the Northern North Sea;

  • provide real-time data for the safe management of daily operations around the platform;

  • provide information to weather forecasting services, and

  • evaluate new instrumentation for wave measurement in order to improve the reliability and associated costs of wave data collection.

Because the largest component m the costs of data collection has been the measurement of wave data, there is a clear need to move towards platform-based systems which offer the potential for reducing maintenance costs The wave sensors that have been deployed at North Cormorant are

  • a Datawell waverider buoy since September 1983,

  • an EM1 Infra-Red Laser Wave Height Monitor since September 1983 (platform-based);

  • a Datawell WAVEC buoy (ref. 1) since April 1986, operational after July 1986;

  • a MIROS microwave radar (ref 2) since March 1986 (platform-based) but operational after December 1986.

The first two are omnidirectional wave sensors while the last two can measure wave height and direction. The need to measure wave direction arises out of the need to establish with confidence the directional wave climate and to use this in reducing the costs of new structures in the area This chapter describes the operating experiences with these sensors and then proceeds to detailed comparisons of the measured parameters.

SENSOR AND SYSTEM PERFORMANCE

The waverider and EM1 laser form part of the North Cormorant Metocean system The two sensors are sampled simultaneously by a data acquisition unit and are then processed identically Spectral analysis is carried out offshore, and the processed parameters are stored, together with quality control flags. The data are quality controlled onshore and stored on a computer databank.

The WAVEC buoy is a pitch-roll buoy (see ref. 1) where the data are processed as described in ref 3. The MIROS is a microwave transmitter and receiver (see ref 2) which measures water particle velocity and uses a first order wave theory to convert this to wave amplitude The sensor has a beam width of 30° and hence samples wave energy from a restricted direction. By rotating the sensor head it is possible to provide information for all directions. The sensor at that stage was unable to distinguish between waves moving towards it and those moving away (referred to as the ‘180-degree ambiguity’).

The data from the four sensors as deployed on North Cormorant all undergo spectral analysis, with differing frequency resolutions and limits. The analysis details are summarized in Table I.

TABLE I Details of calculation of wave parameters(available in full paper)

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