GB2292405A - Offshore structures. - Google Patents

Abstract

An offshore structure and method of repositioning the structure comprises a vertically extending service caisson 11 which contains service items needed between a fixture 10 on the offshore structure and the sea bed SB. Bracing elements 15 are connected between mid-height or lower, on the caisson 11, and a plurality of foundations 13 fixed to the sea bed SB. The effective frontal profile of the structure increases from near the top of the structure to the sea bed SB. In this way, the effective frontal profile for an offshore structure can be such that the environmental forces acting upon it do not vary and are approximately constant despite variations in water depth. In this way, the same structure can be used at a variety of depths without having to reconfigure the structure to accommodate environmental and wave forces at the new site. The structure may be removed from a first site, the length of the caisson adjusted if necessary, the foundations replaced if necessary and the structure then attached to the sea bed at a second site. <IMAGE>

Description

OFFSHORE STRUCTURES The invention relates to offshore structures.

Offshore structures designed to support modules, decks or buildings used for production or drilling of oil, gas or other minerals, generally include a tower structure for supporting the module, deck or building, above the water level, in particular above the anticipated highest wave crest of the water level. The tower structure is generally in the form of a central column or caisson, which contains services such as conductors, risers and the like for establishing communications and functions between the module, platform, deck or building at the top of the tower, collectively, the fixture, and the area adjacent the sea bed, at the bottom of the tower. The tower structure is also known to include a framework for supporting the central column, and a plurality of foundations attached to the sea bed and connected to the framework.

The structures are each capable of use at a particular location where the environmental conditions are known. The structures are specifically designed to suit those conditions. If a different offshore site at a different depth and/or exposed to different environmental conditions is needed, a new tower structure must be designed and constructed.

Examples of known offshore tower structures are disclosed in U.K.

Patent Application GB-A-2 116 237, U.K. Patent Application GB-A-2 136 860 and U.K. Patent Application GB-A-2 267 525. All three of these references disclose offshore structures including tower structures having a central column for supporting a fixture or platform above the water level, a framework for supporting the central column and a foundation of various designs which are suited for anchoring the framework to the sea bed at a particular site. It is known that an offshore structure must be designed to withstand the wave forces that are anticipated at a particular site. None of the references teach how a single tower structure used at one site having one set of environmental conditions and/or depth, can be used at a different site that may be under different environmental conditions and/or depths.

According to one aspect of the present invention there is provided an offshore structure for supporting a fixture above a wave crest and over the sea bed, the sea bed being at any depth within a selected range of depths, comprising: a vertically extending central service caisson having an upper end for supporting the fixture and a lower end for positioning adjacent the sea bed; a plurality of bracing elements connected to the caisson no higher than a mid-height of the caisson; the caisson and bracing elements having a combined effective frontal profile which increases either continuously or discontinuously from the upper end of the caisson to the lower end of the caisson, with shape, dimensions and configuration for the caisson with the bracing elements being selected to produce environmental forces from wave action on the effective frontal profile which is approximately constant for any water depth within the selected range of water depths; and a plurality of foundations for attachment to the sea bed, connected to the caisson through the bracing elements.

According to another aspect of the present invention there is provided a method of utilizing a single offshore structure for reuse at multiple water depths within a selected range of water depths, the offshore structure carrying a fixture above a water crest level, and attaching the fixture to the sea bed, comprising the steps of: designing the offshore structure to have a vertically extending central service caisson having an upper end for supporting the fixture and a lower end for positioning adjacent the sea bed; the structure having a plurality of bracing elements connected to the caisson no higher than a mid-height of the caisson, the caisson and bracing elements having a combined effective frontal profile which increases either continuously or discontinuously, from the upper end of the caisson to the lower end of the caisson. with shape, dimensions and configuration for the caisson with the bracing elements being selected to produce environmental forces from wave action on the effective frontal profile which is approximately constant for any water depth within the selected range of water depth, and a plurality of foundations for attachment to the sea bed, connected to the caisson through the bracing elements; attaching the offshore structure at a first location having a first water depth using foundations suited to the sea bed at the first location; removing the offshore structure from the first location in preparation for attaching the offshore structure to a second location at a second water depth; adjusting the length of the caisson so that the fixture will be above the water crest level at the second location; if needed, replacing the foundations so that they are suited for anchoring to the sea bed at the second location; and attaching the offshore structure at the second location using the foundations suited to the sea bed at the second location.

