WO2013153387A1

WO2013153387A1 - Foundation structures

Info

Publication number: WO2013153387A1
Application number: PCT/GB2013/050927
Authority: WO
Inventors: Hugh Bowerman; Richard SAWKO; Suren NADARAJAH; Alan Clucas
Original assignee: Laing O'rourke Plc
Priority date: 2012-04-13
Filing date: 2013-04-10
Publication date: 2013-10-17
Also published as: GB2501123B; GB201206568D0; GB2501123A

Abstract

A gravity base structure for use as a submerged foundation has a base slab (6) supporting a caisson designed to be filled with ballast. The base slab is surrounded by a porous foundation assembly provided by strip-like radial beams (20) extending away from the caisson to define voids for the reception of rock ballast and each having a footing (22) intended to rest on a sea bed.

Description

FOUNDATION STRUCTURES

Technical Field

The present invention relates to submerged foundation structures which are particularly, but not exclusively, suitable for supporting offshore wind turbines, and more specifically to gravity base structures.

Background Art

Gravity base structures (GBS) are used in the marine environment to support structures such as offshore wind turbines. GBS sit on the seabed and are typically ballasted down such that they have adequate mass to resist the vertical, sliding and overturning forces applied to them. The interface between the GBS and seabed will vary in nature according to the type of GBS structure and the type of seabed. Typically the interface comprises of a large diameter, flat, usually circular slab. The slab sits on the sea bed. Any voids between the slab and seabed are filled with grout.

Concrete caisson foundations of this type are described in PEIRE, KENNETH, NONNEMAN, Hendrik, et al, Gravity Base Foundations for the Thornton Bank Offshore Wind Farm, Terra et Acqua, June 2009, 115, 19-29

A perimeter skirt is often provided which digs into the sea bed, containing the grout and boosting the soil bearing pressure.

With typical GBS there are various problems that need to be addressed:

Seabed surface bearing capacity is often too low to support the GBS. To deal with this and increase bearing capacity, the surface layer may be removed by dredging and replaced with granular backfill;
As the GBS base slab approaches the seabed, there is considerable horizontal out flow of water trapped between the slab and the seabed. The velocity of outflow is such that the seabed can be locally scoured making it uneven;
Flat slabs are heavy. The need to reduce overall GBS weight prior to ballasting (for installation purposes) results in a minimisation of slab diameter. This forces up the bearing capacity required from the seabed, limiting the range of application;
Large diameter flat slabs attract hydrodynamic loading, making handling difficult during marine operations.

EP

2236676

A TIEFBAU GMBH 20101006

is an example of a submerged foundation using a large caisson designed to be filled with ballast and having a large solid base which rests on the sea bed. It shows a caisson in the form of a tank which is internally sub-divided to encourage uniform distribution of the ballast.

ES

2316211

A TORRES 20060109

shows another submerged foundation structure in which a narrow concrete annulus rests on the seabed and connects to a raised central column by means of arched beams.

The existing solutions mentioned above to overcoming the above problems add complexity to the marine operations. This adds both direct costs (extra operations) and indirect costs in the form of time related risk. The present invention aims to overcome these restrictions, extending the range of application of GBS.

Summary of invention

The present invention provides a structure for use as a submerged foundation, comprising a base slab supporting a caisson designed to be filled with ballast; characterised in that the base slab is surrounded by a porous foundation plate intended to rest on a sea bed and comprising interconnected spaced beams.

By eliminating a solid base slab for all but a central caisson the problems of the large bases of the prior art are eliminated as the porous foundation plate provides less resistance to the water during installation as it contains voids between its solid surfaces so that it does not act like a solid plate when being moved through the water. However, when such a porous foundation of interconnected beams is resting on the sea bed it responds structurally as a solid plate. Accordingly, by appropriate use of hydrodynamics, good ground support can be provided without suffering undue resistance during installation.

Preferably the porous foundation plate is provided by spaced beams connected to and extending away from the caisson and each having a footing intended to rest on a sea bed to provide the effect of a solid plate relative to soil when on the sea bed.

Preferably the base slab is positioned above the level of the radial beam footings. By positioning the residual base slightly above the lowermost surfaces of the surrounding radial beams, this prevents the base slab coming to rest on a localised high point in the sea bed.

Preferably the remote ends of the beams are connected to a perimeter beam in order to retain ballast in spaces between the radial beams.

Other advantageous features of the invention are set out in the claims.

