GB2145378A - Semi-submersible platform - Google Patents

Abstract

A semi-submersible platform 20 is supported on columns 30, 34, 35, 36 with pontoons 37, 38 extending between an outboard of the columns. Damper plates are provided by flat surfaces 45, 46 either on top of the outboard section of the pontoons or by plates positioned on the columns above the pontoons to provide heave stabilization and motion phase control in relation to the wave action such that when the platform is in the drilling mode the heave phase of the platform is approximately one hundred eighty degrees out of phase with wave action and in the survival mode heave action of the platform is substantially in phase with wave action. <IMAGE>

Description

SPECIFICATION Semi-submersible platform This invention relates to semi-submersible platforms.

The typical semi-submersible floating platform includes semi-immersed vertical support columns in the form of closed hollow tubes supporting a work platform above water level and connected at the lower end of the columns to submerged horizontal pontoons. In typical sea states this layout offers a large work platform with a minimum of wave induced motion in the operational mode as the major portion of volume is positioned well below the wave exciting forces and the relatively small cross-section of the columns offers a minimal restoring force as the wave front passes.

This motion reduction is essential for the various drilling, oil and gas production, diving and construction support-type operations that offshore platforms are used for.

Bergman Patent No. 4,112,864 teaches distributing the volume of pontoons non-uniformly towards the end to reduce the vertical component of wave induced forces acting between the pontoons and columns in the sub-resonant region. This design aims to reduce motion while in the operating or drilling mode for which heave reduction is most important. The prior art designs, however, ignore the effect when the vessel must be raised to a storm survival mode.

Primarily the reduced heave motions do not allow large waves, seventy feet plus, to clear the underside of the work platform. The only solution has been to lengthen the columns, increasing vessel weight and cost.

The design of Bergman and other prior art semisubmersible columns utilizing pontoons result in a vessel which in both the drilling and the survival mode heaves approximately one hundred eighty degrees out of phase with wave action. Thus, with the wave crest passing the platform the platform, heave motion positions the platform at its lowest level, thus reducing the clearance below the platform.

While damping of heave action is known (see the reference in Bergman to the use of dampers in a copending application), damping has not been utilized to shift the heave motion phase of a semisubmersible to approximately the zero position where the vessel heaves in phase with the wave action so that the vessel rides with the waves in the manner of a cork on the surface to obtain maximum clearance of the platform above wave level.

This permits shortening of the columns and lightening of the overall vessel to clear a given height wave during the survival mode.

It is an object of the present invention to dampen heave action of a semi-submersible platform during the time that the vessel is in the survival mode to provide favorable heave suppression and to shift the phase of heave action from approximately one hundred eighty degrees relative to wave action while in the drilling mode to approximately zero phase difference between heave action of the platform and waves while in the survival mode to permit the plafform to ride with large waves when in a storm survival mode.

Another object is to provide for damping of a semi-submersible platform by providing large flat damping surfaces positioned outboard of the columns and facing upwardly.

Another object is to provide dampers for semisubmersible platforms by forming the outboard pontoons with substantially flat upper surfaces which are large in area relative to the total column horizontal cross-sectional area.

Another object is to provide dampers in a semisubmersible platform by providing outer pontoons with large flat areas which are positioned vertically above the upper surface of the portions of the pontoons extending between columns.

Another object is to provide a semi-submersible platform as in the preceding object in which the upper surfaces of the inner pontoons are above waterline during towing mode and the inner pontoon vertical center of volume is below that of the outboard pontoon sections to increase stability.

Another object is to provide a semi-submersible vessel having outboard pontoons with flat upper surfaces to act as dampers in which the upper surfaces are positioned to substantially reduce the dTT ratio of the outer pontoons during operation of the vessel at survival modes.

Other objects, features and advantages of the invention will be apparent from the drawings, the specification and the claims.

