The present invention relates to a floating and submersible platform, designed for use in the sea as a substation for offshore wind parks but also applicable in oil and gas recovery, of the type that includes multiple lower pontoons arranged in separately or connected, e.g. in a ring, and at least three columns mounted at the lower ends thereof to the pontoons and configured at the upper ends thereof for supporting a platform deck structure.

The invention is the solution, proportions, and technique that enables the concept of a self installing gravity-based platform for shallow and intermediate water depths and that makes it feasible in the present form. The feasibility of the platform has been demonstrated through extensive calculations, simulations, and scale model tests as well as through external verification in an ongoing implementation project.

The floating and submersible platform is so-called "self-installing" and has been designed primarily for water depths of about 15 meters to 60 meters. The platform concept is designed for high voltage direct current (HV-DC) substations for the offshore industry, but is also applicable for other types of offshore platforms regardless of their intended function, such as oil and gas recovery and production.

The platform concept is a gravity based structure (GBS) during operation, and self-floating in temporary stages.

Known and well proven methods, elements and components have been combined into a new concept to arrive at a technically and economically sound and competitive solution for a number of functional requirements indicated below. Also, the invention is a full combination of the proportions of the platform structure, the novel manner in which to ensure continuity between columns and platform deck structure (top side) without limiting the configuration of the top side, as well as the technical solutions for key elements which will be described in more detail.

The prior art in the field of the present invention is self-floating units such as jack-up platforms, which are self-floating and provided with legs that can be lowered to the sea floor and then used for jacking the platform deck up from the water to a level above the waves. Further examples of prior art are gravity base structures (GBS), which are self-floating structures having topsides mounted thereon. Such platforms have generally been used as permanent installations in deep water.

Other structures used are floaters typically including two-pontoon drilling rigs and typical ring-pontoon semi-submersible production platforms. Such platforms include columns and pontoons, but are dimensioned so as to function optimally as floating structures during operation and hence with a ratio of the volume of columns to the volume of pontoons optimized accordingly. Such semi- submersible platforms typically have a ratio of pontoon volume to column volume in the range of 2.4 to 3.5 but which may also exceed 5.

Floating structures must have natural periods that are larger than the periods of the significant energy waves so as to not resonate with the waves. A small footprint GBS should have as small pontoons as possible in order to minimize wave forces and to be able to obtain geotechnical stability.

Hence, the objectives of the present invention can be summarized as follows:

no further securing of the foundation, like piling or anchoring, is necessary, self-installing, i.e. with no use of special vessels or equipment; in sea conditions this means self-floating,

weather tolerant during installation, minimizing the risk of delays caused by weather,

simple removal at the end of its operational life,

tolerant and robust during installation of heavy equipment,

manoeuvrable and with low towing resistance,

may be completed at a yard and carry a complete top side to the field, transportable on typical heavy lift ships,

minimal use of solid ballast on the site,

minimizes substructure when no storage is required,

substructure imposes no restrictions on top side arrangement,

technically and economically competitive for shallow water depths of

15-50 meters,

minimum scouring effect,

minimal need for sea bed preparations, minimal/no need for maintenance/inspection of substructure or substructure systems during operation,

should be able to be constructed at a number of existing yards,

flexible with respect to execution strategy of during construction, can be fully assembled at a single a yard, or individually,

simple, safe and proven marine operations,

satisfy two-compartment damage stability criteria for less risk in the project.

The objectives of the present invention are achieved by a self installing offshore platform according to the independent claim 1 .

Preferred embodiments are set forth with more particularity in claims 2 through 7.

The self installing offshore platform according to the invention will be explained in more detail with reference to the drawings, in which

Figs. 1 a and 1 b show embodiments of a typical HV-DC substation design and a twin pontoon or ring-pontoon GBS, respectively;

Fig. 2 shows alternative designs and configurations using the principles according to the present invention;

Figs. 3a, 3b, and 3c show heave motion transfer functions for previously known platform structures (i.e. prior art) in the different phases; floating on pontoons, floating with the pontoons submerged and descending, and the pontoons located near the sea floor, respectively;

Figs. 4a, 4b, and 4c show natural periods for the platform structure in the different phases; floating on pontoons, floating with the pontoons submerged, and the pontoons located near the sea floor, respectively;

Fig. 5 shows an embodiment of the platform in a stage in which the platform deck structure is transported on a barge moving in between the columns in order to mount the platform deck structure onto the columns;

Figs. 6a and 6b show the platform during transport on a heavy lift vessel;

Figs. 7a, 7b, and 7c show a simplified water ballast system according to the present invention;

Figs. 8a and 8b show a simplified ballast and removal system for solid ballast according to the present invention; Figs. 9a, 9b, 9c, and 9d show steps of exemplary principles for establishing a foundation for the platform;

Figs. 10, 10b, and 10c show an example of how the platforms may be manoeuvred and installed using only towboats;

Figs. 1 1 a, 1 1 b, 1 1 c, and 1 1 d show hydrostatic intact and damaged stability in all phases and all draughts with a full topside weight on;

Fig. 12 shows box tops on the outside of the main topside box; and

Fig. 13 shows a table indicating structural volume ratios for the present platform and prior art platforms.

