SEABED BASE STRUCTURE AND METHOD FOR INSTALLATION OF SAME

Abstract

This publication relates to a shallow water terminal, preferably for storing and loading or unloading hydrocarbons, such as LNG, oil or gas. The base structure comprises a floatable, and removable seabed substructure (10) intended to be supported by a seabed (30), the seabed substructure (10) comprising a base structure (11) provided preferably with an upwards extending wall structure (22), arranged along at least a part of the periphery of the base structure (11), the base structure (10) preferably also being provided with an opening (23) in the wall structure (22) for allowing the floatable module to be berthed in and supported by the seabed substructure (10). The base structure (10) is provided with strong points (24) configured to receive the ends of preinstalled vertical piles (14) for at least temporary support of the base structure (11) during a piling operation for permanent piling of the base structure (10) to the sea bed (30). The publication also relates to a method for piling a base structure on or above a seabed (30).

Claims

1. A shallow water base structure (10), preferably for storing and loading or unloading hydrocarbons, such as LNG, oil or gas, comprising a floatable, and removable seabed substructure (10) intended to be supported by a seabed (30), the seabed substructure (10) comprising a beam structure (11) provided with an upwards extending wall structure (22), arranged along at least a part of the periphery of the base structure (10), characterized in that the upwards extending side walls being provided with buoyancy devices, the base structure (10) being provided with an opening (23) in the wall structure (22) for allowing a floatable module to be berthed in and supported by the seabed substructure (10), the base structure (10) being provided with strong points (24) configured to receive the ends of preinstalled vertical piles (14) for at least temporary support of the base structure (10) during a piling operation for permanent piling of the base structure (10) to the sea bed (30), the strong points (24) being formed by elements extending laterally outwards from the base structure (10).

2. A shallow water base structure (10) according to claim 1, wherein the strongpoints are positioned above sea level (29).

3. A seabed base structure (10) according to claim 1, wherein the strong points (24) are positioned below sea level (29).

4. A seabed base structure (10) according to claim 1, wherein the strong points (24) are arranged on the lower side of beams, cantilevers or sleeves or ducts extending laterally out from the wall(s) (22), preferably above the sea level (29).

5. A seabed base structure (10) according to claim 1, wherein the strongpoints (24) are provided with releasable locking devices for temporarily locking the upper part of a preinstalled pile (14) in fixed position.

6. A seabed base structure (10) according to claim 1, wherein the wall structure (22) is an integrated part of the base structure (10) and that the strongpoint form an integrated part of the base structure (10) or the wall structure (22).

7. A method for installing a base structure (10), according to claim 1, on a seabed (30), the base structure being configured to be supported by the seabed (30) in a piled manner using a number of piles driven into the seabed (30), characterized in that at least two rows of piles (14) are driven into the seabed (30), the distance between the two rows (13,13) and the distance between adjacent piles (14) in a row (13,13) in each being configured to correspond with purpose built strong points (24) on the base structure, whereupon the base structure (10) is towed in between the two rows (13,13) of piles and brought to a position where the strong point (24) are vertically aligned with a corresponding upper pile end, whereupon the base structure (10) is ballasted so the base structure (10) will rest stably on the various piles (14), whereupon the base structure (10) is piled to the seabed (30).

8. Method according to claim 7, wherein the base structure (10) is piled to the sea bed (30) using a number of permanent piles driven into the seabed (30), the top of the piles being rigidly fixed to the base structure (10).

9. Method according to claim 7, wherein the piles (14) supporting the base structure (10) stably and rigidly during the pile operation are removed upon completed process of permanent piling of the base structure (10).

10. Method according to claim 9, wherein the provisional or temporary piles (14) are cut off at the seabed level.

11. Method according to claim 7, wherein the base structure (10) is provided with ballast tanks, using water to adjust weight and buoyancy and the vertical forces and load exposures acting on the temporary piles (14) during installation of the base structure (10).

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0065] One embodiment of the method according to the invention shall be discloses in more detail in the following description below with reference to the enclosed figures, wherein:

[0066] FIG. 1 shows schematically a first stage of the installation procedure, where two rows of aligned piles in spaced relation are established;

[0067] FIG. 2 shows schematically a base structure to be supported by the piles is towed into position between the two spaced apart aligned rows of piles by a towing vessel;

[0068] FIG. 3 shows schematically in perspective seen from below an embodiment of a base structure according to the present invention;

[0069] FIG. 4 shows schematically in perspective an embodiment of the base structure positioned and supported by the piles in aligned position on at least both sides of the base structure;

[0070] FIG. 5 shows schematically and in perspective the base structure in position with a supply vessel moored along one side of the installed base structure; and

[0071] FIG. 6 shows schematically an alternative position of the strong points.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0072] The following description of the exemplary embodiment refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a method for installation of a base structure on a seabed in general and preferably, but not necessarily on a sloped seabed and/or on a seabed with a low bearing capacity.