In the drawings wherein like reference numerals indicate like parts, and wherein illustrative embodiments of this invention are shown: Figure 1 is a perspective view of a conventional semi-submersible design of the current art as used by industry; Figure 2 is a perspective view of one embodiment of a semi-submersible constructed according to the principles of the present invention; Figure 3a is a plan view of a pontoon constructed according to the principles of this invention; Figure 3b is a profile view of a pontoon constructed according to the principles of the present invention; Figure 4a is a profile view of one end of the Figure 3 pontoon shown in relation to the water surface at survival mode; Figure 4b is a profile view of one end of the Figure 3 pontoon shown at operational mode;; Figure 5a is a plot of heave exciting force as a function of d/T in the range of ten to twenty second period seas; Figure 5b is a plot of added mass as a function of dir in the range of ten to twenty second period seas; Figure 5c is a plot of damping as a function of dl T in the range of ten to twenty second period seas; Figures 6a and 6,5 are profile views of a conventional semi-submersible vessel at operating and survival drafts, respectively, showing wave clearance due to phase of vessel motion;; Figures 7a and 7,5 are profile views of a vessel constructed in accordance with this invention at operational and survival drafts, respectively, showing wave clearance due to phase of motion and contrasting with Figures 6a and 6b the relative positions of the platform and waves, particularly in the survival mode even though the columns of Figures 7a and 7b vessels are shorter than those of the Figures 6a and 6b vessel; Figure 8 is a graph illustrating the heave response of a semi-submersible embodying the present invention compared with the heave response of a conventional platform when both platforms are in the operating mode; Figure 9 is a graph illustrating the heave response of a semi-submersible embodying the present invention compared with the heave response of a conventional platform with both platforms in the storm mode;; Figures 1Oa and 10b are profile and plan views, respectively, of another embodiment of a semisubmersible platform constructed according to the principles of the present invention; Figures fla and lib are profile and plan views, respectively, of still another embodiment of a semisubmersible platform constructed according to the principles of the present invention; Figure 12 is a perspective view of still another embodiment of a semi-submersible platform constructed according to the principles of the present invention; and Figures 13 through 15 illustrate the two-step stability transition as a semi-submersible constructed according to the principles of the present invention ballast from transit draft to operating mode with Figures 13a and 13b showing side and end profile views of the vessel in the transit mode, Figures 14a and 14b showing side and end elevation views of the vessel pontoons partially submerged, and Figures 15a and 15b being side and end elevational views of the vessel in the operating mode.

Figure 1 depicts a semi-submersible structure of the type presently used for offshore operations.

The working platform 21 is supported on a plurality of columns 22, 23, 24, 25, 26 and a sixth column which is positioned behind column 23 and not shown. Suitable cross-bracing 27, 28, 29 and 31 may extend between the several columns.

Pontoons 32 and 33 extend parallel to each other and pontoon 32 interconnects the columns 22, 23 and 24 while pontoon 33 interconnects columns 25, 26 and the hidden column.

The pontoon dimension L, extending between the outboard columns 22 and 24 is relatively large as compared to the outboard length LE of the pontoon. As the length LE is relatively small, there is a lack of area of the outboard sections of the pontoons relative to the inboard sections of the pontoons.

In use the vessel of Figure 1 is normally operated at a d/T ratio of 2.0 to 2.5. The dir ratio is illustrated in Figure 1 with T being the vertical distance between the bottom and top of the pontoon and d being the distance from the waterline to T/2 or one-half the vertical dimension of T.

In the survival mode a required air gap of fifty feet plus requires the vessel to be floated at a higher level with the d/T values in the 1.5 to 2.0 range.

With the vessel constructed as shown in the Bergman patent, the d/T ratio for operations would be around 1.875.

Both the platforms shown in Figure 1 and the platform shown in the Bergman patent operate in both the operational and survival modes with the heave of the vessel being approximately one hundred eighty degrees out of phase with wave motion so that the vessel is at its lowest point as the crest of the wave passes under the platform.

The Bergman vessel, by distributing the volume non-uniformly towards the pontoon ends, has shown the possibility of a near cancellation of the vertical components of wave induced forces acting between the pontoons and columns in the sub-resonant region. These designs and all other known designs, however, ignore the effect when the vessel must enter a storm survival mode.