Preferred embodiments of the present invention provide improved structures and techniques for use of the structure at different locations where the water depth differs, and to a certain extent where environmental conditions may also differ, but primarily where the wave climate may not be significantly different. This is achieved by designing the supporting column and bracing for the column with shapes and dimensions proportioned so that the environmental effects from wave actions induce total forces that are approximately constant, despite variations in water depth, within a certain range of water depth; typically between about 20 and 50 metres. The actual water depth at the site need not be known in advance, as long as it is within this range. The structure can be moved from one site then reinstalled at another site.The column may have to be extended or reduced, to ensure that the module or deck is clear of the wave crest and has a sufficient air gap thereover. The foundations to which the structure is connected are also specific to the sea bed conditions at the new site, which may be different from the conditions at the previous site. They may have to be replaced.

The preferred embodiments permit the reuse of the same structure at another location where the water depth can be significantly different (but within the selected range) and the environmental forces acting on the structure will not affect the capacity of the elements to resist those forces. This precludes the need to strengthen the elements, which would involve considerable additional time and expense, to the extent that reuse of the same structure would become uneconomical. It is also necessary, by law, to remove all offshore structures in shallow water in the North Sea, at the operator's expense. The preferred embodiments of the present invention provide the operator with an actual advantage to remove the structure, since now it can be reused at another site. This is particularly important where marginally economically oil field development is involved.

The structure can also be pre-built and stored until it is required. It is then available on short notice. The fact that a single structure can be designed for multiple sea depths makes this possible. There is no cost involved in redesigning the structure to be waveload independent and the size of the elements are economical and efficient. The structure can be built at a time which is advantageous to the operator, for example, when fabrication costs are low, rather than at peak periods when prices are high.

Although the structure would have to be recertified for each location where it is to be used, because the foundations will almost certainly have to be different, careful design according to the criteria embodying the present invention will facilitate recertification without difficulty. To modify the structure for a new water depth and sea bed properties are relatively simple operations and the design calculations and analysis can be reproduced fairly quickly and simply. This also saves design time and costs.

A basis for the present invention is the realization that the wave forces and moments exerted on the structure are greater near the water surface and least near the sea bed. Keeping this in mind, the dimensions and spacial configurations of the elements of the preferred embodiments are selected to be greatest below the middle of the central service caisson, and generally to have an effective frontal profile which increases continuously or discontinuously, from an upper end of the structure, to a lower end thereof.

Specific examples of the offshore structure with constant waveload profile according to embodiments of the present invention, and a method of utilizing the structure at different depths, will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of an offshore structure with support bracing at or about mid-height of the central service caisson and support bracing at or near the bottom end of the central service caisson; Figure 2 is a perspective view of an offshore structure with support bracing at or about mid-height of the central service caisson and with secondary bracing to provide stability to that bracing, and bracing at or near the bottom end of the central service caisson;; Figure 3 is a perspective view of an offshore structure with support bracing at or about mid-height of the central service caisson and support bracing at or near the bottom end of the central service caisson, and bracing connecting the foundations, with the bracing and foundations being arranged around the central service caisson; Figure 4 is a perspective view of an offshore structure with bracing at or near to the bottom of the central service caisson; Figure 5 is a schematic representation of a generic configuration used in an example of the present invention; Figure 6 is a graph plotting water depth against base shear for the configuration of Figure 5; and Figure 7 is a graph similar to Figure 6 but plotting water depth against base moment.

Throughout the figures, the same reference numerals are utilized to designate the same or functionally similar parts.

As can be seen from the figures, the invention can take several forms. There are certain similarities between each configuration and these similarities are associated with the degree of bracing provided and the requirement that the offshore structure exhibits a similar environmental profile. irrespective of the depth of water at the site where the offshore structure is to be located and attached.

The effects of waves on an offshore structure will produce pressures on the elements of the offshore structure. The pressure is dependent on the water particle velocity and acceleration, and decays exponentially with depth. The higher pressures are near the surface and very much lower pressures are near the sea bed. The effect of pressure on an offshore structure is to produce a force and overturning moment due to the effective frontal profile presented to the water particles. By selecting a configuration and dimensions of the elements of the offshore structure, it is possible to arrange for the higher pressures near the water level to act on a part of the offshore structure with a smaller frontal profile closer to the top of the structure.It is also possible to ensure that, with changes in water depth, the pressure acting on the frontal profile, a combined action that is similar for a selected range of water depths, e.g. about 20 to 50 metres, will be produced. It is possible to form an offshore structure that is economical to fabricate and install and has the advantage that it is independent of water depth with respect to the wave forces induced.