[0073] Reference throughout the specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases in one embodiment or in an embodiment in various places throughout the specification is not necessarily referring to the same embodiment.

[0074] The key area for the invention is to provide a quick and safe installation of the storage module with topside equipment where the base structure is stably and rigidly supported during the piling operation of the permanent piles. This is the costly part (90-95%) of the entire installation. By having a pre-installed base foundation, which is stabilized at least by means of piles and leveled in advance to the seabed, then the installation of the storage module can take place within a few hours.

[0075] In addition, the present invention offers the possibility of establishing a seabed terminal on different soil conditions. The density, composition, consolidation and topography of seabed soil may vary significantly for one seabed location to another. This will have direct impact on the load bearing capacity of the seabed soil, and hence the possibility to find a predictable and reliable foundation solution for a seabed structure which shall be supported by the seabed. According to one embodiment, the based foundation may be in the form of a semi-submersible floating body, piled to the seabed. In this case the base substructure can be ballasted as a semi submersible structure and piled to the seabed through the base structure and possibly, but not necessary, the wall structure of the seabed substructure. It is important in these cases to have an efficient transfer of vertical structural forces, it is an advantage that the main structural beams of the base structure and the storage module has mirrored structural interfaces. This means that vertical forces from the bulkheads storage module are preferably transferred directly into the main structural beams of the base structure and into the piling structure and to the seabed. Tests has shown that the piled seabed substructure must tolerate and stand a weight of 100 000-120 000 tons.

[0076] In the Figures, a base structure having upwards extending walls along three sides of the base structure is shown, allowing float in of a floated at one end of the base structure, the upwards extending walls on the remaining sides protecting the floater on the remaining sides while berthed within the U-shaped base structure. It should be appreciated, however, that the base structure may be provided with two opposite open ends, the floater being for example in the form of two more or less parallel vertical wall sections provided with buoyancy, interconnected by a base slab or for example two or more laterally extending box beams interconnecting the two vertical wall sections at their lower end. Such configuration will allow float-in of the floater at either end of the base structure.

[0077] FIG. 1 shows schematically a first stage of the installation procedure, where two rows 13,13 of aligned piles 14 are arranged, the last pile in the row 13 being in process of being forced into the seabed 30 by means of a piling barge 15 with a crane 16 and a pile driving device 17 suspended from the crane 16. During this stage the barge 15 may be moored by means of conventional seabed 30 anchors (not shown) and mooring lines 18 (two of which being shown).

[0078] FIG. 2 shows schematically a base structure 10 being towed into position between the two rows 13,13 of aligned piles to be supported by the piles is towed into position between the two spaced apart aligned rows of piles by a towing vessel 19 and a pair of towing lines 20. The base structure 10 is provided with an outwards projecting cantilever 21,21 extending outwards from the top of the base structure along two parallel top sides, each cantilever 21,21 being configured to rest on top of a corresponding row 13,13 of piles 14. For such purpose the cantilevers 21,21 are provided with strong points 24 (not shown in FIG. 2) dimensioned and configured to carry the weight of the base structure 10 and possible temporarily appearing loads, forces and bending moments introduce at least during the installation stage of the base structure 10 until the base structure is safely piled to the seabed.

