Method and apparatus for stabilizing a floating structure

A semi-submersible platform comprising:

• a deck structure;
• a substructure including a plurality of columns fixed to and extending below said deck structure;
• a plurality of pontoons fixed to at least some of said columns below said deck structure;
• a liquid ballast tank connected to one of said structures;
• means for sensing vertical velocity of said platform;
• and
• pump means for moving liquid to and from said ballast tank, ballast liquid sensor means for detecting the quantity of ballast liquid in said ballast tank, and means responsive to said vertical velocity sensing means and said ballast liquid detecting means for driving said pump means in one direction or another to decrease the difference between the quantity of liquid in said ballast tank with respect to a mean quantity and a quantity related to the sensed vertical velocity of said platform.

A system for diminishing heave displacement of a floating structure in the presence of disburbing waves, said system comprising

• damping means for applying a force to said structure in opposition to forces exerted upon said structure by disturbing waves,
• means for detecting the rate of change of heave displacement,
• means for detecting force applied to said structure by said damping means, and
• means responsive to both of said means for detecting for controlling said damping means to diminish the difference between a function of said rate of change of heave displacement and said force applied by said damping means.

A semi-submersible floating structure comprising:

• deck means;
• a plurality of main columns attached to and extending below said deck means for submersion in water to provide buoyant supporting force for the structure;
• pontoon means attached to said main columns near the lower ends thereof that are adapted to be submerged, said pontoon means having an upper horizontal surface projection which is positioned to remain submerged;
• and
• damping means attached to the structure for exerting damping force thereon substantially in opposition to and as a function of the vertical velocity of the structure attributable to wave motion acting on the plurality of columns and pontoon means, said damping means includes ballast chambers in selected ones of the columns and pontoon means, and includes pump means for controllably introducing and ejecting water from a ballast chamber in relation to the vertical velocity of the structure;
• control means including velocity sensing means for sensing the vertical velocity of the structure;
• ballast sensing means for sensing the volume of water in each ballast chamber relative to a mean volume of water therein;
• and a control loop responsive to the velocity sensing means and to the ballast sensing means for controlling the pump means to vary the actual volume of water in each ballast chamber relative to the mean volume of water therein as a function of the sensed vertical velocity.

A semi-submersible floating structure comprising:

• deck means;
• a plurality of main columns attached to and extending below said deck means for submersion in water to provide buoyant supporting force for the structure;
• pontoon means attached to said main columns near the lower ends thereof that are adapted to be submerged, said pontoon means having an upper horizontal surface projection which is positioned to remain submerged;
• and
• damping means attached to the structure for exerting damping force thereon substantially in opposition to and as a function of the vertical velocity of the structure attributable to wave motion acting on the plurality of columns and pontoon means, said damping means includes ballast chambers in selected ones of the columns and pontoon means, and includes pump means for controllably introducing and ejecting water from a ballast chamber in relation to the vertical velocity of the structure, and for each ballast chamber a control means including first and second presurized air chambers, a first conduit for connecting the first air chamber to the ballast chamber, and a second conduit for connecting the second air chamber to the ballast chamber;
• the pump means includes a pump connected for pumping air from the first air chamber to the second air chamber to maintain a pressure difference in the first and second air chambers, a first valve included in the first conduit, and a second valve included in the second conduit;
• and a control loop including a velocity sensing means and a ballast sensing means for controlling the first and second valves to vary the air pressure in the ballast chamber to vary the actual volume of water in the ballast chamber relative to the mean volume of water therein as a function of the sensed vertical velocity.

Method of reducing heave of a semi-submersible floating structure having a plurality of columns that extend below a deck to provide buoyant supporting force for the structure and having attached to the columns near submerged ends thereof pontoon means which have horizontal surface projections that remain submerged, the method comprising:

• sensing the rate of change of one component of undesirable displacement of the structure;
• exerting damping force on the structure substantially in opposition in relation to and as a function of the sensed rate of change attributable to wave motion acting on the columns and pontoon means;
• and
• sensing the quantity of water contained in a ballast chamber;
• and wherein
• the step of exerting damping force is performed in relation to the rate of change of said one component of undesirable displacement of the structure in selected combination with the difference between a predetermined quantity of water and the actual quantity of water contained within a ballast chamber.