WO2016109962A1

WO2016109962A1 - Construction method for fixing offshore marine platform to a seabed having layers of a soil/clay nature

Info

Publication number: WO2016109962A1
Application number: PCT/CN2015/070348
Authority: WO
Inventors: Carlos Wong
Original assignee: Cbj (Hong Kong) Ocean Engineering Limited
Priority date: 2015-01-08
Filing date: 2015-01-08
Publication date: 2016-07-14

Abstract

A construction method for fixing a foundation of an offshore platform to a seabed is disclosed. The platform is adapted to support wind turbines, bridges and marine buildings thereon. The platform includes at least one floater which has a bottom slab embodied as a downward pointing bottom cone-shaped object. In the method, a concrete bed is cast in the seabed, and the at least one floater is impressed into the concrete bed to form a mirror image, reversed cone indentation of the conic object in the concrete bed. The bottom cone of the at least one floater is aligned to the reversed cone indentation, and the platform is thereafter to couple the bottom cone and the reversed cone indentation together so as to fix the floater to the seabed. By applying the method to a seabed having a thick soft soil/clay layer, a steel tube having a diameter larger than that of the floater is employed, whereby the steel tube is driven into the seabed to the founding level. The steel tube functions as a temporary support or permanent support for the platform in four different ways.

Description

CONSTRUCTION METHOD FOR FIXING OFFSHORE MARINE PLATFORM TO A SEABED HAVING LAYERS OF A SOIL/CLAY NATURE

BACKGROUND

1. Field.

The example embodiment in general relates to a construction method for fixing an offshore marine platform adapted to support wind turbines, bridges and marine buildings thereon to a seabed in a marine environment.

2. Related Art.

A type of commander base on site is needed to support the development of ocean resources that include offshore wind energy, ocean current and tidal energies, ocean fish farms, and even the building of an ocean city, etc. The base may be fixed to the seabed or may be configured so as to float in the water. A drawback of the floating-type of base is that mooring the base for the purpose of anchorage is difficult where the water depth is shallower than 50m. The base drifts aimlessly if the mooring lines are broken as this would pose great danger to the public. Hence, a fixed base is often more desirable and offers greater safety than the floating base. Additionally, a stationery fixed platform is desirable in that it offers riders the comfort similar to living on land, as opposed to the boat living on the floating base.

Applicant’s prior art China Pat. Appl. Ser. Nos. 201210038405.9 and 201200104898.8 both describe a process where a hard seabed or soft materials in the seabed may be dredged, and may be applied to conditions where the bedrock is close to the seabed surface. In near shore waters, especially at an estuary where thick layers of soil and sand have settled, the removal of soft soil materials is simply not feasible. Accordingly, what is needed is a method of fixing an offshore marine platform to a seabed which includes thick layers of soft materials that typically cannot be completely removed.

SUMMARY

An example embodiment is directed to a construction method for fixing a foundation of an offshore platform to a seabed, the platform being adapted to support wind turbines, bridges and marine buildings thereon. The platform includes at least one floater which has a bottom slab embodied as a downward pointing bottom cone-shaped object. In the method, a concrete bed is cast in the seabed, and the at least one floater is impressed into the concrete bed to form a mirror image, reversed cone-shaped indentation of the conic object in the concrete bed. The bottom cone of the at least one floater is aligned to the reversed cone-shaped indentation, and the platform is thereafter to couple the bottom cone and the reversed cone-shaped indentation together so as to fix it to the seabed.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiment will become more fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limitative of the example embodiments herein.

FIG. 1 is a top plan view of a single-floater platform.

FIG. 2 is a left-side elevational view of the platform shown in FIG. 1, the right-side elevational view being a mirror image of the left-side elevatonal view.

FIG. 3 is a top plan view of a multi-floater platform.

FIG. 4 is a left-side elevational view of the platform shown in FIG. 3, the right-side elevational view being a mirror image of the left-side elevatonal view.

FIG. 5 is a side view of a vibro hammer driving a steel tube into the seabed.

FIG. 6 is a cross-sectional view taken from line A-Ain FIG. 5.

FIG. 7 is a cross-sectional view taken from line B-B in FIG. 5 to illustrate the layout of steel brackets in the steel tube.

FIG. 8 is a side view to illustrate how the steel tube is driven to a design depth level in the seabed.

FIG. 9 is a side view provided to illustrate excavation of soft materials within the steel tube.

FIG. 10 is a side view provided to illustrate backfill of sand and gravel in the steel tube.

FIG. 11 is a side view provided to illustrate a procedure of placing a steel ring in the steel tube.

FIG. 12 is a top plan view of the steel ring.

FIG. 13 is a cross-sectional view taken from line C-C in FIG. 11 to illustrate the position of steel ring within the steel tube.

FIG. 14 is a cross-sectional view taken from line D-D in FIG. 13.

FIG. 15 is a side view provided to illustrate a procedure of forming a concrete bed and the reversed cone-shaped indentation therein.

FIG. 16 is a side view provided to illustrate pressing of a mold to form the reversed cone-shaped indentation in the concrete bed.

FIG. 17 is a side view provided to illustrate raising of the mold and cutting of surplus steel tube extending above

FIG. 18 is a side view provided to illustrate transportation of the platform to an installation site by using auxiliary floaters.

