GB2624486A

GB2624486A - Apparatus, system and method for tethering a subsea well assembly

Info

Publication number: GB2624486A
Application number: GB2306003.1A
Authority: GB
Inventors: MacDonald Stewart James; Alexander Gilmour Gourley Charles
Original assignee: Tla Subsea Ltd
Current assignee: Tla Subsea Ltd
Priority date: 2023-04-24
Filing date: 2023-04-24
Publication date: 2024-05-22
Also published as: GB202306003D0

Abstract

A tethering system 32 is configured to tether a subsea well control equipment to a subsea mount arranged on a seabed comprises a load chain L, a first chain block C1 having a first gear ratio, and a second chain block C2 having a second gear ratio. The first chain block C1 is configured to convey the load chain to apply a first tension to the tethering member. The second chain block is configured to subsequently convey the load chain to apply a second tension to the tethering member; wherein the second gear ratio is higher than the first gear ratio.

Description

APPARATUS, SYSTEM AND METHOD FOR TETHERING A SUBSEA WELL

ASSEMBLY

TECHNICAL FIELD

The present invention relates to an apparatus, system and method for tethering a subsea well assembly, particularly a subsea well control equipment of a subsea well assembly.

INTRODUCTION

During its lifespan, a subsea well assembly can operate in a variety of modes, including a construction, production, intervention, de-construction and/or abandonment mode. A subsea well assembly typically comprises a subsea well control equipment during one or more modes of operation. A subsea well control equipment is provided to safeguard the subsea well assembly during one or more mode of operation. For example, the subsea well control equipment may control pressure in the wellbore, control wellbore access, and isolate a wellbore when necessary. The type of subsea well control equipment may vary according to the type of subsea well assembly and type of mode. Examples of subsea well control equipment include a subsea blowout preventer (BOP), subsea intervention lubricator (SI L) or a lower riser package (LRP).

Figure 1 depicts an example of a conventional subsea well assembly 10 in a construction, intervention, de-construction and/or abandonment mode. As shown, the subsea well assembly 10 comprises a tree 16 releasably connected to a wellhead 18 disposed at an upper end of a primary conductor 20 extending into a wellbore 22, a subsea blowout preventer (BOP) 24 releasably connected to the tree 16, and a lower marine riser package (LM RP) 26 releasably connected to the subsea BOP 24. The tree 16 may not necessarily be present, for example, during drilling operations. The subsea BOP 24 is provided as a subsea well control equipment to control pressure in the wellbore 22, control access to the wellbore 22 and isolate the wellbore 22, when necessary, so as to prevent blowouts caused by an uncontrolled release of crude oil or natural gas from the well. The subsea BOP may be part of the subsea well assembly during one or more mode of operation, for example a construction, intervention, de-construction and/or abandonment mode. The tree 16, the subsea BOP 24, and the LMRP 26 are at least substantially vertically arranged or stacked one-above-the-other, and are generally coaxially aligned with the wellhead 18. The subsea well assembly further comprises a riser 28 to connect the subsea well assembly to a floating vessel 12 at the sea surface 14.

During the lifespan, the subsea well assembly is subject to various loads including cyclical loads due to riser movement (for example, from surface vessel motions, wave actions, vortex induced vibrations, or combinations thereof) and environmental loads such as subsea currents. Together, these loads can induce fatigue in one or more component of the subsea well assembly, such as the tree 16 when present, the wellhead 18 and the primary conductor 20. Over time these loads may compromise the integrity of the subsea well assembly. This may be of particular concern due to the configuration, weight, and vertical arrangement of the subsea well assembly components, which present a relatively large surface area for interacting with the subsea current loads. Additionally, the loads can induce bending moments and associated stresses in one or more components of the subsea well assembly, which may, for example, be increased when the relatively tall and heavy combination of a tree and subsea well control equipment are angled relative to vertical. The bending moments and associated stresses further induce fatigue in the subsea assembly.

The increasing size and weight of subsea BOPs and longer drill times that are now commonly employed also increase the risk of subsea well assembly fatigue. In particular, the risk of fatigue when using newer generation subsea BOPs with legacy subsea well assemblies which have a less robust design.

The present invention relates to a solution to mitigate the risk of subsea assembly fatigue described above.

SUMMARY OF INVENTION

Embodiments of the present invention relate to a tethering solution to improve the strength and fatigue performance of subsea well assemblies comprising a subsea well control equipment. The subsea well control equipment may be, but is not limited to, a subsea blowout preventer (BOP), a subsea intervention lubricator (SIL) or a lower riser package (LRP).

A first aspect of the invention provides a subsea tensioning apparatus to tension a tethering member, wherein the tethering member is configured to tether a subsea control equipment to a subsea mount arranged on a sea bed.

The subsea tensioning apparatus is arrangeable, in use, to be coupled to the tethering member.

The subsea tensioning apparatus comprises: a first chain block, wherein the first chain block is configured to apply a first tension to the tethering member; and a second chain block, wherein the second chain block is configured to subsequently apply a second tension to the tethering member.

The tensioning apparatus applies tension to the tethering member in a sequential, two stage tensioning process. The first chain block initially applies the first tension to the tethering member during a first tensioning stage. The second chain block then subsequently applies the second tension to the tethering member, during a second tensioning stage, such that the tethering member is under a final tethering tension.

Preferably, the final tethering tension (combined first tension and second tension) are sufficient for the final tethering tension to at least restrict, preferably inhibit, movement in the tethering member.

By applying the final tethering tension to restrict, preferably inhibit, movement in the tethering member, the tensioning apparatus advantageously restricts, preferably inhibits, movement and lateral bending of the subsea well control equipment. It follows that the tensioning apparatus advantageously enhances the stability and fatigue performance of a subsea well assembly by restricting, preferably inhibiting, movement and lateral bending of the subsea well control equipment.

The tensioning apparatus may be configured to operate in a tension mode to apply tension to the tethering member. To provide the first tensioning stage, the tensioning apparatus may be configured to operate in a first tension mode, wherein the first chain block is configured to initially apply the first tension to the tethering member. To provide the second tensioning stage, the tensioning apparatus may be configured to operate in a second tension mode, wherein the second chain block is configured to subsequently apply the second tension to the tethering member.

Optionally, the tensioning apparatus may comprise a load chain separately conveyable by the first chain block and the second chain block, wherein the first chain block is configured to convey (pay-in) the load chain to apply the first tension to the tethering member and wherein the second chain block is configured to convey (pay-in) the load chain to apply the second tension to the tethering member.

Optionally, the tensioning apparatus may comprise a first coupling and a second coupling.

Optionally, the first coupling and second coupling may be axially aligned and arranged on opposite sides of the tensioning apparatus.

Optionally, the first coupling may be a movable coupling mounted on the conveyable load chain.

Optionally, the second coupling may be a fixed coupling mounted on a fixed portion of the tensioning apparatus.

Optionally, the first coupling and/or the second coupling may be a rotatable coupling to improve the tethering configuration and arrangement of the tensioning apparatus.

Optionally, the first coupling and/or second coupling may be a releasable coupling to allow for attachment and detachment of the tensioning apparatus. The releasable first coupling and releasable second coupling allows for the reuse, retrofitting, removal and/or reinstallation of the tensioning apparatus in one or more subsea well assembly. For example, the first coupling and/or second coupling may comprise a hook. The hook may include a safety catch to allow for the tensioning apparatus to be securely and releasably coupled to the tethering member, and to the subsea well control equipment or subsea mount during use.

Optionally, the tensioning apparatus may be arrangeable to be coupled to the subsea well control equipment and coupled to the tethering member: For example, the tensioning apparatus may be arrangeable in a first arrangement where the first coupling is configured to engage the tethering member and the second coupling is configured to engage the subsea well control equipment.

Alternatively, for example, the tensioning apparatus may be arrangeable in a second arrangement where the first coupling is configured to engage the subsea well control equipment and the second coupling is configured to engage the tethering member.

Optionally, the tensioning apparatus may be arrangeable to be coupled to the subsea mount and coupled to the tethering member: For example, the tensioning apparatus may be arrangeable in a third arrangement where the first coupling is configured to engage the tethering member and the second coupling is configured to engage the subsea mount.

Alternatively, for example, the tensioning apparatus may be arrangeable in a fourth arrangement where the first coupling is configured to engage the subsea mount and the second coupling is configured to engage the tethering member.

Optionally, the first chain block may comprise: a first rotatable chain sheave configured to convey the load chain; the first gear assembly, with a first gear ratio, configured to rotate the first chain sheave; a first actuator configured to actuate the first gear assembly in response to a driving input.

