BACKGROUND OF THE INVENTION

A tension leg platform, or TLP, is a marine platform having economic potential for use in drilling in deep water locations. A typical tension leg platform is similar to a semi-submersible platform by having a number of large, vertical cylinders close to the periphery for providing stability during the transportation phase from shore to the drill site. The platform has a lower structure tieing the cylinders together at the bottom thereof in order to provide some buoyancy. It also has an upper structure tieing the upper ends together, and providing space for equipment, supplies and the like.

At the drill site, the tension leg platform is connected by a plurality of tendons to a heavy anchor secured on the sea bottom. The tendons are vertically oriented, and are always under tension. The tendons are quite slender and flexible, and act essentially as strings to permit the tension leg platform to move relatively freely in the horizontal plane while substantially preventing upward and angular movement. The tension leg platform provides considerable advantage over a semi-submersible platform, because the wellhead may be above water and drilling and work over may be done in the conventional surface fashion. The hydrodynamic behavior of a tension leg platform has certain characteristics during the construction and installation phase which are at odds with the characteristics desired in the on-location phase. These characteristics impose conflicting design requirements. When on-location, it is desirable to reduce the vertical wave forces as much as possible, and thereby to reduce the strength, weight, number and cost of the tendons and the anchor. In order for this to be done, the buoyancy should be well below the surface of the water- Thus the cylindrical columns should be as deep in the water and as small in diameter as possible. Such a configuration, however, results in a very tall structure which is unstable when floating to location, and also has excessive draft during construction. Stability while floating to location is an absolute requirement, with the result that the goal of minimizing the on-location wave forces is normally sacrificed. A further limitation on the prior art tension leg

platform has to do with the buoyancy effect resulting from wave motion. Passage of a wave beyond a column causes that portion of the column normally disposed above the sea surface to be covered, thereby increasing the buoyancy thereof. Those portions normally disposed below the sea surface, on the other hand, suffer from reduced buoyancy when this occurs, on account of the motion of the water particles resulting from the cyclic nature of the wave. The net result is that the buoyancy may be adversely affected by wave action, particularly by waves of substantial amplitude.

The disclosed invention is one which overcomes the conflicting design requirements normally presented by a tension leg platform. The disclosed invention combines the advantages of a jack-up drilling platform with those of a tension leg platform in order .to provide a mobile marine platform which has a first geometry during the construction phase and a second geometry during the on-location phase. In the first geometry, the work platform is closely spaced to a pontoon system floating on the surface of the water and with the pontoon system substantially providing stability because no tendons are connected. In the second geometry, the work platform is spaced a substantial distance above the pontoon on a column carried by the pontoon and stability is substantially provided by the tendons connected to the anchor. The dimensions resulting from this two geometry system permit columns that are much smaller in diameter and extend to a much deeper operating draft than heretofore possible.

A further advantage of the invention involves the relationship between the buoyancy provided by the column

and that provided by the pontoon. It is theoretically possible to size the two elements so that the theoretical wave forces cancel. The disclosed invention permits such dimensional configurations to be more easily taken into account, with the result that full advantage can be obtained of this phenomenon.

OBJECTS AND SUMMARY OF THE INVENTION

The primary object of the disclosed invention is a mobile marine platform having optimum stability during the construction and transportation phases, as well as optimum stability during the on-location phase. A further objective of the invention is a method for shifting the mobile drilling platform from the first geometry, which is suitable for the construction phase, to a second geometry, suitable for the on-site phase. An intermediate geometry may be used during transportation.

