Ocean platform

Claim 2

An exploration or production platform comprising; (a) deck; (b) buoyancy means including a plurality of buoyant columns attached to the deck for keeping the platform afloat at sea with the upper portion of the columns being above the sea surface and the deck spaced from the sea surface; (c) negatively buoyant pontoon means for providing negative buoyancy and having a displaced mass and an added mass of sea water of at least about 45% of the total effective mass of the platform, the added mass being substantially the same for both directions of vertical motion of the negatively buoyant pontoon; (d) flexible tendon means for suspending the negatively buoyant pontoon means from below the balance of the platform to a depth above the sea bottom and where wave induced dynamic forces on the pontoon are small and do not materially contribute to the heave response of the platform at commonly experienced wave periods but where wave induced dynamic forces do act on the pontoon at higher wave periods below resonance of the platform, the negatively buoyant pontoon means being capable of having a negative buoyancy sufficient to keep tension in the tendon means during all sea conditions; and (e) the platform having natural period in heave of at least about 25 seconds.

Claim 3

The platform claimed in claim 1 including: upper pontoon means attached to the columns at their lower ends, the tendon means supporting the negatively buoyant pontoon means below the upper pontoon means.

Claim 4

The platform claimed in claim 2 wherein: the displaced mass and added mass of the negatively buoyant pontoon means is at least about 60% of the effective mass of the entire platform.

Claim 5

The platform claimed in claim 2 wherein: the draft of the columns is from about 25 to about 55 meters and the draft of the negatively buoyant pontoon means is from about 80 to about 150 meters.

Claim 6

The platform claimed in claim 4 wherein: the upper pontoon means has a displaced mass and an added mass of about 20% of the effective mass of the entire platform; and the displaced mass and added mass of the negatively buoyant pontoon means is about 60% of the effective mass of the entire platform.

Claim 7

The platform claimed in claim 2 wherein: the draft of the negatively buoyant pontoon means and the configuration of such pontoon means are such that with wave periods of from about 20 to 25 seconds such pontoon means experience heave forces that materially oppose heave forces on the balance of the platform.

Claim 8

The platform claimed in claim 6 wherein: the draft of the negatively buoyant pontoon means is from about 80 to about 150 meters.

Claim 9

The platform claimed in claim 7 wherein: the upper pontoon means has a displaced mass and added mass of about 20% of the effective mass of the entire platform; and the displaced mass and added mass of the negative buoyant pontoon means is about 60% of the effective mass of the entire platform.

Claim 10

The platform claimed in claim 8 wherein: the negatively buoyant pontoon means is capable of a negative buoyancy of about 5% of the effective mass of the entire platform.

Claim 11

The platform claimed in claim 6 wherein: the upper pontoon means has a displaced mass and an added mass of about 20% of the effective mass of the entire platform.

Claim 12

An exploration or production platform comprising: (a) a deck; (b) buoyancy means including at least four columns in a square pattern attached to the deck for keeping the platform afloat at sea with the upper portion of the columns above the sea surface and the deck spaced from the sea surface; (c) a negatively buoyant pontoon for providing negative buoyancy and a large displaced and added mass, the negatively buoyant pontoon in plan view being generally square shaped with an open center and having a displaced mass and added mass of sea water at least about 60% of the total effective mass of the platform, the added mass being substantially the same for both directions of vertical motion of the negatively buoyant pontoon; (d) flexible tendon means for suspending the negative buoyant pontoon from below the balance of the platform to a depth above the sea bottom and where wave induced dynamic forces on the pontoon are small and do not materially contribute to the heave response of the platform in waves of normally occurring periods, but where wave induced dynamic forces do act on the negatively buoyant pontoon at higher wave periods below resonance of the platform the negative buoyancy of the negatively buoyant pontoon means maintaining the tendons in tension during all sea periods up to about 25 seconds or wave heights of 30 meters; and (e) the platform having a natural period of heave of at least about 25 seconds.

Claim 13

An exploration or production platform comprising: (a) a deck; (b) buoyancy means including at least four columns in a square pattern attached to the deck for keeping the platform afloat at sea with the upper portion of the columns above the sea surface and the deck spaced from the sea surface; (c) a negatively buoyant pontoon for providing negative buoyancy and a large displaced and added mass, the negatively buoyant pontoon in plan view being generally square shaped with an open center and having a displaced mass and added mass of sea water at least about 60% of the total effective mass of the platform, the added mass being substantially the same for both directions of vertical motion of the negatively buoyant pontoon; (d) flexible tendon means for suspending the negative buoyant pontoon from below the balance of the platform to a depth above the sea bottom and where wave induced dynamic forces on the pontoon are small and do not materially contribute to the heave response of the platform in waves of normally occurring periods, but where wave induced dynamic forces do act on the negatively buoyant pontoon at higher wave periods below resonance of the platform the negative buoyancy of the negatively buoyant pontoon means maintaining the tendons in tension during all sea periods up to about 25 seconds or wave heights of 30 meters; and (e) the platform having a natural period of heave of at least about 25 seconds.

