US20020139286A1

US20020139286A1 - Heave-damped caisson vessel

Info

Publication number: US20020139286A1
Application number: US09/821,705
Authority: US
Inventors: James Lee; Steven Byle
Original assignee: Novellent Technologies LLC
Current assignee: Novellent Technologies LLC
Priority date: 2001-03-29
Filing date: 2001-03-29
Publication date: 2002-10-03

Abstract

A floating structure includes an elongate caisson hull and at least one plate set coupled to the hull. The plate set includes multiple heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull.

Description

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to floating structures. More specifically, the invention relates to a deep draft caisson vessel for supporting a deck or other superstructure above a water surface.

2. Background Art

The offshore oil industry employs a variety of floating structures to perform oil exploration, drilling, and production. One type of floating structure is the deep draft caisson vessel, also known as a spar platform. The performance of the spar platform is governed primarily by mass distribution on the platform. FIG. 1 illustrates a

generalized spar platform

100 which includes an elongate caisson hull 102 supporting a deck 104 above a water surface 106. Mooring lines 108 are attached to the hull 102 below the water surface 106 to perform station keeping. Marine risers 110 extend downward from the hull 102 and carry fluids to and from the platform 100. The hull 102 includes an upper buoyant portion 112, an elongated middle portion 114, and a negatively buoyant lower portion 116. The upper portion 112 provides the majority of the buoyancy required to support the weight (W) of the deck 104 above the water surface 106. The lower portion 116 provides a large negatively buoyant mass (M) to counterbalance the weight (W) of the deck 104. The middle portion 114 provides an extended separation (X) between the deck weight (W) and the negative mass (M).

The combined arrangement of the amount of negative mass (M) and the distance of separation (X) positions the combined center of gravity (CG) a distance (Y) below the combined center of buoyancy (CB) for the platform. This arrangement provides desirable stability and pitch characteristics. The

middle portion

114 increases the platform draft (T). The deep draft moves the keel of the hull 102 below a majority of wave-induced loading. The in-place draft is typically in excess of 600 feet. The deep draft also provides a desirable heave natural period by increasing the total mass, i.e., the displacement and virtual mass of the platform. A typical spar platform design will have a heave natural period in the range of 25-30 seconds. To achieve this the hull of the spar has a large ratio of hull draft (T) to hull diameter (D). A typical spar hull has a circular cross-section of a diameter between 70 and 150 feet.

Various designs of spar platforms are known in the art. One design is the conventional or classic spar. FIG. 2A illustrates a prior-art

classic spar

200 which includes an elongate caisson hull 202 and deck 204. The hull 202 has an upper buoyant portion 206 made of variable ballast tanks 208 and permanent buoyancy tanks 210. A center portion 212 of the hull 202 includes an open framed construction which traps a large mass of seawater 214. A lower portion 216 of the hull 202 contains an amount of fixed ballast 218 which places a center of gravity (CG) of the spar 200 below a combined center of buoyancy (CB) of the spar 200. The fixed ballast 218 generally includes iron ore or other dense material. The amount of fixed ballast 218 is typically on the order of 0.5 to 1.0 times the deck weight (W). Mooring lines 220 are attached to the hull through fairleads 222. Helical strakes 224 extend outward from the hull 202. The helical strakes 224 are made of flat plates which extend in a spiral pattern downward along the hull 202. Apertures 226 (shown in FIG. 2B) in the helical strakes 224 permit passage of the mooring lines 220 downward along the hull 202 to the fairleads 222. Risers 228 supported by multiple air cans 230 are disposed within an open well-bay 232 (shown in FIG. 2B) extending through the upper portion 206 of the hull 202. The risers 238 exit through guide tubes 234 in the lower portion 216 of the hull 202.

The deep draft of the

classic spar

200 acts to increase the heave natural period of the deck 204 to a favorable region of 25 to 30 seconds. This is achieved primarily from the increase in virtual mass (M_v) of the structure due to the large mass of water 214 trapped within the center portion 212 of the hull 202. The extended length of the hull 202 is therefore required. For heave motions having periods on the order of heave natural period, the trapped water 214 moves with the hull 202, effectively increasing the apparent total mass of the spar 200. For longer period motions, such as 100 seconds or more, the water may flow in and out of the hull 202. Therefore, the water does not increase the actual mass of the spar 200 for static and quasi-static displacements. The deep draft also lowers the keel draft (T_k), resulting in a reduced wave loading in the vertical direction.

The resulting

spar

200 has very limited wave-induced heave motions.

Generally speaking, heave motions are made of two components, as illustrated in FIG. 5A. The first component is a wave period response, referred to as first order motions. This component includes motions occurring near the peak wave period of the sea in which the spar is located, typically 14-16 seconds for the maximum design hurricane conditions. The magnitude of the first order motions are generally proportional to the magnitude of wave loading at the keel of the hull (T _k). The second component is a long period response, referred to as second order motions. This component includes motions occurring near to the natural period of the floating structure in heave. The magnitude of the second order motions are generally proportional to the damping provided by the hull in the heave direction.