Yet a further object of the disclosed invention is a method for connecting the tendons of the mobile platform to a marine anchor secured to the sea floor. . A mobile marine platform according to the invention comprises horizontally disposed pontoon means for floating on or being submerged" below the surface of a marine environment. At least a first vertically disposed column is secured to the pontoon means and extends upwardly therefrom. A work platform has at least a first opening through which the column extends, and the work platform overlies the pontoon means and is vertically movable relative thereto along the column. Jack means are operably associated with

the work platform and with the column for jacking the work platform relative to the column between a first position wherein the pontoon means float on the surface of the marine environment, and a second position wherein the pontoon means are disposed a substantial distance below the surface of the marine environment. An anchor is provided for being secured to the floor of the marine environment. A plurality of resilient tendons are provided for extending between the pontoon means and the anchor means, and the tendons are under tension when the pontoon means are in the second position so that wave and current induced horizontal movement of the pontoon means and thereby of the column and work -platform is permitted and upward and angular movement thereof is resisted. A well drilling assembly comprises a floating platform comprising pontoon means submerged below the surface of a marine environment, at least a first vertically disposed column is secured to the pontoon means and extends upwardly therefrom and terminates above the marine environment surface, a work platform has at least a first opening through which the column extends and the platform overlies the pontoon means and is movable along the column relative thereto, and jack means are operably associated with the platform and with the column for jacking the platform relative to the column between a first position wherein the pontoon means float on the marine environment surface and a second position wherein the pontoon means are submerged therebelow. An anchor means is secured to the floor of the marine environment, and a plurality of resilient tendons extend under tension between and are secured to the anchor means and the pontoon means so that

wave and current induced horizontal movement of the platform is permitted and upward and angular movement thereof is resisted.

A jack-up tension leg platform comprises a generally horizontally disposed pontoon means for floating on or being submerged below the surface of a marine environment, and the pontoon means are X-shaped in plan. A first vertically disposed cylindrical column is secured centrally to the pontoon means and extends upwardly therefrom. A work platform has a central opening therethrough and through which the column extends, and the work platform overlies the pontoon means and is vertically movable relative thereto along the column. Jack means are operably associated with the work platform and with the column for jacking the work platform between a first position wherein the pontoon means float on the surface of the marine environment and a second position wherein the pontoon means are submerged therebelow.

The method of assembling a tension leg platform comprises the steps of providing a well drilling platform comprising pontoon means floating on the surface of a marine environment, at least a first vertically disposed column is secured to the pontoon means and extends upwardly therefrom, a work platform has at least a first opening through which the column extends and the work platform is movable along the column, and jack means are operably associated with the work, platform and with the column for jacking the column relative to the work: platform between a first position wherein the pontoon means float on the surface of a marine environment and a second position wherein the pontoon means are submerged therebelow, an anchor means is secured to the floor of the

marine environment, and a plurality of resilient tendons are provided. Each of the tendons is secured to the pontoon means so that the tendon means are suspended therefrom in the marine environment and terminate a selected distance above the anchor means. The column is jacked relative to the work platform and thereby causes the tendons to terminate at or below the anchor means. The tendons are connected to the anchor means and the buoyancy of the platform is increased, thereby causing the platform to rise relative to the floor of the marine environment so that a tension is applied to the tendons.

These and other objects and advantages of the invention will be readily apparent in view of the following description and drawings of the above described invention.

DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages and novel features of the present invention will become apparent from the following detailed description of the preferred embodiment of the invention illustrated in the accompanying drawings, wherein: FIGURE 1 is a schematic elevational view of a first embodiment of the mobile marine drilling platform of the invention disposed in a first geometry while floating on the surface of a marine environment;

FIGURE 2 is a fragmentary elevational view of the platform of Figure 1 during installation of the tendons and with the platform in the transportation geometry;

FIGURE 3 is a top plan view of the platform of

Figure 1 in the on-location phase;

FIGURE 4 is a fragmentary bottom perspective view of the platform of Figure 3;

FIGURE 5 is a fragmentary elevational view partially in section disclosing the jacking means of the invention;

FIGURE 6 is a fragmentary perspective view with portions broken away and in section of the jacking system of the invention; FIGURE 7 is a schematic elevational view of the drilling platform of Figure 1 in a second geometry and with the platform in the vertical and horizontally displaced modes;

