Patent Description:
In offshore drilling operations, a large diameter hole is drilled to a selected depth in the sea bed. Then, a primary conductor extending from the lower end of an outer wellhead housing, also referred to as a low pressure housing, is run into the borehole with the outer wellhead housing positioned just above the sea floor/mud line. To secure the primary conductor and outer wellhead housing in position, cement is pumped down the primary conductor and allowed to flow back up the annulus between the primary conductor and the borehole sidewall.

With the primary conductor cemented in place, a drill bit connected to the lower end of a drillstring suspended from a drilling vessel or rig at the sea surface is lowered through the primary conductor to drill the borehole to a second depth. Next, an inner wellhead housing, also referred to as a high pressure housing, is seated in the upper end of the outer wellhead housing. A string of casing extending downward from the lower end of the inner wellhead housing (or seated in the inner wellhead housing) is positioned within the primary conductor. Cement then is pumped down the casing string, and allowed to flow back up the annulus between the casing string and the primary conductor to secure the casing string in place.

Prior to continuing drilling operations in greater depths, a blowout preventer (BOP) is mounted to the wellhead and a lower marine riser package (LMRP) is mounted to the BOP. The subsea BOP and LMRP are arranged one-atop-the-other. In addition, a drilling riser extends from a flex joint at the upper end of the LMRP to a drilling vessel or rig at the sea surface. The drill string is suspended from the rig through the drilling riser, LMRP, and BOP into the well bore. Drilling generally continues while successively installing concentric casing strings that line the borehole. Each casing string is cemented in place by pumping cement down the casing and allowing it to flow back up the annulus between the casing string and the borehole sidewall. During drilling operations, drilling fluid, or mud, is delivered through the drill string, and returned up an annulus between the drill string and casing that lines the well bore.

Following drilling operations, the cased well is completed (i.e., prepared for production). For subsea architectures that employ a horizontal production tree, the horizontal subsea production tree is installed on the wellhead below the BOP and LMRP during completion operations. Thus, the subsea production tree, BOP, and LMRP are arranged one-atop-the-other. Production tubing is run through the casing and suspended by a tubing hanger seated in a mating profile in the inner wellhead housing or production tree. Next, the BOP and LMRP are removed from the production tree, and the tree is connected to the subsea production architecture (e.g., production manifold, pipelines, etc.). From time to time, intervention and/or workover operations may be necessary to repair and/or stimulate the well to restore, prolong, or enhance production.

The invention provides system and method for tethering a subsea blowout preventer (BOP) having the features of independent claims <NUM> and <NUM>. Further embodiments are referred to in the dependent claims.

<CIT> is directed to a method for protecting the integrity of a wellhead which includes the steps of: (<NUM>) affixing a subsea structure to the wellhead; (<NUM>) installing at least a pair of piles into the subsea floor adjacent the wellhead; (<NUM>) extending lines between the subsea structure and the piles; and (<NUM>) tensioning the line such that each of the lines applies a tension to the subsea structure, the line having a tensioning cylinder affixed thereto, said tensioning cylinder having a ratchet rod extending therefrom, and said ratchet rod affixed to either the line or the subsea structure or the pile, wherein the step of tensioning comprises drawing said ratchet rod inwardly of said cylinder when a desired amount of tension is achieved; and fixing a position of said ratchet rod within said cylinder when the desired amount of tension is achieved. Further, an end of each of the lines is affixed to the subsea structure adjacent an upper portion of the subsea structure.

For a detailed description of the invention, reference will now be made to the accompanying drawings in which:.

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to. " Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms "axial" and "axially" generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis.

