Method and apparatus for lowering a subsea structure between the surface and the seabed

The present invention relates to a method and apparatus for lowering a subsea structure between the surface and the seabed and in particular to a method and apparatus for lowering a subsea structure to the seabed by controlling the buoyancy of the structure.

The term "subsea structure" refers to any equipment, tool, machine or other installation to be installed on the seabed, in particular risers, underwater well-head elements on oilfields, or oil processing units.

The lowering and raising of large structures to and from the seabed is difficult because of the large mass of such structures and the depth of water in which they are often required to be located. It is known to lower loads having an apparent weight in water of several hundred tons to the sea bed using a crane mounted on a floating vessel. However, at greater depths, the use of conventional cranes is problematic since, in addition to the load of the subsea structure, the crane must also support the weight of its own cable. The weight of the cable can represent up to 50% of the load capacity of the crane at a depth of 3000 metres. Therefore this method is not practical for deepwater installations.

It is advantageous to reduce the apparent weight of the structure by increasing its buoyancy in the water, consequently reducing the load to be borne by the crane. This can be achieved by buoyancy elements in the form of air tanks attached to the structure. However, such tanks must be strong enough to be capable of withstanding the maximum immersion pressure without imploding or deforming due to the compressibility of the

air contained therein, thus increasing the weight of the tanks and thereby reducing their buoyancy.

Such known buoyancy elements have also been utilised to avoid the need for a crane and allow the structure to be simply sunk to the seabed at a rate controlled by controlling the buoyancy of the structure. A cable or ROV may be used to guide the structure to the correct location on the seabed. Air can be added or displaced from the air tanks as required to vary the buoyancy of the structure. By progressively replacing the air with water, the controlled sinking of the structure can be ensured. However, the compressibility of the air leads to problems in controlling the buoyancy of the structure, particularly at greater depths. The buoyancy elements would be subject to extremely high forces and this must be compensated by increasing the pressure of the gas contained in them.

According to a first aspect of the present invention there is provided a method of lowering a subsea structure to the seabed comprising the steps of:- providing at least one buoyancy element on or connected to the subsea structure, said at least one buoyancy element comprising one or more chambers containing a buoyancy fluid comprising a substantially incompressible fluid having a density less than that of sea water; providing a reservoir for said buoyancy fluid at a location remote from said subsea structure; providing fluid communication between said reservoir and said one or more chambers of said at least one buoyancy element; transferring said buoyancy fluid between said reservoir and said one or more chambers of said at least one buoyancy element to vary the volume of buoyancy fluid within the at least one buoyancy element and

thus vary the overall buoyancy of the subsea structure to thereby initiate and subsequently control the rate of descent of the subsea structure.

In a preferred embodiment, the step of transferring said buoyancy fluid between said reservoir and said one or more chambers of said at least one buoyancy element to vary the volume of buoyancy fluid within the at least one buoyancy element has the effect of varying the volume of said one or more chambers, said one or more chambers being formed from a flexible or elastic material.

The method may comprise the step of initially deploying the subsea structure from a floating vessel by means of a crane or hoist provided on the vessel. Preferably the method comprises the further steps of at least partially filling the at least one or more chamber of the at least one buoyancy element with said buoyancy fluid and releasing the subsea structure from said crane or hoist once sufficient buoyancy fluid has been supplied to the at least one buoyancy element to achieve neutral buoyancy.

The method may comprise mounting a plurality of buoyancy elements onto the subsea structure such that the subsea structure is suspended from said buoyancy elements.

In one embodiment, said reservoir may be provided upon a floating vessel.

In an alternative embodiment, the method may comprise the further step of suspending said reservoir from a hoist or crane provided on floating vessel or mounting the reservoir on an ROV and controlling the depth of the reservoir to thereby maintain the reservoir at a depth substantially

equal to that of the subsea structure during the descent of the subsea structure to the seabed.

Preferably said at least one buoyancy element comprises a plurality of chambers, the step of transferring buoyancy fluid between the buoyancy element and the reservoir comprising sequentially transferring fluid between individual chambers of said at least one buoyancy element and said reservoir.

The method may include the further step of positioning the subsea structure on the seabed using an ROV.

In a preferred embodiment, the method comprises the further steps of deploying a buoyancy fluid recovery means to the seabed, said buoyancy recovery means comprising a chamber for receiving buoyancy fluid and ballast weight means mounted thereon or connected thereto; transferring at least a proportion of the buoyancy fluid from the at least one buoyancy element to said buoyancy fluid recovery means when the subsea structure is in place upon the seabed; and returning said buoyancy fluid recovery means to the surface by virtue of the increased buoyancy imparted thereto by the buoyancy fluid to thereby recover the buoyancy fluid to the surface. The method may further comprise detaching the at least one buoyancy element from said subsea structure and recovering said buoyancy element to the surface once the buoyancy fluid has been substantially removed therefrom. Where a plurality of buoyancy elements are provided on the subsea structure, the method may comprise repeating the abovementioned steps, the buoyancy fluid being removed from each of the buoyancy elements in equal amounts each time, until substantially all of the buoyancy fluid has been removed from the plurality of buoyancy

elements before detaching the buoyancy elements from the subsea structure and recovering them to the surface.