To operate the first chain block in the first tension mode, the first actuator is configured to receive a first driving input. In response to the first driving input, the first actuator actuates the first gear assembly to rotate the first chain sheave by a first rotation. The rotating first chain sheave conveys the load chain to pay-in the load chain, wherein the conveying (paying-in) load chain has a first conveying force and a first conveying rate. The conveying (paying-in) action of the load chain by the first chain block is transmitted to the tethering member coupled to the tensioning apparatus.

The conveying (paying-in) load chain with the first conveying force provides a first tensioning effect on the tethering member, whereby the first tension is applied to the tethering member.

The conveying (paying-in) load chain at the first conveying rate provides a displacing effect on the tethering member, whereby the tethering member becomes taut under the first tension. As such, the first chain block advantageously displaces the tethering member to remove slack when applying the first tension.

In an arrangement of the tensioning apparatus where the tensioning apparatus is coupled to the subsea well equipment, the first tension is applied and the tethering member is displaced in a tensioning direction towards the subsea well control equipment.

In an arrangement of the tensioning apparatus where the tensioning apparatus is coupled to the subsea mount, the first tension is applied and the tethering member is displaced is in a tensioning direction towards the subsea mount.

Optionally, the second chain block may comprise: a rotatable second chain sheave to convey the load chain; a second gear assembly, with a second gear ratio, configured to rotate the second chain sheave; a second actuator configured to actuate the second gear assembly in response to a driving input.

To operate the second chain block in the second tension mode, the second actuator is configured to receive a second driving input. In response to the second driving input, the second actuator actuates the second gear assembly to rotate the second chain sheave by a second rotation. The rotating second chain sheave conveys the load chain to pay-in the load chain, wherein the conveying load chain has a second conveying force and a second conveying rate. The conveying (paying-in) action of the load chain by the second chain block is transmitted to the tethering member coupled to the tensioning apparatus.

The conveying (paying-in) load chain under the second conveying force provides a second tensioning effect on the tethering member already taut under the first tension, whereby the second tension is applied to the tethering member. Following the application of the second tension, the tethering member is under the final tethering tension (combined first and second tension).

The conveying (paying-in) load chain at the second conveying rate also provides a stretching/tightening effect on the tethering member already taut under the first tension, whereby the tethering member becomes sufficiently stiff under the final tension to restrict, preferably inhibit, movement of the tethering member.

The conveying action of the first chain block allows for continuous (non-discrete) displacement of the tethering member to remove slack. By using the conveying action of the first chain block to remove slack in the tethering member, the tensioning apparatus is not limited to tensioning a tethering member of only a specific pre-cut length and allows for the tensioning of tethering members with variable lengths. The tensioning apparatus allows for the tensioning of the tethering member where the subsea mount is locatable at variable distances from the subsea well control equipment.

The conveying action of the first chain block and second chain block allows for high resolution adjustment of the tension of the tethering member. The tensioning apparatus is suitable for use, and re-use, at different subsea well assembly sites.

Optionally, the first actuator of the first chain block may be a mechanically driven actuator. The second actuator of the second chain block may be a mechanically driven actuator.

Alternatively, first actuator may be a hydraulically driven actuator. The second actuator assembly may be a hydraulically driven actuator.

Optionally, the first actuator of the first chain block may be operable by a first driving tool to provide the first driving input. The first actuator may comprise a first interface configured to engage the first driving tool and a first drive shaft to be operably driven by the first driving tool.

Optionally, the second actuator of the second chain block may be operable by a second driving tool to provide the second driving input. The second actuator may comprise a second interface configured to engage the second driving tool and a second drive shaft to be operably driven by the second driving tool.

Optionally, the first driving tool and/or second driving tool may be carried and controlled by a Remotely Operated Vehicle (ROV), and the first interface and/or second interface may be an ROV interface.

Optionally, the first driving tool and the second driving tool may be same driving tool.

For example, the first driving tool and the second driving tool may be a common driving tool carried and controlled by the ROV to drive the first actuator and the second actuator.

Optionally, the first driving input may have a first torque and a first speed.

Optionally, the second driving input may have a second torque and a second speed.

Optionally, the first driving input and the same driving input received by the respective chain blocks are the same. As such, each chain block is actuated by the same driving input torque and the same driving input speed.

Optionally, the first actuator of the first chain block may comprise a first clutch to control the first driving input by the first driving tool.

Optionally, the second actuator of the second chain block may comprise a second clutch to control the second driving input by the second driving tool.

Optionally, the first gear assembly may comprise a plurality of gears configured to provide the first gear ratio. Likewise, the second gear assembly may comprise a plurality of gears configured to provide the second gear ratio.

The mechanical advantage of each chain block is dependent on the gear ratio of each chain block. As such, the conveying force of conveying load by each chain block, and thereby the tension applied to the tethering member by each chain block, is dependent on the gear ratio and the driving input torque of each chain block. Also, the conveying rate of the load chain by each chain block is dependent on the gear ratio and the driving input speed of each chain block.

Therefore, optionally, to optimise the two-stage tensioning process, the first chain block and the second chain block may be configured whereby the second gear ratio is higher than the first gear ratio.

By configuring the second chain block to have a higher gear ratio than the first chain block, the second tension applied to the tethering member by the second chain block is greater than the first tension applied to the tethering member by the first chain block.

Also, by configuring the second chain block to have a higher gear ratio than the first chain block, the first conveying rate of the load chain by the first chain block is faster than the second conveying rate of the load chain of the second chain block.

Hence, the first chain block with the lower first gear ratio is advantageously able to displace the tethering member to remove slack more quickly, and at a lower tension, than the second chain block.

Further, by configuring the second chain block to have a higher gear ratio than the first gear block, the first chain block and second chain block may be actuated by the same driving input (driving input torque and driving input speed) to apply the respective first tension and the second tension.

Optionally, the first chain block may comprise a first brake for the first chain sheave.

The first brake may be configured inhibit back rotation of the first chain sheave as the first tension is being applied to the tethering member during the first tensioning stage. As such, the first brake may at least substantially maintain the first tension applied to the tethering member.

Optionally, the second chain block may comprise a second brake for the second chain sheave. The second brake may be configured to inhibit back rotation of the second chain sheave as the second tension applied to the tethering member during the second tensioning stage. As such, the second brake may least substantially maintain the second tension applied to the tethering member.

Optionally, the tensioning apparatus may be configured to operate in a slack mode to reduce tension in the tethering member. The tensioning apparatus may be configured to operate in a first slack mode, wherein the first chain block may be configured to at least reduce, preferably remove, the first tension applied to the tethering member. The tensioning apparatus may be configured to operate in a second slack mode, wherein the second chain block may be configured to at least reduce, preferably remove, the second tension applied to the tethering member.

Optionally, the first brake may be releasable to allow for reverse rotation of the first chain sheave. Reverse rotation of the first chain sheave reverse conveys the load chain to pay-out the load chain. The reverse conveying (paying-out) of the load chain causes the first tension in the tethering member to be at least reduced. As such, to operate the first chain block in the first slack mode, the first chain block may be configured to release the first brake to at least reduce the first tension previously applied to the tethering member during the first tensioning stage.

Optionally, the second brake may be releasable to allow for reverse rotation of the second chain sheave. Reverse rotation of the second chain sheave reverse conveys (pays-out) the load chain. The reverse conveying (paying-out) of the load chain causes the second tension in the tethering member to be at least reduced. As such, to operate the second chain block in the second slack mode, the second chain block may be configured to release the second brake to at least reduce the second tension previously applied to the tethering member during the second tensioning stage.

Optionally, the tensioning apparatus may comprise a tension monitoring system configured to monitor tension in the tethering member. For example, the tensioning monitoring system may comprise a tension load cell and/or a tension load pin to monitor tension in the tethering member. The tension monitoring system may communicate the measured tension to a remote operator or other third party to allow for tension monitoring, control and quantification of the external loads on the subsea well control equipment, and allow for rapid identification of tethering member failure. The tension monitoring system may communicate the measured tension in the tethering member to the first actuator and/or second actuator to provide loop feedback control.

Optionally, the tensioning apparatus may comprise a fail-safe clutch configured to automatically release at least some of the tension in the tethering member under excessive loading. In this way, the fail-safe clutch may minimise the potential risk of damage to the tensioning apparatus, subsea well control equipment and/or other subsea well assembly components under excessive loading.