<CIT> ('<NUM> patent) describes, in reference to <FIG> and <FIG>, an embodiment of an offshore system <NUM> for interfacing with a wellbore <NUM>. In this embodiment, system <NUM> includes a floating offshore vessel <NUM> at the sea surface <NUM>, a horizontal production tree <NUM> releasably connected to a wellhead <NUM> disposed at an upper end of a primary conductor <NUM> extending into the wellbore <NUM>, a subsea blowout preventer (BOP) <NUM> releasably connected to production tree <NUM>, and a lower marine riser package (LMRP) <NUM> releasably connected to BOP <NUM>. Tree <NUM>, BOP <NUM>, and LMRP <NUM> are vertically arranged or stacked one-above-the-other, and are generally coaxially aligned with wellhead <NUM>. Wellhead <NUM> has a central axis <NUM> and extends vertically upward from wellbore <NUM> above the sea floor <NUM>. In <FIG> of the '<NUM> patent, system <NUM> is shown configured for completion operations, and thus, includes tree <NUM>, however, for drilling operations, tree <NUM> may not be included.

As best shown in <FIG> of the '<NUM> patent, vessel <NUM> is equipped with a derrick <NUM> that supports a hoist (not shown). In this embodiment, vessel <NUM> is a semi-submersible offshore platform, however, in general, the vessel (e.g., vessel <NUM>) can be any type of floating offshore drilling vessel including, without limitation, a moored structure (e.g., a semi-submersible platform), a dynamically positioned vessel (e.g., a drill ship), a tension leg platform, etc. A drilling riser <NUM> (not shown in <FIG> of the '<NUM> patent) extends subsea from vessel <NUM> to LMRP <NUM>. During drilling operations, riser <NUM> takes mud returns to vessel <NUM>. Downhole operations are carried out by a tool connected to the lower end of the tubular string (e.g., drillstring) that is supported by derrick <NUM> and extends from vessel <NUM> through riser <NUM>, LMRP <NUM>, and BOP <NUM>, and tree <NUM> into wellbore <NUM>. In this embodiment of the '<NUM> patent, BOP <NUM> includes an outer rectangular prismatic frame <NUM>.

Still referring to the '<NUM> patent, BOP <NUM> and LMRP <NUM> are configured to controllably seal wellbore <NUM> and contain hydrocarbon fluids therein. Specifically, BOP <NUM> includes a plurality of axially stacked sets of opposed rams disposed within frame <NUM>. In general, BOP <NUM> can include any number and type of rams including, without limitation, opposed double blind shear rams or blades for severing the tubular string and sealing off wellbore <NUM> from riser <NUM>, opposed blind rams for sealing off wellbore <NUM> when no string/tubular extends through BOP <NUM>, opposed pipe rams for engaging the string/tubular and sealing the annulus around string/tubular, or combinations thereof. LMRP <NUM> includes an annular blowout preventer comprising an annular elastomeric sealing element that is mechanically squeezed radially inward to seal on a string/tubular extending through LMRP <NUM> or seal off wellbore when no string/tubular extends through LMRP <NUM>. The upper end of LMRP <NUM> includes a riser flex joint <NUM> that allows riser <NUM> to deflect and pivot angularly relative to tree <NUM>, BOP <NUM>, and LMRP <NUM> while fluids flow therethrough.