The classic spar has an extremely deep keel draft (T _k), typically set at 650 ft for a wide range of hull diameters, displacement, and deck weights. This depth is generally located below as much as 98% or more of the hydrodynamic wave forces for most sea states, as indicated by the magnitude of the wave profile at the keel draft (T_k). The result is extremely small first order heave motions. In fact, there may be only negligible first order heave motion for sea states lower than annual storm conditions. The damping of the spar in heave, however, is generally low. This results in comparatively larger second order motions during storm conditions. The combined heave motions are still generally quite favorable and are dominated by slow, long-period heave.

One concern for the in-place performance of a classic spar arises from its continuous, circular cross-sectional shape. A long slender body oriented vertically in an ocean current may be susceptible to the formation of vortices along the length of the hull. The vortices will induce periodic, potentially large magnitude horizontal excursions of the floating structure. Referring to FIG. 2A, the

helical strakes

224 are designed to impede the formation of vortices. The helical strakes 224 are generally oriented at an angle approximately 30 degrees from vertical. The strakes 224 are arranged such that they spiral down the length of the hull 202. The net affect is to disrupt the horizontal flow of water, imparting some vertical component to the flow. This action tends to disrupt the formation of vortices. The size of each strake 224 may be on the order of 10% of the diameter of the hull 202. Each strake 224 may extend 10-15 feet outward from the hull 202. The helical strakes 224, therefore, increase the drag of the hull 202 to ocean currents and increase the requirements of the mooring system. Also the strakes 224 interfere with the mooring lines 220, requiring the addition of multiple apertures 226 (shown in FIG. 2B) through the strakes 224. In general, despite the negative effects, strakes 224 are considered as essential to the classic spar.

The construction and installation of a classic spar design is eased by its continuous cross-section but complicated by the extended hull length and anti-vortex strake. The classic spar is generally constructed in a horizontal orientation. A support cradle is adapted to support the circular sections of the hull on the construction ways. The height of the support cradle must be high enough such that anti-vortex strake clears the ground. The cost of elevated construction, however, may favor the alternative of leaving the strake off on the bottom side of the hull at the sacrifice of hull symmetry and strake effectiveness. The continuous hull cross-section allows for generally level horizontal floatation of the hull at a relatively shallow draft. The hull could therefore be launched from a construction ways. The circular shape, however, is unstable in roll. Consideration must be taken to augment roll stability to allow horizontal floatation. In general, however, the hull may not be fabricated within wet towing distance of the installation site. Therefore, the hull must be loaded onto a transport vessel.

The spar hull is generally not considered viable for transportation with a launch barge, due to the bending moment induced when the hull is launched from the barge once near the installation site. Instead, a semi-submersible transport ship is employed with the hull floated off near the installation site. The overall length of the classic spar hull may approach 700 feet, adding the length due to free board and deck support legs. This length generally exceeds the capacity of existing semi-submersible transport vessels. As previously discussed, the length of the hull is generally set to achieve the desired heave natural period and is not subject to being shortened. Therefore, the classic spar is typically constructed in two pieces. The individual pieces are transported to a location near the installation site. There the pieces are floated off for mating either in a dry dock or offshore. The two pieces are mated and welded together. The completed hull is then wet towed to the installation site where the hull is upended, moored, and the deck installed. The added cost and time due to the hull mating procedure and dual transportation can be substantial.

An alternative spar design is a truss spar. FIG. 3 illustrates a prior art truss spar 300 which includes a hull having an upper buoyant portion 302 and a lower portion 304. The lower portion 304 includes an amount of fixed ballast 306. The center portion of the truss spar 300 includes an open truss section 308 coupled between the

buoyant portions

302, 304. Large horizontal plates 310, called heave plates, are located at various elevations along the length of the truss 308. These heave plates 310 act to impede the flow of water along a vertical axis of the truss spar 300 and permit flow of water perpendicular to the vertical axis. The heave plates 310 force a large percentage of the water trapped between the plates and

buoyant portions

302, 304 to move with the spar for heave motions having periods on the order of the heave natural period. The net effect of the heave plates 310 is to provide virtual mass (M_v) in the heave direction. The heave plates 310 also provide dynamic damping forces. In contrast to the classic spar design which employs trapped water, there is some movement of water around the heave plates 310, even for shorter period heave motions. The net effect is the addition of velocity-dependent damping forces.

One generally desirable characteristic of a truss construction is that it is not susceptible to vortex formation. The individual truss members will form local vortices disrupted at the nodes and combining to negate the formation of global vortices. As illustrated, the result is a reduced requirement for anti-vortex strakes 312 (compare to strakes 224 in FIG. 2A which extend along the length of the hull 202 for the classic spar). In general, strakes 312 are only necessary in the upper buoyant portion 302, thereby limiting the horizontal drag increase and reducing the interference between the mooring lines 314 and strake 312.

One generally undesirable consequence of replacing the center portion of the

truss spar

300 with a truss is that the keel draft (T_k) is elevated above the lower buoyant portion 304 upwards to the bottom of the upper buoyant portion 302. A typical keel elevation may be in the range of 180 to 250 feet of water depth. This reduced keel depth is subject to larger wave loading, as indicated by the magnitude of the wave profile at the elevated keel draft (T_k). This characteristic acts to increase the first order heave motions for the truss spar 300 (see FIG. 5B). A typical truss spar design might be expected to experience some noticeable heave motion even in fair weather conditions. This more regular motion may result in an increase in fatigue to the

buoyant portions

302, 304, truss section 308, risers 316, and other associated structures. In storm conditions, however, the heave plates 310 of the truss section 308 provide very large damping forces. The magnitude of damping forces induced by the heave plates 310 is proportional to the square of the velocity of the waves. Therefore, under the increased amplitude and velocity induced by storm sea states, the damping forces induced by the heave plates 310 increases exponentially so as to greatly reduce the second order heave motions. The net result is maximum heave motions generally on the order of that for a classic spar design. The motions, however, are dominated by shorter-period first order heave.

