SUCTION ANCHOR OR WELL SUPPORT FOUNDATION FOR USE IN PERMEABLE WATER BOTTOM FORMATIONS

A suction anchor has a skirt open at one end and closed at another end to define an interior volume. At least one conduit nested within or adjacent to the skirt, is open at one end and closed at another end to define an inner volume. A suction line is fluidly connected to the interior volume through a first valve. A second valve is fluidly connected between the inner volume and either the suction line or the interior volume. The first valve and the second valve are operable to cause water flow at respective selected rates along both the skirt and the conduit from a body of water when the interior volume and the inner volume are evacuated and the suction anchor is disposed on the bottom of a body of water.

Not Applicable

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable.

BACKGROUND

This disclosure relates to the field of water bottom suction anchors. More specifically, the disclosure relates to suction anchors or well supports used in permeable water bottom soils or formations.

Suction anchors known in the art have been installed mostly in clay type formations, which have relatively low permeability. More recently, especially motivated by applications connected to offshore wind farming, suction anchors have also been used as foundations in pure sand and mixed (layered clay-sand) formations, which may be relatively permeable.

FIG.1andFIG.2show, respectively, a side cross-section view and a top view of a conventional suction anchor10. The suction anchor10consists of a skirt12, typically having a circular cross-section, and a top11that seals the upper longitudinal end of skirt12. During installation on the water bottom, the tip17of the skirt12first penetrates sediments on the water bottom due to the weight of the suction anchor10, creating a semi permeable seal to the water bottom sediments (formation)3at the bottom end of skirt12. To urge the suction anchor10further into the formation3, a suction pump18is connected to a suction line15that is fluidly connected to an internal water mass4within the skirt12. In the case wherein the formation3consists of sand or mixed sand/clay and is therefore permeable, the under-pressure generated inside the suction anchor10by evacuating water from the suction line15causes water to be displaced from the surrounding body of water1external to the skirt12into the internal water mass4within the suction anchor10. This is indicated inFIG.1by a flow path shown at20. The under-pressure also generates a downward force by reason of the greater pressure of the water1outside the top11that pushes the suction anchor10further into the formation3. Additionally, the water flow around the skirt tip17causes fluidization of the sand, which reduces resistance to further penetration of the skirt12into the sub-bottom.

After reaching the desired penetration depth with the suction anchor10, it is beneficial if the suction line15is sealed, for example using a suction line valve16in order to obtain the maximal load capacity of the suction anchor10.

Conventional suction anchors may have an internal structure, for example an internal support member13, to increase the load capacity of an anchoring point14on the skirt12, where an anchoring chain or the like may be attached. Such internal structures can increase the penetration resistance during deployment of the suction anchor10. The penetration resistance may be decreased by the installation of a water injection line to the bottom edge of the internal member13, in order to fluidize the formation3, e.g., sand, locally, similar to the fluidization that is caused by water flowing along the flow path20.

A well support structure, or Conductor Anchor Node system is based on a suction anchor to form the foundation for subsea oil and gas wells, including water, gas, steam, or other fluid injection wells. One such system is sold under the trademark CAN, which is a registered trademark of Neodrill AS, Stavanger, Norway. The CAN system has some differences with respect to a conventional suction anchor.FIGS.3and4show, respectively, a side cross-section view and a top view of an example CAN system30. Because the CAN system30is used as a well foundation, a conduit31is disposed inside the skirt12of the structure of a suction anchor. This conduit31is attached to a top11disposed on the upper end of the skirt12in a pressure tight manner. During the well construction process, a sub-bottom well is established through the conduit31, which supports well components and acts as a guide for well construction tools. The conduit31may in some cases be additionally supported by internal members32extending between the conduit31and the skirt12. The CAN system30also allows the pre-installation of a conduit known as a conductor (a structural casing) or any other wellbore tubular element as discussed further below.

