Method of erecting a very large diameter offshore column

The method involves erecting a very large-diameter column from a plurality of buoyant columnar members at a selected site in a body of sea water. Each columnar member is floated erect to the site. The first or lowermost columnar member is pulled down and fully submerged to a predetermined depth over the site. The second columnar member is then positioned directly over the lowermost member. The lowermost member is allowed to move up and lift the second member out of the water. The two columnar members are securely interconnected to each other in a watertight fashion. The partial column is then pulled down to a fully submerged condition and the third columnar member is positioned directly over the second member. The partial column is allowed to move up and lift the third member out of the water. A secure watertight connection is made between the third columnar member and the second columnar member. This process of erection is repeated for as many columnar members as may be required to obtain the full length for the desired column. The pulling down of the columnar members can be effected by winches mounted on a platform. The winches have lines on which anchor weights are suspended. The weights are disposed on the sea bed in a suitable pattern around the selected site. The lifting of the columnar members is effected by the inherent buoyancy of the columnar members.

BACKGROUND OF THE INVENTION 
a. Field of the Invention 
This invention generally relates to the offshore construction art and, more 
particularly, to the art of erecting a buoyant, very long, large-diameter 
column and of maintaining the column fully submerged above the sea bottom 
to allow the column to draw cold water from the sea bottom. 
b. The Need for the Invention 
An Ocean Thermal Energy Conversion (OTEC) system can provide an 
inexhaustible electric power source which utilizes the temperature 
difference between the warm surface water and the cold water found at 
lower depths in the oceans to produce electricity. Since the oceans cover 
approximately 70% of the earth's surface, it will be appreciated that they 
can provide a continuously renewable, non-polluting energy source. 
Basically an OTEC system comprises a power plant, a suitable floating 
platform to house the equipment necessary for running the plant, and 
energy conversion and utilization subsystems. The power plant can be based 
on the closed Rankine thermodynamic cycle which utilizes a working fluid 
such as ammonia, capable of evaporating and condensing over a small 
temperature range. Warm sea water is pumped into an evaporator where the 
ammonia liquid is heated and vaporized. The ammonia vapor is fed into a 
turbo-generator where the thermal energy of the vapor is converted into 
mechanical energy, and then into electric power. The vapor leaving the 
turbine is fed into a condenser where it is cooled and condensed into its 
liquid state. The condenser receives cooling water through and from a very 
long pipe column which reaches down into the ocean depths where the 
relatively cold water is found. Condensed ammonia is pressurized and 
returned to the evaporator thereby completing the Rankine cycle. 
It is anticipated that such an OTEC power plant would require approximately 
6 million gallons per minute of warm water flow, and an equivalent volume 
of cold water flow for each 100 MWe of net electric power. 
This invention is concerned only with the construction and erection of the 
cold water pipe column which is required in the above described OTEC power 
plant. The invention, however, can find utility wherever a very large 
diameter offshore column is required to be erected from columnar members. 
Sites for OTEC electricity generating plants are most suitable in latitudes 
not more than approximately 20.degree. north and south of the equator. 
Such sites are available, for example, in the Gulf of Mexico from Florida 
to Texas. Other suitable sites could be near Hawaii and Puerto Rico. 
SUMMARY OF THE INVENTION 
Generally, the invention relates to a method of erecting from buoyant 
columnar members a very long, large diameter column which can be used as 
the submerged cold water pipe in an OTEC electricity-generating plant. 
Each columnar member is made in an onshore fabrication plant. The column 
is provided with buoyancy control means which can include conventional 
ballasting chambers. Anchor weights on the sea bed are suspended from 
winch lines and are used to pull down the columnar members during the 
erection process. 
More specifically, a suitable floating platform is first positioned over 
the selected erection site. A plurality of large anchor weights are 
consecutively suspended on and lowered by the winch lines to the sea 
bottom. The winches are mounted on the deck of the platform. The buoyant 
columnar members are then successively floated erect to the platform. The 
lowermost or first columnar member is pulled down to fully submerge the 
same using the winch lines and the anchor weights. The second columnar 
member is positioned directly over the submerged first columnar member. 