FIG. 19 is a side view provided to illustrate sinking of the platform to sink and sit on the reversed cone-shaped indentation in the concrete bed within the seabed.

FIG. 20 is a side view provided to illustrate installation of a plurality of piles in the floater.

FIG. 21 is a side view provided to illustrate completion of the platform installation.

FIG. 22 is a side view provided to illustrate an optional installation method for a piled foundation using large-diameter raking piles driven into the seabed.

FIG. 23 is a side view provided to illustrate the platform supported on the steel tube.

FIG. 24 is a side view provided to illustrate the steel tube being driven to a design depth in the seabed using a vibro hammer.

FIG. 25 is a side view provided to illustrate excavation of soft materials inside the steel tube to the founding layer.

FIG. 26 is a side view provided to illustrate backfilling of voids left by excavation with sand and gravel.

FIG. 27 is a side view provided to illustrate constructing an underwater concrete bed and the reversed cone-shaped indentation therein.

FIG. 28 is a side view provided to illustrate a platform supported by a replacement sand column inside the steel tube.

FIG. 29 is a side view provided to illustrate the steel tube being driven to a design depth by a vibro hammer.

FIG. 30 is a side view provided to illustrate excavation of unsuitable materials at a top soft soil layer, replaced by a suitable soil layer for soil property improvement.

FIG. 31 is a side view provided to illustrate soil improvement inside the steel tube.

FIG. 32 is a side view provided to illustrate forming of the underwater concrete bed and reversed cone-shaped indentation therein.

FIG. 33 is a side view provided to illustrate a completed installation of the platform supported on a dense soil column inside the steel tube, after soil improvement.

Parts List

1. Sea level

2. Seabed/seafloor

3. Bed rock level

4. Soft soil/clay layer

5. Founding layer

6. Excavation of soft soil/clay

7. Sand and gravel bed layer

8. Pressure grout pipe

9. Improved soil layer

10. Replacement sand and gravel layer

11. Platform

14. In-situ concrete

15. Piling plant

21. Floater

22. Floater wall

31. Auxiliary floater/semi-submerged barge

32. Platform structural strut

33. Platform sinking

41. Vertical piles

42. Raking piles

44. Concrete plug

48. Pile head

50. Pile cap (inside floater)

51. Pile cap (inside platform)

101. Steel tube

102. Weld line between segments of steel tube

103. Steel brackets

104. Steel ring

105. Vibro hammer

106. Reversed cone making mold

107. Underwater concrete bed

108. Reversed cone-shaped indentation in concrete bed

109. Cut line of steel

DETAILED DESCRIPTION

As used herein, the phrase “present invention” should not be taken as an absolute indication that the subject matter described by the term "is covered by either the claims as they are filed, or by the claims that may eventually issue after patent prosecution； while the term "present invention" is used to help the reader to get a general feel for which disclosures herein are believed as maybe being new, this understanding, as indicated by use of the term "present invention, " is tentative and provisional and subject to change over the course of patent prosecution as relevant information is developed and as the claims are potentially amended.

Reference throughout this specification to "one example embodiment" or "an embodiment" means that a particular system, method, feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one example embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Further, the particular systems, methods, features, structures or characteristics may be combined in any suitable manner in one or more example embodiments.

The term “and/or” may be understood to mean non-exclusive or； for example, A and/or B means that: (i) A is true and B is false； or (ii) A is false and B is true； or (iii) A and B are both true.

As used in this specification and the appended claims, the singular forms "a, " "an, " and "the" include plural referents unless the content clearly dictates otherwise. The term "or"is generally employed in its sense including "and/or"unless the content clearly dictates otherwise.

In the drawings, identical reference numbers identify similar elements or acts. The size and relative positions of elements in the drawings are not necessarily drawn to scale.

Unless the context requires otherwise, throughout the specification and claims that follow, the word "comprise" and variations thereof, such as "comprises" and "comprising, " are to be construed in an open, inclusive sense, that is, as "including, but not limited to. "

As used in the specification and appended claims, the terms "correspond, " "corresponds, " and "corresponding" are intended to describe a ratio of or a similarity between referenced objects. The use of "correspond" or one of its forms should not be construed to mean the exact shape or size.

As used herein, the term “floater” refers to a floating structure in a body of water on which a wind power turbine, marine building, and/or bridge may be mounted thereon.

As will be described in further detail hereafter, an example embodiment of the present application addresses the challenges discussed above in the background section. Additionally, the example marine platform and its construction method (s) therefor may include a piling support arrangement or foundation comprising a plurality of pilings that are inserted through the inside of the hollow floater and down into the soft soil/clay layer of a seabed to connect the floater to bedrock, thereby accomplishing piling installment tasks without requiring an expensive piling vessel at sea.

In general, innovative techniques exemplified by the example embodiments may further include replacing the usual solid vertical pier or column used in a conventional offshore platform with the hollow, cylindrical, vertically arranged buoyancy tube (floater) which provides buoyancy, so that a single floater or a plurality of interconnected floaters forming the marine platform may float in a body of water. The buoyancy provided by the floaters reduces the bearing pressure on the founding stratum in a temporary state or in a permanent state. By employing a unique cone matching technique as part of an installation and construction method to be described hereafter, the bottom of the floater can easily be fixed to a concrete bed on the seabed. As the floater interior is a relatively large space, a piled foundation for the marine platform can be realized by installing small diameter piles through the inside of the floater and down into bedrock, and by constructing the pile cap therein, which secures upper ends of the piles within the floater.

