The objective of the field measurement program was to provide accurate data for flow velocities in the seabed boundary layer and other parameters having influence on the flow boundary layer, especially storm waves, suspended sediment concentrations and bottom roughness. Measurements were carried out using special instrumentation developed in this project at the Edda (70 m depth) and Odin (102 m depth) fields in the North Sea during the period November 1984 through January 1985. Horizontal current velocity components were measured at 6 levels from 0.1 m up to 5 m above the sea bottom along with other environmental parameters. The overall data recovery achieved was approximately 75%, and better than 90% for the acoustic current meters in the boundary layer.
The measuring period included several storms. Maximum significant wave heights of near 8 m and current speeds slightly greater than 40 cm/s were measured. Considerable suspended sediment concentrations were recorded during storm periods. In addition, some bottom ripple activity was recorded at Edda.
Though the storm activity was not as great as had been expected, a large body of useful high quality data was recorded. The data constitute one of the most detailed measurements of the ocean storm environment at one location made to date, and may also have considerable application outside the PIPESTAB project.
The hydrodynamic forces on a pipeline depend on the steady and wave induced oscillatory water particle motions near the sea bottom. The velocity profile close to the bottom depends on many factors including wave action, stratification, moveable bed effects, and topography. To evaluate a model that predicts the velocity profile (boundary layer), it is therefore important to use a complete set of simultaneously measured physical parameters that influence the velocity profile. These parameters can then be used to distinguish between the different phenomena and to reveal their relative importance for the velocity profile. Previous investigations [1] - [6] had indicated potentially significant effects on the boundary layer due to the presence of large waves and/or suspended sediment concentrations. Actual measurements under such conditions were very limited and the implications for pipeline design were unclear.
In order to better assess the flow boundary layer a field measurement program was planned and carried out as part of the PIPESTAB project. The field measurement program was designed to determine the vertical velocity profile by measuring the horizontal velocity components (u, v) at 6 levels along a vertical axis between 0.10 and 5.0 m above the sea bottom. The measurements at 5.0 m above sea bottom were carried out in order to relate the measurements in the bottom boundary layer to a "free stream" velocity. Current measurements were also carried out at two intermediate depths. This was done in order to record the variation in the mean current over the water depth and to compare these velocities to the velocities measured 5.0 m above sea bottom.
A continuous sampling rate of 1 Hz was used for the velocity recordings throughout the whole field recording period, lasting from 12 Nov. 1984 to 26 Jan. 1985. To study the effect of the sediment in suspension, light transmission equipment was installed at two levels near the sea bottom recording light transmission continuously with a sampling rate of 1 Hz during the entire recording period. The suspended load not only has influence on the boundary layer velocity profile, but also may have influence on the sea bed sediment profile.