Referring to Figure 1, there is shown a module, platform, deck or building 10, which we will term a fixture, and which is supported by a central service caisson 11. The central service caisson 11 is a tube of relatively thin wall material and of constant or varying diameter.

The central service caisson 11 contains piping, drilling caissons and conductors, umbilicals or any other services required for the facilities contained in the fixture 10, or to enable the production or drilling of oil, gas or other minerals or for any other activity for which the offshore structure is to be used. Caisson 11 must be long enough to support fixture 10 above a wave crest on water level WL.

The wave loading on the items inside the central service caisson 11 is minimized by locating them inside the tube. The central service caisson 11 is supported by one or more bracing elements 15, connected to the central service caisson 11, at or about mid-height MH and to a plurality of foundations 13. The central service caisson 11 is supported by one or more bracing elements 12 and 16, located at or about the bottom end of the central service caisson 11. The bracing elements 12 and 16 can be arranged to form one or more trusses. The bracing elements 12 and 16 are connected to the foundations 13. The foundations 13 can be piles, spread footings or of the suction pad type, or a combination of these types, for attaching the structure to the sea bed SB.

Referring to Figure 2, the central service caisson 11 is supported by one or more bracing elements 15 connected to the central service caisson at or about mid-height and to the foundations 13. The central service caisson is supported by one or more bracing elements 16 located at or about the bottom end of the central service caisson. The bracing elements 15 and 16 can be arranged to form one or more trusses.

The bracing elements 15 and 16 are braced by elements connecting them together or to the central service caisson, or to braces 14 connecting the foundations 13. The bracing elements 15 and 16 are connected to the foundations 13. The foundations 13 can be piles. spread footings or of the suction pad type, or a combination of these types.

Referring to Figure 3, the central service caisson 11 is supported by one or more bracing elements 15 connected to the central service caisson 11 at or about mid-height and to the foundations 13.

The central service caisson 11 is supported by one or more bracing elements 12 located at or about the bottom end of the central service caisson 11. The bracing elements 12 can be horizontal or pitched at an angle and the end of the central service caisson 11 can be at a considerable distance from the sea bed. The bracing elements 14 connect the foundations 13. The bracing elements 12 and 15 are connected to the foundations 13. The foundations 13 can be piles, spread footings or of the suction pad type, or a combination of these types.

Referring to Figure 4, the central service caisson 11 is supported by one or more bracing elements 12 connected to the central service caisson 11 at or about the bottom end and to the foundations 13. The bracing elements 12 can be horizontal, angled, or pitched at an angle and the end of the central service caisson 11 can be at a considerable distance from the sea bed. The bracing elements 12 are of a constant or varying cross-section. The foundations 13 can be piles, spread footings or of the suction pad type, or a combination of these types.

Returning to Figure 1, the asymmetric structure was selected for supporting the mid-height of the caisson 11 using the angled bracing elements 15, from one side of the fixture and caisson, and the angled bracing elements 12 and 16 which are connected preferentially to the lower end of the caisson, and on an opposite side of the fixture and caisson. Foundations 13 are three in number and include hollow central tubes 17 with funnel-shaped upper ends, of the type for receiving a pile (not shown) which can be anchored through the pipe 17 to fix the foundation to the sea bed SB.

In the embodiment of Figure 2, the asymmetric lower bracing element 16 is used in conjunction with the angled bracing elements 15 and the further cross bracing elements 12 and 14.

Figure 3 is a symmetrical structure of four foundations 13 connected to each other by bracing elements 14, and connected to the lower end of the caisson 11 with bracing elements 12. The angled bracing elements 15 further connect the mid-height of the caisson 11 to the foundations 13.

Figure 4 uses heavy bracing elements 12 to connect the foundations 13 to the lower end of the caisson 11 with no direct bracing elements connected between the foundations and the mid portion of the caisson. If needed, stability of the fixture above wave crest can be increased by using cables (not shown) connected between an upper end of the caisson 11, and the sea bed. These have very small effective frontal profiles so they do not greatly affect the constant configuration of the offshore structure.