Optionally, the tensioning apparatus may comprise a frame to rigidly support the first chain block and the second chain block. The frame may protect the tensioning apparatus from damage during use and allow for ease of handling.

Optionally, the tensioning apparatus may comprise a buoyancy aid to aid the buoyancy and control the orientation of the tensioning apparatus during use in subsea conditions. For example, the buoyancy aid may be coupled to the frame.

The buoyancy aid may maintain the orientation of the tensioning apparatus while subsea to help avoid twisting of the tethering member. The buoyancy aid may maintain the orientation of the tensioning apparatus so that the actuator interface is in a generally vertical plane relative to the seabed. In this orientation, a driving tool carried by ROV may be able to more easily interface with the chain block.

Optionally, to aid the mechanical advantage of the tensioning apparatus, the tensioning apparatus may comprise at least one pulley around which the load chain is looped. For example, the tensioning apparatus may comprise a compound pulley system comprising a fixed pulley and a moveable pulley.

A second aspect of the invention provides a subsea method of tensioning a tethering member, wherein the tethering member is tethering a subsea well control equipment to a subsea mount, the method comprising: providing a subsea tensioning apparatus, wherein the subsea tensioning apparatus comprises a first chain block, a second chain block, a load chain configured to be separately conveyed by the first chain block and the second chain block; coupling the subsea tensioning apparatus to the subsea well control equipment or the subsea mount; coupling the subsea tensioning apparatus to an end of the tethering member; actuating the first chain block to convey (pay-in) the load chain to apply a first tension to the tethering member; after applying the first tension, actuating the second chain block to convey (pay-in) the load chain to apply a second tension to the tethering member.

Optionally, the tensioning apparatus is the tensioning apparatus according to the first aspect of the invention.

Optionally, actuating the first chain block may comprise: driving the first actuator with a first driving input to actuate the first gear assembly with the first gear ratio; actuating the first gear assembly with the first gear ratio to rotate the first chain sheave by a first rotation; rotating the first chain sheave by the first rotation to convey the load chain with a first conveying force and at a first conveying rate; conveying the load chain with a first conveying force and at the first conveying rate so as to apply the first tension to the tethering member.

Optionally, actuating the second chain block may comprise: driving the second actuator with a second driving input to actuate the second gear assembly with the second gear ratio; actuating the second gear assembly with the second gear ratio to rotate the second chain sheave by a second rotation; rotating the second chain sheave by the second rotation to convey the load chain with a second conveying force and at a second conveying rate conveying the load chain with the second conveying force and at the second conveying rate so as to apply the second tension to the tethering member.

A third aspect of the invention provides a subsea tensioning system for tethering a subsea well control equipment to a subsea mount, the system comprising a tethering member arrangeable to tether the subsea well control equipment to the subsea mount; a subsea tensioning apparatus arrangeable to be coupled to the tethering member and to apply tension to the tethering member, wherein the tensioning apparatus comprises: a first chain block to apply a first tension to the tethering member; a second chain block to subsequently apply a second tension to the tethering member.

Optionally, the system may comprise the subsea mount arrangeable on a sea bed in a spaced relationship to the subsea well control equipment.

Optionally, the subsea mount may comprise a third coupling.

Optionally, when the tensioning apparatus is arranged to be coupled to the subsea well control equipment and coupled to the tethering member, the first end portion of the tethering member may be coupled to the first coupling or the second coupling of the tensioning apparatus, and the second end portion of the tethering member may be coupled to the third coupling of the subsea mount.

Optionally, when the tensioning apparatus is arranged to be coupled to the subsea mount and coupled to the tethering member, the second end portion of the tethering member may be coupled to the first coupling or the second coupling of the tensioning apparatus, and the first end portion of the tethering member may be coupled to the fourth coupling of the subsea well control equipment.

A fourth aspect of the invention provides a subsea method of tethering under tension a subsea well control equipment, the method comprising: providing a subsea mount; providing a tethering member; providing a tensioning apparatus, wherein the subsea tensioning apparatus comprises a first chain block, a second chain block, a load chain configured to be separately conveyed by the first chain block and the second chain block; arranging the subsea mount on the sea bed in spaced relationship to the subsea well control equipment; coupling the tensioning apparatus to the subsea well control equipment or the subsea mount; coupling an end of the tethering member to the tensioning apparatus; coupling a respective other end of the tethering member to the respective other of the subsea mount or the subsea well control equipment; actuating the first chain block to convey the load chain to apply a first tension to the tethering member; after applying the first tension, actuating the second chain block to convey the load chain to apply a second tension to the tethering member.

Optionally, wherein the tensioning apparatus may be a tensioning apparatus according to the first aspect of the invention.

It will be appreciated that any feature described herein as being suitable for incorporation into one or more aspects or embodiments of the present invention is intended to be generalisable across any aspect or embodiment of the present

disclosure.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate presently exemplary embodiments of the disclosure, and together with the general description given above and the detailed description of the embodiments given below, serve to explain, by way of example, the principles of the disclosure.

Figure 1 shows a schematic view of a conventional example of a subsea well assembly with a subsea blowout preventer (BOP); Figure 2A shows a schematic view of an example of a subsea well assembly with a BOP and tensioning system according to the present invention; Figure 2B shows a schematic view of an example of a subsea well assembly with a subsea intervention lubricator (SIL) and tensioning system according to the present invention; Figure 2C shows a schematic view of an example of a subsea well assembly with a lower riser package (LRP) and tensioning system according to the present invention; Figure 3 shows a view of an example of a subsea tethering system according to the present invention, where a subsea tensioning apparatus is coupled to a subsea blowout preventer (BOP) and a tethering member is coupled to the subsea tensioning apparatus and a subsea mount to tether the subsea BOP to the subsea mount; Figure 4 shows a view of an alternative example of a subsea tethering system according to the present invention, where a subsea tensioning apparatus is coupled to a subsea mount and a tethering member is coupled to the subsea tensioning apparatus and a subsea blowout preventer (BOP) to tether the subsea BOP to the subsea mount; Figure 5A shows a front-on view of an example subsea tensioning apparatus according to the present invention; Figure 5B shows a perspective view of the example of the subsea tensioning apparatus shown in Figure 5A; Figure 5C shows a side-on view of the example of the subsea tensioning apparatus shown in Figure 5A; Figure 5D shows a top view of the subsea tensioning apparatus shown in Figure 5A; Figure 5E shows a cross-sectional view along A-A' of the first chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 5F shows a cross-sectional view along B-B' of the second chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 6A shows an enlarged front-on view of the first gear assembly of the first chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 63 shows an enlarged perspective view of the first gear assembly of the first chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 6C shows a bottom view of the first gear assembly of the first chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 6D shows rear view of the subsea tensioning apparatus shown in Figure 5A with the first gear assembly cover omitted; Figure 7A shows an enlarged front-view of the second gear assembly of the second chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 73 shows an enlarged perspective view of the second gear assembly of the second chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 7C shows a bottom view of the second gear assembly of the second chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 7D shows an enlarged front-on view of the subsea tensioning apparatus shown in Figure 5A with the second gear assembly cover omitted; Figure 8 shows an view of a first example of a subsea tethering system according to the present invention, where the subsea tethering system is tethering a subsea BOP to a subsea mount; Figure 9 shows an enlarged view of the subsea tensioning apparatus of the subsea tensioning system of Figure 8; Figure 10 shows a view of a second example of a subsea tethering system according to the present invention, where the subsea tethering system is tethering a subsea BOP to a subsea mount; Figure 11 shows a view of a third example of subsea tethering system according to the present invention, where the subsea tethering system is tethering a subsea BOP to a subsea mount; Figure 12 is a view of a fourth example of a subsea tethering system according to the present invention, where the subsea tethering system is tethering a subsea BOP to a subsea mount; Figure 13 shows a perspective view of an example of a sea mount of a tensioning system according to the present invention; Figure 14A shows a view of an example of a brake for a chain block of the subsea tensioning apparatus shown in Figure 5A; Figure 143 shows a perspective view of the brake for the chain block shown in Figure 14A; Figure 15 shows an example of a method for tensioning a tethering member according to the present invention, whereby the tethering member is tethering a subsea well control equipment to a subsea mount; Figure 16A and 169 show an example of a method for tethering a subsea well control equipment under tension according to the present invention.

DETAILED DESCRIPTION

The present invention relates to a tensioning apparatus, system and methods to tether under tension a subsea well control equipment of a subsea well assembly.

Examples of subsea well control equipment include, but not limited to, a subsea blowout preventer (BOP), a subsea intervention lubricator (SIL), and a lower riser package (LRP).