Figure 2 illustrates a semi-submersible platform in accordance with the present invention which has all of the advantages of the Bergman platform plus the additional advantage of shifting the heave phase from one hundred eighty degrees out of phase with wave action as in Bergman, to approximately zero degrees between heave of the vessel and wave action so that the vessel will ride with the waves in the manner of a cork.

The vessel includes a platform which is indicated in dashed lines at 20 supported upon the four columns 30, 34, 35 and 36. These columns are substantially capable of providing buoyancy necessary for supporting the vessel and the pontoons indicated generally at 37 and 38 will be substantially filled with liquid when the platform is in the operational mode.

Suitable bracing, such as shown at 39 and 41, may extend between the several columns.

The two pontoons 37, 38 are identical and a description of one will be understood to apply to the two pontoons.

An inboard pontoon section 42 extends between the two columns 35 and 36 and outboard pontoon sections 43 and 44 extend beyond the two columns 35 and 36. If desired, the outboard pontoon section 44 may have an upwardly and outwardly inclined lower surface section 44a and a like upwardly and outwardly inclined section 43a may be provided on pontoon section 43. These surfaces extend above and below the water line when the platform is in the towing mode and reduce resistance to towing of the platform in the conventional manner.

To reduce resistance in towing it is preferred that the bottom of the pontoon 38 between the inclined surfaces 43a and 44a be formed as a continuation of the bottom of the columns 35 and 36 to present a smooth bottom surface to reduce resistance in towing.

In accordance with this invention the outer pontoon sections 43 and 44 are larger than the inboard pontoon section 42. They further are so positioned that the inner pontoon's vertical center of volume and hence its center of buoyancy is below that of the outer pontoon sections 43 and 44. To accomplish the above, the upper surface 45 of outer pontoon section 43 and the upper surface 46 of the outer pontoon section 38 will be positioned at a higher elevation than the upper surface 47 of the inner pontoon section 42.

The upper surfaces 45 and 46 of the outer pontoon sections are flat or substantially flat to provide for maximum damping actions. Thus, in the illustrated form of the invention the two surfaces 45 and 46 are formed in single planes except that the edges may be chamferred, as at 48 and 49, if desired. The sum of the area of the flat surfaces 45 and 46 is about twenty percent greater than the horizontal cross-sectional area of the two columns 35 and 36 and is also greater than the area of the flat surface 47 of the inner pontoon. Thus, twice the length of the outer pontoon LE times the beam BE of the outer pontoon is greater than the length L, times the beam B1 of the inner pontoon 42.

To obtain the lower center of inner pontoon section the upper surface 47 of the inner pontoon will be positioned substantially below the upper surfaces 45 and 46 of the outer pontoons. However, it is preferred that the upper surface 47 of the inner pontoon be above the waterline when the vessel is in the transit mode and for this purpose it may be desirable to reduce the beam of the inner pontoon to a width less than the beam of the two outer pontoons, as shown in the drawings.

When this is done, it is preferred that the columns have their side walls converging towards each other so that the sides of the pontoons will be a smooth, substantially continuous surface. For this purpose the side walls 35a and 35b of column 35 converge inwardly of the vessel towards the column 36 and the side walls 36a and 36b of column 36 converge inwardly of the vessel towards column 35. With this relationship the side walls of the column may mate with the side walls of the inboard and outboard pontoon sections to provide smooth flowing lines. This permits the vertical center of volume g, the inner pontoon to be positioned substantially lower than the center of volume g, of the outer pontoons.This results in positioning of the upper surfaces of the outer pontoons in closer proximity to the water surface to utilize the sharp increases in damping and added mass that result at shallow submergence depths while keeping the buoyant force center as low as possible when the vessel is in operational mode and the pontoons are in a region of reduced heave excitation forces.

Thus, the design provides for a low center of buoyancy while providing for positioning the tops of the outboard pontoon sections at a high level to take advantage of high excitation forces at shallow depths.