[0079] The seabed base structure 10 comprises a inwards projecting cantilever and/or beam structure 11 and upwards extending wall structures 22 arranged along at least a part of the periphery of the base structure 10. The wall structure 22 forms an integrated part of the cantilever and/or beam structure 11, together forming a seabed base structure 10. Both the cantilever and/or beam structure 11 and wall structure 22 are provided with buoyancy devices (not shown). Such buoyancy means may be in the form of tanks and compartments in the cantilever and/or beam structure 11 and in the upwards extending wall structure 22. The embodiment of the seabed substructure 10 shown in FIG. 1 is provided with a bottom beam structure in longitudinal and transverse direction, forming upwards open compartments in the base structure 10 between the cantilevers 11/beams. The compartments may be closed at the lower end by a bottom slab or the compartments may optionally be open downwards, providing access to the permanent piles (not shown) in case the base structure 10 is in an elevated position more or less above the seabed 30. Said longitudinal and transverse beams or walls may serve as a supporting, strengthen surface for supporting a floatable storage module to be floated in between the upwards extending wall structure 22, over the base structure 10 and ballasted to rest on said surface. The upwards extending wall 22 extends along three sides of the base structure 10 and is provided with an opening 23 in the wall structure for introducing a floatable storage module (not shown in FIG. 2) in over the base structure 10. The storage module may be removable arranged on top of the base structure 10/internal cantilever 11 and possibly the beams within the wall structure, together forming a seabed unit.

[0080] The seabed base structure 10 is provided with floating buoyancy and has means for ballasting (not shown) and is intended to be placed on or just above the seabed 30, supported by a number of permanent (not shown in FIG. 2) or optionally, also resting on the seabed partly due to gravity, fixed by means of said permanent piles. The upward extending wall structure 22 of the substructure 10 has perforations or ducts/sleeves through the wall structure for optional and/or additional piling, and also there are perforations in the base structure 11 for receipt of the permanent piles, intended to be driven into the seabed soil. The ducts and accessories for receiving the piles are described in the PCT/NO2015/050156, which hereby is incorporated by the reference and will thus not be described in further detail. A vessel 16 with machines and tools for piling are moored next to the wall structure 2, similar to the one disclosed in FIG. 1 may be used for the piling operations. As indicated in Figure, permanent piles may be arranged both in longitudinal and transverse direction along the foot of the three walls 22 along the submerged front beam beneath the opening of the cantilever and/or beam structure 11, and along the internal cantilever 11 and/or walls or beams forming the upwards open compartments there between. In such way the entire footprint or at least parts of the footprint may be provided with permanent piles for supporting the base structure 10 properly and safely. The number of piles used and their position, diameter and length depend on the weight to be supported and on the seabed soil condition.

[0081] An advantage according to the present invention is that the seabed base structure 10, constituting a part of the seabed unit for floating modules, such as a floatable LNG storage unit or barge according to the invention, can be lowered down to installed offshore or near shore, be removed, be moved and be replaced to form new individual configurations as required using known techniques.

[0082] FIG. 3 shows schematically in perspective seen from below an embodiment of a base structure 10 according to the present invention. As shown the lower side of the cantilevers 21,21 are provided with strong points 24 configured, designed and dimensioned to receive the upper ends of the temporary piles 14, supporting the base structure at least until a sufficient number of permanent piles are piled through the ducts 25 in the inwards projecting cantilever and/or beams 11 and fixed to said parts. As shown in FIG. 3, the upwards projecting walls 22 are interconnected by beams 26 forming upwards open cells 27 without a top or a bottom slab, configured together with the cantilevers 11 to support a floating unit, configured to be de-ballasted and to rest on said parts of the base structure 10.

[0083] Along the outer edge of the upper, outwards projecting cantilever devices 28 serving as fenders may be arranged, fendering between the cantilever and the side of a vessel to be moored along the side of the base structure.

[0084] FIG. 4 shows schematically in perspective an embodiment of the base structure 10 positioned and supported by the temporary piles 14 in aligned position along at least both sides of the base structure 10. The permanent piles may now be installed by forcing the piles through the ducts 25 down into the seabed 30 to sufficient depth for stably supporting the base structure temporarily. The base structure 10 may be permanently fixed to the seabed 30 by said permanent piles, while the base structure 10 is stably fixed in position and supported by means of the rows the temporary piles 14. As indicated in FIG. 6, showing schematically an alternative position of the strong points 24, the strongpoint is made as an integrated part of the vertical walls 22, projecting laterally outwards from the wall 22 and may be positioned either above or below the sea level 29.

[0085] FIG. 5 shows schematically and in perspective the base structure 10 in position with a supply vessel 30 moored along one side of the installed base structure 10. The Figure shows a stage where the base structure is resting firmly on the temporary piles 14o its own weight and possibly any additional weight due to any ballast water, the weight being sufficiently larger than the buoyancy of the base structure. At such stage the process of establishing the permanent piling system as further disclosed in PCT/NO2015/050156 may start, said publication being included by the reference with respect to the build-up of the permanent piling arrangement and the method for establishing a proper piling of the base structure.

