A Semi for All Seasons
Authors: Haselton F.R. 
Published: October 2008

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.