The construction and installation of a

truss spar

300 are eased by the ability to reduce hull length but complicated by the horizontal floatation characteristics of a discontinuous hull. The use of heave plates 310 in the truss spar 300, to provide heave virtual mass (M_v) and heave damping forces, results in a hull design wherein the motion characteristics are not entirely dependent upon overall hull length. The length of the hull may thereby be reduced to fall within the limits of existing semi-submersible transport ships. For certain configurations, this aspect might permit the construction of the hull in one piece. The hull is, however, of two different construction types, which may require construction at separate fabrication yards. Additionally, the horizontal floatation characteristics of the hull may act to greatly complicate the float-off and other installation procedures. The buoyancy of the horizontally oriented hull is discontinuous along its length. The result is a deeper horizontal floatation draft and a natural tendency to float at an angle. A floatation tank 318 attached to the lower portion 304 of the hull 302, may be employed to reduce the floatation angle. The combination of increased horizontal floatation draft and angle may, nonetheless, make the float-off procedure infeasible. This would again require that the two pieces of the hull be mated offshore as with the classic spar design.

As illustrated above, prior-art spar platform designs each have their relative advantages and disadvantages which might be susceptible to combination. One prior-art alternative spar platform design combines a continuous caisson hull with heave plates. As illustrated in FIG. 4A, the ring-

plate spar

400 comprises a hull 402 supporting a deck 404. The hull 402 comprises a buoyant upper portion 406 and a lower portion 408 comprising an amount of fixed ballast 410. There is no central portion of the hull. Instead, continuous circular heave plates 412 are located at several elevations along the length of the hull 402. The overall draft of the hull 402 can be reduced due to the increased added mass (M_v) (indicated by the dotted lines) and damping provided by the heave plates 412. Unlike with the truss spar (300 in FIG. 3), the keel draft (T_k) remains at the base of the lower portion 408 of the hull 402. The keel draft (T_k) can therefore remain deeply submerged below the majority of wave loading, as indicated by the magnitude of the wave profile.

The construction and installation of the ring-

plate spar

400, however, place restrictions on the allowable hull length. As most clearly shown in FIG. 4B, the heave plates 412 extend radially outward from the caisson hull 402, generally to a diameter 50-100% larger than the diameter (D) of the hull 402. This arrangement generally makes horizontal construction and transportation practically infeasible. This limitation eliminates many of the advantages sought by the hybrid design. The hull 402 must instead be fabricated vertically. Economical fabrication is generally not considered compatible with elevated construction. Vertical construction may also place an absolute limit on allowable overall hull length. The limited hull length reduces the keel draft (Tk) at the sacrifice of performance. Another consequence of a limited keel draft (Tk) is a non-linear increase in the amount of fixed ballast 410 required to provide adequate stability and pitch characteristics. To achieve similar characteristics, the amount of fixed ballast 410 may have to be increased to multiples of the deck weight. The large increase in fixed ballast increases the required buoyancy, also increasing the hull steel to provide the buoyancy, further increase weight. For these and other reasons, vertical construction is generally considered undesirable for elongate structures.

As can be appreciated from the foregoing discussion of prior art structures, a spar platform design would be highly desirable which combined the advantages of a deep draft caisson and heave plates design without sacrificing the advantages of horizontal construction and transportation. It is highly desirable to employ a caisson hull having a deep keel draft. FIG. 5A illustrates the generalized heave motion response of a spar platform having low damping but with deep keel draft, such as the prior-art classic spar platform ( 200 in FIG. 2A). The low wave-induced hydrodynamic heave forces result in small first order heave motions, but the low damping results in large second order heave motions despite low wave loading. It is also highly desirable to employ heave plates in combination with a caisson hull. FIG. 5B illustrates the generalized heave motion response of a spar platform having high damping but with a shallow keel draft, such as the prior-art truss spar platform (300 in FIG. 3). The damping provided by the heave plates results in small second order heave motions. Heave plates also provide virtual mass to increase the heave natural period or allowing a reduction in hull draft without performance decrease. The reduced keel draft, however, increases the wave loading and results in relatively large first order heave motions.

FIG. 5C illustrates the generalized heave motion response of a spar platform having both high damping and a deep keel draft. A spar platform successfully combining both the reduced wave loading of a deep keel draft and the increased damping from heave plates results in superior overall heave motions, having a relatively small first and second order motion response. Prior art structures, however, have faced difficulties combining these two desirable features. In general, the addition of heave plates has come at the cost of greatly reduced keel submergence. In other configurations, combining a deep draft caisson and heave plates added substantial complications to the fabrication, construction, and installation of the floating structure. Further, these floating structures also may encounter difficulty and high cost in installation due to multiple section construction and offshore hull mating, or may encounter high elevation vertical construction and the resulting draft limitations.