According to a further aspect of the present invention there is provided an apparatus for lowering a subsea structure to the seabed comprising:- at least one buoyancy element provided on or connectable to the subsea structure, said at least one buoyancy element comprising one or more chambers containing a buoyancy fluid comprising a substantially incompressible fluid having a density less than that of sea water; a reservoir for said buoyancy fluid, said reservoir being provided at a location remote from said subsea structure; fluid communication means for transferring said buoyancy fluid between said reservoir and said one or more chambers of said at least one buoyancy element; and control means for controlling the operation of the fluid communication means whereby buoyancy fluid can be transferred between the one or more chambers of the at least one buoyancy element and the reservoir to vary the volume of buoyancy fluid within the buoyancy element and hence control the rate of descent of the subsea structure.

In a preferred embodiment, the one or more chambers of the at least one buoyancy element may be formed from a flexible or elastic material to permit the volume thereof vary to accommodate the volume of buoyancy fluid contained therein.

In one embodiment the reservoir is provided upon a floating vessel.

In an alternative embodiment, the reservoir is provided upon a subsea unit suspended from a floating vessel on a crane or winch or mounted upon an ROV, whereby the reservoir can be maintained at a depth substantially

equal to the depth of the subsea structure as the subsea structure descends to the sea bed.

Preferably the apparatus further comprises a floating vessel having a winch or crane for initially deploying the subsea structure into the water, a supply of said buoyancy fluid, and fluid transfer means for initially transferring said buoyancy fluid into the at least one buoyancy element to provide the subsea structure with neutral buoyancy to permit detachment of the subsea structure from said winch or crane once it has been deployed into the water.

The subsea structure is preferably suspended beneath a plurality of buoyancy elements.

Preferably the apparatus further comprises a buoyancy fluid recovery means, comprising a chamber for receiving buoyancy fluid and having ballast weight means mounted thereon or connected thereto, the buoyancy fluid recovery means being deployed onto to the seabed at or adjacent the location whereat the subsea structure is to be installed, whereby at least a proportion of the buoyancy fluid from the at least one buoyancy element can be transferred to said buoyancy fluid recovery means when the subsea structure is in place upon the seabed, said buoyancy fluid recovery means subsequently returning to the surface due to the increased buoyancy imparted thereto by the buoyancy fluid to thereby recover the buoyancy fluid to the surface.

The apparatus may further comprise an ROV for positioning the subsea structure upon the seabed.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Fig. 1 shows a first stage of a method of lowering a subsea structure to the seabed according to an embodiment of the present invention;

Fig. 2 illustrates a second stage of the method;

Fig. 3 illustrates a third stage of the method;

Fig. 4 illustrates a fourth stage of the method;

Fig. 5 illustrates a fifth stage of the method;

Fig. 6 illustrates a further stage of the method;

Fig. 7 illustrates a seventh stage of the method; and

Fig. 8 illustrates a further stage of the method.

The present invention provides an improved method and apparatus for deploying a large subsea structure 1 to deepwater seabed location from a floating vessel 2.

As illustrated in Fig. 1 , in a first step, a buoyancy fluid recovery unit 10 is deployed to the seabed comprising an empty storage vessel 12 and a clump weight 14. The buoyancy fluid recovery unit 10 is lowered to the seabed via a crane 4 mounted on the floating vessel 2.

Next, as illustrated in Fig. 2, the subsea structure 1 is lowered into the water from the floating vessel 2 via a suitable crane. An ROV (remotely operated vehicle) 20 is also deployed from the vessel 2 to monitor and assist the operation and, in particular, to assist in the final location of the subsea structure 1 upon the seabed.

As illustrated in Fig. 3, next four buoyancy elements 30 are attached symmetrically to the subsea structure 1 (this step may take place while the subsea structure is still on the vessel 2) and filled from the vessel with a substantially incompressible buoyancy fluid having a density less than that of sea water, whereby the subsea structure is supported beneath the buoyancy elements. A sufficient volume of buoyancy fluid is supplied into each buoyancy elements to counter the weight of the subsea structure to achieve neutral buoyancy.

Each buoyancy element 30 comprises a plurality of separate chambers, each formed from a flexible or elastic material whereby the volume of each chamber can expand to accommodate the required volume of buoyancy fluid. No other liquid or gas is contained with the chambers. The substantially incompressible nature of the buoyancy fluid means that the buoyancy elements 30 can be made from relatively thin and light material compared to prior art air tanks, thus reducing the weight of the buoyancy elements and increasing the buoyancy thereof.