The presence of the conduit31causes several complications during the installation process of a CAN system in permeable formation3, such as sand, sandy clay and interspersed sand and clay. The installation mechanics of a CAN system in permeable formation rely on water being drawn from the surrounding water1, through the formation3along the outer wall of the skirt12, around the tip17of the skirt12into the interior of the skirt12, upward inside the skirt12into the internal water mass4below the top11. This is indicated by a flow path shown at20. From there, the water will be drawn through the suction line15, by a suction pump18and discharged back into the surrounding water mass1. The water flow around the skirt tip17fluidizes the sand, which decreases the penetration resistance of the CAN system30into the formation3. As mentioned above, the CAN system30comprises an additional conduit31nested in the skirt12. The conduit31provides a second possible flow path which water may traverse during the CAN system installation process. This second possible flow path along the conduit31is downward inside of the conduit31, around the conduit tip33and upward into the internal water mass4. This is indicated as a flow path shown at34. In practice, it is not possible to predict which flow path (20or34) will be traversed by moving water. It can be detrimental to the installation process if the path of least resistance is along flow path34. In this case only the relatively small circular length of the circumference of conduit31, at the tip33, will experience fluidization, whereas the larger circular length of circumference of the skirt12, at the tip17, will experience no fluidization. This may minimize the reduction of the total resistance to movement formed by the sum of tip area of the skirt tip17and the conduit tip33, and as a result the installation of the CAN system30to the target depth may fail.

SUMMARY

One aspect of the present disclosure relates to a suction anchor. A suction anchor according to this aspect of the disclosure has a skirt open at one end and closed at another end to define an interior volume. A conduit is nested within the skirt or is adjacent to the skirt. The conduit is open at one end and is closed at another end to define an inner volume. A suction line is fluidly connected to the interior volume through a first valve. A second valve is fluidly connected between the inner volume and either the suction line or the interior volume. The first valve and the second valve are operable to cause water flow at respective selected rates along both the skirt and the conduit from a body of water when the interior volume and the inner volume are evacuated and the suction anchor is disposed on the bottom of a body of water. At least one of the first valve and the second valve has a variable orifice.

Some embodiments further comprise a conductor nested within the conduit. The conductor comprises a wellhead housing and a conductor pipe extending from the wellhead housing through the conduit.

In some embodiments, the conduit and the conductor pipe are connected at respective longitudinal ends by a conductor anchor.

In some embodiments, the inner volume is defined within an interior of the conductor pipe.

In some embodiments, the cap comprises a conductor running tool disposed in the conductor pipe.

Some embodiments further comprise a pressure sensor in fluid communication with the inner volume and the interior volume, the pressure sensor arranged to generate signals corresponding to a difference between fluid pressure in the inner volume and fluid pressure in the interior volume.

In some embodiments, the second valve is in fluid communication between the inner volume and the interior volume.

In some embodiments, the second valve is in fluid communication between the inner volume and the suction line.

In some embodiments, at least one of the first valve and the second valve has a variable orifice.

A method for affixing a suction anchor having a conduit nested in or adjacent to the suction anchor to the bottom of a body of water according to another aspect includes lowering the suction anchor so that a skirt is in contact with the bottom of the body of water. Pressure is reduced in an interior volume defined within the skirt and external to the conduit. Pressure is reduced in an inner volume defined by the interior of the conduit. The reducing pressure in the inner volume and the reducing pressure in the interior volume are performed at respective rates such that water cross flow between the interior volume and the inner volume is minimized.

In some embodiments, the respective rates are controlled by operating at least one variable flow valve.

A suction anchor according to another aspect of this disclosure comprises a skirt open at one end and closed at another end to define an interior volume. At least one conduit is nested within the skirt or disposed adjacent to the skirt, the conduit open at one end and temporarily closed at another end to define an inner volume. Means for evacuating the inner volume and the internal volume is operable to cause water flow along both the skirt and the conduit from a body of water by separately controlling rates of evacuation from the inner volume and the internal volume when the suction anchor is disposed on the bottom of a body of water.

Some embodiments further comprise a conductor nested within the conduit, the conductor comprising a wellhead housing and a conductor pipe extending from the wellhead housing through the conduit.

In some embodiments, the conduit and the conductor pipe are connected at respective longitudinal ends by a conductor anchor.

In some embodiments, the inner volume is defined within an interior of the conductor pipe.

In some embodiments, the cap comprises a conductor running tool temporarily disposed in the conductor pipe.

Some embodiments further comprise a pressure sensor in fluid communication with the inner volume and the interior volume, the pressure sensor arranged to generate signals corresponding to a difference between fluid pressure in the inner volume and fluid pressure in the interior volume.