SUMMARY OF INVENTION

In one aspect, the invention relates to floating structure which comprises an elongate caisson hull and at least one plate set coupled to the hull. The plate set comprises a plurality of heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull.

In another aspect, the invention relates to a floating structure which comprises a deep draft caisson hull having a generally prismatic shape and a plurality of heave plates forming a discontinuous ring about a circumference of the hull.

In another aspect, the invention relates to a floating structure which comprises a buoyant hull having a diameter and a vertical axis and an array of heave plates attached about the diameter of the hull. The array of heave plates fit within an imaginary bounding box in a horizontal plane centered about a vertical axis of the hull. The bounding box has sides of length no greater than 120% of the diameter of the hull.

In another aspect, the invention relates to a method of construction which comprises constructing a caisson hull in a horizontal orientation upon a support structure which allows a keel of the hull to be elevated a distance above a ground level and attaching a plurality of heave plates to the caisson hull at one or more locations along a length of the hull.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a generalized profile of a prior art spar platform. [0028]
FIG. 2A is a vertical cross-section of a prior-art classic spar. [0029]
FIG. 2B is a horizontal cross-section of the prior-art classic spar shown in FIG. 2A. [0030]
FIG. 3 is a vertical cross-section of a prior-art truss spar. [0031]
FIG. 4A is a vertical cross-section of a prior-art ring-plate spar. [0032]
FIG. 4B is a horizontal cross-section of the prior-art ring-plate spar shown in FIG. 4A. [0033]
FIGS. [0034] 5A-5C illustrate generalized heave-direction response amplitude operators for lightly- and highly-damped caisson vessels of various configurations.
FIG. 6 is a three dimensional outboard profile of a heave-damped caisson vessel according to an embodiment of the invention. [0035]
FIG. 7A is an inboard profile of the heave-damped caisson vessel shown in FIG. 6. [0036]
FIG. 7B is a vertical cross-section of the heave-damped caisson vessel shown in FIG. 7A. [0037]
FIG. 8A is a top view of a fabrication site arrangement for the heave-damped caisson vessel shown in FIG. 7A. [0038]
FIG. 8B is a front view of the fabrication site arrangement shown in FIG. 8A. [0039]
FIGS. [0040] 9A-C illustrate a buoyancy diagram, side view, and end view of a floatation arrangement for a heave damped spar platform in accordance with an embodiment of the invention.
FIGS. [0041] 10A-H illustrate end views of alternative horizontal arrangements and designs of heave plates in accordance with embodiments of the invention.
FIGS. [0042] 11A-C illustrate outboard profiles and a top view of alternative vertical arrangement of heave plates in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described with reference to the accompanying drawings. [0043]
FIG. 6 shows a heave-damped [0044] caisson vessel 600 with associated heave damping structures in accordance with an embodiment of the invention. The heave-damped caisson vessel 600 comprises an elongate caisson hull 602. In the illustrated embodiment, the hull 602 has an octagonal cross-section. The hull 602 is of sufficient length to provide a deeply submerged keel draft (T_k). A desirable keel draft (T_k) is typically in excess of 400 feet to place it below the majority of hydrodynamic wave forces and may be advantageous to exceed 600 feet in certain embodiments. An array of heave plates 604 is attached to the hull 602. The plate array 604 includes multiple plate sets 606 comprising four individual heave plates 608. The individual heave plates 608 are generally triangular in shape and located in diagonal comers 610 of the octagonal cross-section of the hull 602. The four individual heave plates 608 of each plate set 606 are located at a single elevation and form a discontinuous symmetric pattern about a vertical axis of the heave-damped caisson vessel 600.
Preferably, the uppermost plate set [0045] 606 is set at a depth (T_p) on the order of two times the maximum wave height. For the Gulf of Mexico design hurricane, for example, this depth may be on the order of 140 feet. Cross-braced truss structures 612 reinforce the individual heave plates 608 and cross-connect between individual heave plates 608 of vertically-spaced plate sets 606. A desirable vertical spacing between plate sets 606 will vary between specific embodiments, but may be established by conventional model testing and computational methods familiar to the art. Anti-vortex strakes 614 are attached to the hull 602 at an elevation above the array of heave plates 604. The strakes 614 include flat plates oriented approximately 30 degrees from the vertical of the heave-damped caisson vessel 600. The strakes 614 are also placed in the diagonal comers 610 of the hull 602 and form a symmetric pattern about the vertical axis of the hull 602. The location and discontinuous patterns of the heave plates 608 and strakes 614 about the circumference of the hull 602 leave four clear sides 616 to the hull 602. Mooring lines 618 run downward along the clear sides 616 of the hull 602 and extend out through fairleads 620, which are attached to the hull 602 below a water surface 622.
FIG. 7A shows an inboard profile of the heave-damped caisson vessel [0046] 600 (previously shown in FIG. 6). The buoyant hull 602 supports a deck 704 above the water surface 622. The hull 602 includes an upper portion 708, a center portion 710, and a lower portion 712. The upper portion 708 includes variable ballast tanks 714 and permanent buoyancy tanks 716. The center portion 710 includes an open framed construction trapping a large mass of seawater 718. The lower portion 712 includes an amount of negatively buoyant fixed ballast 720. The amount of fixed ballast 720 is arranged such that a combined center of gravity (CG) is located below a combined center of buoyancy (CB) for the caisson vessel 600. Mooring lines 722 are attached to the hull 602 through fairleads 724. Riser 726 supported by a plurality of air cans 728 are disposed within an aperture 730 extending through the upper portion 708 of the hull 602. The risers 726 exit through guide tubes 732 in the lower portion 712 of the hull 602. Deck legs 734 connect the deck 704 to the hull 602.
The array of plate sets [0047] 604 is attached to the hull 602. As previously described, each plate set 606 includes four individual heave plates 608. Each heave plate 608 is a flat construction and has a generally triangular shape. The heave plates 608 are located in four diagonal comers (742 in FIG. 7B) of the octagonal hull 602. In this arrangement the four individual heave plates 608 of a single plate set 606 form a symmetric pattern about a vertical axis of the hull 602 bounded by an imaginary square shape. The individual plates 608 of a plate set 606 are located at a single elevation. Multiple plates sets 606 are disposed at various elevations along the hull 602. The truss structure 612 reinforces the individual heave plates 608 and interconnects individual heave plates 608 of vertically-adjacent plate sets 606. The array of heave plates 606 forces a percentage of the water trapped between vertically-adjacent individual heave plates 608 to move with the caisson vessel 600 for heave motions on the order of the heave natural period. The net effect of the array of plate sets 606 is to provide virtual mass (M_v) in addition to that provided by the water 718 trapped within the center portion 710 of the hull 602. The heave plates 608 also provide dynamic damping forces.