Referring to the drawings, a self installing offshore platform 1 is shown. Platform 1 comprises lower pontoons arranged in a separately or connected, e.g. in a ring 5, and at least 3 columns 10 mounted at the lower end regions thereof to pontoons 5 and mounted at the upper end regions thereof to a platform deck structure 15. For the embodiments of platform 1 shown, a ratio between the volume of pontoons 5 and volume of columns 10 is in the range of 0.8 to 1 .5. The result of this volume ratio can be seen in Figs. 3a, 3b, and 3c and Figs. 4a, 4b, and 4c, showing, respectively, heave movement transfer functions for prior art floating production and drilling rig semi-platforms and the present platform concept. The drawings show that it is a distinct difference in the floating motion characteristics between existing semi-submersibles and the platform of the present invention. In this respect, reference is also made to table 13, which shows the ratios of the volume of pontoons to volume of columns for five different versions of the present platform concept (that is, the uppermost five rows of the table), and compared with six prior art floating semi-structures (that is, the lowermost six rows of the table). The volume ratios of pontoons/columns vary from 0.83 to 1 .38 for the present platform concepts whereas the volume ratios for the prior art structures varies from 2.45 to 5.55.

Platform deck structure 15 is further provided with an outer box top 20 in the lower region of the platform deck structure for connection between columns 10, whereby continuity between columns 10 and platform deck structure 15 is ensured. The outer box top is closed at all 6 faces, which meet deck levels at the top and bottom and then transfers all forces from the columns above into the deck and side bulkhead of the deck box through an in-plane continuous force path.

A transitional element 12 is also provided in the upper end regions of columns 10. Columns 10 could have a tapered configuration. Referring to Figs. 7a, b, c as well as Figs. 8a and b, platform 1 is shown provided with a simplified water ballast and solid ballast system, respectively, which advantageously does not require maintenance during the operational life of the platform.

The platform concept is self-floating and provides sufficient stability to support the topside fully installed and commissioned and tested in dock/at-quay, from the assembly yard to the offshore location during towing in the sea, with no need for post-installation of modules or equipment. The ballast system as mentioned above is used for ballasting down platform 1 to land at the sea floor for operation.

Installation is then carried out with no dependence on crane vessels or other specialized vessels, and the extent of expensive offshore hook-up and commissioning is limited to the parts that can only be tested in real operating conditions for the platform systems, such as high voltage tests for HV-DC substations and oil and gas production from real wells.

The motion response during installation, due to environmental actions, wave currents, and wind is favourable and makes the concept more weather tolerant during installation than prior art concepts. The concept is prepared so that the solid ballast can be removed and the ballast system is operable with no maintenance during operation at the end of the operating life of the platform. Moreover, the foundation concept makes sure that during de-ballasting, the platform has sufficient buoyancy to be pulled away from the sea floor with no need for cutting poles, etc. This concept allows the platform to be easily removed and towed to a decommission yard at the end of its operating life. Skirts may be used, being long or short depending on the conditions on site, but in most cases the platform is dimensioned in such a manner that the mudline moment is small and uplift does not take place, and there is hence no need for suction skirts and therefore no need to apply overpressure in the skirt chambers for removal. The platform concept is flexible and may be constructed and assembled locally. It can be constructed in other parts of the world than the operation site and transported, either fully assembled or in multiple parts for on-site assembly.

The platform concept is scalable to any size and topside/equipment weight and does not depend on crane vessel availability or any other constraints.

The weather resistance during installation, independence of special vessels during installation, scalability, and flexibility of the project make the concept robust, competitive, and represent low risk both technically and economically.

The platform concept of the present invention may be considered to be a combination of a floater and a GBS, being self-floating in temporary phases and providing bottom fixed gravity-based stability during operation. The platform exhibits excellent floater characteristics during temporary phases, minimal towing resistance when at pontoon draft, and favourable motions during installation when the pontoons are submerged.

The foundation area for use in the case of gravity-based stability when the platform is bottom fixed may be adapted to the conditions on site. On a sandy sea bed, a minimum area to obtain a vertical pre-stressing of the ground that increases the earth shear strength is advantageous, with a smaller submerged volume also being advantageous with respect to environmental impact and reduced need for solid ballast, and in this regard, the concept herein differs from a conventional large foundation area for GBS.