In some embodiments, the means for evacuating comprises a pump, a first valve in fluid communication between an inlet of the pump and the internal volume and second valve in fluid communication between the inner volume and the interior volume.

In some embodiments, the second valve is in fluid communication between the inner volume and the inlet of the pump.

In some embodiments, at least one of the first valve and the second valve has a variable orifice.

In some embodiments, the means for evacuating comprises a first pump having an inlet in fluid communication with the inner volume and a second pump having an inlet in fluid communication with the internal volume.

In some embodiments, at least one of the first pump and the second pump is a variable speed pump.

Other aspects and possible advantages will be apparent from the description and claims that follow.

DETAILED DESCRIPTION

FIG.5andFIG.6show example embodiments of a conductor anchor node (CAN) system30according to the present disclosure. To enable a successful installation of the CAN system30in a permeable formation3, for example, that consists of sand or mixed sand-clay layers, it must be possible to actively influence where a water flow path is established when enclosed volumes4and5within the CAN system30are evacuated, e.g., by a pump18. The present example embodiment of a CAN system30may comprise a skirt12, which may have an open end12A for insertion into formation3on the water bottom2. The other end of the skirt12may be closed by a top11similar to that described with reference toFIG.3. A conduit31may be nested within the interior of the skirt12and may form an opening through the top11. In some embodiments, the conduit31may be mounted external to the skirt12rather than nested within the skirt12. Furthermore, multiple conduits31may be mounted in or external to the skirt12. The top11is coupled to the skirt12and the conduit31to sealingly close an interior volume4that will be filled with water during installation of the CAN system30.

In the present example embodiment, selectively establishing a water flow path during evacuation of the interior volume4may be obtained by introducing a conduit cap40to one end of the conduit31to seal the inner volume5of conduit31from the surrounding water1. The other end of the conduit31may be open to enable movement of the conduit31and the remainder of the CAN system30(including the skirt12) into the formation3during installation.

The conduit cap40may comprise a cap vent valve41that can provide several functions. Firstly, the cap vent valve41may be in an open position to enable fluid flow when the CAN system30is lowered from an installation vessel into the surrounding water1. The cap vent valve41being open allows any trapped air inside the conduit31to escape to the surrounding water1. A suction line valve16, or a vent hatch (not shown) with increased cross-section, has a similar function, namely to vent air trapped inside of the skirt12, that is, in the interior volume4during the installation process. Secondly, the cap vent valve41may be closed to fluid flow to ensure that the inner volume5of the conduit31is fluidly isolated from the surrounding water1during the suction phase of installation.

The interior of the conduit cap40may be fluidly connected to the suction line15through a valve, which in the present embodiment may be an adjustable orifice valve, identified herein as a cap suction valve42. Thus, both the interior volume4in the skirt12and the inner volume5in the conduit31may be selectively opened to the suction side of the pump18used to evacuate the enclosed volumes, namely, the interior volume4and the inner volume5, to urge the CAN system30into the formation3. The cap suction valve42allows selectively and variably applying suction, and therefore under-pressure, to the inner volume5separately and controllably from suction separately and controllably applied to the interior volume4. By selecting a suitable amount of opening of the cap suction valve42it is possible to establish both possible water flow paths, the flow path (20inFIG.3) along the exterior of the skirt12and the flow path (34inFIG.3) along the interior of the conduit31at the same time.

Although the present example embodiment contemplates a single suction pump18connected at its inlet to the cap suction valve42and the suction line valve16, and the cap suction valve42is described as a variable flow opening valve, the same effect, namely, controllable suction applied to the inner volume5and to the interior volume4, may be obtained by any combination of fixed and variable flow opening features for the suction line valve16and the cap suction valve42. It is also within the scope of this disclosure to have separate pumps (not shown) connected at their respective inlets to the suction line valve16and the cap suction valve42. Such pumps (not shown) may be single speed, multiple speed or variable speed to effect the same result, namely, to cause water movement into the interior volume4and the inner volume5to traverse both flow paths (20and34inFIG.3) simultaneously.

