Spar buoy construction having production and oil storage facilities and method of operation

In a floating structure including oil storage capacity and adapted to be anchored by catenary mooring lines at a subsea well location, the combination of:

• an elongated hull means vertically positionable in water;
• said hull means including an oil storage chamber means for storing oil and extending for a major portion of the height of said floating structure;
• a plurality of vertically extending ballast chamber means within said hull means and extending from the bottom portion of the oil storage chamber means to above the top portion of the storage chamber means;
• means in the hull means for introducing oil into the oil storage chamber means and for removing water therefrom as oil enters said storage chamber means;
• and means for regulating the amount and location of ballast in the ballast chamber means to maintain a center of gravity of the entire mass at a selected location above the bottom of the hull means and at a selected distance from the center of buoyancy of the structure while the amount of oil in the oil storage chamber means changes.

In a method of maintaining constant draft and constant selected distance between center of gravity and center of buoyance to provide stable motion characteristics of a spar buoy construction including a structure of fixed weight, equipment and stores thereof of variable weight, a vertically disposed oil storage chamber of selected volume and of prismatic shape, a plurality of variable ballast chambers of selected volume and of prismatic shape extending at least for the height of the oil storage chamber, and having inlet and outlet means for inflow and outflow of oil and water, said oil storage chamber being initially filled with water and establishing a center of gravity position and a selected draft of the spar buoy construction in water;

• the steps of:
• causing oil to flow into said oil-storage chamber while displacing water therefrom;
• causing water to flow into initially empty variable ballast chambers until the aggregate weight of the water in the ballast chambers and the weight of oil and water in the oil storage chamber is equivalent to the initial aggregate weight of the water-filled oil chamber;
• and controlling the amount of water entering certain of said variable ballast chambers to maintain the center of gravity of the spar buoy construction at the said selected position.

In a floating structure including production facilities, oil-storage facilities, and adapted to be associated with a riser system connected with a subsea well installation, the combination of:

• an elongated hull means;
• means for vertically positioning said hull means in water;
• said hull means including a longitudinal passageway means, oil storage means of prismatic shape and surrounding the passageway means, a plurality of separate variable ballast means of prismatic shape surrounding said central passageway means, and extending from the bottom of the oil storage means to above the oil storage means;
• means for filling said oil storage means with water and for removing said water as said oil storage means is filled with oil;
• means for inflow and outflow of water from said variable ballast means;
• a riser system extending through said central passageway means for connection to said production facilities, and to provide flow of oil from said subsea wellhead to said oil storage means;
• said riser system including buoyant means for maintaining tension on said riser system, said buoyant means being located in said passageway means and providing relative movement between the upper end of said riser system and the passageway means of the hull means;
• said oil storage means when filled with water establishing a center of gravity of the entire structure and a selected draft;
• said inflow of oil to said oil storage means and outflow of water therefrom with resultant change in the center of gravity of the entire mass being compensated for by varying the water volume in at least certain of said variable ballast means whereby the center of gravity of the entire mass remains substantially in the same position, the draft remains substantially constant, and relative movement of the riser system in the hull means is stabilized.

In a stabilized spar buoy construction for deep sea operations having submerged oil storage capacity, above surface construction facilities, and a riser system connecting subsea well installations with the production facilities, the combination of:

• means for minimizing heave response and maintaining substantially constant draft of said spar buoy construction including
• an elongated submerged hull means having a selected volume and a selected water plane area,
• mooring means connecting the bottom portion of the hull means with the sea bottom,
• said spar buoy construction having a center of buoyancy;
• means including oil storage chambers and variable ballasting chambers in said hull means for establishing and maintaining a constant center of gravity of said construction at a selected distance K below said center of buoyancy;
• means for passing said riser system through said hull means, maintaining tension on said riser system, and minimizing pitch motions in the riser system including
• a submerged float chamber means having pitch oscillations of the same amplitude as the hull means,
• the bending stresses in said riser system between the sea floor and the float chamber means being minimized by maintaining said selected distance K under different load conditions.