During drilling, completion, production, and workover operations, cyclical loads due to riser vibrations (e.g., from surface vessel motions, wave actions, current-induced VIV, or combinations thereof) are applied to BOP <NUM>, wellhead <NUM>, and primary conductor <NUM> extending from wellhead <NUM> into the sea floor <NUM>. Such cyclical loads can induce fatigue. This may be of particular concern with subsea horizontal production tree architectures (e.g., system <NUM>) due to the relatively large height and weight of the hardware secured to the wellhead proximal the mud line (i.e., tree, BOP, and LMRP). For example, in this embodiment, the hardware mounted to wellhead <NUM> proximal the sea floor <NUM>, production tree <NUM> and BOP <NUM> in particular, is relatively tall, and thus, presents a relatively large surface area for interacting with environmental loads such as subsea currents. These environmental loads can also contribute to the fatigue of BOP <NUM>, wellhead <NUM>, and primary conductor <NUM>. If the wellhead <NUM> and primary conductor <NUM> do not have sufficient fatigue resistance, the integrity of the subsea well may be compromised. Still further, an uncontrolled lateral movement of vessel <NUM> (e.g., an uncontrolled drive off or drift off of vessel <NUM>) from the desired operating location generally over wellhead <NUM> can pull LMRP <NUM> laterally with riser <NUM>, thereby inducing bending moments and associated stresses in BOP <NUM>, wellhead <NUM>, and conductor <NUM>. Such induced bending moments and stresses can be increased further when the relatively tall and heavy combination of tree <NUM> and BOP <NUM> is in a slight angle relative to vertical. Accordingly, in this embodiment, a tethering system <NUM> is provided to reinforce BOP <NUM>, wellhead <NUM>, and primary conductor <NUM> by resisting lateral loads and bending moments applied thereto. As a result, system <NUM> offers the potential to enhance the strength and fatigue resistance of BOP <NUM>, wellhead <NUM>, and conductor <NUM>.

Referring again to <FIG> and <FIG>, in this embodiment, tethering system <NUM> includes a plurality of anchors <NUM>, a plurality of pile top assemblies <NUM>, and a plurality of flexible tension members <NUM>. One pile top assembly <NUM> is mounted to the upper end of each anchor <NUM>, and one tension member <NUM> extends from each pile top assembly <NUM> to frame <NUM> of BOP <NUM>. As will be described in more detail below, each pile top assembly <NUM> includes a tensioning system <NUM> that can apply tensile loads to the corresponding tension member <NUM>. In this embodiment, each tensioning system <NUM> is a winch, and thus, may also be referred to as winch <NUM>. Each winch <NUM> can pay in and pay out the corresponding tensioning member <NUM>.

Each tension member <NUM> includes a first or distal end 160a coupled to frame <NUM> of BOP <NUM>, and a tensioned span or portion <NUM> extending from the corresponding winch <NUM> to end 160a. As best shown in <FIG>, each distal end 160a is coupled to frame <NUM> of BOP <NUM> at a height H measured vertically from the sea floor <NUM> and at a lateral distance D measured radially and horizontally from central axis <NUM>. In this embodiment, four uniformly circumferentiallyspaced anchors <NUM> and associated tension members <NUM> are provided. In addition, in this embodiment, height H of each end 160a is the same, lateral distances D to each end 160a is the same. For most subsea applications, lateral distance D is preferably between <NUM> and <NUM> feet, and more preferably about <NUM> ft. However, it should be appreciated that lateral distance D may depend, at least in part, on the available connection points to the frame <NUM> of BOP <NUM>. As will be described in more detail below, each height H is preferably as high as possible but below LMRP <NUM>, and may depend on the available connection points along frame <NUM> of BOP <NUM>.

As best shown in <FIG>, a tensile preload L is applied to each tensioned span <NUM>. With no external loads or moments applied to BOP <NUM>, the actual tension in each span <NUM> is the same or substantially the same as the corresponding tensile preload L. However, it should be appreciated that when external loads and/or bending moments are applied to BOP <NUM>, the actual tension in each span <NUM> can be greater than or less than the corresponding tensile preload L.