In some embodiments, a first pressure sensor or gauge P1may be arranged to measure pressure in the inner volume5, and a second pressure gauge P2may be arranged to measure pressure in the interior volume4. The pressure sensors or gauges P1, P2may be substituted by a differential pressure sensor arranged to measure pressure difference between the inner volume5and the interior volume4. In such embodiments, any or all of the suction line valve16, the cap suction valve42and one pump18or a second pump (not shown) may be operated to maintain a pressure in the inner volume5that is the same as or is within a predetermined difference of the pressure in the interior volume4. By maintaining such pressures or pressure difference, water flow other than along the two paths (20and34inFIG.3) may be minimized. In such way, movement of the CAN system30into the formation3may be optimized in the presence of permeability in the formation3.

In some embodiments, a way to decrease the penetration resistance at the conduit tip33and at the lower edge of the internal member32is to install a water injection system (not shown) as described in the Background section with reference toFIG.1.

An alternative arrangement, and referring specifically toFIG.6, to establish suction in the conduit's enclosed water mass (i.e., inner volume5) is to connect the cap suction valve42to the internal water mass enclosed by the skirt12(i.e., interior volume4) directly. In this arrangement an internal flow path43is established proximate the top11. The embodiment shown inFIG.6may have the advantage that the suction line15and the cap suction valve42can be designed as separate elements, while using only a single pump18.

Another example embodiment of a CAN system60according to the present disclosure is shown inFIG.7. In this embodiment, a conductor50, with a low pressure wellhead housing51and a conductor pipe52, may be pre-assembled into the CAN system60by nesting the conductor50into the skirt12through the conduit31in a workshop or other land-based facility prior to installation of the CAN system60on the water bottom. Thus, the conductor50may be pre-installed instead of being installed in the already emplaced CAN system60by a drilling unit or other water-borne or water bottom supported construction device. The conductor50may be mounted proximate its bottom end to proximate the bottom end of the conduit31using a conductor anchor54. An annular space between the outer diameter of the conductor50and the inner diameter of the conduit31may be filled with void filling material such as cement55or other filling medium suitable for such purpose, for example and without limitation, epoxy. The upper end of the conductor50may be formed into a low pressure wellhead housing51, which is open prior to further well construction. A cap is needed to seal the conductor50at its upper end in order to effect suction installation of the CAN system60into the formation3. Sealing may be effected by using a conductor running tool53sealingly engaged to the interior surface of the low pressure wellhead housing51. In this embodiment, the cap vent valve41may be fluidly connected to conduit the inner volume5via a cement port (not shown) in the conductor50. The cap suction valve42may be connected to the interior volume5in a similar manner.

FIG.8shows an oblique view of the embodiment inFIG.7to better illustrate the relative positions of the various components with reference to the centerline of the CAN system60.FIG.9shows an elevation view of the embodiment ofFIG.8.FIG.10shows a top view of the embodiment ofFIG.8, andFIG.11is a cross-section along line11-11′ of the view inFIG.10. In this embodiment the suction line valve (16inFIG.6) is replaced by a receptacle56, as shown inFIG.11. The suction pump (not shown) may be connected by a stab that inserts into the receptacle56during the suction process. The closing function of suction line valve (16inFIG.6) may be substituted by a sealing plug (not shown) that seals the receptacle56pressure tight after the installation is completed.

During removal of the CAN system (30or60) from the formation3some similarities apply. In order to remove the CAN system (30or60) from the formation3, to establish fluid flow paths such as at20and34(but in reverse direction as indicated in the drawings) the conduit cap40may be installed on the conduit31again. Similar to the installation process described above it may be an advantage to also apply over-pressure in the inner volume5and in the internal volume4. Other elements of the system, such as the cap vent valve41, the cap suction valve42, internal flow path43, etc. may be used to adjust or balance the applied over-pressure between the internal and inner volumes,4and5, respectively.

In light of the principles and example embodiments described and illustrated herein, it will be recognized that the example embodiments can be modified in arrangement and detail without departing from such principles. The foregoing discussion has focused on specific embodiments, but other configurations are also contemplated. In particular, even though expressions such as in “an embodiment,” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the disclosure to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. As a rule, any embodiment referenced herein is freely combinable with any one or more of the other embodiments referenced herein, and any number of features of different embodiments are combinable with one another, unless indicated otherwise. Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible within the scope of the described examples. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.