In rough seas pitch of the platform must be controlled and this is accomplished by the large outboard pontoon sections. It will be appreciated by comparing the length L, with the length LE that the outboard pontoons extend outwardly from the columns a distance almost as great as the distance between columns 35 and 36. Thus, the flat top pontoon surfaces 45 and 46 extend for a substantial distance outboard of the platform. The damping effect of these outboard damping surfaces 45 and 46 is exerted a substantial distance from the platform to not only control heave of the vessel but also to dampen pitch of the vessel and prevent the outboard pontoon sections from coming out of the water when the vessel is in survival mode, even though the design survival mode for the vessel is at a shallower draft than for conventional semi-submersible platforms.

The d/T ratio for vessels constructed in accordance with this invention will have approximately a ratio of 1.1 at operational draft and .75 at survival draft, as best illustrated in Figures 4a and 4b. Due to the damping action provided by the upper surfaces of the large outboard pontoon sections, the vessel when in the survival mode wjll ride with the waves much in the manner of a cork and the phase of the vessel will be substantially the phase of the waves as contrasted with one hundred eighty degrees out of phase, as in the prior art.

This.relationship is illustrated in Figures 6a, 6b, 7a and 7b. The height L, of the platform above the bottom of the pontoons of the prior art vessel is greater than the height L2 of a vessel constructed in accordance with this invention, as illustrated in a comparison of Figure 6a and 7a. At operational depths the pontoons and columns of both vessels will cooperate in the same general manner to cause the vessel to heave at approximately one hundred eighty degrees out of phase with wave action. It will be noted that the center 51 of the prior art vessel shown in Figure 6a is at substantially the same level as the center 52 of the vessel constructed in accordance with this invention shown in Figure 7a when the two vessels are in the operational mode.Due to the shorter columns, the platform 38 does not have as much clearance as does the conventional vessel, but this is unimportant in the operational mode as ample clearance between the waterline and the platform is present.

In the survival mode shown in Figure 6b the center 51 of the conventional vessel is below the waterline while the center 52 of the vessel of this invention, as shown in Figure 7b, is positioned above the mean waterline raising the pontoons to a position where the outboard pontoon sections are subject to greater excitation forces to increase the effect of damping, resulting in a shift in phase to cause the vessel to be in phase with the waves and ride with the waves, as illustrated in Figure 7b.

Figures 5a, 5b and Sc illustrate, respectively, the magnitude of heave excitation, added mass and damping force coefficients with respect to depth ratio, i.e., d/T. The shaded region signifies the major energy range of sea states. The ten S-curve represents a ten second period sea, and twenty S, a twenty second sea. The hatched areas A and B depict the d/T ratio of a conventional semi-submersible in the operating zone and in the survival zone, respectively.

A shift from A to B gives about a three percent increase in heave added mass, a twenty-five per cent increase in damping, and a twenty percent increase in excitation force. These are relatively minor changes. The C and D lines denote them dir ratio of a semi-submersible constructed according to the principles of the present invention at operating and survival drafts, respectively. A shift from C to D gives about a fifteen percent increase in added mass and a seventy percent increase in damping.Lines E and F represent the adjusted dir values 1.3 and 0.9 giving a thirty percent increase in excita tion forces from operation to survival drafts. dir has been adjusted to account fer the greater difference between the pontoon upper surfaces and their volume center in this stepped hull form than is normal in a conventional pontoon.

Of greatest significance is the difference in heave added mass, damping and excitation forces between a conventional semi-submersible and a semi-submersible constructed according to the principles of the present invention. For a given operating condition, added mass has increased by fifteen percent allowing a fifteen percent larger column cross-section for a constant natural frequency. This increases stability and load capacity with no loss due to motions. Damping has increased in magnitude by two hundred percent, raising it from an insignificant force to one of major importance. This seventy percent change in heave damping force between operating and survival drafts combined with the magnitude of the initial damping at operating draft now gives a major phase shift in heave motion versus wave height elevation and brings the heave of the vessel into phase with wave form.

Figure 8 plots heave amplitude/wave amplitude versus wave period in seconds. There is shown in curve 53 the heave response curve for a vessel constructed in accordance with this invention and in curve 54 the heave response curve for a conventional vessel when both vessels are in the operating mode. It is noted that there is an improvement in the heave response in the range of eight to fifteen seconds and a substantial reduction for the fifteen to eighteen second wave periods and at resonant frequency there is also a reduction in the amplitude.

