ABSTRACT

The stress exerted on the seabed by currents in a turbulent oceanic bottom boundary layer influences many aspects of offshore operations. It is important that those stress components containing a vertical velocity term be quantified so that, combined with sediment properties, rates and intensities of erosion, deposition, and scour may be accurately estimated.

An autonomous profiling system (BLIPS) is being developed to measure physical and optical properties in ocean bottom boundary layers. System sensors include electromagnetic and rotor-type current meters, temperature and pressure measuring devices, transmissometers, and water sample bottles. The sensors are affixed in a vertical array to a bottom-supported instrument frame at heights ranging from 0.25 to 5.00 m above its base. The instrumentation measures and records on high-density magnetic tape high-frequency (5 Hz) fluctuations of material properties, including three components of current velocity a s well as mean values. The simu1taneous measurement of near-bottom oceanographic and sedimentologic properties allows the determination of flow sediment interactions and provides data necessary for the validation of theoretical models of particle-laden flow.

BLIPS is a third-generation system that has improved capability over existing profilers. Field tests in the Gulf of Mexico are scheduled for early 1985.

INTRODUCTION

The pattern and intensity of oceanic currents are of interest to all elements of the offshore petroleum industry because of the forces these motions exert on structures such as platforms, riser systems, and pipelines. Using equipment and techniques developed for oceanographic use over the past few decades, current measurements are routinely conducted before and during installation of offshore facilities. These measurements, however, seldom extend into the lowermost part of the benthic boundary layer, where large velocity gradients set the level of bottom stress. As a result, near-bottom forces are not estimated as accurately as the fluid forces higher in the water column. Furthermore, measurement programs rarely document sediment re-suspension events, although those geologic phenomena are known to have an effect on the level of shear stress at the seabed. Although knowledge of bottom shear stresses is essential for evaluating drag on bottom structures and scour about structural components that are in contact with the bottom, standard current-monitoring techniques fail to provide the required information.

In this paper, a state-of-the-art autonomous boundary-layer interaction profiling system (BLIPS) designed and being built at Coastal Studies Institute specifically for oceanic benthic boundary layer studies is described. A background summary of the theory dictating the characteristics of individual components, together with practical considerations for overall system configuration, is included. The profiling system provides the capability to document benthic boundary layer structure and processes necessary for an understanding of the fluid dynamical forces acting at the seabed. Measurements of this type can form the basis for improved offshore facilities design, construction, and operation requirements, and at a savings in cost over requirements based on conservative estimates of the relevant parameters.

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