FIGURE 8 is a fragmentary elevational view of the pontoon and tendon system of the invention;

FIGURE 9 is a cross-sectional view taken along the line 9-9 of Figure 7 and viewed in the direction of the arrows;

FIGURE 10 is a perspective view of the anchor of the system with the tendons installed;

FIGURE 11 is an elevational view with portions shown in phantom of a second embodiment of the invention floating on the surface of a marine environment in a first geometry; FIGURE 12 is a fragmentary elevational view of the platform of Figure 11 during the tendon connection phase; FIGURE 13 is a top plan view of the platform of Figure 11;

FIGURE 14 is a schematic elevational view of the platform of Figure 11 in a second geometry and with the

platform in the vertical and horizontally displaced modes; and,

FIGURE 15 is a fragmentary cross-sectional view disclosing an alignment system used with the platform of Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

Mobile marine well drilling platform PI, as best shown in Figure 1, comprises a pontoon assembly 10 floating on the surface 12"of marine environment 14. Cylindrical column 16 is secured to pontoon assembly 10 and extends upwardly therefrom. Work platform 18 is mounted above pontoon assembly 10 and about column 16 on account of opening 19 and is longitudinally movable relative thereto. Derrick 20 is mounted to work platform 18 adjacent column 16, while quarters 22 are positioned on work platform 18 on an opposite side thereof adjacent to column 16. Derrick 20 is movable on rails 21.

Pontoon assembly 10, as best shown in Figures 3, 4 and 9, is X-shaped in plan and has equiangularly disposed legs 24, 26, 28 and 30 extending uniformly outwardly from the base of column 16. Each of the legs 24, 26, 28 and 30 is rectangular in plan and cross-section, and the legs 24, 26, 28 and 30 have uniform dimensions, although other configurations are possible. Each of the legs 24, 26, 28 and 30 terminates in an end member 32, 34, 36 and 38, respectively, which is disposed generally transverse to the length dimension of the associated leg.

The legs 24, 26, 28 and 30 likewise have upper members 40, 42, 44 and 46, respectively, which lie on a

common plane and are uniformly spaced from parallel lower members 48, 50, 52 and 54, respectively. Side leg members 56 and 58 extend between upper and lower members 40 and 48 and between column 16 and end member 32, while side leg members 60 and 62 complete leg 26. Likewise, leg 28 has side leg members 64 and 66, while leg 30 has side leg members 68 and 70. The legs 24, 26, 28 and 30 are hollow and provide a multi-compartment ballast tank system for admitting sea water through valve 72 or causing same to be expelled as a result of air pressure supplied by a compressor or otherwise.

A plurality of tendon receiving connector assemblies 74 are secured to each of the end members 32, 34, 36 and 38 adjacent the associated lower surfaces thereof, as best shown in Figures 8 and §. The tendon receiving connector assemblies 74 are all substantially identical, and permit pivotal movement of a secured tendon 76. Those skilled in the art understand that the tendons 76 are long, multi-element tubular assemblies which are resilient but substantially non-elastic in the vertical direction. Tlje tendon receiving connector assemblies 74 are uniformly spaced apart both relative to each other, and to the associated end members from which they depend.

It can be noted in Figure 2 that a pivot connector 78, provided by a resilient bushing or the like, is mounted within or at the top of each of the tendon receiving connector assemblies 74 and is secured to the associated tendon 76 in order to permit pivoting of the tendons 76 during flexing thereof as may be caused by wind, wave or current conditions. Cylindrical column 16 has an annular chamber 80