Winches <NUM> are positioned proximal to the sea floor <NUM>, and ends 160a are coupled to frame <NUM> of BOP <NUM> above winches <NUM>. Thus, each span <NUM> is oriented at an acute angle α measured upward from horizontal. Since portions <NUM> are in tension and oriented at acute angles α, the tensile preload L applied to frame <NUM> of BOP <NUM> by each span <NUM> includes an outwardly oriented horizontal or lateral preload L<NUM> and a downwardly oriented vertical preload Lv. Without being limited by this or any particular theory, the lateral preload L<NUM> and the vertical preload Lv applied to BOP <NUM> by each tension member <NUM> are a function of the corresponding tensile load L and the angle α. For a given angle α, the lateral preload L<NUM> and the vertical preload Lv increase as the tensile load L increases, and decrease as the tensile load L decreases. For a given tensile load L, the lateral preload L<NUM> decreases and the vertical preload Lv increases as angle α increases, and the lateral preload L<NUM> increases and the vertical preload Lv decreases as angle α decreases. For example, at an angle α of <NUM>°, the lateral preload L<NUM> and the vertical preload Lv are substantially the same; at an angle α above <NUM>°, the lateral preload Ll is less than the vertical preload Lv; and at an angle α below <NUM>°, the lateral preload Ll is greater than the vertical preload Lv. In embodiments described herein, angle α of each span <NUM> is preferably between <NUM>° and <NUM>°, and more preferably between <NUM>° and <NUM>°.

The lateral preloads L<NUM> applied to frame <NUM> of BOP <NUM> resist external lateral loads and bending moments applied to BOP <NUM> (e.g., from subsea currents, riser <NUM>, etc.). To reinforce and stabilize BOP <NUM>, wellhead <NUM>, and primary conductor <NUM> without interfering with an emergency disconnection of LMRP <NUM>, each height H is preferably as high as possible but below LMRP <NUM>, and may depend on the available connection points along frame <NUM> of BOP <NUM>. In this embodiment, ends 160a are coupled to frame <NUM> proximal the upper end of BOP <NUM> and just below LMRP <NUM>. By tethering frame <NUM> of BOP <NUM> at this location, system <NUM> restricts and/or prevents BOP <NUM>, tree <NUM>, wellhead <NUM>, and primary conductor <NUM> from moving and bending laterally, thereby stabilizing such components, while simultaneously allowing LMRP <NUM> to be disconnected from BOP <NUM> (e.g., via emergency disconnect package) without any interference from system <NUM>.

Referring again to <FIG> and <FIG>, the tensile preload L in each span <NUM> is preferably as low as possible but sufficient to pull out any slack, curve, and catenary in the corresponding span <NUM>. In other words, the tensile preload in L in each span <NUM> is preferably the lowest tension that results in that span <NUM> extending linearly from the corresponding winch <NUM> to its end 160a. It should be appreciated that such tensile loads L in tension members <NUM> restrict and/or prevent the initial movement and flexing of BOP <NUM> at the onset of the application of an external loads and/or bending moments, while minimizing the tension in each span <NUM> before and after the application of the external loads and/or bending moments. The latter consequence minimizes the potential risk of inadvertent damage to BOP <NUM>, tree <NUM>, and LMRP <NUM> in the event one or more tension members <NUM> uncontrollably break.

In general, each tension member <NUM> can include any elongate flexible member suitable for subsea use and capable of withstanding the anticipated tensile loads (i.e., the tensile preload L as well as the tensile loads induced in spans <NUM> via the application of external loads to BOP <NUM>) without deforming or elongating. Examples of suitable devices for tension members <NUM> can include, without limitation, chain(s), wire rope, and Dyneema® rope available from DSM Dyneema LLC of Stanley, North Carolina USA. In this embodiment, each tension member <NUM> comprises Dyneema® rope, which is suitable for subsea use, requires the lowest tensile preload L to pull out any slack, curve, and catenary (~ <NUM> ton of tension), and is sufficiently strong to withstand the anticipated tensions.

Referring now to <FIG>, an alternate tethering system <NUM> includes a plurality of anchors <NUM>, a plurality of tensioning systems <NUM>, a plurality of flexible tension members <NUM>. A tension member <NUM> is connected to the top of each of the plurality of anchors <NUM> and extends from each anchor <NUM> to a tensioning system <NUM> mounted on frame <NUM> of BOP <NUM>. Each tensioning system <NUM> includes a winch or spool <NUM> that can pay in and pay out the corresponding tension member <NUM> and a gripping mechanism <NUM> to engage the tension member <NUM>. A winch refers to a reel having sufficient tensioning capacity to apply to the tension member <NUM> a tensile preload L as discussed in the description of <FIG>. A spool refers to a reel having a tensioning capacity to apply at least a tension sufficient to avoid sagging of the tension member <NUM>, but the tension member <NUM> may remain slack. A spool may, however, have a tensioning capacity larger than the tension required to prevent sagging and may have as much tensioning capacity as a winch.