Figure 2A depicts a first example of a subsea well assembly 10' according to the present invention. In the example depicted, the subsea well assembly is arranged for construction, intervention, de-construction and/or abandonment mode, and the subsea well control equipment comprises a subsea blowout preventer (BOP) 24. The subsea well assembly 10' further comprises a tree 16 releasably connected to a wellhead 18 disposed at an upper end of a primary conductor 20 extending into a wellbore 22. The subsea BOP 24 is releasably connected to the tree 16, and a lower marine riser package (LMRP) 26 releasably connected to the subsea BOP 24. The tree 16, subsea BOP 24, and LMRP 26 are vertically arranged or stacked oneabove-the-other, and are generally coaxially aligned with wellhead 18. The subsea well assembly also further comprises a riser 28, wherein the riser 28 connects the subsea well assembly 10' to a floating vessel 12 at the sea surface 14. The tree 16 may not necessarily be present, for example, during drilling operations. The wellhead 18 has a central axis and extends vertically upward from wellbore 22 above the seabed 30.

A second example of a subsea well assembly 10" according to the present invention is depicted in Figure 2B. In this example, the subsea well assembly is arranged for intervention and/or abandonment mode, and the subsea well control equipment comprises a subsea intervention lubricator (SIL) 24'. The SIL 24' is releasably connected to the tree 16. A riser 28 extends from the SIL 24' to a tool catcher 44. A wireline 46 extends between the tool catcher 44 and a floating vessel 12 at the sea surface 14. The purpose of such well assemblies is to allow a downhole tool to be deployed from the wireline 46 into the subsea well, for example, to enhance production or to measure conditions in the well.

A third example a subsea well assembly 10" according to the present invention is depicted in Figure 2C. In this example, the subsea well assembly is an open water intervention riser system arranged in construction, intervention and/or abandonment mode, and the subsea well control equipment comprises a lower riser package (LRP) 24" releasably connected to the tree 16. An emergency disconnect package (EDP) 48 is arranged above and releasably connected to the LRP (24"). The subsea well assembly further comprises a riser 28 extending between the EDP 48 and a floating vessel 12 at the sea surface 14. The riser 28 may comprise a dual bore riser as shown, or mono bore riser.

To minimise load fatigue, the subsea well assemblies 10', 10", 10-further comprises one or more tethering system 32 for tethering the subsea well control equipment under tension.

For the sake of clarity and brevity, the tensioning apparatus and system according to the present invention are described below in connection to a subsea well assembly comprising a subsea BOP, where the subsea BOP is tethered under tension to a subsea mount. However, it will be appreciated that the tensioning apparatus and systems described below may be deployed and used with subsea well assemblies comprising any other suitable subsea well-control equipment, including, but not limited to, a SIL or LRP.

As shown in the examples depicted in Figures 3, 4, 8 to 12, the tethering system 32 according to the present invention comprises a subsea mount 34 arranged on a sea bed 30 in spaced relationship to the BOP 24, a tethering member 36 to tether the BOP to the subsea mount, and a subsea tensioning apparatus 38 to tension the tethering member.

An optimal layout for the tensioning of the subsea BOP 24 may comprise four subsea tensioning systems 32 that are arranged equi-spaced around the well and secured to the subsea BOP at each corner with a tethering member angle of 45 degrees. The radial distance from the well is a function of the tethering member attachment point on the subsea BOPs and is typically around 20m. However, any suitable number and arrangement of tensioning systems may be utilised to provide a desired tethering effect of the subsea BOP.

As shown in an example of the tethering system 32 depicted in Figure 3, the tensioning apparatus 38 may be secured to the subsea BOP 24. The tethering member 36 may be coupled to the tensioning apparatus 38 and the subsea mount 34 so as to tether the subsea BOP 24 to the subsea mount 34. For example, as shown in Figure 3, an upper end portion 36a of the tethering member may be coupled to the tensioning apparatus 38 and a lower end portion 36b of the tethering member may be coupled to the subsea mount 34.

Alternatively, as shown in an example of the tethering system 32 depicted in Figure 4, the tensioning apparatus 38 may be secured to the subsea mount 34. The tethering member 36 may be coupled to the tensioning apparatus 38 and the subsea BOP 24 so as to tether the subsea BOP 24 to the subsea mount 34. For example, as shown in Figure 4, the lower end portion 36b of the tethering member may be coupled to the tensioning apparatus 38 and the upper end portion 36a of the tethering member may be coupled to the subsea BOP 24.

The subsea tensioning apparatus is configured to tension the tethering member whereby the tethering member is taut (without slack) and under a tensile load that is sufficient to restrict, preferably prevent, undesired movement and lateral bending of the subsea BOP when subjected to external loads during one or more mode of operation of the subsea well assembly. As such, undesired movement and lateral bending of the subsea BOP due to the external loads is impeded, and the overall fatigue performance of the subsea well assembly is improved.

In the present invention, the tensioning apparatus comprises a first chain block Cl configured to apply a first tension to the tethering member 36. The tensioning apparatus further comprises a second chain block C2 configured to apply a second tension to the tethering member 36.

The tensioning apparatus 38 is configured to operate in a first tension mode whereby the first chain block Cl is configured to initially apply the first tension to the tethering member 36. Any slack in the tethering member is removed by the application of the first tension to the tethering member. Subsequent to the first tension mode, the tensioning apparatus is configured to operate in a second tension mode whereby the second chain block 02 is configured to sequentially apply the second tension to the tethering member. As such, the tensioning apparatus 38 operates in the first tension mode and second tension mode to apply tension to the tethering member 36 in a two stage tensioning process. Together, the combined first tension and second tension subject the tethering member to a final tension.

Figures 5A to 5F depict a schematic arrangement of an example of the tensioning apparatus 38 according to the present invention which may be used to apply tension to a tethering member 36 that is tethering a subsea BOP 24 to a subsea mount 34.

As seen in Figures 5A to 5F, the tensioning apparatus 38 comprises a first chain block Cl, a second chain block C2, and a load chain L. As explained above, the first chain block Cl and second chain block C2 are configured to be independently and separately conveyable in the respective first tension mode and second tension mode. As such, the load chain L is independently and separately conveyable by the first chain block during the first tension mode and by the second chain block during the second tension mode.

As shown in the example of the first chain block Cl depicted in Figure 5E, the first chain block Cl may comprise first chain sheave 100, a first gear assembly 110 and a first actuator 120.

To operate the first chain block Cl in the first tension mode, the first actuator 120 is configured to receive a first driving input. In response to receiving the first driving input, the first actuator 120 is configured to actuate the first gear assembly 110.

When actuated, the first gear assembly is configured to drive the first chain sheave 100 to rotate. When rotating, the first chain sheave is configured to convey the load chain L to pay-in the load chain.

The first actuator 120 may comprise a first interface 122 and a first drive shaft 124.

The first interface 122 is configured to receive a first driving tool (not shown), whereby the driving action of the first driving tool provides the first driving input to the actuator. The first driving input has a first driving input torque and a first driving input speed. The first interface may comprise teeth, apertures, protrusions and/or indents to enhance the engagement of the first driving tool with the first interface. The first interface may be configured to releasably engage the first driving tool to allow for the engagement and dis-engagement of the first driving tool.

If the first driving tool is carried by a Remotely Operated Vehicle (ROV), the first interface 122 may comprise a ROV first interface. For example, an ROV may be employed during the installation, maintenance, deconstruction and abandonment modes of the subsea assembly. The ROV may include multiple arms for manipulating objects, and a subsea camera for viewing the subsea operations.

Streaming video and/or images from the cameras may be communicated to the surface or other remote location for viewing on a live or periodic basis.

In the example depicted in the Figures 5A to 5F, the first interface 122 is an ROV first interface comprising an ROV first bucket. Such ROV buckets are well known in the art. Alternatively, the ROV first interface may comprise any suitable interface known in the art, such as a hydraulic Hot Stab connection, a Manual Connection (e.g. D handle, T-bar or Fishtail), or a OD direct drive coupling.

As shown in Figure 5E, the first drive shaft 124 may comprise a first end 126 and a second end 128, where the first drive shaft 124 extends from the first interface 122, through a central bore of the first chain sheave 100 and to the first gear assembly 110. The first end 126 of the first drive shaft 124 extends into the first interface 122 and is coupled thereto. Hence, when the first driving tool engages the first actuator, the first drive shaft 124 is rotatable under the driving action of the first driving tool.