defined by outer wall 82 and inner cylindrical member 84 having central opening 86, as best shown in Figure 9. Cylindrical column 16 and its annular chamber 80 are closed at the base thereof by end member 88, as best shown in Figure 4, and at the upper end by end member 90, as best shown in Figure 3. In this way, annular chamber 80 likewise provides a ballast tank system which, through valve 91 and in combination with pontoon assembly 10, permits the buoyancy of platform PI to be regulated as may be needed. Figure 1 illustrates the platform PI in'the construction geometry. In this geometry, the work platform 18, which interiorly may include crew quarters, storage and the like, is lowered in order to be adjacent pontoon assembly 10 so that floatational stability is provided mainly by the pontoon assembly 10. Once deeper water is reached, then platform PI may be appropriately ballasted so that pontoon assembly 10 is submerged, and platform 18 floats on surface 12, as best shown in Figure 2. Stability in this transportation geometry is important because it is common for the platform PI to be constructed at the shore, and to thereafter be towed or otherwise transported to the drill site. The drill site can be some distance from the shore, with the result that stability must be maintained for quite some period and distance, particularly during storms. Figure 7 discloses the platform PI in the on- _l ocation geometry. In this geometry, the work platform 18 has been raised to an operating elevation along the column 16 relative to the pontoon assembly 10. Stability is provided by appropriate control of the ballasting of the chamber 80 and the pontoon assembly 10 so that the tendons 76, having

substantial length and secured to and extending between pontoon assembly 10 and anchor 92, have a sufficient tension applied thereto. It can be noted in Figure 7 that the platform assembly PI, as clearly illustrated on the right side thereof, has been horizontally shifted as a result of wave Wl . The string-like effect of the tendons 76 perm ts the platform PI to be horizontally shifted but substantially prevents upward and angular displacement, and also causes the platform to be returned to the vertical orientation illustrated on the left side of Figure 7.

Figure 2 illustrates the platform PI in the transport¬ ation and installation geometry as the tendons 76 are suspended from hook 94 carried by crane assembly 96 secured to crane mount 95. Installation of the tendons 76 in the associated tendon receiving connector assemblies 74 can proceed fairly straightforwardly, because the pontoon assembly 10 is rotatable relative to the work platform 18. There is no pressure between the work platform 18 and the column 16, because both are floating on account of their own buoyancy. Receipt of a tendon 76 in one of the tendon receiving connector assemblies 74 and securement thereto by its pivot connector 78 can be rapidly effected. Rotation of the pontoon assembly 10 or the work platform 18 can then occur, in order to align the crane assembly 96 with the next tendon receiving connector assembly 74.

Once the tendons 76 have all been mounted in their respective tendon receiving connector assemblies 74, then it is necessary to have the downwardly suspended ends thereof secured to the anchor 92 _r followed by jacking of the platform PI into the second geometry illustrated in Figure

7. Figures 5 and 6 illustrate the jack assembly J utilized for shifting the work platform 18 relative to the column 16 in order to cause the suspended ends of the tendons 76 to be positioned at or below the anchor 92, as well as to jack the work platform 18 to the elevated operating position once the tendons have been appropriately secured to the anchor 92 and the pontoon assembly 10 and chamber 80 deballasted.

The jack assembly J is positioned on the top of the work platform 18 and comprises an upper annular yoke 98 mounted about the column 16 and free to slide along the column 16. A lower annular yoke 100 is similarly mounted about column 16 and is free to slide relative to the column 16. The yokes 98 and 100 have uniform inner and outer diameters, and the lower yoke 100 is secured to upper wall 104 of work platform 18. It is necessary that one of the yokes be secured to the work platform 18, while the other be free to move relative to the work platform 18.

A plurality of pin receiving openings 106 are equiangularly disposed about the column 16 for substantially the length thereof. The openings 106 are also disposed in a series of layers, with each layer having the same number of openings 106, and with the openings 106 of each layer longitudinally aligned with the openings 106 of the other layers. Each of the openings 106 is closed interiorly of the column 16 by a pin closure 108 in order to prevent the entry of water into chamber 80, as well as to prevent leakage from the chamber 80 and to maintain the proper ballast. In this way, each of the pin receiving openings 106 resembles a detent which is adapted for receiving an associated pin or jack element 110 or 112

carried by the yokes 98 and 100, respectively. There are sixteen sets of pins 110 and 112 disposed about the yokes 98 and 100 for assuring positive secure ent of the work platform 18 in a selected geometry, as well as to minimize the load on any individual pin 110 or 112 during jacking. While I have illustrated the pins 110 and 112 as being manually operable, those skilled in the art will understand that numerous mechanisms can be provided for driving the pins into the openings 106, as well as for removing them therefrom. It is preferred, however, that each of the pins 110 and 112 extend generally perpendicular to the axis of column 16 in order to minimize bending forces.