The floater as previously noted is a hollow, cylindrical member capped at its upper and lower ends by slabs. The lower end may be tapered； for example, the bottom slab may be conically-shaped (as a single cone shape or as a plurality of cone-shapes) with the apex of the cone pointing downward toward the seabed/seafloor. The floater has a buoyancy capable of allowing the floater itself to float, and compensates part or all of the dead weight of the marine platform when the platform is deposited in a body of water (marine environment) . In one example, the marine platform includes only a single, vertically aligned, hollow cylindrical floater including a bottom slab configured so as to have a single cone shape (or multiple cone object shapes) with the apex pointing downward. The tapered or cone-shaped bottom slab of the floater is desirable so that bearing pressure on the founding stratum or founding layer can be minimized to a small value； this is desirable for gravity type floaters. This tapered out floater can accommodate raking piles (piles installed at an angle) .

As discussed above, the construction method described hereafter employs a cone matching technique to fix the marine platform into the seabed. At the corresponding location of where the floater (or floaters) of the platform is to be secured in the seabed, the bottom cone-shaped slab of the floater, which points vertically down toward the seafloor, contacts and is secured to a mirror-image of the cone shape of the bottom slab, or “reversed cone shape” that has been impressed or formed into the concrete bed within the seabed (the concrete bed having been made by a mass concrete deposit in a pothole formed by removing soft materials on the seabed, exposing the founding layer) .

As an example, the method may include, at the corresponding location of where the cone-shaped bottom slab of the floater is to be attached in the seabed, excavating, dredging or sucking away soft material such as a soft soil layers to expose a firm stratum or founding layer of material in the seabed that can withstand the expected load of the marine platform. The marine platform is then floated in position and at the same time the concrete bed is prepared by filling the pothole (s) left by the excavation in the seabed with concrete from construction vessels using a tremie concrete pipe down to the seabed according to established underwater concreting technology. In an example, the quantity of concrete used for forming the concrete bed should be such that the cone shape of the bottom slab may be completely immersed and covered up by the concrete bed.

Prior to the concrete setting in the pothole to form the concrete bed, the marine platform is lowered down within the marine environment (water) by adjusting its buoyancy with water in-take until the cone-shaped bottom slab (s) of the floater (s) are completely immersed within the still-wet concrete bed within the seabed. The orientation and level of the marine platform is maintained until the concrete starts to set, i.e., starts to harden. At that point, high pressure water is used to flush separate the two faces of the cone-shaped bottom slab (s) from the concrete bed, and the platform is thereafter raised off the concrete bed, thereby revealing an indented, mirror-image, reversed cone indentation or shape formed in the concrete bed by the cone-shaped bottom slab (s) of the floater (s) .

Once the concrete of the concrete bed has reached its design strength, the platform is lowered again so that the cone-shaped bottom slab of the floater contacts the reversed cone-shaped indentation in the concrete bed. Level and orientation of the marine platform is maintained, with any gap (s) formed between opposed faces of the bottom slab reversed cone-shaped indention in the concrete bed cement grouted via pipes pre-installed within the interior body of the floater； this completes installation of the platform in the seabed of the marine environment.

In an example, a pressure piping system may be installed in the floater to deliver high pressure water jets from a high pressure water source, and cement grout from a grout source through openings that are formed in the bottom of the floater. Pumping machinery may be located inside the floater, or from outside in the construction vessels.

According to another example, a piling support arrangement may be provided for the marine platform (to accommodate deeper water depths, for example) . Piling may be added to the platform foundation in cases where the concrete founding stratum (concrete bed) for the cone-shaped bottom slab of the floater cannot resist further loads imposed on the founding stratum. In providing the piling support arrangement, generally a plurality of small diameter piles are installed through the inside space of the floater with their lower ends to be secured in bedrock； raking piles may be installed if necessary, and a pile cap is then cast within the lower end of the floater to secure the upper ends of the piles.

Once the piles are secured in bedrock, the floater interior is dewatered by pumping, or by adding concrete at the bottom of the floater to form a concrete plug which prevents water from entering； the interior of the floater thus realizes a dry working environment. Then, upper ends of the piles are cut to a desired level and made ready for a pile cap casting according to established procedures.

General concepts of the example embodiment having been described above, the following FIGS. 1-33 should be referred to for describing an example method of fixing an offshore marine platform adapted to support wind turbines, bridges and marine buildings thereon to a seabed which includes a thick layer of soft materials within a marine environment. The example method suits water depths in a range of about between 3m to 50m.

According to the example embodiment, an offshore marine platform 11 includes a deck made up by a beam-and-slab deck that is supported by at least one floater 21. The floater 21 has a given wall thickness 22 and includes a bottom slab configured as a bottom cone 23 with its apex pointing down.

FIGS. 1 and 2 illustrate a single floater platform in a plan view and a sectional view, whereby the platform 11 is supported by a single floater 21. The stability of the platform 11 may be facilitated by connecting it to one or more adjacent platforms, as best shown in FIG. 33. The adjacent platforms are connected to each other with in-situ concrete so as to form a multi-floater platform, as shown in FIG. 28 for example.