The second end 128 of the first drive shaft 124 is coupled to a pinion gear 112 of the first gear assembly 110.

The gear assembly 110 comprises a plurality of gears having any suitable configuration to provide a desired first gear ratio. For example, the first gear assembly 110 may have a first gear ratio of 5.2:1.

Figures 6A to 6D depict a schematic arrangement of an example of a first gear assembly 100 according to the present invention. As seen in Figures 6A-6D, the first gear assembly 110 may comprise the pinion gear 112, a first pair reduction gears 114a, 114b, an associated second pair of reduction gears 116a, 116b and a sheave drive gear 118. The pinion gear 112 meshes with the first reduction gears 114a, 114b. The first reduction gears 114a, 114b are arranged in parallel, on opposing sides of the pinion gear 112, with the pinion gear 112 positioned in between. Each first reduction gear and associated second reduction gear is mounted on a respective reduction drive shaft. The second reduction gears 116a, 116b are arranged on the respective reduction shafts to the rear of the associated first reduction gears. The first reduction gears 114a, 114b and second reduction gears 116a, 116b are arranged in tandem on the respective reduction shafts, where the second reduction gears 116a, 116b are arranged to the rear of the first reduction gears 114a, 114b. The sheave drive gear 118 meshes with the second reduction gears 116a, 116b. The second reduction gears 116a, 116b are arranged in parallel, on opposing sides of the sheave drive gear 118, with the sheave drive gear positioned in between. The sheave drive gear 118 is coupled to the first chain sheave 100 so as to rotate the first chain sheave in use.

With this arrangement, rotation of the pinion gear 112 by the first drive shaft 124 causes the first reduction gears 114a, 114b to rotate, which in turn cause the associated second reduction gears 116, 116b to rotate, which in turn cause the sheave drive gear 118 to rotate, and thereby cause the first chain sheave 100 to rotate.

As depicted in the Figures, the first chain sheave 100 may comprise a rotatable wheel over which the load chain L is looped. The first chain sheave may comprise teeth, apertures, protrusions and/or indents to engage the load chain.

As the first chain sheave 100 rotates, it conveys the load chain L that is looped over the first chain sheave 100, paying-in the load chain. The conveying (paying-in) load chain has a first conveying force and a first conveying rate. The first conveying force of the load chain is dependent on the first driving input torque and the first gear ratio.

The first conveying rate of the load chain is dependent on the first driving input speed and the first gear ratio.

In the first tension mode, when the first chain block Cl is actuated by the first driving input provided by the first driving tool, the chain block Cl transmits the first driving input to the load chain L via the first drive shaft 124, the first gear assembly 110 and the first chain sheave 100.

The second chain block 02 has the same general construction as the first chain block Cl. As depicted in Figure 5F, the second chain block C2 may comprise second chain sheave 200, a second gear assembly 210 and a second first actuator 220.

To operate the second chain block 02 in the second tension mode, the second actuator is configured to receive a second driving input. In response to receiving the second driving input, the second actuator 220 is configured to actuate the second gear assembly 210. When actuated, the second gear assembly is configured to drive the second chain sheave 200 to rotate. When rotating, the second chain sheave is configured to convey the load chain L to pay-in the load chain.

The second actuator 220 may comprise a second interface 222 and a second drive shaft 224.

The second interface 222 is configured to receive a second driving tool (not shown), whereby the driving action of the second driving tool provides the second driving input to the actuator. The second driving input has a second driving input torque and a second driving input speed. The second interface may comprise teeth, apertures, protrusions and/or indents to enhance the engagement of the second driving tool with the second interface. The second interface may be configured to releasably engage the second driving tool to allow for the engagement and dis-engagement of the second driving tool.

If the second driving tool is carried by a Remotely Operated Vehicle (ROV), the second interface 222 may comprise a ROV second interface.

In the example depicted in the Figures 5A to 5F, the second interface 122 is an ROV second interface comprising an ROV second bucket. Such ROV buckets are well known in the art. Alternatively, the ROV second interface may comprise any suitable interface known in the art, such as a hydraulic Hot Stab connection, a Manual Connection (e.g. D handle, T-bar or Fishtail), or a QD direct drive coupling.

As shown in Figured 5F, the second drive shaft 224 may comprise a first end 226 and a second end 228, where the second drive shaft extends from the second interface 222, through a central bore of the second chain sheave 200 and to the second gear assembly 210. The first end 226 of the second drive shaft 224 extends into the second interface 222 and is coupled thereto. Hence, when the second driving tool engages the second actuator, the second drive shaft 224 is rotatable under the driving action of the second driving tool. The second end 228 of the second drive shaft 124 is coupled to a pinion gear 212 of the second gear assembly 210.

The gear assembly 210 comprises a plurality of gears having any suitable configuration to provide a desired second gear ratio. For example, the second gear assembly 210 may have a second gear ratio of 43.3:1.

Figures 7A to 7D depict a schematic arrangement of an example of a second gear assembly 200 according to the present invention. As seen in Figures 4A to 7D, the first gear assembly 110 may comprise the pinion gear 212, a first pair reduction gears 214a, 214b, an associated second pair of reduction gears 216a, 216b and a sheave drive gear 218. The pinion gear 212 meshes with the first reduction gears 214a, 214b. The first reduction gears 214a, 214b are arranged in parallel, on opposing sides of the pinion gear 212, with the pinion gear 212 positioned in between. Each first reduction gear and associated second reduction gear is mounted on a respective reduction drive shaft. The second reduction gears 216a, 216b are arranged on the respective reduction shafts to the rear of the associated first reduction gears. The first reduction gears 214a, 214b and second reduction gears 216a, 216b are arranged in tandem on the respective reduction shafts, where the second reduction gears 216a, 216b are arranged to the rear of the first reduction gears 214a, 214b. The sheave drive gear 218 meshes with the second reduction gears 216a, 216b. The second reduction gears 216a, 216b are arranged in parallel, on opposing sides of the sheave drive gear 218, with the sheave drive gear positioned in between. The sheave drive gear 218 is coupled to the second chain sheave 200 so as to rotate the second chain sheave in use.

With this arrangement, rotation of the pinion gear 212 by the first drive shaft 224 causes the first reduction gears 214a, 214b to rotate, which in turn cause the associated second reduction gears 216, 216b to rotate, which in turn cause the sheave drive gear 218 to rotate, and thereby cause the second chain sheave 200 to rotate.

As depicted in the Figures, the second chain sheave 200 may comprise a rotatable wheel over which the load chain L is looped. The second chain sheave may comprise teeth, apertures, protrusions and/or indents to engage the load chain.

As the second chain sheave 200 rotates, it conveys the load chain L that is looped over the second chain sheave 200, paying-in the load chain. The conveying (paying-in) load chain has a second conveying force and a second conveying rate. The second conveying force of the load chain is dependent on the second driving input torque and the second gear ratio. The second conveying rate of the load chain is dependent on the second driving input speed and the second gear ratio.

In the second tension mode, when the second chain block C2 is actuated by the second driving input provided by the second driving tool, the second chain block C2 transmits the second driving input to the load chain L via the second drive shaft 224, the second gear assembly 210 and the second chain sheave 200.

In the present invention, the tensioning apparatus 38 may further comprise a frame to rigidly support one or both chain blocks Cl and 02. The frame may comprise a plurality a struts, and have an open structure. The frame may be configured to protect the tensioning apparatus from damage during use. The frame may allow for ease of handling during construction, deconstruction and transportation. As shown in an example depicted in Figures 5A to 5D, the tensioning apparatus may comprise a first frame Fl to rigidly support the first chain block Cl, and a second frame F2 to rigidly support the second chain block C2. The first chain block Cl and second chain block 02 are mounted horizontally along the frame with a gap space therebetween The tensioning apparatus 38 may comprise a buoyancy aid B to aid the buoyancy and control the orientation of the tensioning apparatus during use in subsea conditions. The buoyancy aid may maintain the orientation of the tensioning apparatus while subsea to help avoid twisting of the tethering member. The buoyancy aid may maintain the orientation of the tensioning apparatus so that the first chain block and the second chain block are accessible. In an example shown in Figures 5A to 50, the tensioning apparatus may comprise a buoyancy aid B coupled to a side-strut of frame.

To enhance the mechanical advantage of the tensioning apparatus 38, the tensioning apparatus may further comprise one or more pulley about which the load chain L is looped. Each pulley may comprise one or more pulley wheel. As shown in the example depicted in the Figures 5A to 50, the tensioning apparatus may comprise a compound pulley comprising a first movable pulley P1 and a second fixed pulley P2. As the load chain L is conveyed, the movable pulley P1 moves with the load chain. The second fixed pulley P2 is mounted on the frame between the first chain block C1 and the second chain block C2. In the example depicted in the Figures 5A to 50, each pulley defines three pulley wheels (M.

