INTRODUCTION

Acoustic instrumentation for obtaining water current velocities over a depth profile evolved by applying range-gating techniques to ship Doppler speed logs (Joyce et al., 1982). However, the errors in navigational position and pitch/roll motions of the vessel limit the accuracy that can be achieved unless compensated for by other sensors (Crocker, 1983). This spurred the development of self-contained acoustic Doppler current profilers (ADCPs), so that by mounting the ADCP on the seabed in an upward-looking configuration such sources of error could be avoided (Pettigrew and Irish, 1983). This development also allowed mounting such instruments on offshore production platforms (Gordon, 1982), and with oblique beam axes as in the CUMEX study (Gordon and Skorstad, 1985).

This paper described applications where ADCPs have been used in a downward-looking configuration, mounted from semisubmersible vessels being used for exploration drilling. Since a semisubmersible is subject to some motion, not all errors due to motion of the ASCP transducer can be avoided. However, the deployment methods described here allowed some decoupling from the semisubmersible motion, and the remaining motion-induced errors were generally within acceptable limits, as discussed later. Also, used in the direct reading mode, the instrument is freed from constraints on power supply and data-recording capacity.

Immediate and recent historical data on current profiles can be of great benefit while conducting exploration drilling, especially on the West Shetland Continental Slope (see Fig.1). Here, the upper Atlantic water, above about 500 m, flows generally parallel with the shelf break towards the north-east, modulated by the shelf tidal currents with major axes also roughly aligned with the shelf break, so that during weak residual flows reversal of the total flow can occur. Below this, in the Faroe-Shetland Channel, cold water from the Norwegian Sea flows out to the Atlantic.

Changes in the depth of the boundary between these flows can cause interaction of the currents on the Continental Slope. Also perturbations in response to storms can cause inertial currents with semi-diurnal oscillations or quasi-steady residual currents (Heaps, 1985). Interactions of these currents can set up shelf wave motions propagating to the north-east which can develop into eddies, causing current reversals to the south-west (Gould, 1984).

Therefore, while the general current conditions to be expected in the area can be reasonably well predicated, the actual conditions experienced at a particular site may be quite complex and unpredictable; hence the benefit in monitoring the current profile during the drilling operation. Unexpected problems with the riser catenary, with station-keeping of the vessel and mooring line tensions, or with ROV operations can be related to the known current conditions, and appropriate decisions made. Also, trends in the current can be deduced to assist in scheduling critical operations such as ‘spudding in’ the drill string, and pulling off or reconnecting the riser to the wellhead.

Fig.1 Topography and UK lease blocks on West Shetland Hebrides Shelf, Currents data acquired for operators by OES; CONSLEX and JIP data (available in full paper)

This content is only available via PDF.
You can access this article if you purchase or spend a download.