The [0048] anti-vortex strakes 614 are attached to the diagonal comers (742 in FIG. 7B) of the hull 602, fitting within the aforementioned imaginary square shape. The strakes 614 include flat-plate constructions oriented approximately 30 degrees from vertical. The strakes 614 disrupt the formation of vortices along the length of the hull 602 by imparting a vertical component to the horizontal water flow. Water flowing around one side of the hull 602 is deflected downward. Water flowing around the other side of the hull 602 is deflected upwards. This disruption in flow impedes vortex formation. The overall requirement for the strakes 614 is generally reduced. The comers of the octagonal hull 602 impede vortex formation to some degree. The array of heave plates 606 and truss structure 612 further act to impede vortex formation. If required, however, the strakes 614 could be placed at multiple elevations along the hull 602 to further disrupt vortex formation.
The keel draft (T[0049] _k) of the caisson vessel 600 is deeply submerged. In addition to displacement, the total mass of the caisson vessel 600 in heave includes both the added mass (M_v) contributed by the trapped water 718 and the added mass (M_v) contributed by the heave plates 608. The net effect is to extend the heave natural period of the caisson vessel 600 for a given keel draft (T_k), or in the alternative allow the keel draft (T_k) to be reduced while maintaining an equivalent heave natural period. The motion behavior of the caisson vessel 600 provides the advantages of both deep keel draft and the damping provided by heave plates, as generally represented previously in FIG. 5C. In contrast to prior-art hybrid spar designs, however, embodiments of the invention may be adapted for horizontal fabrication using conventional shipyard infrastructure.
FIG. 8A shows the caisson vessel [0050] 600 (previously shown in FIGS. 6-7B) at a fabrication site. In FIG. 8B, the hull 602 of the caisson vessel 600 is placed upon a construction ways 804. The construction ways 804 are fixed to the ground 806. Launch beams 808 elevate the caisson vessel 600 a distance (X) above the construction ways 804. The construction ways 804 are spaced such that they align with load bearing points in the hull 602, typically at bulkheads 810. Individual heave plates 608 are attached to the diagonal comers 742 of the hull 602 to form plate sets 606. The plate sets (606 in FIG. 8A) are placed at various elevations along the length (L) of the hull 602 to form an array of plate sets (604 in FIG. 8A). Anti-vortex strakes 614 are also attached to the hull 602 in the diagonal comers 742. The hull 602 can be constructed in this horizontal orientation without the attached heave plates 608 and strakes 614 interfering with the construction ways 804 or ground 806. Mooring line fairleads 620 are attached to the four clear sides 824 of the hull 602. On the clear side 824 nearest to the ground 806, the fairleads 620 fit within a gap (G) between launch beams 808. After partial or completed construction, the hull 602 may be launched.
FIG. 9A shows a generalized floatation graph which illustrates buoyancy and weight distribution for the heave-damped caisson vessel [0051] 600 (previously shown in FIGS. 6-8B) in a horizontal floatation condition. The elongate caisson structure of the hull 602 provides continuous buoyancy. Hull weight is unevenly distributed. Referring to FIG. 9B, the majority of the structural weight of the caisson vessel 600 is attributed to the upper hull portion 708. The center hull portion 710 and lower hull portion 712 contribute less structural weight. An amount of fixed ballast 720 is placed in the lower hull portion 712 to achieve level floatation. The octagonal shape of the hull 602 provides a stability to roll motions of the hull 602. The continuous waterplane provided by the elongate hull 602 results in a relatively shallow horizontal keel draft (T_h). As previously discussed, the use of heave plates 608 permits reduction of the keel draft (T_k) in the installed vertical orientation, which reduces the overall hull length (L) in the horizontal floatation orientation. The level floatation characteristic, combined with the shallow horizontal keel draft (T_h), and the reduced overall hull length (L) may be employed to permit transportation within the draft and length limits of existing semi-submersible transport ships. Further, the launch beams (808 in FIG. 9C) may be retained on the hull 602 to provide a load spreading and bearing surface upon the deck of the transport ship.
Those skilled in the art will appreciate that other embodiments of the invention can be devised which are within the scope of the invention. The following is a discussion of some of those variations which are possible while still permitting horizontal fabrication of the heave-damped caisson vessel. [0052]
FIG. 10A shows the [0053] hull 602 resting on construction ways 804 and launch beams 808. The launch beams 808 elevate the hull 602 above the ground 806 a distance (X), as previously described in FIG. 8B. The extent of elevation (X) varies with the specific infrastructure employ by a fabrication yard. For example, the elevation (X) may be as low as 3 feet or in excess of 10 feet. In general, to retain economical construction, the elevation (X) is generally to approximately 10% of hull diameter (D). On the lower side 1010 of the hull 602, this places a limit on the placement of structures, such as heave plates 608 and strakes 614, so as not to interfere with the ground 806 or construction ways 804. Functional aspects of heave plates 608 and strakes 614 design further limit placement by strongly favoring symmetry. Symmetry is general preferable about the vertical axis of the hull (+), such that placement is symmetric both the X and Y axis. This shall be referred to as single-axis symmetry. It is further preferable that the X and Y axis are symmetric to each other, such that a 90 degree rotation of the X and Y axis would not change the configuration. This will be referred to as dual-axis symmetry.
To achieve dual-axis symmetry, the lower limit on [0054] heave plate 608 and strake 614 placement forms an imaginary line which is mirrored on four sides to form an imaginary square 1016. The comers 1018 of the imaginary square 1016 mark the diagonal comers 742 of the hull 602 for heave plate 608 and strake 614 placement. The flat sides 1022 of the imaginary square 1016 mark the clear sides 824 of the hull 602 for placement of less obtrusive structures such as fairleads, mooring lines, etc. (not shown) dimensions of the imaginary square 1016 are limited by elevation (X), generally restricted to 10% of hull diameter (D). Preferably, the sides 1022 of the square 1016 are no longer than 1.2(D) for reduced elevations (X). The heave plates 608 and strakes 614 are designed to fit within the envelope bounded by the imaginary square 1016.
In alternate embodiments, the [0055] hull 602 may have cross-sections other than octagonal. Where the hull cross-section is not octagonal, a bounding box other than a square may result. In such embodiments, bounding box and symmetry may instead be referenced by tangents to the hull. As illustrated in FIG. 10B, a bounding box 1026 can be formed by finding the tangent 1028 to a side of the hull and adding allowance for elevation (X) generally up to 10% of hull diameter. Specific embodiments employing this reference scheme are illustrated in FIGS. 10C-10H.