The winch lines are then slackened to allow the inherent net positive 
buoyancy of the first member to completely lift the second columnar member 
above the water surface. The two columnar members are then securely 
interconnected in a watertight fashion, by any conventional means, in a 
dry atmosphere. 
The two-member column is then pulled down to allow the third columnar 
member to become positioned directly over the second member, and the above 
steps are repeated until the entire column becomes fully erected. The 
winch lines are then transferred from the winches to the submerged, 
fully-erected column, and the suspended anchors will maintain the buoyant 
column in its desired position between the top and bottom surfaces of the 
water body.

DETAILED DESCRIPTION OF THE INVENTION 
Throughout the drawings the same or similar reference characters will be 
used to designate the same or similar parts to facilitate the 
understanding of the description. 
The following words are frequently used herein: "bed" means the floor or 
bottom of the sea; "trim" when used as a verb means the act of adjusting 
the net buoyancy of a columnar member or of a plurality of columnar 
members as between positive and negative buoyancy, and the words derived 
from "trim" will have the same connotations. 
A "columnar member" generally means a tubular member, pipe or conduit which 
can be made of any suitable material such as concrete, steel, etc. The 
preferred material is light-weight reinforced concrete. By inherent 
positive buoyancy is meant the ability of individual columnar members or 
of the fully erected column to move from a fully submerged position up to 
and above the water surface depending on the extent (sometimes expressed 
in %) of positive buoyancy involved, and trimming from this inherent 
positive buoyancy condition. 
It will be understood that in a practical embodiment of a columnar member, 
water-tight compartments or chambers are provided and means such as pipes, 
valves, etc., are used to controllably flood or empty such compartments 
during the column erection process. Since such trimming is widely used in 
the offshore construction art, no detailed description thereof will be 
further provided herein. 
A. DEPLOYMENT OF THE ANCHOR WEIGHTS 
The erection process requires a conventional platform 10 (FIGS. 1-3) which 
preferably is a semi-submersible platform such as is used in the offshore 
oil drilling business. The platform typically includes a plurality of 
pontoons 15 interconnected to vertical columns 14. The platform defines a 
generally rectangular slot 12 on the periphery of which are operatively 
positioned a plurality of winches 13, preferably of the wire line type. 
A suitable barge 16 (FIGS. 4, 4A, 5) is used to transport a plurality of 
large clump weight anchors 17 having sheaves 18. Barge 16 should be 
capable of being partially submerged and such barges are well known in the 
art. Barge 16 moves into slot 12 and carries the first weight 17. The 
barge is positioned so that weight 17 lies under a winch 13 (FIGS. 6-7). 
The winch's wire line 19 is looped through sheave 18 on weight 17 and its 
end is removably secured to a padeye 20 on platform 10. The end of barge 
16 carrying weight 17 is submerged (FIGS. 8-9) to a depth sufficient to 
completely submerge the weight. Wire line 19 is then tensioned with winch 
13 to support weight 17 (FIG. 10) which allows barge 16 to refloat and 
move away from platform 10 for bringing a second weight 17 (FIG. 11). In 
the meantime, the first weight 17 has been lowered to the bed 21. Barge 16 
returns to slot 12 with the second weight 17, and the process is repeated 
until a desired plurality of weights 17 are positioned on bed 21 (FIGS. 
12-13) in a predetermined pattern, preferably in a circular pattern with 
the weights 17 being equally spaced apart (FIG. 13). 
B. ERECTION OF THE COLD WATER COLUMN 
The first or lowermost columnar member 30 is floated upright or erect 
towards platform 10 (FIGS. 14-15) for positioning within slot 12 (FIG. 
16). The means for maneuvering member 30 can be any suitable conventional 
means and therefore they are not illustrated in the drawings. Member 30 
has a net positive buoyancy and has a portion thereof submerged and a 
portion extending above the mean water level 9. A plurality of padeyes 31 
are mounted on the bottom end of member 30 and after it its centered 
within slot 12 the bitter ends of wire lines 19 are attached to the 
padeyes 31 (FIG. 17). Thereafter the wire lines are pulled (FIG. 18) to 
fully submerge member 30 (FIG. 18). 