FIGS. 3 and 4 illustrate a multi-floater platform in a plan view and a sectional view respectively, whereby platform 11 is supported by multiple floaters 21； in this example four (4) floaters 21 are employed. These figures show a stage where the platform 11 may be transported using an auxiliary floater 31.

With reference to FIGS. 5-33, the example construction method to fix the platform foundation to a thick soft soil layer in a seabed is now more fully described. A large diameter steel tube 101 configured so as to be larger than the diameter of the floater 21 is vibro driven (element 105 representing the action of a vibro hammer) into the soft soil/clay layer 4 of the seabed 2 down to the founding layer 5. The steel tube 101 may be a single unit, or multi-units welded together. FIG. 5 shows a typical example where a weld line 102 exists between two segments. FIG. 6 shows a cross-sectional view taken along line A-Ain FIG. 5, and is provided to show a section of the steel tube 101. For clarity, the thickness of the floater 21 is not shown, only a single line is drawn.

According to the example method of fixing the platform 11 foundation in the thick soft soil layer 4 of the seabed 2, the steel tube 101 may offer four (4) different functions in different designs: (1) as a temporary support to the offshore platform 11 to accommodate self-weight and construction loads during a construction period while the permanent support relies on a plurality of

piles

41, 42； (2) as a permanent support for the platform 11 from the construction period to a service period； (3) as a container for replacement of the soft soil 4 inside the steel tube 101 with a sand and gravel column 10； and (4) as a container for carrying out soil improvement inside the tube 101 to an improved soil column having adequate bearing capacity, provided that the material (such as sandy soil/clay) is suitable for modification.

Accordingly, the following description of the example construction method may be classified into four types, based on the differing functions of the steel tube 101.

1. Steel tube 101 used as a temporary support.

Initially in this method, the steel tube 101 is prepared (FIG. 5) . Preparation of the steel tube 101 includes (a) welding several layers of steel brackets 103 around the location of the tube 101 where an underwater concrete bed 107 will be cast. The brackets 103 are distributed about the circumference of the tube 101, as best shown in FIG. 7. The brackets 103 are used to support a steel ring 104 (FIG. 11) . As the driven depth of the steel tube 101 may vary, it is necessary to weld more than one layer of brackets 103 to cover any variation in the driven depth.

A vibro hammer 105 may be used to drive the steel tube 101 into the soft soil/clay layer 4 (FIG. 5) down to the founding layer 5 (FIG. 8) , which is the design depth for the steel tube 101. At the level (depth) , the surface friction of the steel tube 101 effectively carries the platform 11’s self-weight and any construction loads thereon during the construction period. Further, at this level (depth) , as best shown in FIG. 8, any soft soil/clay inside the steel tube 101 is excavated 6 (FIG. 9) . Also, a top layer of the soft soil/clay materials at the outer perimeter around the steel tube 101 is removed, and a layer of a sand and gravel bed 7 (FIG. 10C) is then backfilled inside the steel tube 101, after which a layer of anti-erosion stone is backfilled in the outer side of the steel tube 101.

A level where a layer of brackets 103 is to support the steel ring 104 is determined, then those brackets 103 are cut above the location for the selected layer (FIG. 11) . The strength of the steel ring 104 with brackets 103 offers a steel structure having a strength to support the self-weight of the platform 11 and any construction load thereon.

As best shown in FIG. 15, a tremie concrete pipe may then be used, according to known technologies, to pour concrete underwater and onto the sand and gravel bed 7 inside the steel tube 101 so as to cast a concrete bed 107 having an adequate thickness to form a reversed cone-shaped indentation 108 that is a mirror image of the shape of the bottom cone 23 of the floater 21.

To form the reversed cone-shaped indentation 108 in the concrete bed 107, a reversed cone mold 106 may be used； its level and alignment over the concrete bed 107 is adjusted (FIG. 15) . Alternatively, instead of using the reversed cone mold 106, the bottom cone 23 of the floater 21 or another steel mold may be used to form the indention 108 in the concrete bed 107. For purposes of explanation only, the following steps are described where the reversed cone mold 106 is employed to form the indentation 108 in the concrete bed 107.

Prior to an initial set of the concrete bed 107, the reversed cone mold 106 is sunk or lowered into the still-wet concrete which is to form the concrete bed 107 so as to be completely buried in the wet concrete. Its position and verticality is maintained until the concrete has completely set (FIG. 16) , thereby the reversed cone-shaped indentation 108 in the concrete bed 107. Any surplus steel tube 101 extending above a cut line 109 for the concrete bed 107 is cut and removed (FIG. 17) . If mold 106 proves difficult to raise, pre-installed pressure pipes within the mold 106 may be used to inject water into the two surfaces to separate the mold 106 from the concrete bed. Alternatively, the surfaces of the mold 106 may be coated with a debonding agent such as mold oil to debond the mold 106 from the concrete bed 107.