FIG. 10C illustrates an octagonal hull cross-section. The [0056] heave plates 608 may be designed flush, as illustrated by the solid lines, where the construction elevation is substantially zero. Where the construction elevation is greater than zero the amount of area occupied by the heave plates 608 can be increased substantially by outward extension, as illustrated by the dotted lines 1012. The increased plate area will act to increase the virtual mass provided by the heave plates in the installed condition. Both ground clearance and dual-axis symmetry are maintained in this manner.
Another feature illustrated in FIG. 10C are [0057] perforations 1030 drilled through the heave plates 608. These perforations 608 reduce to some degree the effectiveness of the individual heave plate 608 to produce virtual mass by allowing water to pass through the heave plate. However, movement of water through the perforations 1030 provides additional damping forces. The number, sizing, and placement of the perforations 1030 may vary. Advantageous perforation design may be determined by conventional model testing and computer modeling techniques known to the art. In general, it may advantageous to place perforations 1030 in the heave plates 608 closer to the water surface. The majority of damping is generated by wave action rather than hull motion. Heave plates 608 farther away from the water surface may be left without perforations to retain full virtual mass. This combination design permits versatility in the design of added mass and damping to suit the conditions at a particular installation location.
FIG. 10D illustrates a similar heave plate design, order, and arrangement to that of FIG. 10C as applied to a [0058] prismatic hull 1032. Both a flush and extended heave plate 608 configurations can be achieved with a non-octagonal cross-section while maintaining dual-axis symmetry and ground clearance. Similar principles also apply in application to a Greek cross 1034 hull cross-section as illustrated by FIG. 10E. Additional heave plate area is realized in the recesses 1036 of the diagonal corners 1038 of the Greek cross 1034 geometry. Flush or extended heave plate configurations are achieved without loss of dual-axis symmetry or ground clearance.
Dual-axis symmetry may be relaxed for certain embodiments. Especially for applications involving highly direction sea states and currents, an embodiment may allow limited dual-axis asymmetry. Single-axis symmetry should be maintained, however. [0059]
Lack of single-axis symmetry will couple heave and pitch motions. The total system mass will be moved away from the vertical axis of the hull and damping forces will apply off center. The net result will be that heave motions induce pitching moments. [0060]
FIG. 10F illustrates the embodiment of FIG. 10C including an additional 10%D extension of the [0061] heave plates 608 along the X-axis. This extension acts to greatly increase heave plate area and the resulting virtual mass. Ground clearance and single axis symmetry are maintained in both the flush and extended configurations. The loss of dual-axis symmetry may result in limited coupling of heave and yaw under certain conditions. This performance reduction, however, may be outweighed by the increased virtual mass and damping provided in a designers judgment.
A hexagonal hull cross-section loses dual-axis symmetry as well. As illustrated in FIG. 10G, a large plate area can be achieved with a [0062] hexagonal hull 1040 cross-sectional area. Both the flush and extended configuration maintains clearance and single-axis symmetry. The extended configuration uses the tangential method to determine the bounding box 1026 for heave plate 1012 and strake 1014 placement. Alternative single-axis symmetry embodiments may be desirable at a designers option. FIG. 10H illustrates a decagonal hull 1042 cross-section having relatively small heave plate area. The bounding box 1026 is determined using the tangential method. Such a configuration may be desirable where the virtual mass requirement is low but the damping requirement is high. As illustrated, the heave plates 1012 include multiple perforations 1030 to increase damping forces.
Flexibility is also achieved in vertical placement of the plates sets ([0063] 606 shown in FIG. 6-7B). As illustrated in FIG. 11A, the individual heave plates 608 within each plate set 606 may be placed at more than one elevation. As shown, each heave plate 608 of a plate set 606 is placed a distance (dZ) below the previous plate around the circumference of the hull 602. The result is a spiral staircase-like arrangement of heave plates 608 forming a helical pattern about the hull 602. Such an arrangement may further disrupt vortex formation.
The [0064] individual heave plates 608 themselves might also be oriented away from horizontal alignment. As illustrated in FIG. 11B, the individual heave plates 608 are aligned at an angle to the horizontal plane such that water flowing around one side of the hull 602 is deflected upwards and water flowing around the other side of the hull 602 is deflection downwards. In this manner, the individual heave plates 608 may further act to disrupt vortex formation. Care should be taken not to couple heave and yaw. The overall array pattern should not form a fan pattern causing rotation with heave. In both the embodiments illustrated in FIGS. 11A and 11B, symmetry about the vertical axis is maintained as illustrated by the top view of FIG. 11C applicable to both embodiments.
The invention may provide general advantages. First, the invention has relatively small first and second order heave motions. The caisson vessel has both a deeply submerged keel draft and heave plates. The deep keel draft reduces wave loading, reducing first order heave motions. The heave plates provide damping forces to reduce second order heave motions. [0065]
Second, the invention can be constructed and transported horizontally. The heave plates and strakes are placed within a bounding box at the diagonal corners of the hull. These structures thereby do not interfere with the ground or construction ways during construction or the ship deck during transportation. The horizontal construction with limited hull elevation is generally substantially more economical than vertical construction. [0066]
Third, the invention achieves a large amount of virtual mass. The extended hull traps a large amount of water within the hull. The virtual mass provided by the trapped water combines with virtual mass provided by the heave plates. The result is that the vessel can be designed with an elongated heave natural period for a given hull draft. [0067]
Fourth, the invention allows for transportation within the draft and length limits of existing semi-submersible transport ships. The increased virtual mass provided by the heave plates allows a reduction in hull length. The continuous caisson hull provides shallow draft, level horizontal floatation. These characteristics combine to fall within the allowable limits imposed by existing transport ship designs. [0068]
Fifth, the invention eliminates interferences between the heave plates, strakes, and mooring lines. The invention provides multiple sides clear of strakes and heave plates. Mooring lines and fairleads may places along the clear sides to avoid interference and reduce design complications. [0069]
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. [0070]