In each of the embodiments, the total surface area of the structure facing the oncoming waves or wave force, that is, the effective frontal profile (EFP) of the structure, increases either continuously or discontinuously, as the sea depth along the caisson 11 varies.

To proportion a structure to have a constant wave load profile for a particular range of water depths, the method described herein will enable the size of the profile width to be determined at the various elevations.

For the determination of the wave properties, it is assumed that Stokes Fifth Order Gravity Wave Theory can be applied. For the calculation of the wave forces, Morison's Equation has been assumed to be appropriate.

A single vertical monopile like that shown in Figure 5 is used to represent the structure. The variation of the hydrodynamic pressures along the length can be obtained for a certain water depth, wave height and period. By defining the profile width, the unit loading can be obtained, and the total forces acting, by integrating over the length from sea bed to surface.

If this is repeated for another water depth or for several water depths, it is possible to obtain the profile width that will give an approximately constant wave load for the range of water depths selected. It may be necessary to choose the best solution in a least square sense to give a profile width for a particular elevation along the length of the monopile.

The calculation method is as follows: 1. For a reference water depth, usually taken as the upper water depth, and wave height and wave period, calculate the pressures along the length of the monopile, using a particular wave theory, usually Stokes Fifth Order, and Morison's Equation.

2. Integrate the pressure along the length of the monopile from sea bed to surface.

p = J p* dy where: P = total pressure p = pressure at elevation Y y = Y/Drer Y = distance from sea bed to a particular elevation D ref = the instantaneous water depth from sea bed to surface 3. Determine the theoretical profile width for the reference water depth for various elevations along the length of the monopile.

d ref = J (p*k/P} where: d ref = profile width at elevation Y P = total pressure p = pressure at elevation Y k = width dimension to give dref correct size proportion 4. Repeat the procedure for another water depth except evaluate the theoretical profile width as follows: dy = (Dref/D)2*; (P+k/P) where: D ref = the reference instantaneous water depth D = instantaneous water depth P = pressure at elevation Y k = width dimension to give Dre, correct size proportion 5. There will be a theoretical profile width for each water depth and for each location along the monopile length. The theoretical profile width will vary by a small amount for each case.By fitting a curve of the profile width versus the elevation for water depth, it is possible to average the values at a particular elevation. Where the water depth is less than the reference water depth, the profile width at the surface is taken for elevation above the surface. A curve can then be fitted to the average theoretical profile widths versus elevation. The values calculated by this curve are the actual profile widths to be used. The curve used can be of the form indicated below: dy = Ao + J (Y) + A2 * Y where: dy = actual profile width at elevation Y Ao A1. A2 - coefficients In the following are examples of the evaluation of the actual profile widths calculated by the method described.

Example 1 is for a monopile with the wave height and period constant for the range of water depths.