Platform 11 is floated to the site using an auxiliary floater 31； the floater 31 being attached to platform 11 via platform structural struts 32. The bottom cone 23 of the floater 21 is aligned so as to be directly over the now-formed reversed cone-shaped indentation 108 in the concrete bed 107 (FIG. 18) . As shown in FIG. 19, platform 11 is then sunk or lowered in the water so that the bottom cone 23 seats naturally in the reversed cone-shaped indentation 108, even if there may be position error. A plurality of pre-installed pressure pipes in the floater 21 may be used to inject pressurized cement grout/concrete into a gap between the two cone faces, sealing the gap. At this stage, the platform 11 is now fixed to the soft soil/clay layer 4 in the seabed 2

The loading takes a path from the platform 11 → to the floater 21 → to the concrete bed 107 → to the steel ring 104 → to the steel bracket 103 → to the steel tube 101, and is finally resisted by the friction imparted by the soft soil/clay layer 4 acting on the steel tube 101 surface. The horizontal load is carried by the floater 21 to the steel tube 101 and finally is resisted by the lateral resistance of the soft soil/clay layer 4.

Referring to FIG. 20, and in an example, a plurality of

piles

41, 42 configured as a piled foundation having a pile head 48 and pile cap reinforcement may be installed inside the floater 21, the

piles

41, 42 being configured to carry all dead and live loads of the platform 11 in a servicing period thereof. Specifically, a plurality of piles 41 and raking piles 42 are vertically installed from the deck of the platform 11 through the floater 21. Referring to FIG. 21, once installed, a layer of concrete is poured down to the bottom of the floater 21 to form a concrete plug 44. Plug 44 prevents or terminates any further water from coming in. The inside of the floater 21 is then dried, and workers are sent down into the floater 21 to cut any surplus lengths of piles and modify the pile head 48 to fit in the pile cap reinforcement cage (as is known) , and to connect steel bars to pre-installed steel bar sockets in the floater wall 22 so as to fix the pile cap reinforcement. Concrete is then poured to cast the pile cap 50, thereby forming a fixed piled foundation.

Alternatively, as shown best in FIG. 22, only the piles 41 are vertically installed in the floater 21, and the raking piles 42 are driven downward from the deck of platform 11 to the seabed 2, with a corresponding pile cap 51 being cast in the platform 11 deck level. Optionally, in order to avoid the ingress of water in the floater 21 after the bottom slab of the floater 21 has been penetrated during piling installation, a steel casing may be pre-installed at each piling location from the bottom slab of floater 21 to the platform 11 deck level above the sea level 1, with the casing being supported at intervals inside the floater 21. In this arrangement, water can enter the steel casing but not the floater 21. At the completion of piling installation, the lower part of the casing is filled with concrete, which enables water-tight integrity even if the

piles

41, 42 are cut to the bottom level of the floater 21. Thereafter, the same procedures mentioned above are employed to cast the pile cap 50 in a dry working environment.

2. Steel tube 101 used as a permanent structure.

In this embodiment of the method, the steel tube 101 is prepared by welding several layers of steel brackets 103 at a level where the concrete bed 107 is expected to be located. The brackets 103 are distributed evenly around the steel tube 101’s circumference. The brackets 103 are used to support the steel ring 104. Several layers of brackets 103 may be welded to account for any level error.

The vibro hammer 105 then is used to drive the steel tube 101 into the soft soil/clay layer 4 until the tube 101 reaches the designed founding layer 5. The driven depth should allow the development of an adequate frictional resistance in the steel tube 101 that is greater than the loads on the floater 21 from installation to service, or in other words, the allowable friction resistance developed between the soil/clay layer 4 and the steel tube surface 101 can support the platform 11 from the construction period to the service period for the applied loads.

Once steel tube 101 is at the design depth, excavate 6 soft soil/clay inside the tube 101, and excavate the top part of soft soil/clay layer 4 present at the external realm of the tube 101. A sand and gravel bed layer 7 is the backfilled inside the tube 101, and gravel stone backfilled at the external realm of the pipe 101 for erosion protection.

The level where the layer of brackets 103 is to be located in order to support the steel ring 104 is then determined, portions of surplus brackets 103 extending above the selected layer are then cut, and the steel ring 104 is installed on the brackets 103. The brackets 103 and ring 104 form a steel structure with adequate strength to support the platform 11 loads from construction to service periods.

The remaining steps to form the reversed cone-shape indentation 108 in the concrete bed 107 and then to lower down the platform 11 so as to fix the bottom cone 23 of the floater 21 to the formed reversed cone-shape indentation 108 are the same as described above with regard to FIGS. 15 through 19, and are thus not repeated here for purposes of brevity. As shown in FIG. 20, the load path initially from the offshore platform 11 to the final resistance imparted by the frictional force developed in the steel tube 101 surface by the lateral pressure imparted by the soil is the same as in the previous embodiment and hence not repeated here for purposes of brevity. In this example embodiment, the steel tube 101 supports the platform 11 loads from construction to service periods； hence, the design life of the steel tube 101, steel ring 104 and bracket 103 should be at least as long as the design life of the platform 11 or even longer, and should be corrosion protected and have a sacrificial thickness.