In Figure 9 the heave amplitude/wave amplitude is plotted again against wave period in seconds and shows at curve 55 a vessel in accordance with this invention and at curve 56 a conventional vessel when operating in the storm mode. While the vessel of this invention has a slightly greater heave amplitude in the ten to fifteen second range, there is a substantial decrease in heave amplitude between fifteen seconds and the resonant frequency of the vessels. This is due to the extreme damping of the vessel constructed in accordance with this invention. This reduction of the extreme motions in the larger seas combined with a substantially zero degree phase angle allows the improved vessel of this invention to ride with the storm seas analogous to a cork following the water surface.

Figures 10a and 10b illustrate a symmetrical starlike distribution of the pontoons 57, 58 and 59. It will be noted that the outboard sections of the pontoons are again provided with flat top surfaces, have a higher center of buoyancy and are larger than the columns 40, 50 and 60 and that the sections of the pontoons outboard of the columns are also larger than the sections inboard of the columns.

Figures lla and llb illustrate another form of platform in which three columns 61, 62 and 63 are joined by three triangularly arranged inner pontoon sections 64, 65 and 66. Again, outer pontoon sections 67, 68 and 69 are provided outboard of the columns. The center of buoyancy, the areas, etc., are as defined hereinabove.

Figure 12 shows a still further form of the invention. In this vessel the platform 71 is supported on four columns 72, 73, 74 and 75. The columns 72, 75 are interconnected by the inner pontoon section 76 and outboard pontoon sections 77 and 78 extend beyond the columns 72 and 75. In like manner an inboard pontoon section 79 is provided between the columns 73 and 74 and outboard columns 81 and 82 extend beyond the columns 73 and 74. The construction of the vessel thus far described parallels that shown in the above identified Bergman patent.

In accordance with this invention, dampers 83, 84, 85 and 86 are supported on the columns 72, 73, 74 and 75, respectively. These dampers provide in their outboard sections the damping areas and functions discussed hereinabove and additionally the dampers are continued through a three hundred sixty degree circle for convenience of construction.

The application of the damper plates 83, 84, 85 and 86 to a vessel of the type shown in Figure 12 would be effective to shift the phase of the vessel to bring it back into phase with wave action. It would, however, not permit the vessel to be constructed with the shorter columns, as with the vessel shown in Figure 2. With the vessel of Figure 12, the damper plates do not extend out to the end of the outboard pontoons. Thus, their effectiveness in controlling pitch of the vessel would be reduced and with this reduced pitch control the survival depth of the vessel would parallel that of the prior art and the d/T ratio would be larger than with the Figure 2 form of this invention.The use of the damper plates, however, would shift the phase of heave of the vessel and thus the vessel while riding deeper in the water would be riding with the waves in the manner of a cork to provide greater clearance below the platform as compared with the prior art vessels. While the pontoons would be at a lower survival level, the plates would be positioned approximately ten to fifteen feet above the top of the pontoons and would be in the regions where they would be exposed to greater damping forces to obtain the desired shift in phase and, of course, a reduction in heave as compared to the vessel without these damping plates.

Figures 13a, 13b, 14a, 14b, 15a and 15b illustrate the two-step stability transition as a semi-submersible constructed according to the principles of this invention ballast from a transit draft to operating mode. Stability is a function of volume, weight, vertical centers of volume and weight, and water plane inertia. This inertia is a function of water plane area and the area position in relation to the centerline axis; the more area or the farther away from the axis this area is positioned, the greater stability a vessel will possess. In Figure 13a and 13b a semi-submersible constructed according to the principles of this invention with a stepped deck in shown floating at transit draft above the water line.

With the vessel afloat the water plane area pf the outboard pontoons is L, times W, times four. The area of the columns is L2 times W2 times four. The area of the inner pontoon is L3 times W2 times two.