Upper water depth 50 m Lower water depth 25 m Wave Height 16 m Wave Period 12.5 sec Water Density 10.5 k/N/cu m Current Velocity at surface 2.5 m/sec Marine Growth Allowance 0.05 m EXAMPLE 1 water depth = 58 50 water elevation t 59.478 yld pressure theoretical theoretical actual actual constant constant profile horiz profile horiz profile horn:: width shear width shear width shear 0 1.47 0.950 1.256 4.000 0.1 5.41 1.823 33.49 1.881 35.75 4.000 81.86 02 6.22 1.955 65.48 2.205 71.04 4.000 138.34 0.3 7.06 ' 2.083 79.87 2.481 92.86 4.000 157.96 0.4 8.08 2.228 9727 2.730 117.70 4.000 180.12 0.5 9.39 2.402 120.60 2.964 148.35 4.000 207.81 0.6 11.09 2.611 153.15 3.186 187.81 4.000 243.62 0.7 13.35 2.864 199.76 3.399 239.95 4.000 290.72 0.8 16.36 3.171 287.95 3.605 310.29 4.000 353.43 0.9 20A5 3.544 369.80 3.805 406.74 4.000 437.89 26.04 4.000 52521 4.000 541.09 4.000 552.98 2.614 661.39 406.41 1912.59 0.5343 2151.59 2644.74 water depth = 45 water elevation = 54.676 y/d pressure theoretical theoretical actual actual constant constant profile horiz profile horiz profile horiz width shear width shear width shear 0 1.87 1.256 1.256 4.000 0.1 6.17 2283 44.94 1.850 37.64 4.000 87.90 0.2 7.04 2.439 85.49 2.157 72.76 4.000 144.52 0.3 7.95 2.592 103.31 2.417 94.09 4.000 164.00 0.4 9.06 2.766 124.87 2.652 118.23 4.000 186.06 0.5 10.47 2.974 153.67 2.871 147.89 4.000 213.62 0.6 12.31 3.225 193.71 3.079 185.86 4.000 249.20 0.7 14.74 3.529 250.76 3.279 235.81 4.000 295.88 0.8 17.98 3.897 333.75 3.472 302.82 4.000 357.85 0.9 22.35 4.346 466.96 3.660 394.25 4.000 441.02 1 28.30 4.890 643.75 3.843 520.94 4.000 553.93 673.80 239122 2110.29 2694.01 EXAMPLE 1 jcontlnuedl water depth - 40 wat.r el.vatlon = 49.943 y/d pressure theoretical theoretical actual actual constant constant profile horse profile horiz profile horb width shear width shear width shear 0 2.39 1.677 1256 4.000 0.1 7.12 2.895 61.51 1.820 39.87 4.000 95.06 0.2 8.07 3.082 113.64 2.109 74.89 4.000 151.88 0.3 9.08 3.267 136.19 2.352 95.83 4.000 171.32 0.4 10.30 3.480 163.53 2.573 118.46 4.000 193.52 0.5 11.86 3.735 200.11 2.778 148.42 4.000 221.34 0.6 13.90 4.043 250.94 2.972 185.43 4.000 25731 0.7 16.59 4.417 323.27 3.159 234.00 4.000 304.52 0.8 20.17 4.871 428.32 3.339 299.04 4.000 367.20 0.9 25.01 5.423 584.02 3.514 387.63 4.000 451.30 1 31.60 6.096 819.65 3.685 510.15 4.000 565.43 694.99 3081.17 2094.73 2778.84 water depth = 35 water elevation = 45.307 y/d pressure theoretical theoretical actual actual constant constant profile horiz profile horse profile horiz width shear width shear width shear 0 3,10 2262 1.256 4.000 0.1 8.35 3.714 86.11 1.789 42.63 4.000 103.72 0.2 9.40 3.942 154.23 2.060 77.71 4.000 160.88 0.3 10.53 4.172 183.51 2.287 98.45 4.000 180.66 0.4 11.92 4.438 219.37 2.493 121.89 4.000 203.46 0.5 13.71 4.759 267.62 2.684 150.66 4.000 232.23 0.6 16.05 5.150 334.98 2.865 187.50 4.000 269.64 0.7 19.15 5.625 431.22 3.039 235.97 4.000 318.95 0.8 23.30 6.205 571.53 3206 301.02 4.000 384.68 0.9 28.92 6.913 780.36 3.368 389.86 4.000 47320 1 36.60 7.777 1097.70 3.526 513.04 4.000 593.75 730.41 4126.63 2118.73 2921.17 EXAMPLE 1 lcontinuedl water depth = 30 water elevation t 40.804 yld pressure theoretical theoretical actual actual constant constant profile horiz profile horiz profile hori: width shear width shear width shear 0 4.09 3.081 1.256 4.000 0.1 9.99 4.818 123.90 1.757 4630 4.000 114.91 02 1120 5.100 214.73 2.010 81.76 4.000 172.96 0.3 12.51 5.391 254.14 2.222 102.67 4.000 193.53 0.4 14.16 5.735 303.30 2.413 126.46 4.000 217.70 0.5 16.30 6.153 370.35 2.501 155.89 4.000 248.64 0.6 19.14 6.667 464.99 2.758 193ss7 4.000 28927 0.7 22.93 7.297 601.65 2.919 24423 4.000 343.33 08 28.04 8.070 803.07 3.073 31237 4.000 416.01 09 36.02 9.019 1106.19 3.223 406.17 4.000 514.74 1 44.65 10.183 1571.99 3.369 53723 4.000 65025 700.05 5814.30 2206.94 3161.33 water depth = 25 water elevation = 36.484 yld pressure theoretical theoretical actual actual constant constant profile horiz profile horse profile horiz width shear width shear width shear 0 5.59 4.238 1256 4.000 0.1 12.39 6.307 185.80 1.725 51.83 4.000 131.24 02 13.82 6.661 310.45 1.961 88.46 4.000 19126 0.3 15.43 7.039 366.07 2.158 110.20 4.000 213.47 0.4 17.50 7.496 437.50 2.334 13529 4.000 240.35 0.5 20.24 8.061 536.99 2.498 166.78 4.000 275.45 0.6 23.92 8.763 680.11 2.653 208.01 4.000 322.30 0.7 28.92 9.635 890.70 2.800 263.50 4.000 385.61 0.8 35.77 10.716 1207.50 2.943 339.75 4.000 472.07 0.9 45.28 12.058 1695.01 3.081 446.46 4.000 591.45 1 58.62 13.718 2462.80 3.215 59820 4.000 758.22 894.21 8772.93 2408.47 3581.42 EXAMPLE I icontlnuedi water depth summary of summary of summary of theoretical actual constant total shear total shear total shear 50 1912.59 2151.59 2644.74 45 2391.22 211029 2694.01 40 3081.17 2094.73 2778.84 35 4126.63 2118.73 2921.17 30 5814.30 2206.94 3161.33 25 8772.93 2408.47 3581.42 The summary of the actual mud line shear forces gives 2200 kN approximately for the range of water depths.