3. Steel tube 101 used as a temporary container for soft soil/clay layer 4 replacement.

Referring to FIGS. 24-27, in the embodiment of the method, and once the steel tube 101 has reached its design depth (founding layer 5) , excavate 6 soft soil/clay inside the tube 101, and backfill the excavated area with a suitable replacement sand and gravel layer 10 inside the pipe 101. Once this has been accomplished, the remaining steps in this example construction method to form the reversed cone-shape indentation 108 in the concrete bed 107 and then to lower down the platform 11 so as to fix the bottom cone 23 of the floater 21 to the formed reversed cone-shape indentation 108 are the same as previously described above with regard to FIGS. 15 through 19, and are thus not repeated here for purposes of brevity. However, the load path differs from the previous example embodiments. Specifically, and in reference to FIG. 28, the load path in this example is from the offshore platform 11 → to floater 21, → thereafter to the concrete bed 107 → and then to the replacement sand column 10 by end bearing. Horizontal load is resisted by the coupling of the faces of bottom cone 23 to faces of indentation 108 formed in the concrete bed, transferring the load to the steel tubes 101 resisted by the lateral pressure imparted thereto by the soft soil/clay layer 4 in seabed 2.

4. Steel tube 101 as a temporary container for soil/clay improvement of the bearing capacity.

In this example method, and referring now to FIGS. 29 to 33, once the steel tube 101 is at its design depth (founding layer 5) , excavate 6 any unsuitable material in the top layer of soil inside the tube 101, and thereafter initiate a soil improvement process using known technologies, thereby creating an improved soil layer 9.

Once this has been accomplished, the remaining steps in this example construction method to form the reversed cone-shape indentation 108 in the concrete bed 107 and then to lower down the platform 11 so as to fix the bottom cone 23 of the floater 21 to the formed reversed cone-shape indentation 108 are the same as previously described above with regard to FIGS. 15 through 19, and are thus not repeated here for purposes of brevity. The load path in the example embodiment is from the offshore platform 11 → to floater 21, → thereafter to the concrete bed 107, → then to the improved soil layer 9 by end bearing. As best shown in FIG. 33, horizontal load is resisted by the coupling of the faces of bottom cone 23 to faces of indentation 108 formed in the concrete bed 107, transferring the load to the steel tubes 101 resisted by the lateral pressure imparted thereto by the soft soil/clay layer 4 in seabed 2.

For the non-piled foundation in the second through fourth example embodiments above, the load transfer relies on the coupling of the floater bottom cone 23 to the reversed cone-shaped indention 108 in concrete bed 107. At the end of the platform 11 service life or decommission at an early stage, the platform 11 can be reclaimed by using auxiliary floaters 31 to raise the platform 11 and to float it out of the site, to thereafter dismantle all superstructures. A new or refurbished platform 11 can then be installed at the same location.

The following are examples for the same platform in two different locations.

Work Example 1. A 50m×50m platform is supported by 4×8.8m diameter floaters； platform self-weight is 8,500 tons, total weight to support all dead and live loads of a building thereon is 12,000 tons. The foundation is arranged as a piled support in a permanent state, and water depth at the site is 8m. The following geological data for this example is shown in Table 1 below.

As is known in the industry, a standard penetration test (SPT) is used in this test example to determine SPT-N values at differing depth and strata conditions. The SPT is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. The main purpose of the test is to provide an indication of the relative density of granular deposits, such as sands and gravels from which it is virtually impossible to obtain undisturbed samples. The soil strength parameters which can be inferred are approximate, but may give a useful guide for engineers in ground conditions where it may not be possible to obtain borehole samples of adequate quality like gravels, sands, silts, clay containing sand or gravel and weak rock.

In general, the test uses a thick-walled sample tube, with an outside diameter of 50 mm and an inside diameter of 35 mm, and a length of around 650 mm. This is driven into the ground at the bottom of a borehole by blows from a slide hammer with a mass of 63.5 kg (about 140 lb) falling through a distance of 760 mm (about 30 in) . The sample tube is driven 150 mm into the ground and then the number of blows required for the tube to penetrate each 150 mm (about 6 in) chunk or penetration into the soil up to a depth of 450 mm (about 18 in) is recorded.

The sum of the number of blows required for the second and third 6 inches of penetration is termed the "standard penetration resistance"or the "N-value". This blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae.

Table 1 -Bored Log Standard Penetration Test (SPT) Blow value (SPT N-value)

From Table 1, it can be seen that the first 5m of the seabed is marine mud that cannot resist the self-weight of the platform 11 during installation. By using the steel tube 101 as a temporary support, and ignoring the marine mud, a 10m diameter steel tube 101 has the allowable load carrying capacity as calculated in the following equation (1) :

Allowable friction force between soil and steel plate is calculated by the formulae

Fs ＝ 1.0 N (kPa) where N is the SPT N-value [Eq. 1]

Table 2 -Bored Log SPT Blow value (SPT N-value) –Use of Steel tube as support

The 10m diameter steel tube 101 is founded on the gravel level, with its top end 2m above sea level 1, 8m in the water and 49m in the soft soil/clay layer 4 of the seabed 2； this shows a total length of 2+8+ (51.25-5) +3 ＝ 59.25 m. As seen in table 2, the load carrying capacity at the gravel layer is 11738 tons, which after deducting the platform 11 self-weight of 8, 500 tons and the steel tube weight of 300 tons, indicates that the remaining 2935 tons may be mobilized for supporting a construction load, which is expected to be adequate for supporting small-diameter piling operations and the piling plant.