A second shorter columnar member 32 (FIGS. 19, 19A) also having a net 
positive buoyancy, is then positioned within slot 12 directly over member 
30 (FIG. 20). The wire lines 19 are slackened to allow member 30 which has 
a sufficient positive buoyancy, to lift member 32 out of and above the 
water surface 9 so that a proper interconnection 62 (FIG. 29) therebetween 
can be made above the water surface (FIG. 20). 
This process is repeated for another short member 32, then for larger 
standard size columnar members 33, and for the top or uppermost member 34 
(FIGS. 22-23) until the entire column 40 is erected, and fully submerged 
by the winch lines 19. 
Thus, first the connected columnar members are lowered by winching and made 
ready to receive an additional columnar member, then the winch lines are 
slackened to allow the buoyancy of the assembled columnar members to lift 
the additional columnar member. Each columnar member is provided with 
buoyancy chambers 29 (FIGS. 26-29) for controlling the net buoyancy of the 
columnar members during the erection process as well as the net buoyancy 
of the fully erected column 40. In the erected column 40, the buoyancy 
chambers 29 are in fluid communication with each other. For the purpose of 
illustration only, such fluid communication is illustrated by pipes 63 
(FIG. 29). In this fashion, column 40 can be ballasted and deballasted, as 
required. This process allows the trimming of the buoyancy in connection 
with the pulling exerted by the winch lines 19. 
The top member 34 has a plurality of circumferentially spaced padeyes 35 
(FIG. 23) and the bitter ends of the wire lines 19 are transferred from 
the winches 13 and secured to these padeyes 35 (FIG. 24). 
The fully erected column 40 is submerged at a desired distance above the 
sea bed 21 and is anchored thereto by the weights 17 through the wire 
lines 19. Platform 10 is now removed and substituted with an OTEC platform 
50 to which the erected column 40 is coupled by a completely pivotable 
joint 51 so as to decouple the movements of platform 50 from the erected 
column 40. 
The lowermost member 30 is positioned at a sufficient distance above the 
sea bed 21 to prevent silt from being drawn into the column 40. It is 
desired also to provide a screen 60 to filter debris out from the pumped 
cold water through column 40 into the platform 50. 
C. ILLUSTRATIVE EMBODIMENT 
For columnar members having say an internal diameter of approximately 150 
feet, the lowermost columnar member 30 may have a length approximately 100 
feet and a 50% net buoyancy. The columnar member 32 may be about 30 feet 
long and have a 50% net buoyancy. Therefore, member 30 should be submerged 
by winching to about 20 feet below the mean water level 9 and have 
sufficient positive buoyancy to lift the member 32 completely out of the 
water, so that the necessary connection 62 (FIG. 29) can be made 
therebetween above the water level. Such connections 62 should be 
sufficiently rigid and watertight. 
The third columnar member 32 and also have a 30 foot length. It is lifted 
out of the water by the buoyancy of the first and second members 30, 32 to 
about 10 feet above the water level. The net buoyancy of the connected 
columnar members is trimmed by the addition or removal of sea water to or 
from the interconnected chambers 29. 
Columnar members 33 may have a length of 50 feet and have a 25% net 
buoyancy. The connected members (30, 32, 32) are lowered by winching to 
about 40 to 45 feet below the water level 9 to accept the first 50-foot 
columnar member 33. The process is then repeated for all the members 33 
and the last or top member 34. 
An important aspect of the invention is the use of the weights 17 on the 
sea bed 21 as anchors for pulling down the buoyant columnar members and 
the fully erected column. The completed column will be about 500 to 1,000 
feet above the sea bed 21. The columnar members are interconnected to make 
water-tight connections therebetween which will withstand tension and 
bending loads. The lower portion of the fully erected column 40 has a 
negative buoyancy, while the upper portion has the required positive 
buoyancy. In this manner the column will remain erect. Ballast is added to 
the lowermost portion and buoyancy is added to the uppermost portion 
during the erection process.