ABSTRACT 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. INTRODUCTION 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. DAMPLNG TECHNIQUE TODAY 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.
ABSTRACT A new conceptual design for a semi-submersible that offers dynamic platform control in all six degrees of freedom as well as a doubling of remobilization speed is presented. The six degree of freedom control will enable non-moored station keeping and yaw orientation as well as the practical elimination of sea-induced platform heave, roll and pitch motions. These advantages accrue from two major departures from conventional semi-submersible design; the platform support columns are streamlined and tapered, and the submerged hulls are more submarine shaped and equipped with fore and aft ?wrap around? propellers whose blades are collectively as well as cyclically changeable in pitch. Submarine model experimental data are presented in addition to an extrapolation of Naval Research and Development Center (NSRDC) model data for a 300' X 300' platform presenting speed vs power and stability predictions. INTRODUCTI ON The design of present semi submersibles is inherently limited by their support columns' reaction to sea-induced hydrodynamic forces. The fact that the columns provide all static stability in heave, roll and pitch necessarily requires that they have a large cross-section area and therefore react adversely to wave motions. It is obvious that these platforms are stable with reference to the averaged sea surface else they would roll or pitch over. Unfortunately the average sea surface is, more frequently than not, made up of mountains and valleys which, at least once or twice a year, is described as a lI one hundred year storm ll Although the stormy North Sea is not typical of all fields currently being worked, there are potentially productive areas in deeper waters where platform motions may slow down or stop productive work. In addition to the adverse induced motions attributed to the support columns, there are the penalties associated with their hydrodynamic drag during transit and the hydrodynamic drag in the presence of a current when on station. These effects are generally mitigated by ballasting as high as possible; however, as the submerged longitudinal bodies approach the surface, their wave-making drag increases. Ideally, these bodies would be a ALBACORE, THRESHER, or later submarine shape, submerged to a depth of one or more diameters with platform support columns of zero cross-section. In short, present platforms are adversely affected by sea conditions and have low transit speeds. PLATFORM DEVELOPMENT With due regard for the laws of physics, if a wish list of semi-submersible platform characteristics were prepared by operators, it would surely contain the following: Maximum stability in all sea conditions. No necessity for bottom mooring. High transit speed. Dynamic positioning on station is currently successfully practiced in three degrees of freedom, yaw, surge, and sway, through the expedient of multiple thrusters. Thus, platform orientation may be aligned either up or down seas to minimize drag on the submerged hulls. These thrusters may also be used for transit. In high sea use, however, the other three degrees of freedom; heave, pitch, and roll, the troublesome ones, are mitigated only by the sheer mass and passive damping of the platform.