US20040141811A1

US20040141811A1 - Ballast deployment apparatus and method for installing and retrieving said apparatus

Info

Publication number: US20040141811A1
Application number: US10/348,141
Authority: US
Inventors: Karel Karal
Original assignee: ABB Offshore Systems AS
Current assignee: Aibel AS
Priority date: 2003-01-20
Filing date: 2003-01-20
Publication date: 2004-07-22

Abstract

An apparatus for controlled deployment of ballast material onto a subsea location, preferably a subsea structure, comprising at least one chute for conveying said ballast material, said chute having a first end and a second end; at least one buoyancy element associated with the chute such that the chute may assume a generally vertical orientation in the water, supported by said buoyancy element; a support element associated with said chute and adapted for cooperation with an installation weight, said chute second end being adapted for connection to an element associated with the seabed. A method for installation of said apparatus comprising bringing the chute into a generally vertical orientation in the water above said subsea location, lowering portions of said installation weight onto said support element, causing said ballast deployment apparatus to move towards the seabed, until said apparatus has reached a desired depth in the water; connecting said chute second end to said element associated with the seabed; and withdrawing said installation weight from said support element, whereupon deployment of ballast material through said chute may commence.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of subsea operations. More specifically, the invention relates to an apparatus for the deployment of ballast material to a subsea location, preferably onto a structure or structures on the seabed, and a method for installing and retrieving said apparatus.

2. Description of the Related Art

Installation of structures on the seabed, such as for example laying of subsea pipelines in connection with exploitation of hydrocarbons, often require deployment of ballast materials onto these structures. These structures may require ballast materials, which in most cases comprise rocks or gravel, for a number of reasons, such as increasing the weight (and thus stability), preventing scour, enhancing the seabed capacity, providing a permeable layer, and/or for mitigating environmental impacts on shipping and fisheries. One particular application of ballast materials is to cover pipelines on the seabed in order to protect them and increase stability. One problem associated with deployment of ballast materials, is to accurately control that it is deposited in the desired location.

Deployment of ballast material is today normally accomplished from vessels dumping the material directly into the sea, or (in deeper waters) through chutes in order to better control where the ballast material actually lands. Known techniques and equipment have practical limits. Today's chutes are not practical for very deep waters, as the chute's free end is susceptible to moving considerably due to water currents and other perturbations and due to the considerable length and weight of such chutes. The known chute therefore limits the depth to which this method is feasible or economical.

In order to ensure accurate deployment of ballast materials in very deep waters, one of today's techniques is to lower the ballast material in large “baskets,” which is a time-consuming and costly operation, and often requires a separate vessel in addition to the ballast deployment vessel.

Thus, there is a need for a method and an apparatus which extend the depth limits without a significant cost increase and allows the use of less specialized vessels to perform the operation, thus also reducing the associated costs.

The present invention solves that need, in that it provides a novel apparatus for the deployment of ballast material to the seabed, and an associated installation and retrieval method. Brief Summary Of Certain Inventive Aspects

These and other objects and features of the invention are provided by an apparatus for controlled deployment of ballast material to a subsea location, preferably onto a subsea structure, comprising

at least one chute for conveying said ballast material, said chute having a first end and a second end;

at least one buoyancy element associated with the chute such that the chute may assume a generally vertical orientation in the water, supported by said buoyancy element; and

a support element associated with said chute and adapted for cooperation with an installation weight.

Alternative embodiments of the invented apparatus are defined by dependent claims 2-12.

The method for installing the apparatus according to the invention, comprises:

bringing the chute into a generally vertical orientation in the water above the subsea location, said first end being on or near the surface of the water and said second end pointing towards the subsea location;

lowering portions of said installation weight onto said support element, causing said ballast deployment apparatus to move towards the seabed, until said apparatus has reached a desired depth in the water;