Brief description of drawings

In order that the invention may be well understood, an embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:

Figure 1 is a vertical sectional view of a foundation structure in accordance with the invention;

Figure 2 is a plan view of the structure of Figure 1;

Figure 3 shows a section through two radial beams of the structure of Figure 1; and

Figure 4 is a section through a portion of the structure taken through the radial beams after the structure has been placed on the sea bed and voids between the beams filled with ballast in order to illustrate the design of the width and spacing of the footings in dependence on the sea bed bearing conditions.

Description of embodiment

The gravity base foundation structure 2 shown in Figures 1 and 2 is a concrete structure having a cylindrical column 4 which passes through a base slab 6. A cone 8 surrounds the column and is joined to a periphery of the base slab 6 by a vertical wall 10. The column 4 has an upper part 12 which projects above the cone 8, a lower part 14 beneath the base slab and an intermediate part 16 above the base slab to a junction with the cone 8. Although the column is shown as cylindrical, it will be appreciated that other cross-sections may be used.

The lower part 14 is surrounded by a porous foundation assembly or plate constructed from a plurality of strip- type beams 20 connected to the lower part 14 of the column and extending radially away from the column. Each of the beams 20 has a broader footing 22 at its base as shown in Figure 3 designed to rest on the sea bed. The beams 20 are connected at their remote ends 24 to a perimeter beam 30. This perimeter beam is shown as being circular but it will be appreciated that other cross-sections, such as straight-sided polygonal shapes may be used. As well as the annular porous foundation illustrated it could take a cruciform, square or other shape and/or be made of a grillage. A slotted foundation plate could also be used.

The base slab 6, cone 8, wall 10 and column part 16 form the outer boundaries of a volume or caisson, which is totally sealed from water ingress. Note that in an alternative design in which a wall of column part 16 is not solid, the upper part of the column may be included within the caisson. Said volume defines a buoyancy tank, which is filled with water or sand infill to act as ballast to hold the structure 2 on the sea bed after it has been positioned. The column may also be filled with a ballast of sand or water.

The radial beams 20 are arranged like the spokes of a wheel. Between each pair of beams is a void 32. Figure 3 shows a section through two adjacent radial beams. Each beam 20 has a footing 22 of breadth B. Radial beams 20 are spaced at a distance S at their remote ends. The porosity of the foundation is defined by the ratio (S-B)/S. Breadth B and porosity are adjusted to suit the soil conditions. The volume of the voids 32 between the beams which can be used to add ballast 34 after the structure has been placed on the sea bed relative to the total ballast volume of the structure, is a function of seabed conditions, but would typically be in the order of 60%. The voids 32 can be considered porous to water, but impervious to soil.

The height H of beams 20 is designed to take the bending moments and shear forces imposed by the seabed soil bearing on footing 22. These forces increase towards the intercept with the caisson wall 10, hence H also increases. This results in a sloping upper surface 26 to beams 20.

Figure 4 shows another sectional view through the radial beams 20. Under each footing 22 is shown a bulb 40 defining a failure envelope associated with a soil type. An intended feature of the design of the structure is that the bulbs 40 overlap or abut as indicated at 42. In this situation the foundation acts as if it were a solid slab, i.e., global GBS failure under an overturning moment can be determined assuming the foundation is a solid slab.

Figure 4 also shows the voids between radial beams 1 being backfilled with a ballast 34 of rock. This contributes to the scour protection. A proportion of the rock mass also acts on the top of the footings 22 via rock arching 44. Perimeter beam 30 acts to contain the rock to stop it spreading beyond the structure.

The proportions and dimensions of the caisson and radial beams 20, and the densities of the materials used, are selected to satisfy the following criteria:

The centre of gravity of the GBS lies below the centre of buoyancy of the GBS. This enables the GBS to float in a stable manner;
The volume enclosed by the caisson is sufficiently large that, when filled with sand and combined with any contributing rock ballast 34, the net weight of the GBS provides the necessary over-turning and sliding resistance to resist the applied forces;
The caisson and column have sufficient capacity (thickness; material strength; buckling stability) to resist the external hydrostatic pressure generated at highest astronomical tide when the GBS is sitting on the seabed;
The caisson and column 4 have sufficient capacity (thickness; material strength; tensile) to resist internal pressures resulting from saturated ballast fill to the top of upper cylinder 12 with external water level at lowest astronomical tide;
Over-turning moments and shear forces arising from the attached super-structure combined with wave and current loading from the GBS can be distributed from cone 8 into radial beams 20 and thence into the seabed;

Further Features and Advantages

The voids between the beams 20 allow water to flow through. This reduces the effect of hydrodynamic loading whilst the GBS is towed to location. It also significantly eliminates the wash-out that may occur with a solid foundation as the GBS approaches the sea-bed.