Example 2 is similar to the example above except that the wave height and period has been taken to be that expected for the particular water depth over the range.

Water Depth Wave Height (Hw) Wave Period 50 X 16.00 m 12.5 Sec 45 1 15.75 m 12.5 sec 40 m 15.50 m 12.5 sec 35 m 15.00 m 12.5 sec 30 m 14.50 m 12.5 sec 25 m 14.00 m 12.5 sec EXAMPLE 2 water depth t 50 water obvatlon = 59.478 y/d pressure theoretical theoretical actual actual constant constant profile hori: profile hori: profile hori:: width shear width shear width shear 0 1.47 0.950 1.280 4.000 0.1 5.41 1.823 33.49 1.989 37.60 4.000 81.84 02 6.22 1.965 65.48 2.324 74.97 4.000 138.31 0.3 7.06 2.083 79.87 2.597 97.48 4.000 157.92 0.4 8.08 2.228 9727 2.838 122.71 4.000 180.08 0.5 9.39 2.402 120.60 3.058 153.57 4.000 207.77 0.6 11.09 2.611 153.15 3.265 193.06 4.000 243.57 0.7 13.35 2.854 199.76 3.460 245.00 4.000 290.66 0.8 16.36 3.171 267.95 3.647 314.76 4.000 353.36 0.9 20.45 3.544 369.80 3.826 410.07 4.000 437.80 1 26.04 4.000 525.21 4.000 542.31 4.000 552.86 2.614 661.39 406.41 1912.59 0.5136 2191.52 2644.17 water depth = 45 water elevatIon = 54.498 y/d pressure theoretical theoretical actual actual constant constant profile horn: : profile horiz profile hori: width shear width shear width shear 0 1.81 1.262 1280 4.000 0.1 6.06 2.308 44.37 1.956 38.64 4.000 85.83 0.2 6.92 2.466 84.85 2.273 75.19 4.000 141.52 0.3 7.81 2.620 102.29 2.532 96.77 4.000 160.57 0.4 8.89 2.795 123.51 2.760 120.76 4.000 182.05 0.5 1027 3.004 151.75 2.968 149.89 4.000 208.79 0.6 12.05 3.254 190.89 3.163 186.88 4.000 243.22 0.7 14.40 3.557 246.47 3.347 235.19 4.000 286.28 0.8 17.53 3.925 327.05 3.522 299.57 4.000 347.97 0.9 21.73 4.370 446.27 3.691 386.85 4.000 427.89 1 27.46 4.912 626.33 3.855 507.01 4.000 536.10 656.09 2343.60 2096.73 2622.24 EXAMPLE 2 Icontinuedl water depth r 40 water elevation = 49.574 y/d pressure theoretical theoretical actual actual constant constant profile horiz profile horiz profile horn: width shear width shear width shear 0 225 1.698 1.280 4.000 0.1 6.87 2.967 59.97 1.922 39.85 4.000 90.38 0.2 7.79 3.160 111.48 2.222 75.58 4.000 145.26 0.3 8.74 3348 133.53 2.465 96.30 4.000 163.86 0.4 9.90 3.563 159.99 2.680 119.19 4.000 184.82 0.5 11.37 3.819 195.10 2.876 146.84 4.000 210.91 0.6 13.28 4.127 243.54 3.059 181.78 4.000 244.44 0.7 15.79 4.500 312.00 3.231 227.18 4.000 28823 0.8 19.12 4.951 410.71 3,396 287.40 4.000 346.06 0.9 23.58 5.498 555.92 3.554 358.56 4.000 42326 1 29.63 6.163 779.91 3.707 479.98 4.000 527.44 656.78 2956.15 2022.76 2624.67 water depth = 35 water elevation = 44.536 yld pressure theoretical theoretical actual actual constant constant profile horiz profile horiz profile horiz width shear width shear width shear 0 2.74 2.340 1280 4.000 0.1 7.73 3.931 81.94 1.885 40.26 4.000 93.24 0.2 8.71 4.173 148.61 2.167 74.48 4.000 146.43 0.3 9.73 4.411 176.55 2.395 93.94 4.000 16428 0.4 10.97 4.683 210.02 