These areas together all provide the water plane area of the vessel afloat. During submergence, as shown in Figure 13b, the water plane area of the inner pontoon will first submerge to a point below the water line while the water plane area of the columns and the outer pontoons remain above the waterline. Figure 15a illustrates the vessel with the water plane area of only the columns being present as both pontoons are below the water line. The advantage of this invention is that as the vessel is ballasted to sink from the transit mode of Figure 13a to the operating mode of Figure 15a, a stability transition occurs after the upper surface of the inner pontoons have submerged but before the upper surfaces of the outboard pontoons submerge.

Instead of a water plane area suddenly reducing from the 13a configuration to the 15a configuration, as with a conventional semi-submersible, the water plane area will reduce first by the inner pontoon area as shown in Figure 13b. A typical value of D, that is, the difference in height of the step provided by the inner pontoon and the outer pontoons of six feet would give a few hours in this transition stage. Then as the vessel sank further and the outer pontoons, go below the surface of the water, the water plane would reduce to that conventionally provided by just the columns. This stepped transition in water plane area will provide a much more stable platform than the conventional platform.

It will be appreciated from the above that the use of flat damping plates in shallow water during the survival mode of the vessel shifts the phase of the vessel to conform to wave action. By using the flat top outboard pontoons as damping surfaces pitch is controlled permitting the relatively shallow survival mode and the reduction in column height.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims (9)

1. A semi-submersible vessel comprising, a platform, a plurality of columns extending downwardly from said platform and having sufficient buoyancy to substantially support the vessel, and pontoons having inboard sections extending between said columns and outboard sections extending beyond said columns, the top surface of said outboard pontoon sections all being substantially flat to provide damping plates and having a total area of at least approximately twenty percent greater than the sum of the horizontal cross-sectional area of all columns, said total top surface area of the outboard pontoon sections being greater than the total top surface area of said inner sections of said pontoons, said inner pontoon sections' center of volume being below the outer pontoon sections' center of volume.

2. The vessel of Claim 1 wherein the top surface of the outboard pontoons is at an elevation giving a d/T ratio of the outer pontoon sections of approximately 1.1 when at design operational draft and d/T ratio of approximately 0.75 when at design survival draft.

3. The vessel of Claim 1 wherein the top surfaces of said inner sections of said pontoons are substantially flat.

4. A semi-submersible vessel comprising, a platform, a plurality of columns extending downwardly from said platform and having sufficient buoyancy to substantially support the vessel, and pontoons having inboard sections extending between said columns and outboard sections extending beyond said columns, the top surface of said outboard pontoon sections all being substantially flat to provide damping plates and having a total area of at least approximately twenty percent greater than the sum of the horizontal cross-sectional area of all columns, said total top surface area of the outboard pontoon sections being greater than the top surface area of said inboard sections of said pontoons, said inner pontoon sections having their top surface lower than the top surface of said outer pontoon area but above the waterline when the vessel is in the transit mode, said inner pontoon sections' center of volume being below the outer pontoon sections' center of volume.

5. The vessel of Claim 4 wherein the top surface of the outboard pontoons is at an elevation giving a d/T ratio of the outer pontoon sections of approximately 1.1 when at design operational draft and a d/T ratio of approximately 0.75 when at design survival draft.

6. The vessel of Claim 4 wherein the top surfaces of said inner sections of said pontoons are substantially flat.

7. A semi-submersible vessel comprising, a platform, a plurality of columns extending downwardly from said platform and having sufficient buoyancy to substantially support the vessel, pontoons having inboard sections extending between said columns and outboard sections extending beyond said columns, said outboard pontoon sections having a greater volume than said inboard pontoon sections, and flat horizontally extending damping plates carried by said columns and having a total area outboard of said columns of at least approximately twenty percent greater than the sum of the horizontal cross-sectional area of said columns and greater than the total top surface area of said inner sections of said pontoons, said damping plates being located vertically above the top surface of said inner pontoon sections.

8. The vessel of Claim 7 wherein said damping plates are located vertically above the top surface of said outboard pontoon sections.

9. A semi-submersible vessel substantially as hereinbefore described with reference to and as shown in the accompanying drawings.