Claims

What is claimed is:

1. A floating structure comprising:

an elongate caisson hull;

at least one plate set coupled to the hull, the plate set comprising a plurality of heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull.

2. The floating structure of claim 1 wherein the individual heave plates are contained within an imaginary bounding box formed by a square centered on a vertical axis of the hull having a side length no greater than 120% of a diameter of the hull.

3. The floating structure of claim 1 wherein a cross-section of the hull is octagonal and the individual heave plates are located at one or more elevations in four diagonal corners of the hull.

4. The floating structure of claim 1 wherein a cross-section of the hull is Greek cross and the heave plates are located within recesses of the cross.

5. The floating structure of claim 1 wherein the heave plates are located at more than one elevation.

6. The floating structure of claim 1 further comprising a plurality of support structures reinforcing the heave plates, the support structures being located at multiple locations along the length of the hull and forming a pattern which disrupts the flow of water in a horizontal direction so as to impede formation of global vortices.

7. The floating structure of claim 1 wherein one or more of the individual heave plates further comprise one or more perforations adapted to increase damping forces provided by the heave plates.

8. A floating structure comprising:

a deep draft caisson hull having a generally prismatic shape;

a plurality of heave plates forming a discontinuous ring about a circumference of the hull.

9. The floating structure of claim 8 wherein the discontinuous ring of heave plates a pattern generally symmetric about a vertical axis of the floating structure.

10. The floating structure of claim 8 wherein a cross-section of the hull is octagonal and the heave plates are located at one or more elevations in four diagonal corners of the hull.

11. The floating structure of claim 8 wherein a cross-section of the hull is a Greek cross and the heave plates are located within recesses of the cross.