Suitable buoyancy fluids are polymerised low molecular weight hydrocarbons, such as methanol. In order to further reduce the density of the fluid, glass microspheres may be added. Such material preferably has a density of between 500 and 600 kg/m3 (the density of sea water is around 1027 kg/m3). However, any liquid having a density less than that of sea water may be suitable. Preferably the buoyancy fluid also has a

low viscosity such that it is easily movable (such as by one or more pumps) between the buoyancy element and an external reservoir, and such fluids include known low viscosity gels and the like.

As illustrated in Fig. 4, a subsea fluid reservoir 40 is subsequently deployed from the vessel 2 and connected to the buoyancy elements 30 whereby buoyancy fluid can be selectively transferred between each chamber of the buoyancy elements 30 and the subsea fluid reservoir 40 to adjust the volume of buoyancy fluid within the buoyancy elements and thus control the overall buoyancy of the subsea structure.

The subsea fluid reservoir 40 includes a fluid storage chamber, one or more pumps and a manifold distribution system and is connected to the floating vessel 2 via a control umbilical 42. At deployment, the storage chamber of the subsea fluid reservoir is nearly empty. The subsea fluid reservoir 40 is deployed from the floating vessel on a winch so that it can be maintained at substantially the same depth as the subsea vessel during its descent to the seabed. Alternatively, the subsea fluid reservoir may be mounted on an ROV.

As illustrated in Fig. 5, to initiate descent of the subsea structure, a small amount of buoyancy fluid is transferred from each buoyancy element 30 to the subsea fluid reservoir 40 to reduce the volume of the buoyancy fluid in the buoyancy elements 30 and hence the buoyancy thereof.

During the descent of the subsea structure, due to the increasing pressure gradient and vertical pressure stratification of most oceans, small additional amounts of buoyancy fluid will need to be transferred between the buoyancy elements 30 and the subsea fluid reservoir 40 to maintain a net downward force on the subsea structure.

The subsea fluid reservoir 40 and the ROV 20 are lowered at a rate equivalent to the rate of descent of the subsea structure to maintain them all at substantially the same depth during the descent.

The buoyancy elements 30 may be arranged such that each transfer of fluid between each buoyancy element and the subsea fluid reservoir may correspond to the total volume of an individual chamber of the buoyancy element 30. The pressure of the seawater surrounding the buoyancy element may be utilised to force the buoyancy fluid out of the respective chamber and into the subsea fluid reservoir. Alternatively a pump may be used to achieve the desired fluid transfer.

As illustrated in Fig. 6, as the subsea structure 1 approaches the seabed, the ROV 20 is used to manoeuvre the subsea structure 1 into the correct position. Once the subsea structure 1 reaches the seabed, further buoyancy fluid is transferred from each buoyancy element 30 into the subsea fluid reservoir 40 to firmly ground the subsea structure onto the seabed.

Next, as illustrated in Fig. 7, the subsea fluid reservoir is connected to the previously deployed buoyancy fluid recovery unit 10 and approximately 25% of the buoyancy fluid remaining each buoyancy element 30 is transferred to the storage vessel 12 of the buoyancy fluid recovery unit 10. The subsea fluid reservoir 40 is disconnected from the buoyancy elements 30 and returned to the surface with the buoyancy fluid recovery unit, as illustrated in Fig. 8. This process is repeated until all of the buoyancy fluid is removed from the buoyancy elements and subsequently the buoyancy elements are recovered by the ROV.

While the present invention has described the use of four buoyancy elements, more or less may be used depending upon the mass of the subsea structure to be deployed. Reducing the number of buoyancy elements used will reduce the time taken for recovery (by reducing the number of trips required for the buoyancy fluid recovery unit). However, the maximum size of each buoyancy element will be limited by the practicalities of handling such components in the offshore and subsea environment.

The incompressibility of the buoyancy fluid solves the problems usually encountered when using a gas inside flexible buoys and allows the use of lightweight, flexible buoys in deepwater applications. The use of a subsea fluid reservoir and buoyancy fluid recovery unit avoids the need for long pipelines extending from a surface vessel which are prone to damage and leaks and require powerful pumps and low viscosity fluids. However, where practical, it is envisaged that the fluid reservoir may be provided upon the floating vessel with a direct connection to the buoyancy elements being maintained during the descent of the subsea structure to the seabed This method may be used where the volume of buoyancy fluid required to be transferred between the buoyancy elements and the reservoir exceeds the capacity of the subsea fluid reservoir.

Various modifications and variations to the described embodiment of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.

For example, it is envisaged that buoyancy elements containing a buoyancy fluid as described above may be used to reduce the effective weight of a subsea structure lowered to the seabed by means of a crane.

In a further alternative embodiment, a further means for equilibrating the load may be used in addition to the buoyancy fluid. For example, catenary chains extending from the surface vessel to the subsea structure in a loop below the structure, or an assembly of ballast weights or a direct connection to the vessel (such as a cable extending from the vessel to the structure) may be used to control the effective weight and hence the rate of decent of the subsea structure.