Construction method referred to as “Steel tube as temporary support”

Work Example 2. In this work example, the foundation is formed by the replacement of soil material (i. e. , replacement sand and gravel column 10) inside the steel tube 101 without piling. Here, the load is transferred from the floater wall 22 to the replacement material to the founding layer 5 in the seabed 2.

First, it is necessary to locate the founding layer 5 from the bored log of Table 1. The founding layer 5 is determined at 58m where the rock is weathered rock with N>200. The steel tube 101 is driven down to a depth of 53m in the seabed 2, thus the total length of the steel tube 101 is 2+8+53 ＝ 63m. Since this penetration depth is substantial, the steel tube 101 has to sustain the lateral soil pressure after the material inside the tube 101 is removed (water pressure is balanced between the inner and outer faces) . The steel tube 101 should be designed to take the max lateral soil pressure, or optionally, the thickness of the tube 101 should vary at different depth levels.

The max lateral soil pressure is given by Equation (2) :

P ＝ Ko×P×H, [Eq. 2]

where Ko is the soil lateral pressure coefficient, P is the specific weight of soil in water and H is the depth of the soil. In this example, Ko ＝ 0.5, P ＝ 10kN/m3 and H ＝ 56m, hence

P ＝ 0.5×10×56 ＝ 280 kPa

Assuming the thickness of the steel tube 101 at its bottom is t ＝ 24mm, the radius of the steel tube 101 is r＝5m, the circumferential stress in the steel tube becomes:

which is the stress that can be taken by the steel.

The allowable bearing pressure offered by weathered rock is calculated by Equation (4) :

Q ＝ 5N, [Eq. 4]

where Q is the allowable bearing pressure (kPa) and N is the SPT-N value (N>200) , thus

Q ＝ 5×200 ＝ 1000 kPa

In the example construction method referred to a “Steel tube as temporary container for replacement of material inside the pipe” or “Steel tube as temporary container for modification of material inside the tube” , the replacement or modified material should be able to support a total load of (8500+12000) ＝ 20, 500 tons. The replacement material can be designed with optimal sand-stone grading to satisfy the load and displacement requirements.

Lastly, the capacity of the founding layer 5 is determined. The area of the sand/stone column is 3.1416×10²/4 ＝ 78.54 m2. Thus, the total capacity of the founding layer 5 is:

Total capacity ＝ 4 (#of supports per platform) ×1000×78.54 ＝ 314, 160 kN.

Since 31, 416 tons > 20, 500 tons, the capacity is adequate.

The floater-supported platform 11 described herein offers several social-economic benefits. As the platform 11 rises above sea level 1, water flows freely under the platform 11； this can replace the conventional reclamation platform to provide substantially improved environmental protection, since the conventional reclamation platform totally kills the wetlands and destroys the breeding grounds of marine creatures. Additionally, the platform 11 described herein offers better economics since the conventional reclamation platform cannot be applied to deep water operations, whereas the example platform 11 may be configured for placement in water depths in a range of about 3-50m deep. Moreover, the floaters 21 are ideal as a fish shelter. The platform 11 can be manufactured in a factory in standard form, providing large-scale production possibilities that will lower its manufacturing cost even further.

The example embodiment having been described, it is apparent that such may have many varied applications. For example, the method of fixing the offshore platform 11 to the seabed 2 as disclosed herein is not limited to the specific example embodiment described above. Various changes and modifications thereof may be effected by one skilled in the art without departing from the spirit or scope of protection. For example, elements and/or features of different illustrative embodiments could be combined with each other and/or substituted for each other within the scope of this disclosure.

The present invention, in various embodiments, configurations, and aspects, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, sub-combinations, and subsets thereof. Those of skill in the art will understand how to make and use the present invention after understanding the present disclosure. The present invention, in various embodiments, configurations, and aspects, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments, configurations, or aspects hereof, including in the absence of such items as may have been used in previous devices or processes, e. g. , for improving performance, achieving ease and\or reducing cost of implementation.

The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the invention are grouped together in one or more embodiments, configurations, or aspects for the purpose of streamlining the disclosure. The features of the embodiments, configurations, or aspects of the invention may be combined in alternate embodiments, configurations, or aspects other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment, configuration, or aspect. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the invention.

Moreover, though the description of the invention has included description of one or more embodiments, configurations, or aspects and certain variations and modifications, other variations, combinations, and modifications are within the scope of the invention, e. g. , as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments, configurations, or aspects to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.

Claims

A construction method for fixing a foundation of an offshore platform adapted to support wind turbines, bridges and marine buildings thereon to a seabed, the platform having at least one floater, the at least one floater having a bottom slab embodied as a downward pointing bottom cone-shaped object, the method comprising:

casting a concrete bed in the seabed,

impressing the at least one floater into the concrete bed to form a mirror image, reversed cone-shaped indentation of the conic object in the concrete bed,

aligning the bottom cone of the at least one floater with the reversed cone-shaped indentation, and

lowering the platform to couple the bottom cone and reversed cone-shaped indentation together so as to fix the platform to the seabed.
The method of claim 1, wherein a steel mold with an exterior profile similar to the profile of the bottom cone is employed in lieu of the at least one floater to form the reversed cone-shaped indentation of the conic object in the concrete bed.
The method of 2, wherein impressing further includes:

adjusting a level and alignment of the at least one floater or of the steel mold and, prior to an initial setting of the concrete to form the concrete bed,

lowering the at least one floater or steel mold so as to completely immerse them in the concrete bed,

maintaining the level and alignment of the at least one floater or steel mold until the concrete is set, and

raising the at least one floater or steel mold so as to form the reversed cone-shaped indentation in the concrete bed.
The method of claim 3, wherein lowering the platform to couple the bottom cone to the reversed cone-shaped indentation further includes:

floating in the platform to an installation venue,

aligning the bottom cone with the reversed cone-shaped indentation,

lowering the platform so that the at least one floater seats naturally into the reversed cone-shaped indentation in the concrete bed to couple the two cones, and

injecting cement grout using a plurality of pre-installed pressure pipes in the at least one floater to fill a gap between opposing faces of the cones.
The method of claim 4, wherein casting further includes:

driving, to a given depth level in the seabed, a steel tube having a diameter larger than that of the at least one floater,

welding a plurality of layers of steel brackets on the steel tube around a level of the top face of the concrete bed,

removing soft materials from the soft soil layer within the steel tube,

backfilling the steel tube with sand,

cutting selected steel brackets that extend above the to-be-cast concrete bed,

placing a steel ring on a top layer of the steel brackets,

pouring concrete using a tremie concrete pipe to form the concrete bed,

impressing the at least one floater into the concrete bed to form the mirror image reversed cone-shaped indentation therein, and

cutting any surplus steel tube which extends above the concrete bed.
The method of claim 5, wherein

a penetration length of the steel tube is adapted to mobilize a skin friction present between a surface of the steel tube and the soft soil layer,

an allowable friction resistance is greater than the weight of the at least one floater or the total weight of the platform including construction loads,

the steel ring and steel brackets have a strength adapted to carry the weight of the platform and construction loads, the method further comprising:

installing a plurality of piles which are configured as a piled foundation having a pile head and pile cap reinforcement inside the at least one floater, the piles configured to carry all dead and live loads of the platform in a servicing period thereof.
The method of claim 6, wherein installing the plurality of piles includes:

boring through the bottom slab of the at least one floater to install a plurality of vertical piles and a plurality of raking piles,

pouring a layer of concrete to form a concrete plug in the bottom slab to terminate any ingress of water,

drying the at least one floater,

cutting any surplus pile length which extends above the at least one floater,

modifying the pile head of the foundation,

fixing the pile cap reinforcement, and

concreting the pile cap.
The method of claim 6, wherein installing a plurality of piles as a piled foundation further includes:

boring through the bottom slab of the at least one floater to install a plurality of vertical piles,

pouring a layer of concrete to form a concrete plug in the bottom slab to terminate any ingress of water,

drying the at least one floater,

cutting any surplus pile length which extends above the at least one floater,

modifying the pile head of the foundation,

fixing the pile cap reinforcement,

concreting the pile cap,

installing a plurality of raking piles from a deck level of the platform, and

casting the pile cap in the deck level.
The method as recited in claims 7 or 8, wherein installing a plurality of piles as a piled foundation further includes:

pre-installing steel casing for each pile between the bottom cone to the platform deck level, the steel casing having intermediate support from a wall of the at least one floater, and

boring through the bottom slab of the at least one floater to install the vertical piles.
The method of claim 5, wherein

a penetration length of the steel tube is adapted to mobilize a skin friction present between a surface of the steel tube and the soft soil layer,

an allowable friction resistance is greater than all load combinations from construction phase to service state,

a steel structure composed of the steel ring and steel brackets has a strength adapted to carry all load combinations from construction phase to service state, and

the steel structure has a design life not shorter than the platform and is configured to be corrosion protected.
The method of claim 4, wherein casting further includes:

driving, to a given depth level in the seabed, a steel tube with a diameter larger than that of the at least one floater,

excavating all materials within the steel tube,

backfilling the steel tube with graded sand-stone having an allowable bearing capacity that matches a load imparted by the platform,

pouring concrete using a tremie concrete pipe to form the concrete bed,

impressing the at least one floater into the concrete bed to form the mirror image reversed cone-shaped indentation therein, and

cutting any surplus steel tube extending above the concrete bed.
The method of claim 4, wherein casting further includes:

driving, to a given depth level in the seabed, a steel tube with a diameter larger than that of the at least one floater,

excavating any top layer materials to modify the interior of the steel tube,

backfilling the steel tube with graded sand-stone having an allowable bearing capacity that matches a load imparted by the platform,

carrying out soil improvement in soil strata contained within the steel tube,

pressure cementing the grout to fill voids so as to form a solid column inside the steel tube upon hardening thereof,

pouring concrete using a tremie concrete pipe to form the concrete bed,

impressing the at least one floater into the concrete bed to form the mirror image reversed cone-shaped indentation therein, and

cutting any surplus steel tube extending above the concrete bed.
The method as recited in any of claims 10 through 12, wherein

the platform is decommissioned before or at the end of service life,

disposal of the platform is initiated with dismantling of the platform’s superstructure,

a plurality of auxiliary floaters are used to lift the platform from the concrete bed so as to be reclaimed, and

a prospective new or refurbished platform is adapted to be re-installed in the same location.