Interdigitated between the pins 110 of the yoke 98 and the pins 112 of the yoke 100 are a plurality of hydraulic cylinder and piston assemblies 114. The cylinders 116 thereof are secured to the yoke 100, and the pistons 118 thereof are secured within the yoke 98. In this way, extension and retraction of the pistons 118 will cause corresponding movement of the yokes 98 and 100 along the column 16, provided that the pins 110 or 112 of one of the yokes 98 and 100 are removed from the associated openings 106.

Movement of the -work platform 18 along the column 16 can be readily accomplished by appropriate manipulation of the pins 110 and 112, in cooperation with extension and retraction of the cylinder and piston assemblies 114. Because the cylinder and piston assemblies 114 are secured to and extend between the yokes 98 and 100, then the weight of the work platform 18 will be carried by the yoke 98 as the yoke 100 is moved in response to operation of the cylinder and piston assemblies. In this way, once the pins 110 have been

properly secured in their associated openings 106, then the pins 112 can be removed from their openings and the piston 118 caused to retract relative to the cylinder 116 so that the yoke 100 is caused to move toward the yoke 98, and thereby the platform 18 to be moved upwardly. Once the platform 18 has been moved sufficiently upwardly, then the pins 112 are inserted into their associated openings, and the pins 110 removed, thereby permitting the yoke 98 to again be moved upwardly relative to the yoke 100. After a sufficient distance, then the pins 110 are again inserted into their associated openings, and the process is repeated. Those skilled in the art wjlll understand that the work platform 18 can similarly be moved downwardly through like cooperative action of the pins 110 and 112 with the cylinder and piston assemblies 114.

Anchor 92, as best shown in Figure 10, is, preferably, a torus comprised of concrete having a hollow interior to which is connected valve 120. The anchor 92, because of the buoyancy provided by its hollow interior, will float on the surface of marine environment 14 and can therefore likewise be towed from shore to the drill site. Although configurations other than a torus are possible, I prefer a torus because it provides maximum strength for withstanding the pressures applied at extreme depths. Opening of the valve 120 will cause the anchor 92 to be filled with water such that the decreased buoyancy will cause the anchor 92 to sink to the sea floor 122, as best shown in Figure 7*. Anchor 92 may then be secured to the sea floor by suitable pilings, subterranean anchors and the like 124, although the weight of anchor 92 in certain instances may be sufficient

to preclude the need for pilings 124.

Anchor 92 has girders 126 and 128 extending in spaced parallel relation from one side to the other. Beams 130 and 132 extend between the girders in order to form a box-like structure 134 through which risers 136 extend. The risers 136 extend upwardly from box structure 134 into aperture 86 of column 16 and are used to extract oil or gas, supply drilling fluids and the like. Preferably, each of the risers 136 has a pivot joint 138 slightly below end member 88, as best shown in Figure 4, in order to permit the risers 136 to pivot as the platform PI is shifted, as best shown in Figure 7, and similar pivot joints may also be provided at anchor 92. Anchor 92 preferably has pivot joints 140, provided by resilient bushings or the like, for connecting the lower terminal end of each tendon 76 with the anchor 92. This prevents the respective tendon 76 from breaking during horizontal movement of the platform PI.

Figure 15 discloses an alignment mechanism particularly useful during rotation of the work platform P during installation of the tendons 76. In this regard, ring 142 is secured to column 16 and fork 144 is slidably received within member 146 secured to platform 18. The fork 144 is movable generally transverse to the axis of column 16 with the result that engagement of the tines around the ring 142 prevents the column 16 from tipping or otherwise being shifted out of proper alignment.