Claim 14

The platform claimed in claim 11 including; an upper pontoon that in plan view is generally square shaped with an open center; the columns attaching to the upper pontoon at its corners, the tendon means supporting the negatively buoyant pontoon below the upper pontoon.

Claim 15

The platform claimed in claim 12 wherein: the upper pontoon, the buoyant pontoon, and each of the columns has a cylindrical cross section and a characteristic diameter, the diameter of the upper pontoon being about 60% of the diameter of each of the columns and the diameter of the negatively buoyant pontoon being about 95% of the diameter of each of the columns.

Claim 16

The platform claimed in claim 12 wherein: the upper pontoon has a displaced mass and an added mass of about 20% of the effective mass of the entire platform.

Claim 17

The platform claimed in claim 14 wherein: the negatively buoyant pontoon is capable of a negative buoyancy of about 5% of the effective mass of the entire platform.

Claim 18

The platform claimed in claim 15 wherein: the draft of the columns is from about 25 to about 55 meters and the draft of the negatively buoyant pontoon is from about 80 to about 150 meters.

Claim 19

The platform claimed in claim 14 wherein: the draft of the columns is from about 25 to about 55 meters and the draft of the negatively buoyant pontoon is from about 80 to about 150 meters.

Claim 20

The platform claimed in claim 11 wherein: the negatively buoyant pontoon and each of the columns has a cylindrical cross section and a characteristic diameter, the diameter of the negatively buoyant pontoon being about 110% of the diameter of each of the columns.

Claim 21

The platform claimed in claim 18 wherein: the displaced mass and added mass of the negatively buoyant pontoon is about 80% of the effective mass of the entire platform.

Claim 22

The platform claimed in claim 19 wherein: the draft of the columns is from about 25 to about 55 meters and the draft of the negatively buoyant pontoon is from about 80 to about 150 meters.

Claim 25

The method claimed in claim 21 wherein: the lowering step is done by winches.

Claim 26

The method claimed in claim 21 including supporting the tendons on the columns before lowering and guiding them on the columns during lowering.

Claim 27

An improvement in an exploration or production platform of the type having a plurality of buoyant columns that support a deck above the water surface and which piece the sea surface and upper pontoons at the base of the columns and below the sea surface, the exploration or production platform having a heave response curve that has a secondary maximum resulting from wave induced forces acting on the pontoons at periods below the platform's resonant period, the improvement comprising: a negatively buoyant suspended pontoon below the upper pontoons and having an effective mass constituted of its actual mass and its added mass, the added mass being the same in both vertical directions; flexible tendon means suspending the suspended pontoon below the upper pontoons to a depth where wave forces at the secondary maximum do not materially act on it but where wave forces at higher wave periods below resonance of the platform do materially act on it; and the effective mass of the suspended pontoon being sufficient to increase the natural period of heave.

Claim 28

An improvement in an exploration or production platform of the type having a plurality of buoyant columns that support a deck above the water surface and which piece the sea surface and upper pontoons at the base of the columns and below the sea surface, the exploration or production platform having a heave response curve that has a secondary maximum resulting from wave induced forces acting on the pontoons at periods below the platform's resonant period, the improvement comprising: a negatively buoyant suspended pontoon below the upper pontoons and having an effective mass constituted of its actual mass and its added mass, the added mass being the same in both vertical directions; flexible tendon means suspending the suspended pontoon below the upper pontoons to a depth where wave forces at the secondary maximum do not materially act on it but where wave forces at higher wave periods below resonance of the platform do materially act on it; and the effective mass of the suspended pontoon being sufficient to increase the natural period of heave.

Claim 29

The platform claimed in claim 24 wherein the effective mass of the suspended pontoon is at least about 65% of the effective mass of the entire platform.

Claim 30

The platform claimed in claim 25 wherein the displaced mass and added mass of the upper pontoons is about 20% of the effective mass of the entire platform.

Claim 31

The platform claimed in claim 26 wherein the suspended pontoon has a negative buoyancy mass of at least 5% of the effective mass of the entire platform.