In the present invention, the tensioning apparatus 38 may further comprise a first coupling and a second coupling. As shown in the example depicted in Figures 5A to 50, the first coupling may comprise a first hook H1, which is attached to the first movable pulley P1. In this way, as the load chain L is conveyed under actuation of the actuator, the first hook H1 and the first movable pulley are also conveyed along with the load chain L. As such, the first hook H1 may be referred to as a "movable hook". The second coupling may comprise a second hook H2, which is attached to the second pulley P2. Since, the second pulley P2 is fixed, the second hook H2 is also static. As such, the second hook H2 may be referred to as a "fixed hook". In this example, the first hook H1 and the second hook H2 are axially aligned and arranged on opposite sides of the chain block C. In the example, each hook H1 and H2 includes a safety catch that is biased in the closed position to allow for the chain block to be securely and releasably coupled to the tethering member and one of the subsea BOP or subsea mount during use.

The chain block C may additionally or alternatively comprise any other suitable coupling means.

The first coupling and the second coupling allow the tensioning apparatus 38 to be coupled to the tethering member and one of the subsea BOP or the subsea mount, depending on the arrangement of the chain block. As such, the tensioning apparatus provides an interconnection between the tethering member 36 and one of the subsea BOP 24 and the subsea mount 34, depending on the arrangement of the tensioning apparatus.

In operation, with the tensioning apparatus 38 coupled to the tethering member 36, the conveying action of the load chain L is transmitted to the tethering member to apply tension to the tethering member.

As shown in Figures 3, 8, 9 and 10, the tensioning apparatus 38 may be attached to the subsea BOP 24 and the tethering member 36.

In an example of the tensioning system 32 depicted in Figures 8 and 9, the tensioning apparatus 38 may be arranged such that it is attached to the subsea BOP via hook H2 and attached to a first end portion 36a of the tethering member 36 via hook H1. A second end portion 36b of the tethering member 36 is attached to the subsea mount 34 to tether the subsea BOP 24 to the subsea mount 34. When the tensioning system 38 is operating in the initial first tension mode, the first chain block Cl is actuated by the first driving input to convey (pay-in) the load chain L in a direction towards the subsea BOP 24. As the load chain L is conveyed, the conveying action is directly transmitted to the tethering member coupled to hook H1 and the first tension is thereby applied to the tethering member 36 in a tensioning direction towards the subsea BOP 24. When the tensioning system 38 is operating in the subsequent second tension mode, the second chain block C2 is actuated by the second driving input to further convey (pay-in) the load chain L in the direction towards the subsea BOP 24. As with the first chain block Cl, the conveying action of the load chain L by the second chain block C2 is directly transmitted to tethering member coupled to hook H1, whereby the second tension is applied to the tethering member (already under the first tension) in the same tensioning direction towards the subsea BOP 24.

Conversely, in the example of the tethering system 32 depicted in Figure 10, the tensioning apparatus 38 may be arranged such that it is attached to the subsea BOP 24 via hook H1 and attached to the first end portion 36a of the tethering member via hook H2. A second end portion 36b of the tethering member is attached to the subsea mount 34 to tether the subsea BOP 24 to the subsea mount 34. When the tensioning apparatus 38 is operating in the initial first tension mode, the first chain block Cl is actuated by the first driving input to convey (pay-in) the load chain L in a direction towards the subsea mount 34. As the load chain L is conveyed, the conveying action is indirectly transmitted to the tethering member 36 coupled to the hook H2, via the tensioning apparatus 38. As a result, the first tension is applied to the tethering member in a tensioning direction towards the subsea BOP 24. Likewise, when the tensioning apparatus is operating in the subsequent second tension mode, the second chain block C2 is actuated by the second driving input to further convey (pay-in) the load chain L in the direction towards the subsea mount 34. The conveying action of the load chain L is indirectly transmitted to the tethering member 38, via the tensioning apparatus, to apply the second tension to the tethering member 38 (already under the first tension) in the same tensioning direction towards the subsea BOP 24.

In an alternative arrangement as shown in Figures 4, 11 and 12, the tensioning apparatus 38 may be attached to the subsea mount 34 and the tethering member 36.

In an example of the tethering system 32 depicted in Figure 11, the tensioning apparatus 38 may be arranged such that it is attached to the subsea mount 34 via hook H1 and attached to a second end portion 36b of the tethering member via hook H2. The first end portion 36a of the tethering member is attached to the subsea BOP 24 to tether the subsea BOP 24 to the subsea mount 34. When the tensioning apparatus 38 is operating in the initial first tension mode, the first chain block Cl is actuated by the first driving input to convey (pay-in) the load chain L in a direction towards the subsea BOP 24. As the load chain L is conveyed, the conveying action is indirectly transmitted to the tethering member 36 coupled to hook H2, via the tensioning apparatus, and the first tension is thereby applied to the tethering member in a tensioning direction towards the subsea mount 34. Likewise, when the tensioning apparatus 38 is operating in the subsequent second tension mode, the second chain block 02 is actuated by the second driving input to convey (pay-in) the load chain in the direction towards the subsea BOP 24. The conveying action of the load chain L is indirectly transmitted to the tethering member 36, via the tensioning apparatus, to apply the second tension to the tethering member (already under the first tension) in the same tensioning direction towards the subsea mount 34.

Conversely, in an example depicted in Figure 12, the tensioning apparatus may be arranged such that the tensioning apparatus is attached to the subsea mount 34 via hook H2 and attached to a second portion 36b of the tethering member via hook Hi.

The first end portion 36a of the tethering member is attached to the subsea BOP 24 to tether the subsea BOP 24 to the subsea mount 34. When the tensioning apparatus 38 is operating in the initial first tension mode, the first chain block Cl is actuated by the first driving input to convey (pay-in) the load chain L in a direction towards the subsea mount 34. As the load chain L is conveyed, the conveying action is directly transmitted to the tethering member coupled to hook H1, and the first tension is thereby applied to the tethering member in a tensioning direction towards the subsea mount 34. When the tensioning apparatus 38 is operating in the subsequent second tension mode, the second chain block C2 is actuated by the second driving input to further convey (pay-in) the load chain in direction towards the subsea mount 34. As the load chain is conveyed by the second chain block, the conveying action is directly transmitted to the tethering member coupled to hook H1 to apply the second tension to the tethering member (already under the first tension) in the same tensioning direction towards the subsea mount 34.

As shown in Figure 13, the subsea mount 34 may include a plurality of anchor points for attachment of the tethering member 36 or the tensioning apparatus 38 to the subsea mount. The plurality of anchor points may be positioned at a plurality of locations on the subsea mount to accommodate tethering members of different lengths. The anchor points may be padeye connection points 40 and/or bollard connection points 42 of the type known in the art.

When the first chain block Cl conveys the load chain L to apply the first tension to the tethering member, the conveying action of the load chain transmitted to the tethering member will remove any slack in the tethering member. In the example depicted in Figures 8, 9 and 10, the conveying action of the load chain L by the first chain block Cl may displace the first end portion 36a of the tethering member in the tensioning direction towards the subsea BOP 24, removing slack in the tethering member. In the examples depicted in Figures 11 and 12, the conveying action of the load chain L by the first chain block Cl may displace the second end portion 36b of the tethering member in the tensioning direction towards the subsea mount 34, removing slack in the tethering member.

The mechanical advantage of each chain block is dependent on the gear ratio of each chain block. As such, the conveying force of the load chain as it is conveyed by each chain block, and thereby the tension applied to the tethering member by each chain block, is dependent on the gear ratio and driving input torque of each chain block. Also, the conveying rate of the load chain as it is conveyed by each chain block is dependent on the gear ratio and the driving input speed of each chain block.

To optimise the two-stage tensioning process of the tensioning apparatus in the present invention, the first chain block Cl and second chain block C2 are configured whereby the second gear ratio of the second chain block is higher than first gear ratio of the first chain block Cl.

By configuring the second chain block C2 to have a higher gear ratio than the first chain block Cl, the second conveying force of the load chain as it is conveyed by the second chain block is greater than the first conveying force of the load chain as it is conveyed by the first chain block Cl. It follows that the second tension applied to the tethering member by the second chain block is thereby greater than the first tension applied to the tethering member by the first chain block. For example, the first chain block Cl may have a first gear ratio whereby the first tension applied to the tethering member by the first chain block Cl falls within the range of approximately 5,000N to 20,000N. The second chain block may have a second gear ratio whereby the second tension applied to the tethering member by the second chain block results in a combined first and second tension (final tension) of up to 500,000N.