INSTALLATION OF PLATFORM PI

The platform PI, when configured in the geometry

of Figure 1, has sufficient stability to permit the platform PI to be towed for a substantial distance from the shore. Once the water is sufficiently deep, then appropriate ballasting submerges the pontoon assembly 10 to the geometry of Figure 2, and platform PI remains in this geometry for the rest of the distance to the drilling location. Similarly, the anchor 92 likewise has sufficient buoyancy to permit it to be towed. Once the platform PI has reached the location, however, then it is necessary to transform it to the on-site geometry illustrated in Figure 7. The jacking system J permits this transformation to occur with relative ease, without sacrificing the -stability attributable to a tension leg platform.

The anchor 92 is first caused to sink within marine environment '14 until it comes to rest on sea floor

122. Pilings 124 may then installed to secure the anchor 92 to the sea floor. After the anchor 92 is appropriately secured to the sea floor 122, then the tendons 76 are installed. Figure 2 illustrates the platform PI during installation of the tendons 76 through utilization of the crane assembly 96. Each of the tendons 76 is appropriately secured within its associated tendon receiving connector assembly 74 and caused, .thereby, to hang downwardly in marine environment 14. The tendons 76 are normally comprised of a series of interconnected pipe-like assemblies which together, necessarily, are quite long because the tension leg platform PI is normally most suitable for water depths in excess of 500 feet. Once all tendons 76 have been appropriately

secured by pivot connectors 78 within their associated tendon receiving connector assemblies 74, then the pontoon assembly 10 may be j cked downwardly relative to surface 12 in order to cause the lower, suspended terminal ends of the tendons 76 to be disposed at or below the pivot joints 140 of the anchor 92. Appropriate manipulation of the pins 110 and 112 in combination with the cylinder and piston assemblies 114 will jack the column 16 and its pontoon assembly 10 relative to work platform 18, with the result that the pontoon assembly will be sufficiently further submerged.

The lower, suspended terminal ends of the tendons 76 may then be connected to the pivot joints 140. Once all ^' tendons 76 have been appropriately secured to the anchor 92, then the platform PI is shifted to the geometry of Figure 7. . This is accomplished ^" by an upward stroke of the jacking assembly J in order to pull the platform 18 down into the water, thus creating a tension in the tendons 76. The specified initial design tension may then be achieved by pumping ballast from the pontoon assembly 10 and the column 16, as well as by simultaneously raising the platform 18 through appropriate manipulation of the jacking system J because the tendons 76 prevent the pontoon assembly 10 and column 16 from being moved vertically upward. Once the platform 18 has been jacked clear of the marine environment 14, and the ballast in the pontoon assembly 10 adjusted for proper initial tension of the tendons 76, then the platform 18 may be raised to its final design elevation through use of the j cking system J. Once at the operating elevation, then platform 18 is secured by the pins 112 of yoke 100 to column 16.

The single column platform PI is an advantageous configuration because wind, wave and current forces are reduced on account of the reduced surface area of the single column 16. Furthermore, because of the central orientation of the column 16 relative to the pontoon assembly 10, the upper portions of the risers 136 are not exposed to wave forces because they pass upwardly through the aperture 86, and this aperture is normally submerged. The single column platform PI is furthermore advantageous because of the X-shape of the pontoon assembly 10, and its ability to widely space the various groups of tendons 76. The tendons 76 are connected to the pontoon assembly 10 outwardly thereof such that, when the platform PI is caused to be horizontally shifted on account of wave Wl, as best shown in Figure 7, then the tendons 76 are not struck or otherwise contacted by the pontoon assembly or related parts of the platform PI. This is also attributable to the fact that the tendon receiving connector assemblies 74 are positioned along the respective lower surfaces of the legs 24, 26, 28 and 30, and furthermore permit pivoting as may be required.

