The SEA RATCHET* is a device which will substantially reduce wave induced motions as well as increase deck loading flexibility.

Slung deeply beneath existing semis using chains, the ratchet action results from its upward only damping action entraining a hundred thousand tons of apparent mass. Filmed scale model tests are shown.


All relative motions between drilling platforms and the earth are undesirable with the single exception of forward translation during mobilization. Semis present a logical attempt to reduce heave, pitch and roll motions through the use of large size and mass coupled with small water plane area. The heave motions which remain are reduced even further through the use of drill string heave compensators. There occur, however, periods when the sea state overrides the present capabilities to deal with heave and operations must shut down. Unfortunately, daily costs do not shut down and, in some areas, significant annual down-time losses mount to millions of dollars. What recourses have semi users for reducing these losses? There are three: larger platforms, creating a larger ratio of platform mass to wave energy, active six degree of freedom hydrodynamic controls, as suggested last year in paper 2030; and more effective passive damping. The size of present semis is not likely to increase enough to significantly reduce wave induced motions because of construction limitations as well as costs. Active control systems, while feasible for future design semis, can do nothing for existing platforms or those already planned. A retro-fitable damping system may offer a solution and is the subject of this paper.


Nearly all semis embody damping techniques most generally in the form of the flat deck surfaces of the submerged hulls. -Unfortunately, these surfaces are neither large enough nor deep enough to significantly reduce heave, particularly in very heavy weather. Structural as well as stability considerations preclude longer columns and more deeply submerged pontoons. If adequate damping surfaces could be sufficiently removed from the sea surface yet rigidly attached to the semi, their effectiveness would be greatly improved. One such system has been proposed and scale tested as reported in OCEAN INDUSTRY, August 1971, by Jack R. Hilder, Jr., of Special Offshore Services. Its mechanical complexity coupled with the difficulty of retrofitting existing semis probably has hindered its acceptance in the community despite the improved model test results indicated in the article.

A brief review of basic wave theory will reveal- that the vertical component of water wave particle motion decays exponentially with depth such that for every A* increase in depth below the mean water surface the motion is halved. Unfortunately, higher waves are associated with longer wave lengths and longer periods while a semi's heave response generally degrades with longer periods. As any submariner can verify, virtually all vertical surface induced motion ceases below 100 meters, even in the most violent storms.

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