Hence, given that the tethering member will become taut under the first tension, the first chain block Cl with the lower gear ratio is thereby able to remove slack in the tethering member 36 at a lower first tension, and before the second chain block C2 with the higher gear ratio applies the second higher tension to the tethering member.

Also, by configuring the second chain block C2 to have a higher gear ratio than the first chain block Cl, the first conveying rate of the load chain as it is conveyed by the first chain block Cl is faster than the second conveying rate of the load chain as it is conveyed by the second chain block C2.

Hence, it follows that the first chain block Cl with lower gear ratio is able to displace the tethering member to remove slack more quickly than the second chain block.

The first chain block Cl with the lower first gear ratio is advantageously able to remove slack in the tethering member 36 relatively more quickly, and at a relatively lower tension than the second chain block C2. The second chain block C2 with higher second gear ratio is advantageously able to subsequently apply the relatively higher tension to the tethering member 36 with relatively less displacement of the tethering member than the first chain block Cl.

Moreover, by configuring the second chain block to have a higher gear ratio than the first chain block, the first chain block and second chain block may be actuated by the same driving input (same driving input torque and the same driving input speed) to apply the lesser first tension with the faster first conveying rate of the load chain, and to apply the greater second tension with the slower second conveying rate of the load chain. For example, the first chain block Cl may have a first gear ratio of 5.2:1 whereby on receiving a driving input torque of 55 Nm (40.5 ft.lbf), the first chain block Cl is able to apply a first tension of up to 20,000 N to the tethering member. The second chain block C2 may have a second gear ratio of 43.3:1, whereby on receiving a driving input torque of 55 Nm (40.5 ft.lbf), the second chain block C2 is able to apply a second tension of up to 200,000 N. Also, the first chain block Cl may have a first gear ratio of 5.2:1, whereby on receiving a driving input speed of 25 rpm, the first chain block Cl is able to convey the load chain by approximately 360mm in approximately 20 seconds. The second chain block C2 may have a second gear ratio of 43.3:1, whereby on receiving a driving input speed of 25 rpm, the second chain block C2 is able to convey the load chain L by approximately 360mm in approximately 2 minutes and 43 seconds.

Advantageously the mechanical advantage provided by the gear ratio of each chain block Cl and C2, and any pulley, enables a high overall tension to be generated with a relatively low input torque. In this way. the tensioning apparatus of the present invention can be used to apply and maintain high tensile loads in the tethering member with a relatively low input torque. This is particularly advantageous when operating under subsea conditions.

The tension (combined first tension and second tension) applied by the tensioning apparatus is sufficient to at least restrict, preferably prevent, undesired movement of the tethering member when subjected to external loads during one or more mode of operation of the subsea well assembly. As such, undesired movement and lateral bending of the subsea BOP 24 (and any other type of subsea well control equipment) due to the external loads is impeded, and the overall fatigue performance of the subsea well assembly is improved. For example, the tension (combined first tension and second tension) applied by the tensioning apparatus to the tethering member may fall in the range of approximately 10,000 N to 500,000 N, depending on the subsea well assembly and external loads acting on the subsea assembly and tethering member. Advantageously, the tensioning apparatus may have a mechanical advantage where a tension of up to 200,000 N can be applied to the tethering member with a driving input torque to each chain block of 55 Nm (40.5 ft. I bf).

In this way, the tensioning apparatus is configured to apply a tension to the tethering member so as to reinforce and/or stabilize the subsea BOP 24, tree 16, wellhead 18 and primary conductor 20 by restricting movement of subsea BOP 24. As a result, the tensioning apparatus 38 according to the present invention improves the strength and fatigue resistance, and reduces bending moments, of the subsea BOP 24, the tree 16 (when present), wellhead 18 and primary conductor 20.

Each chain block Cl, C2 of the tensioning apparatus 38 may further comprise a brake 130 configured to prevent back-rotation of the respective load chain sheave 100, 200. An example of a brake for a chain block is depicted in Figures 13A to 13B. In this example, the brake comprises a ratchet wheel and pawl mechanism disposed adjacent to the respective load chain sheave 100, 200. A pair of pawls 132 may be provided on opposite sides of the ratchet wheel 134, and arranged to engage one of the tooth valleys of the ratchet wheel 134 and thereby stop the ratchet wheel 134 from rotating backward. This braking effect ensures the tension applied to the tethering member by the chain block is maintained and the operational safety of a chain block is maintained. Such braking structures are described in GB2501515B, the content of which is incorporated by reference in its entirety. However, the brake may include any suitable braking structure known in the art.

The tensioning apparatus 38 may be configured to operate in a slack mode to reduce tension in the tethering member 36. The slack mode allows for the adjustment of tension previously applied to the tethering member during the tension mode, for example, depending on the external loads and/or mode of operation of the well system. The slack mode also allows for the removal of tension applied to the tethering member, for example, when the tensioning apparatus is not in use or being de-constructed.

The tensioning apparatus 38 may be configured to operate in a first slack mode to reduce the first tension in the tethering member 36. In the first slack mode, the first chain block Cl may be configured to reverse the first tension applied to the tethering member during the first tension mode. The first chain block may be operated in the first slack mode to allow slack in the tethering member.

The tensioning apparatus 38 may be configured to operate in a second slack mode to reduce the second tension of the tethering member 36. In the second slack mode, the second chain block C2 may be configured to reverse the second tension applied to the tethering member during the second tension mode.

Following the sequential application of the first tension and second tension to the tethering member, the tensioning apparatus may be configured to initially operate in a second slack mode to reverse the second tension applied to the tethering member, and subsequently operate in a first slack mode to reverse the first tension applied to the tethering member.

Optionally, the brake 130 of first chain block Cl and/or the second chain block C2 may be released to allow the respective chain block to operate in a slack mode. For example, each chain block may further comprise a clutch system (not shown) to allow the pinion shaft to disengage from the pawl system. This allows for reverse rotation of the load chain sheave. Reverse rotation of the load chain sheave reverse conveys (pays out) the load chain. The reverse conveying of the load chain causes the tethering member to move in a slackening direction, opposite to the tensioning direction, and the tension in the tethering member is at least reduced.

The tensioning apparatus may further comprise a tensioning monitoring system to monitor the tension of the tethering member. The tensioning monitoring system may comprise communication means to communicate the measured tension to a remote operator and/or location to enable tension monitoring, quantification of external loads on the BOP and/or rapid identification in the event of a potential tethering member failure. The tensioning monitoring system may comprise a tension load cell TLC. The load cell may be an acoustic or visual display load cell. As tension is applied to the tethering member the tension load cell is compressed and the change in tension is identified.

A example of method M according to present invention for tensioning a tethering member, where the tethering member is tethering a subsea well control equipment to a subsea mount is shown in Figure 15. In a first step Ml, a subsea tensioning apparatus according to the present invention is provided. The subsea tensioning apparatus comprises a first chain block with a first gear ratio, a second chain block with a second gear ratio, and a load chain configured to be separately conveyed by the first chain block and the second chain block. The second gear ratio is greater than the second gear ratio. In a second step M2, the subsea tensioning apparatus is coupled, with a coupling, to a subsea well control equipment or the subsea mount. In a third step M3, the subsea tensioning apparatus is coupled, with another coupling, to an end of the tethering member. In a fourth step M4, the first chain block is actuated, by a first driving input, to convey the load chain to apply a first tension to the tethering member. Applying the first tension removes any slack from the tethering member. In a fifth step M5, after the fourth step M4, the second chain block is actuated, by a second driving input, to convey the load chain to apply a second tension to the tethering member.

Due to the second gear ratio of the second chain block being greater than the first gear ratio of the first chain block, the second tension applied by the second chain block is greater than the first tension applied by the first chain block. Also, the first chain block conveys the load chain to apply the first lower tension to the tethering member, and remove slack in the tethering member at a faster first conveying rate and the second chain block conveys the load chain to apply a second higher tension to the tethering member at a slower second conveying rate.

The combined first tension and second tension is sufficient to restrict, preferably inhibit, movement of the tethering member under subsea loads.