2.595 115.32 4.000 184.40 0.5 12.54 5.007 25427 2.778 141.01 4.000 209.43 0.6 14.58 5.398 315.11 2.949 173.32 4.000 241.55 0.7 17.26 5.871 400.70 3.109 215.12 4.000 283.43 0.8 20.78 6.444 523.58 3.263 270.34 4.000 338.61 0.9 25.50 7.139 703.44 3.410 344.54 4.000 412.10 1 31.88 7.983 972.04 3.552 445.76 4.000 511.01 646.79 3786.25 1914.08 2584.49 EXAMPLE 2 Icontinuedi water depth t 30 water elevation = 39.585 y/d pressure theoretical theoretical actual actual constant constant profile hori: profile horiz profile horiz width shear width shear width shear 0 3.40 3.296 1280 4.000 0.1 8.55 5.320 115.39 1.848 40*99 4.000 96.99 02 9.92 5.632 203.80 2.110 73.82 4.000 148.62 0.3 11.04 5.941 240.46 2.323 9221 4.000 165.96 0.4 12.41 6.298 284.55 2.509 112.38 4.000 185.65 0.5 14.15 6.725 343.01 2.678 136.62 4.000 210.23 0.6 16.41 7242 423.49 2.836 167.10 4.000 241.88 0.7 19.37 7.869 536.85 2.985 206.54 4.000 283.22 0.8 23.30 8.631 699.73 3.127 258.65 4.000 337.80 0.9 28.57 9.556 938.37 3,263 328.68 4.000 410.61 1 35.70 10.682 1295.00 3.394 42424 4.000 508.71 648.01 5080.66 1841.23 2589.67 water depth = 25 water elevatIon = 34.767 yid pressure theoretical theoretical actual actual constant constant profile horiz profile hori: profile horn: width shear width shear width shear 0 438 4.766 1.280 4.000 0.1 10.43 7.372 169.71 1.809 42.49 4.000 102.79 02 11.63 7.784 290.92 2.053 7428 4.000 153.32 0.3 12.91 8.202 341.34 2.249 91.94 4.000 170.57 0.4 14.49 8.689 402.85 2.420 111.41 4.000 190.46 0.5 16.52 9.278 485.25 2.577 134.94 4.000 215.57 0.6 19.18 9.997 599.67 2.722 164.71 4.000 248.17 0.7 22.70 10.875 76221 2.858 203.48 4.000 291.09 0.8 27.40 11.948 997.82 2.988 255.04 4.000 348.19 0.9 33.74 13.258 1346.40 3.113 324.84 4.000 424.97 1 42.41 14.864 1872.98 3.233 420.83 4.000 52929 668.93 7269.15 1823.96 2674.41 EXAMPLE 2 [continuedl water depth summary of summary of summary of theoretical actual constant total shear total shear total shear 50 1912.59 2191.52 2644.17 45 2343.60 2096.73 2622.24 40 2956.15 2022.76 2624.67 35 3786.25 1914.08 2584.49 30 5080.66 1841.23 2589.67 25 7269.15 1823.96 2674.41 The summary of the actual mud line shear forces gives 2000 kN approximately for the range of water depths.

If the profile width is kept constant then the mud line shear force ii 2600 kN approximately for the range of water depths.

If the real structure is not a monopile but consists of braces, other members and appurtenances, provided the summation of the projected profile widths at each elevation are comparable with the monopile actual profile widths, then the real structure will exhibit the same properties ss the monopile. As various parts of the real structure will be located at different positions in the wave and at different orientations, there will be some deviation from the constant wave load expected. The wave climate will also vary for different sites, even with the same water depth.