connecting said chute second end to said element associated with the seabed; and

withdrawing said installation weight from said support element,

whereupon deployment of ballast material through said chute may commence.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in more detail, with reference to the accompanying schematic drawings, where like parts have been given like reference numbers. [0020]
FIG. 1 is a sectional drawing, showing the principles of the invented apparatus during deployment of ballast materials; [0021]
FIG. 2 is a sectional drawing, showing the invented apparatus used in a combination with a standard ballast deployment equipment, such that the depth is extended beyond present limits; [0022]
FIG. 3 shows the apparatus being transported to an offshore installation site; [0023]
FIG. 4 shows a cross-section of the chute in an optional arrangement that is introduced for transport/tow under special conditions (e.g. extensive wave action, long towing distances); [0024]
FIG. 5 is an illustration of the first stage of installing the invented chute; [0025]
FIGS. 6, 7 and [0026] 8 are sectional drawings, showing the installation of the apparatus in deep water;
FIG. 9 shows a typical chute joint (optional); [0027]
FIG. 10 is essentially similar to FIG. 5, but illustrates the chute having one joint. [0028]

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

FIG. 1 shows one embodiment of the invented [0029] ballast deployment apparatus 1 in an installed position over a structure 3 on a subsea location on the bed 4 under the surface 19 of a body of water, in the process of deploying ballast material 2 on the structure 3. The ballast deployment apparatus 1 comprises a chute 5 and any number of buoyancy elements 6. The chute 5 has a first end and a second end. For illustration purposes, the chute 5 is shown displayed with a “cut line” to give an impression that the chute length normally is considerably greater than what is illustrated. Such “cut lines” also appear in FIGS. 2, 6, 7 and 8. The first end, which is the upper end when the apparatus is in the installed position, may have a funnel shape in order to ease the filling of ballast material 2 into the chute. The buoyancy elements 6 are generally attached near the chute's first end, may be of any number and have any suitable shape. In the described embodiment, only two buoyancy elements are shown for clarity of illustration.
The overall buoyancy of the [0030] buoyancy elements 6 is greater that the overall submerged weight of the ballast deployment apparatus 1, so that the excessive buoyancy can be applied for station-keeping of the apparatus, in an alternative embodiment of the invention, as shown in FIG. 2. In this embodiment, the station-keeping at the desired depth is achieved by a tether 7 that connects the apparatus 1 to a structure 3 on the seabed location. The tether 7 is preferably attached to or near the chute's second end, in a known fashion and method to be described below. The connection between the chute and the buoyancy element is moment free in the vertical plane so that upending of the buoyancy element, if desirable, is achieved by weight from the chute only. This is achieved by hinging the chute to the buoyancy element in a point distant from the center of buoyancy of the element in its initial position, i.e. before upending.
FIG. 2 also illustrates the use of the invented [0031] ballast deployment apparatus 1 in combination with a standard ballasting (rock dumping) vessel 8 equipped with a standard chute 9 that conveys ballast material 2 into the chute 5. This arrangement allows using conventional equipment and reaching deep waters.
FIG. 3 is an illustration of how the [0032] ballast deployment apparatus 1 may be towed to an offshore installation site by a tug 10 and a towline 20. In this position, the chute 5 is sealed at both ends by suitable end caps (not shown). Water is thus prevented to ingress the chute interior and the entire chute remains buoyant for the transportation.
It is also in principle conceivable to transport such chute on a reel aboard a vessel, and deploy the chute by conventional off-reeling techniques. [0033]
FIG. 4 shows an optional arrangement in which at least one [0034] buoyancy pipe 11 is fixed to the ballast deployment apparatus 1 and its chute 5 by known means; e.g. straps 12. In this arrangement the chute 5 is not sealed and thus in all phases flooded and the buoyancy pipe 11 is gas-filled (e.g. air) during tow.
The [0035] buoyancy pipe 11 may be opened and closed selectively, such that displacing of water from the pipe is feasible. Hence, positive buoyancy of the apparatus can be regained, resulting in emerging of the apparatus to the surface and assuming a horizontal position suitable for removal of the apparatus upon completion of the ballasting operation.
In a case when the [0036] buoyancy pipe 11 is not used, such as in a recovery scenario, the apparatus is towed between two tugs where the towline from the leading tug is connected to the lower end of the chute while the towing line from the trailing tug is connected to the upper end. In another scenario, if no re-use is planned, or in a case of a short/light chute, a chute with hinge(s) on the apparatus is towed by one tug with the towline connected to the lower end.
The [0037] buoyancy pipe 11 may extend the entire length of the chute 5 as one piece, or may comprise a number of buoyancy pipes 11 fixed to the chute 5 at given intervals.
FIG. 5 shows upending of the [0038] ballast deployment apparatus 1 as shown in FIGS. 1 and 2, for example, as the first stage of its installation. The chute 5 is opened at both ends (e.g. by removing said end caps), whereby the chute 5 is flooded by water. The ballast deployment apparatus 1 gradually rotates about a support (hinge) on the buoyancy elements 6, which remain floating (on or near the water surface) and take the submerged weight of the chute 5. In the final equilibrium position the ballast deployment apparatus 1 is generally vertical, free-hanging down into the water.