The single column platform PI has improved performance not only because of the reduced surface area provided by the column 16, but also because the pontoon assembly 10 is appropriately dimensioned relative thereto to minimize turbulence, wave forces and the like which would occur as a wave Wl of substantial magnitude passed beyond the platform PI. As earlier noted, the relative dimensions of the column 16 to the pontoon assembly 10 can be appropriately selected so that the oppositely oriented buoyancy effects caused by the wave Wl are cancelled in

a manner which enhances and increases stability.

FOUR COLUMN EMBODIMENT

Figures 11-14 illustrate a four column marine platform assembly P2 which utilizes a jack system J2, which corresponds to the jack system J, for shifting the pontoon assembly 148 and work platform 150 between the construction geometry of Figure 11 the transportation and installation geometry of Figure 12 and the on-site geometry of Figure 14. Platform P2 has columns 152, 154, 156 and 158 extending upwardly from pontoon assembly 148. The pontoon assembly 148 is a centrally open system, resulting from the interconnection of peripherally disposed hollow tubular legs 160, ], _, 62, 164 and 166 and has valve 167 for ballast contral purposes.

Tendon receiving connector assemblies 168, which correspond to the tendon receiving connector assemblies 74 of the platform PI, are secured to the rounded, corner portions of the pontoon assembly 148 adjacent the associated lower surface thereof. The tendons 170, which correspond to the tendons 76 of the platform PI, are pivotally secured by pivot connections 172 within the associated tendon receiving connector assemblies 168, In this way, as with the platform PI, the tendons 170 are outwardly disposed relative to the pontoon assembly 148 in order to prevent them from being struck or otherwise contacted by the pontoon assembly 148 as the platform P2 is shifted from the normal, floating position illustrated to the left in Figure 14, to the horizontally shifted position illustrated on the right side of Figure 14.

Platform P2 has a well derrick 174 which is secured to the work platform 150. The work platform 150 has a central opening 176, as best shown in Figure 13, in order to permit risers 178 to extend upwardly from anchor 180, which corresponds to anchor 92 of platform Pi, to the top of work platform 150. Because the legs 160, 162, 164 and 166 are spaced apart relative to each other, then there is a central opening in the pontoon assembly 148 which avoids the need for a pivot connector for the risers 178 prior to their passing through the pontoon assembly 148.

The platform P2 is connected to the anchor 180 much as discussed with the platform PI, with the exception that the platform 150 cannot rotate relative to the pontoon assembly 148 because of the four columns 152, 154, 156 and 158 which are fixed to and extend upwardly from the pontoon assembly 148. Once on location, however, the tendons 170 are secured within the associated tendon connector assemblies 168 and the pontoon assembly 148 is jacked by a plurality of jacking systems J2, each of which corresponds with the jack system J of the platform PI, and each of which is disposed about one of the columns 152, 154, 156 and 158 and carried by the platform 150.

Figure 12 discloses the work platform 150 after it has been jacked intermediate the construction orientation of Figure 11, and the operating orientation of Figure 14. In this transportation and installation geometry the tendons 170 are illustrated during the process of being lowered toward the anchor 180 in order to permit connection thereto.

Much as with the platform PI, the platform P2 has the work platform 150 normally disposed a substantial

distance above the surface 182 of marine environment 184, as best shown in Figure 14. Should a wave W2 pass through the platform P2, then the tendons 170 will again permit the platform P2 to be horizontally shifted relative to the normal vertical orientation, but will prevent upward and angular displacement. The columns 152, 154, 156 and 158 are sized relative to the pontoon assembly 148, as with the platform PI, in order to have the oppositely oriented buoyancy effects of the wave W2 cancelled in a manner which minimizes stress on the tendons 170.

While this invention has been described as having a preferred design, it is understood that it is capable of further uses, adaptations and/or modifications of the invention following in general the principle of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbefore set forth, and fall within the scope of the invention of the limits of the appended claims.