The concept differs from typical semi-submersible platforms in its relative proportions between columns and pontoons, as the concept includes smaller and lower pontoons that are more optimized for the in-place conditions when bottom foxed, with the most advantageous motion characteristics in the floating phases being maintained through the use of the semi-cancellation principle.

Platform deck structure 15 is provided with a central box which can be freely positioned independently of columns 10. External areas (i.e. those located outside columns 10) may be closed by secondary structures or as part of the main structure in order to facilitate accommodation of topside equipment. Structural continuity for columns 10 into vertical bulkheads and topside deck is achieved through the use of outer box tops 20. The concept allows freedom of layout for the central box, but in a manner that ensures structural continuity. The distance be- tween columns 10 is sufficient to allow a large barge, typically having a width of 36-40 meters, to be manoeuvred in between the columns for wet deck mating. The platform concept is also adapted so that the platform can be transported on a typical heavy load vessel having a width of 40-50 meters.

The platform concept also makes sure that hydrostatic stability at all draughts, with the topside installed, will be obtained. Additionally, damage stability will also be achieved with sufficiently small tank volumes in separated tanks. In order to keep the environmental loads on the structure as small as possible, i.e. the submerged volume is kept at a minimum, and this is achieved through the use of tapered columns and as small as possible pontoons.

From model tank experiments it has been found that the proportions of columns and having a low and small pontoon minimize the hydrodynamic forces when the platform is fixed to the sea floor. The proportions of the columns and pontoons is also maintained so that a vertical dynamic pressure on the pontoons results in a moment in an opposite wave phase relative to the moment from horizontal loads, and hence a cancellation effect for the mudline moment is obtained that in most cases ensure that there is no upward lift and hence no need for suction skirts. The foundation area is sufficiently large, but not too large, to obtain a certain vertical compression of the ground and thereby geotechnical stability against mudline forces and moments.

Minimal scouring is achieved through the use of rounded pontoon ends, but as an alternative, ring pontoons of quadrilateral or circular shapes could be used.

The slender pontoons 5 may be stiffened by using of permanent stays between the pontoons and also between the pontoons and columns in the lower region of the platform.

A water ballast system has been developed that uses simple caissons for each tank and one for water intake. Filling is accomplished using a submersible pump in the inlet caisson having a hose into the caisson to fill the tank. Emptying is accomplished by lowering a submersible pump into the tank to be emptied.

This simple system is robust, safe, cost efficient and will be operable at the end of the operating life of the platform with no maintenance during the operational phase. Also, the water ballast system is configured so as to be able to receive water from utility vessels in order to accelerate the ballasting process.

In this, the present concept differs from a permanent floater having a permanent and numerous more complicated ballast systems requiring continuous maintenance.

A solid ballast system has been developed which is able to receive a slurry mix of water and sand or crushed rock at one or more single docking points and includes an internal distribution system with a minimum of bends in the internal piping, and which uses water tanks as precipitation tanks for overflow water treatment before discharge into the sea.

A hatchway system for providing access to submersible mud pumps has also been developed. This is intended to be used for removing solid ballast for decommissioning at the end of the operational life of the platform. This combination is novel.

Three different foundation concepts have been developed for the concept.

1 . Skirts that penetrate the upper soil layer and with grouting by using a concrete mixture, of cavities in the skirt chamber for uniform foundation contact. This is the solution used for most GBS applications.

2. Gravel bed plus ribs on the pontoon underside combining foundation and wave-wash protection. Referring to Figs. 9a, b, c, and d, the gravel bed installation sequence is shown. Ribs are mounted on the underside of the pontoons in order to achieve the necessary friction between the steel and gravel.

3. Gravel bed plus skirt. Using longer skirts at the periphery, which will penetrate a gravel bed and into more consolidated earth layers and oppose geo- technical fracture mechanisms to deep layers, is an option that will reduce the need for solid ballast. An optimal shape for hydrostatic stability is obtained by using sufficiently large columns, with a sufficient spacing, but keeping the volume to a minimum by using tapered columns.

A redundant structural system has been designed, having a better than normal continuity of load-bearing members, having reduced steps in rigidity of local members, and having more than typical redundant capacity in possible load paths which are still predictable, using standard design and analysis approaches. A combination of plate members has been used through the use of girders, beams, trusses, and plate stiffeners in order to complete an efficient main structural system with the need of subdivisions, either water-tight in the substructure or air-tight for air conditioning of topside rooms.

The platform concept has been designed for simple and reliable marine operations. In this regard, reference is made to Figs. 10a and b.