Construction of the Ocean Horizontal Array Turbulence Study (OHATS)
Support Structure

This computer drawing of the Air-Sea Interaction Tower (ASIT) and OHATS array shows the vertical legs of the array support structure in light blue with brackets in darker blue and pink (Created by Megan Carroll).

     The overall array design was based on a National Center for Atmospheric Research (NCAR) experiment performed over land in the San Joaquin Valley of southern California. Since this array would be deployed over the open ocean, its supporting structures had to withstand high winds and sea spray commonly encountered at the Martha's Vineyard Coastal Observatory (MVCO). With that in mind, existing parts from the NCAR experiment were used and incorporated into the OHATS project for unpredictable ocean conditions.
     The scientist objectives of OHATS required that a horizontal array of nine sonics be deployed at two levels in the atmospheric boundary layer near the ocean surface. The required horizontal separation was 23 inches (0.6m) for a total span of 183 inches (4.6m). The same separation was required between the two levels.
The width of the tower platform where the structure was to be suspended, often referred to as the diving board, was only 54 inches (1.4m). That proved to be one of the major limiting design factors. Because the design needed to incorporate two horizontal arrays with nine sonics each, the width on either side would have to be about 64 inches (1.6m). This would require assembly of the outside sonics before placement of the horizontal structure.
Another concern was the actual amount of working space on the tower. Each of the 18 sonics had to be brought up to the platform separately to avoid possible damage. The sonics were then attached to a boom and carefully clamped to the horizontal structure once in place. Because of this, it was important to design a system that would require as few people as possible to assemble and deploy.

Vertical Support Towers

Vertical support towers were needed to extend the array down from the platform and over the water. Factors considered when selecting sections for these vertical legs were:

" Horizontal array weight and placement
" Maximum wind speed of 50 knots
" Placement of the structure high enough above water to avoid potentially dangerous waves

The tower sections used were fabricated in 8-ft lengths which required us to weld several together to reach the necessary length of 27 ft. This length would allow positioning of the array 17 ft and 23 ft (6.5m and 7m) below the tower - or 16.4 ft and 18 ft (5m and 5.5m) above mean sea level.

Written by Megan Carroll
  Brackets designed by WHOI engineers link the vertical legs to the tower and help to keep the array stable while suspended over the ocean (Created by Megan Carroll).