FIGS. 6, 7 and [0039] 8 illustrate further steps of the installation procedure in the embodiment where the deployment apparatus is to be installed at a certain depth.
In FIG. 6, the [0040] ballast deployment apparatus 1 is hanging freely in the water, generally in a vertical orientation. An installation weight (e.g. a chain) 14 is being lowered into the chute. At a convenient location in the chute 5, between the buoyancy elements 6 and the chute's second end, a support element 13 is installed. This support element 13, which will support the installation weight (chain) 14 but allow the ballast material 2 to pass by, may conveniently be in the form of a grating or other such device.
FIG. 7 shows how the continued feeding of the [0041] installation chain 14 into the chute causes the chain to accumulate on the support element (grating) 13, which in turn causes the ballast deployment apparatus 1 to descend further down into the water. In another embodiment, the chain may also be fitted with a plug (not shown) that stops the chain when in contact with a recess in the chute located at the desirable depth
In FIG. 8, the [0042] ballast deployment apparatus 1 has reached a position above the seabed 4, said position being controlled by the installation chain in relation to the buoyancy elements 6. The structure 3 on the seabed is equipped with a tether 7 that is tensioned by a float 15. This tether-and-float combination may be any suitable element or elements associated with the seabed. In one embodiment, the tether itself is buoyant, thus abolishing the need for the float 15. The structure 3 is intended to be covered by ballast material.
The [0043] ballast deployment apparatus 1 with buoyancy elements 6 is lowered and positioned above the structure 3 so that a free end of the tether 7 may be attached to the chute second end in a known manner, automatically or manually, by a remotely operated vehicle 16 or other equipment or diver. Upon completion of this operation, the installation chain 14 is retracted and retrieved, and the ballast deployment apparatus 1 is ready for the use.
Upon completion of the ballast deployment operation, the installation chain is lowered back into the [0044] chute 5, the tether 7 unloaded and disconnected, and the entire ballast deployment apparatus 1 may be moved, by means of the installation chain 14, to another structure/location for deployment of ballast material. Finally, the ballast deployment apparatus 1 can be retrieved to floating in upright orientation by removing the installation chain from the chute.
One constraint in installing very long chutes [0045] 5 (for large water depths) is the bending moment that is generated during the upending and/or retrieval of the chute (see FIG. 5). In order to eliminate this constraint, the chute 5 may be divided into two or more sections (5 a, 5 b), and joined by a corresponding number of joints 17. This is illustrated in FIGS. 9 and 10. In order to prevent disruptions of the ballast material deployment, the joints 17 may be covered by a flexible cladding 18.
Such jointed chute arrangement facilitates the use of [0046] longer chutes 5, thus achieving greater water depths in which the ballasting deployment can be performed. As the bending moments are proportional to the square of the length between the ends of the joints, the use of one of several joints efficiently increases the feasible depth. Another constraint for the practical length may be related to the length of the available facilities for fabrication and launching into floating in water.
In the described embodiment, the [0047] ballast material 2 is shown as a granulate material, such as rock or gravel. The invented ballast deployment apparatus 1 is, however, also suitable for subsea deployment of other materials, such as concrete or any fluid. Also, while the ballast material is shown as falling to the structure on the seabed due to gravity, the invented ballast deployment apparatus 1 is also suitable for situations where the deployment of materials is effected in part or completely by means of an applied pressure.
Also the invented apparatus is suitable as a component in a power plant in open oceans, the chute extracting energy from temperature differential between cold water in deep water and warm water in surface (in tropic areas). Cold water flows in the chute to the surface for heat exchange with warm water. [0048]
Furthermore, the invented apparatus may find use for transmission of free gas to the surface and controlled discharge, e.g. during blow-out or natural seepage of hydrocarbon gases, from the seabed. In this case, a guide funnel on the lower end of the chute may be necessary. [0049]
Although the figures show the [0050] structure 3 as a flat element, it should be understood that the structure may be of any shape or form, e.g. circular in the case of a pipeline. The structure may be resting on or partially into (trenched) the seabed 4.
Although the [0051] support element 13 in the described embodiment is shown as a grating inside the chute, and the installation weight 14 is shown as a chain to be lowered onto the grating, the support element may have other forms and shapes, and may be mounted on the chute exterior. Similarly, the installation weight should not be restricted to a chain, but may comprise any device for transferring weight onto the support element.
It should be understood that the invention is not limited to any particular dimension. For example, the [0052] chute 5 may any suitable length, the buoyancy elements 6 are equally dimensioned to suit the particular application. Furthermore, although the chute 5 is shown with a circular cross section, a person skilled in the art will understand that the invention is not limited to chutes having a such shape.
The foregoing description and the embodiments of the present invention are to be construed as mere illustrations of the application of the principles of the invention. The foregoing is not intended to limit the scope of the claims, but the true spirit and scope of present invention is defined by the claims. [0053]