12. The floating structure of claim 8 wherein the heave plates are located at more than one elevation.

13. The floating structure of claim 8 wherein the hull has a positively buoyant upper portion and a negatively buoyant lower portion, such that a combined center of gravity for the floating structure is located below a combined center of buoyancy for the floating structure.

14. The floating structure of claim 8 further comprising a plurality of support structures reinforcing the individual heave plates, the support structures are located at located at multiple locations along the length of the hull and form a pattern adapted to disrupt the flow of water in a horizontal direction so as to impede formation of vortices.

15. The floating structure of claim 8 wherein one or more of the individual heave plates further comprise one or more perforations adapted to increase damping forces provided by the heave plates.

16. A floating structure comprising:

a vertical columnar hull having a diameter and an outer shell having at least four sides,

one or more heave plates attached to the hull wherein the heave plates are contained within an imaginary box formed by lines parallel to tangents to the four sides, wherein each parallel line is located within a distance of 10% of the hull diameter from the tangent.

17. The floating structure of claim 16 wherein a cross-section of the hull is octagonal and the heave plates are located at one or more elevations in four diagonal comers of the hull.

18. The floating structure of claim 16 wherein the heave plates form an array comprising one or more plate sets, the plates sets comprising a pattern of individual heave plates symmetric about a vertical axis of the floating structure.

19. The floating structure of claim 16 wherein the individual plates of a plate set are located at more than one elevation.

20. The floating structure of claim 16 wherein the hull has a positively buoyant upper portion and a negatively buoyant lower portion, such that a combined center of gravity for the floating structure is located below a combined center of buoyancy for the floating structure.

21. The floating structure of claim 16 wherein the array of heave plates further comprises a plurality of support structures reinforcing the individual heave plates, the support structures adapted to impede formation of vortices induced by movement of water about the hull.

22. The floating structure of claim 16 wherein one or more of the individual heave plates further comprise one or more perforations adapted to increase damping forces provided by the heave plates.

23. A floating structure comprising:

a buoyant hull having a diameter and a vertical axis, and

an array of heave plates attached about the diameter of the hull, wherein

the array of heave plates fit within an imaginary bounding box in the horizontal plane that is centered about a vertical axis of the hull, the bounding box having sides of length no greater than 120% of the diameter of the hull.

24. The floating structure of claim 23 wherein the bounding box is a square.

25. The floating structure of claim 23 wherein the hull has an octagonal cross-section and the heave plates are located at one or more elevations in diagonal corners of the hull.

26. The floating structure of claim 23 wherein the array of heave plates is composed of one or more plate sets, the plates sets comprising a pattern of individual heave plates symmetric about a vertical axis of the floating structure.

27. The floating structure of claim 23 wherein the individual plates of a plate set are located at more than one elevation

28. The floating structure of claim 23 wherein the hull has a positively buoyant upper portion and a negatively buoyant lower portion, such that a combined center of gravity for the floating structure is located below a combined center of buoyancy for the floating structure.

29. The floating structure of claim 23 wherein the array of heave plates further comprises a plurality of support structures reinforcing the individual heave plates, the support structures adapted to impede formation of vortices induced by horizontal movement of water about the hull.

30. The floating structure of claim 23 wherein one or more of the individual heave plates further comprise one or more perforations adapted to increase damping forces provided by the heave plates.

31. A method of construction comprising:

constructing a caisson hull in a horizontal orientation upon a support structure which allows a keel of the hull to be elevated a distance above a ground level; and

attaching a plurality of heave plates to the caisson hull at one or more locations along a length of the hull.

32. The method of construction of claim 31 wherein the caisson hull has an octagonal crosssection and the heave plates are polygonal in shape and adapted to attach in a symmetric pattern in diagonal comers of the octagon without interference with the support structure or ground.

33. The method of construction of claim 31 wherein the heave plates form an array, the array comprising one or more sets comprising a plurality of individual heave plates, wherein the individual heave plates form a generally symmetric pattern about a central axis of the hull.

34. The method of construction of claim 31 wherein the caisson hull and heave plates are adapted to provide substantially horizontal floatation of the hull sufficient to enable the hull to be launched as a single piece.

35. The method of construction of claim 31 wherein the horizontal floatation occurs at a draft of less than thirty feet.

36. The method of construction of claim 31 wherein the keel elevation is less than ten feet.

37. A method of increasing the heave natural period of a floating structure comprising: attaching a plurality of heave plates to a buoyant hull having a draft in excess of 300 ft, wherein:

the heave plates are attached at one or more elevations and arranged so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull, and

the heave plates are located vertically along a length of the hull so as to increase the added mass of the hull in the heave direction.

38. The method of claim 31 wherein more than one heave plate is placed in a general vertical line relative to one another, wherein the heave plates are spaced a distance apart vertically so as to increase the virtual mass of the floating structure in the heave direction.

39. The method of claim 31 wherein the heave plates comprise generally flat plates having a substantially horizontal orientation so as to impeded the flow of water in a vertical direction along the hull but so as to permit the flow of water in the horizontal direction about the hull.

40. The method of claim 31 wherein a cross-section of the hull is octagonal, and the heave plates comprise triangular geometry and are located at one or more elevations in four diagonal corners of the hull.