The porosity of the GBS base permits a larger diameter base to be made for a given weight. As base diameter increases, the required soil strength for a given overturning moment reduces. Thus the structure of the invention enables GBS to be used on weaker soils than an equivalent weight solid slab GBS.

When placed on an uneven seabed, the foundation may sit on the locally high areas. It is anticipated that only one or two radial beams will be in contact. Due to the concentration of pressure at these local areas, there will be a local failure of the seabed until adjacent radial beams come into contact with the seabed and themselves apply sufficient bearing pressure to (i) reduce the load on the initially contacting beams and (ii) bring about the bearing conditions illustrated in Figure 4.

If the solid central part of the foundation were to sit on a locally high area, the whole foundations could pivot about this point. To prevent this happening, the central base slab area is raised relative to the underside of the radial beams, see Figure 1.

Raising the base slab 6 also enables water to flow laterally under the base of the structure. This helps to prevent wash-out of non-cohesive surface deposits.

The foundation is designed to operate without requiring a grouting operation under the base. Such grouting is an additional offshore operation. During de-commissioning such grout may bond to the underside of the GBS preventing re-floating.

The use of dense aggregate such as iron ore rock to fill the voids between the beams can result in enough on-bottom weight to make it unnecessary to fill the cone with anything other than water. Since rock dumping is a required activity for scour protection, this saves an additional offshore operation. If the inside of the structure is only filled with water, it also permits inspection by remotely operated vehicles (ROV) of all the critical internal surfaces of the foundation.

An alternative to the use of a dense aggregate rock fill 34 is to use a greater depth of ordinary density rock, but to pile this up over the foundation;

The strip beams 20 would advantageously be constructed from pre-stressed concrete. The proportions of the beams are similar to deep bridge beams, hence the technology is developed and understood for developing these elements.

The cone 8 and wall 10 could advantageously be made from pre-cast 'petals’. These would be assembled into position and post-tensioned together. The use of a ‘blind’ coupler at the base of strands used for post-tensioning prevents this caisson anchor being subject to seawater exposure. Petals could either be bi-linear in section, as shown in Figure 1, or curved;

The upper part 12 of the column 4 may be made of pre-cast concrete ring segments post-tensioned together.

In general the upper parts of the GBS would be made using a light weight aggregate concrete with the lower parts made using normal weight aggregate concrete. This helps ensure the centre of gravity lies below the centre of buoyancy

The upper part 12 of the column 4 may be made from a steel tube to reduce the upper weight and lower the centre of gravity.

The lower part 16 of column 4 may be made up of column sections instead of being a solid tube, the space between the column sections allowing any sand ballast to pass to the lower, outer volume of the caisson.

Decommissioning of the foundation requires only removing any internal ballast, removing the external rock dump and pumping out the water within the buoyancy tank. The foundation can thus be re-floated and towed to an appropriate site for disposal or re-use.

Claims

A structure (2) for use as a submerged foundation, comprising a base slab (6) supporting a caisson (8, 10) designed to be filled with ballast; characterised in that the base slab (6) is surrounded by a porous foundation plate (14, 20, 30) intended to rest on a sea bed and comprising interconnected spaced beams (20).
A structure as claimed in claim 1, wherein the porous foundation plate comprises beams (20) connected to and extending away from the caisson and each having a footing (22) intended to rest on a sea bed.
A structure as claimed in claim 2, wherein the base slab (6) is positioned above the level of the beam footings (22).
A structure as claimed in any one of the preceding claims, wherein the remote ends of the beams are connected to a perimeter beam (30) in order to retain ballast in spaces (32) between the beams (20).
A structure as a claimed in any one of the preceding claims, wherein a height (26) of the beams increases towards an intercept with the caisson.
A structure as a claimed in any one of the preceding claims, wherein a width of the footings (22) of the beams is designed in dependence on a soil type at the sea bed, such that a failure envelope (40) beneath each beam abuts or overlaps with that of each adjacent beam so that the foundation plate responds as if it were a solid slab.