<FIG> illustrates one embodiment of an anchor <NUM> to which a tension member <NUM> is connected. Anchor <NUM> may be a driven piling, a clump weight, a suction piling, plate anchor, or any other structure used to affix a base to the sea floor. The top of the anchor <NUM> includes a connecting ring <NUM>, or other feature that provides a location for affixing an end of a tension member <NUM>. Tension member <NUM> includes a releasable connector <NUM> that allows an ROV to selectively engage and disengage the tension member <NUM> from the anchor <NUM>.

The gripping system <NUM> is coupled to the BOP frame <NUM> or be part of or mounted on the BOP <NUM>. <FIG> illustrate one embodiment of a tensioning system <NUM> including a frame <NUM>, gripping system <NUM>, winch reel <NUM>, and ROV interface <NUM>. The tensioning system <NUM> may be removeably connected to the BOP frame <NUM> at connection points <NUM>. An ROV, or other equipment, can be used to install each tensioning system <NUM> onto the BOP frame <NUM> subsea by flying the tensioning system <NUM> into place and connecting it to the connection points <NUM>. The ROV can then be used to rotate winch reel <NUM> paying in or out tensioning member <NUM> as needed. Once the tensioning member <NUM> is properly installed, gripping system <NUM> can be activated to grip the tensioning member <NUM> and maintain any tension in the tension member <NUM>. Gripping system <NUM> can be any type of gripping system that can apply a fixing force to the tensioning member <NUM>, such as a hydraulic slip system, a locking ring, or a brake mechanism. The hydraulic slip is used for paying in or out tension member <NUM>. The hydraulic slip includes one or more hydraulic cylinders and a slip or gripper that may have a first configuration for paying in the tension member <NUM>, applying to the tension member <NUM> a tensile preload L as discussed in the description of <FIG>, preventing pay out of the flexible tension member <NUM>, and/or maintaining any tension in the tension member <NUM>. The one or more hydraulic cylinders and the slip or gripper may have a second configuration for paying out the tension member <NUM>.

Referring now to <FIG>, an alternative tensioning system <NUM> is shown that includes a frame <NUM> and gripping system <NUM> connected to a BOP frame <NUM> at connection points <NUM>. The winch assembly <NUM> is located remote from the tensioning system <NUM>, such as at the surface on the drilling or service vessel. Winch assembly <NUM> is used to apply pay in or out to tension member <NUM> as needed to achieve the desired tension. Once the tension member <NUM> is properly tensioned, gripping system <NUM> can be activated to grip the tension member <NUM> and maintain any tension in the tension member <NUM>.

Claim 1:
A system (<NUM>) for tethering a subsea blowout preventer (BOP) (<NUM>), the system comprising:
an anchor (<NUM>) disposed about the subsea BOP;
a flexible tension member (<NUM>), wherein the flexible tension member has a first end including a releasable connector engaged to the anchor, wherein the flexible tension member extends horizontally and vertically from the first end to a second end to impart a lateral preload and a vertical preload to the subsea BOP; characterized in that the system comprises:
a gripping system (<NUM>, <NUM>) mounted on the subsea BOP, wherein the gripping system is configured to selectively engage the flexible tension member to prevent pay out of the flexible tension member; and further comprises a spool (<NUM>) coupled to the second end of the flexible tension member; and
a hydraulic cylinder coupled to the gripping system, wherein actuation of the hydraulic cylinder causes paying in or out the flexible tension member.