Claims

What is claimed is:

1. An apparatus for controlled deployment of ballast material to a subsea location, comprising

2. The apparatus of claim 1, wherein said support element is fixed to the chute interior below the location of said buoyancy elements.

3. The apparatus of claim 2, wherein said support element comprises a grating or similar element which does not prevent the conveyance of said ballast material while supporting the installation weight.

4. The apparatus of claim 1, wherein said ballast elements are located at or near said first end.

5. The apparatus of claim 1, wherein said chute first end has a funnel-shape.

6. The apparatus of claim 1, wherein the chute is associated with at least one buoyancy pipe generally oriented parallel to said chute.

7. The apparatus of claim 6, wherein the buoyancy pipe extends the length of said chute.

8. The apparatus of claim 1, wherein a plurality of chutes are joined by flexible joints.

9. The apparatus of claim 8, wherein said joints are covered by a flexible cladding.

10. The apparatus of claim 1, wherein said chute second end is adapted for connection to an element associated with the seabed.

11. The apparatus of claim 10, wherein said element associated with the seabed is a tether connected to a subsea structure.

12. The apparatus of claim 10, wherein said element associated with the seabed is a separate mooring on the seabed.

13. A method for installing an apparatus for the controlled deployment of ballast material to a subsea location, said apparatus comprising

at least one buoyancy element associated with the chute such that the chute may assume a generally vertical orientation in the water, supported by said buoyancy element;

a support element associated with said chute and adapted for cooperation with an installation weight. and

said chute second end being adapted for connection to an element associated with the seabed,

said method comprising the steps of:

bringing the chute into a generally vertical orientation in the water above said subsea location, said first end being on or near the surface of the water and said second end pointing towards the subsea location;

lowering portions of said installation weight onto said support element, causing said ballast deployment apparatus to move towards the seabed, until said apparatus has reached a desired depth in the water;—connecting said chute second end to said element associated with the seabed; and

withdrawing said installation weight from said support element,

whereupon deployment of ballast material through said chute may commence.

14. The method of claim 13, wherein said portions of said installation weight is lowered into said chute, onto said support element which is fixed inside the chute.

15. The method of claim 14, wherein said installation weight is lowered into a grating or similar element which does not prevent the conveyance of said ballast material, but supports the installation weight.

16. The method of claim 13, wherein said element associated with the seabed is a tether connected to a subsea structure.

18. The method of claim 13, wherein said element associated with the seabed is a separate mooring on the seabed.