Deep water offshore apparatus

In an offshore apparatus for use over one or more seafloor wellheads for drilling and/or producing an oil field, the combination of;

• a hull means adapted to be partially submerged in the sea and having a bottom end portion at a depth where wave energy is attenuated;
• means for achieving a selected natural period for said apparatus depending upon the expected wave conditions at the locality of the oil field comprising:
• a downwardly extending frame means connected to said bottom end portion and extending into a depth beyond significant wave action,
• said frame means having a plurality of vertically spaced horizontal plates defining bays having windows open for relative transverse movement of water through said bays and for entrapment of water in said bays upon relative vertical motion of said frame means and water;
• a keel assembly having ballast means of selected weight;
• and anchor means connected to said apparatus.

An apparatus for minimizing heave, pitch, and roll motions of buoyant deep draft offshore apparatus comprising:

• frame means including a plurality of vertically arranged bays defined by horizontal plates vertically spaced a selected distance related to the aspect ratio of a plate and by open windows between said plates,
• said windows providing transparency to ocean current in a horizontal direction to minimize drag loads,
• said plates defining a substantially water impervious barrier to entrap water in said bays in a vertical direction relative to said frame means, thereby increasing the effective mass of the offshore apparatus, and
• said frame means being connected to said apparatus and extending downwardly therefrom, whereby the natural period of said apparatus is modified to minimize heave, pitch, and roll motions of said apparatus.

In a buoyant offshore apparatus for drilling and production of an oil field, said apparatus including a taut anchor system which includes:

• anchor means comprising an anchor box for holding ballast,
• a downwardly projecting peripheral skirt on said box for penetrating the sea floor,
• said box having side walls,
• means for holding an anchor member at a selected angle to direct mooring forces through a juxtaposed portion of said skirt,
• and means for securing said anchor member in said holding means against relative movement therewith.

A device for use with an offshore apparatus having a plurality of risers, comprising:

• a substantially vertical frame adapted to extend downwardly from the offshore apparatus;
• a plurality of substantially horizontal vertically spaced plates associated with the frame, the plates being substantially water impervious and having a plurality of riser openings defined therein;
• and
• a plurality of windows between adjacent plates that allow water to flow horizontally between adjacent plates.

An offshore apparatus, comprising:

• a hull adapted to be partially submerged in the sea and having a bottom end portion at a depth where wave energy is attenuated;
• and
• control means for achieving a selected natural period for the offshore apparatus corresponding to expected wave conditions, the control means including,
• a downwardly extending frame associated with the bottom end portion of the hull and extending to a depth beyond significant wave action, and
• a plurality of vertically spaced substantially horizontal plates associated with the frame and defining bays which trap water therein upon relative vertical motion of the frame and water, the bays having windows that allow transverse movement of water through the bays.