[0086] As shown in FIG. 5 piling operations of the permanent piles may be performed more or less simultaneously by means of a piling barge 15, similar to the one disclosed in FIG. 1 and by movable cranes 31 with associated piling devices 17, for example similar to the one disclosed in FIG. 1.

[0087] Upon completed piling operation of the two parallel rows 13,13 of the temporary piles 14, a base structure 10 is towed into position between the two rows 13.13 by means of a towing vessel 19 until the strong points 24 along the lower surface of the outwards projecting cantilevers 21,21 are in aligned position above the corresponding temporary piles 14, whereupon the base structure 10 is ballasted so that the base structure 10 is lowered down onto a respective pile 14 and exerting a downwards acting force or weight on to the piles 14, the piles taking more or less the total vertical weight of the base structure 10. Each strong point may have a recess sufficiently deep to allow an upper part of the pile ends to be inserted into said recess. The strong points may also be provided with a releasable locking mechanism for temporarily locking the joint between the upper end of the pile 14 and the strong point 24.

[0088] Once the base structure is sufficiently secured and fixed in a proper position, the permanent piling operation for example all in accordance with the method, system and arrangement described in PCT/NO2015/050156 may be initiated. Upon completed permanent piling operation, the temporary piles may be cut off, for example at seabed level or at a depth where the pile ends do not represent any hazard to the operation of the piled base structure 10.

[0089] The base structure 10 is provided with a system (not shown) for ballasting and is preferably made from steel, although also other materials can also be used such as concrete. It should be appreciated that the storage module 10 according to the present invention also may be provided with means, such as loading systems, cranes, winches etc. on top of the storage module. When the storage module arrives at the site, it is mated with the seabed substructure or base structure 10. During this mating operation, the floating module is manoeuvred in through the opening at one end of the base structure and in between the two parallel upwards extending side wall structures 22. The floating storage module is guided on top of the base structure 10, within the wall structure 22. The floating module is the ballasted so that it rests stably on the base of the seabed substructure 10, forming a seabed assembled unit.

[0090] The permanent pipe arrangement for ballasting and for rigidly fixing the piles to the base structure 10 may be of a type as described in PCT/NO2015/050156, the part of said application relating to the pile system hereby being incorporated by the reference. Once a permanent pile is driven into its intended depth in the seabed soil, a annulus between the external surface of the pile and the surface of the duct wall may be grouted by injecting grout from a grout producing plant (not shown) through a grout supply line. Said grout supply line may have its outlet at the lower end of the duct. As a consequence of such outlet position, injected grout from the supply line will be pressed upwards through the annulus until the injected grout exits at the top of the duct. In order to prevent the grout from being forced downwards and oy of the annulus and into the interface between the lower surface of the bottom plate of the base structure and the seabed 30, a ring formed stopping seal is arranged, having contact surface against the outer surface of the pile around its entire circumference. The stopping seal may be in the form of a circular hose with cylindrical cross section, or as a semi-circular body, both free ends of the semi-circular body being sealing fixed to the surface of the duct, extending around the entire circumference of the duct, providing a fluid tight seal. The interior void of the seal is fluid contact with a pressurized source (not shown) through a fluid supply line, securing supply of a pressurized fluid to the interior of the seal at the start-up of the grouting process, causing the stopping seal to expand, and possibly relieving the fluid pressure upon completed grouting process.

[0091] According to one embodiment of the invention, sixty one permanent piles having a diameter of 2.2 m and e length of 50 m are required in order to sustain the maximum environmental design loads. These piles are inclined with a 5 angle from the vertical in order to reduce the ground effect. In this context, it should be appreciated that where piles supporting the base structure are positioned close to each other a simple and conservative approach mat be to reduce the oiling capacity to approximately of a single pile capacity, when considering load cases.

[0092] It should be appreciated that the piles may extend vertically down into the seabed 30 or, they may be arranged inclined with respect to the vertical, either in same direction, inwards or outwards, or a combination of the same.

[0093] The seabed substructure may also be provided with a harbour section, configured for allowing vessels to moor alongside the harbour section. The construction material may be concrete or steel or a combination of both. The harbour section is fixed to and built into at least one of the vertically extending walls, so that all forces and loads is taken by the seabed substructure and transferred to the piles. Moreover, the harbour section may preferably be arranged on the opposite side(s) of the prevailing direction of wind and/or waves, providing a shelter for the vessel(s) moored along the harbour section.