Provided the upper bound wave climate is selected, the design of the real structure will be satisfactory.

Example 3 is illustrated by Figure 5, The following data WM observed and resulted in the relatively constant base shear and base moment results, at different depths, illustrated in Figures 6 and 7.

Desren Data Hw s 16.0m Tassoc s 12.5 sec Surf current = 2.5 m/sec Stoke's 5th wave Cd g 0.7 Cm = 2.0 Mar. growth = 0.050 m (thickness)

Claims (13)

1. CLAIMS 1. An offshore structure for supporting a fixture above a wave crest and over the sea bed, the sea bed being at any depth within a selected range of depths, comprising: a vertically extending central service caisson having an upper end for supporting the fixture and a lower end for positioning adjacent the sea bed; a plurality of bracing elements connected to the caisson no higher than a mid-height of the caisson; the caisson and bracing elements having a combined effective frontal profile which increases either continuously or discontinuously from the upper end of the caisson to the lower end of the caisson, with shape, dimensions and configuration for the caisson with the bracing elements being selected to produce environmental forces from wave action on the effective frontal profile which is approximately constant for any water depth within the selected range of water depths; and a plurality of foundations for attachment to the sea bed, connected to the caisson through the bracing elements.
2. 2. An offshore structure according to claim 1, in which services are provided between the fixture and the sea bed, the services extending through the central service caisson so that the effective frontal profile of the central service caisson accommodates the services to protect the services from environmental forces and to reduce a frontal profile of the services to equal the effective frontal profile of the caisson.
3. 3. An offshore structure according to claim 1 or claim 2, including at least one bracing element connected directly between the caisson, near mid-height of the caisson, and at least one of the foundations.
4. 4. An offshore structure according to claim 3, including at least one additional bracing element connected between at least one of the foundations and the lower end of the caisson.
5. 5. An offshore structure according to any one of the preceding claims, wherein each of the foundations comprises a pile, a spread footing or a suction pad for securing the caisson to the sea bed.
6. 6. An offshore structure according to any one of the preceding claims, wherein the fixture comprises a module, a building, a deck or a platform, the caisson being sufficiently long to maintain the fixture above the wave crest.
7. 7. An offshore structure according to any one of the preceding claims, including three or more foundations, at least two of the foundations being connected by angled bracing elements to the midheight of the caisson on one side of the fixture, and at least one of the foundations being connected by bracing elements to a lower end of the caisson and on an opposite side of the fixture.
8. 8. An offshore structure according to claim 7, including cross bracing elements between the angled bracing elements that are between the mid-height of the caisson and the at least two foundations.
9. 9. An offshore structure according to claim 1 or claim 2, wherein at least one of the bracing elements is connected between two of the foundations.
10. 10. An offshore structure according to claim 1 or claim 2, wherein each of the foundations is connected to the caisson by bracing elements that are only connected to the lower end of the caisson.
11. 11. An offshore structure substantially as hereinbefore described and illustrated in the accompanying drawings.
12. 12. A method of utilizing a single offshore structure for reuse at multiple water depths within a selected range of water depths, the offshore structure carrying a fixture above a water crest level, and attaching the fixture to the sea bed, comprising the steps of: designing the offshore structure to have a vertically extending central service caisson having an upper end for supporting the fixture and a lower end for positioning adjacent the sea bed; the structure having a plurality of bracing elements connected to the caisson no higher than a mid-height of the caisson, the caisson and bracing elements having a combined effective frontal profile which increases either continuously or discontinuously, from the upper end of the caisson to the lower end of the caisson, with shape, dimensions and configuration for the caisson with the bracing elements being selected to produce environmental forces from wave action on the effective frontal profile which is approximately constant for any water depth within the selected range of water depth, and a plurality of foundations for attachment to the sea bed, connected to the caisson through the bracing elements; attaching the offshore structure at a first location having a first water depth using foundations suited to the sea bed at the first location; removing the offshore structure from the first location in preparation for attaching the offshore structure to a second location at a second water depth; adjusting the length of the caisson so that the fixture will be above the water crest level at the second location; if needed, replacing the foundations so that they are suited for anchoring to the sea bed at the second location; and attaching the offshore structure at the second location using the foundations suited to the sea bed at the second location.
13. 13. A method of utilizing a single offshore structure for reuse at multiple water depths, the method being substantially as hereinbefore described.