An example of a method N according to the present invention for tethering a subsea well control equipment under tension is shown in Figure 16a and 16b. In a first step Ni, a tensioning system according to the present invention is provided. The tensioning system comprises a subsea mount, a tethering member, and a tensioning apparatus comprising a first chain block with a first gear ratio, a second chain block with a second gear ratio and a load chain separately conveyable by the first chain block and the second chain block, wherein the second gear ratio is greater than the first gear ratio. In a second step N2, the subsea mount is arranged on the sea bed in spaced relationship to the subsea well control equipment. In a third step N3, the tensioning apparatus is coupled, with a coupling, to the subsea well control equipment or the subsea mount. In a fourth step N4, an end of the tethering member is coupled, with a coupling, to the tensioning apparatus. In a fifth step N5, a respective other end of the tethering member is coupled, with a coupling, to the respective other of the subsea well control equipment or the subsea mount. In sixth step N6, the first chain block is actuated, by a first driving input, to convey the load chain to apply a first tension to the tethering member. Applying the first tension removes any slack from the tethering member. In a seventh step N7, after the sixth step N6, the second chain block is actuated, by a second driving input, to convey the load chain to apply a second tension to the tethering member.

In general, the tethering member 36 may comprise or consist of any elongate flexible member suitable for subsea use and capable of withstanding the anticipated tensile loads without deforming or elongating. Non-limiting examples of suitable tethering members include chain(s), wire rope, and Dyneema® rope available from DSM Dyneema LLC of Stanley, North Carolina USA. In the particular embodiments shown, the tethering member comprises Dyneema® rope, which is suitable for subsea use and is sufficiently strong to withstand the anticipated tensions.

In the context of the present invention, a sea bed 30 may be understood to mean any subsea surface that allows for the arrangement of the subsea mount in spaced relationship to the subsea well control equipment to achieve a tethering effect as the tethering member extends from the subsea well control equipment to the subsea mount.

The tensioning apparatus of the present invention may apply a tension to the tethering member during one or more mode of operation of the subsea well assembly. For example, the tensioning apparatus may apply tension to the tethering member during the installation of the subsea well assembly (construction), during drilling (production), intervention, during deconstruction and/or abandonment of the subsea well assembly.

It should be appreciated that tethering apparatus and tethering members described above may be deployed and installed on an existing subsea mount or subsea well control equipment. Moreover, tensioning apparatus, tethering members and subsea mounts can be independently retrieved and reused at different locations, and different subsea well assemblies, as required.

It will also be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitafive sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS1. A subsea tensioning apparatus for tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount arranged on a sea bed, the subsea tensioning apparatus comprising: a load chain; a first chain block having a first gear ratio, wherein the first chain block is configured to convey the load chain to apply a first tension to the tethering member; and a second chain block having a second gear ratio, wherein the second chain block is configured to subsequently convey the load chain to apply a second tension to the tethering member; wherein the second gear ratio is higher than the first gear ratio.
2. The subsea tensioning apparatus of claim 1, wherein: the first chain block is configured to be actuated by a first driving input to convey the load chain to apply the first tension to the tethering member; the second chain block is configured to actuated by a second driving input to convey the load chain to apply the second tension to the tethering member; and wherein the first driving input and the second driving input are the same.
3. The subsea tensioning apparatus according to claim 1, wherein the first chain block comprises: a rotatable first chain sheave configured to convey the load chain; a first gear assembly, with the first gear ratio, configured to rotate the first chain sheave; a first actuator configured to actuate the first gear assembly; wherein, in response to the first actuator receiving a first driving input, the first actuator is configured to actuate the first gear assembly to rotate the first chain sheave by a first rotation; and wherein, when rotating by the first rotation, the first chain sheave is configured to convey the load chain, whereby the conveying load chain has a first conveying force and a first conveying rate.
4. The subsea tensioning apparatus according to claim 3, wherein the first chain block further comprises a first brake assembly configured to prevent back rotation of the first chain sheave.
5. The subsea tensioning apparatus according to claim 4, wherein the first brake assembly is configured to be releasable to allow back rotation of the first chain sheave; wherein, when back rotating, the first chain sheave is configured to reverse convey the load chain to at least reduce the first tension.
6. The subsea tensioning apparatus according any of claims 3 to 5, wherein the first actuator is operable by a first driving tool to provide the first driving input.
7. The subsea tensioning apparatus according to any of claims 3 to 6, wherein the first actuator is a mechanically driven first actuator or a hydraulically driven first actuator.
8. The subsea tensioning apparatus according to any of claims 3 to 7, wherein the second chain block comprises: a rotatable second chain sheave configured to convey the load chain; the second gear assembly, with the second gear ratio, configured to rotate the second chain sheave; a second actuator configured to actuate the second gear assembly; wherein, in response to the first actuator receiving a second driving input, the second actuator is configured to actuate the second gear assembly to rotate the second chain sheave by a second rotation; and wherein, when rotating by the second rotation, the second chain sheave is configured to convey the load chain, whereby the conveying load chain has a second conveying force and a second conveying rate.
9. The subsea tensioning apparatus according to claim 8, wherein the first driving input received by the first actuator and the second driving input received by the second actuator are the same.
10. The subsea tensioning apparatus according to claim 8 or 9, wherein the second chain block further comprises a second brake assembly configured to prevent back rotation of the second chain sheave.
11. The subsea tensioning apparatus according to claim 10, wherein the second brake assembly is configured to be releasable to allow back rotation of the second chain sheave; wherein, when back rotating, the second chain sheave is configured to reverse convey the load chain to at least reduce the second tension.
12. The subsea tensioning apparatus according any of claims 8 to 11, wherein the second actuator is operable by a second driving tool to provide the second driving input.
13. The subsea tensioning apparatus according to any of claims 8 to 12, wherein the second actuator is a mechanically driven second actuator or a hydraulically driven second actuator.
14. The subsea tensioning apparatus according to any preceding claim, further comprising a first coupling and a second coupling, wherein the first coupling is a movable coupling mounted on the conveyable load chain and the second coupling is a fixed coupling mounted on a rigid portion of the tensioning apparatus.
15. The subsea tensioning apparatus according to claim 14, wherein: the tensioning apparatus is arrangeable in a first arrangement where the first coupling is configured to engage the tethering member and the second coupling is configured to engage the subsea well control equipment; or the tensioning apparatus is arrangeable in a second arrangement where the first coupling is configured to engage the subsea well control equipment and the second coupling is configured to engage the tethering member; or the tensioning apparatus is arrangeable in a third arrangement where the first coupling is configured to engage the tethering member and the second coupling is configured to engage the subsea mount; or the tensioning apparatus is arrangeable in a fourth arrangement where the first coupling is configured to engage the subsea mount and the second coupling is configured to engage the tethering member.
16. The subsea tensioning apparatus according to any preceding claim further comprising a frame configured to support the first chain block and the second chain block.
17. The subsea tensioning apparatus according to any preceding claim further comprising a buoyancy aid to aid buoyancy of the subsea tensioning apparatus.
18. The subsea tensioning apparatus according to any preceding claim further comprising a tension monitoring system configured to monitor tension in the tethering member
19. A method of tensioning a tethering member, wherein the tethering member is configured to tether a subsea well control equipment to a subsea mount, the method comprising: providing a subsea tensioning apparatus, wherein the subsea tensioning apparatus comprises a first chain block having a first gear ratio, a second chain block having a second gear ratio, a load chain configured to be separately conveyed by the first chain block and the second chain block, and wherein the second gear ratio is higher than the first gear ratio; coupling the subsea tensioning apparatus to the subsea well control equipment or the subsea mount; coupling the subsea tensioning apparatus to an end of the tethering member; actuating the first chain block to convey the load chain to apply a first tension to the tethering member; after applying the first tension, actuating the second chain block to convey the load chain to apply a second tension to the tethering member, wherein the second tension is higher than the first tension.
20. The method according to claim 21, wherein: actuating the first chain block comprises: driving the first actuator with a first driving input to actuate the first gear assembly with the first gear ratio; actuating the first gear assembly with the first gear ratio to rotate the first chain sheave by a first rotation rate; rotating the first chain sheave by the first rotation rate to convey the load chain; conveying the load chain with a first conveying force and at a first conveying rate to apply the first tension to the tethering member; and actuating the second chain block comprises: driving the second actuator with a second driving input to actuate the second gear assembly with the second gear ratio; actuating the second gear assembly with the second gear ratio to rotate the second chain sheave by a second rotation rate; rotating the second chain sheave by the second rotation rate to convey the load chain at a second conveying rate; conveying the load chain with a second conveying force and at the second conveying rate to apply the second tension to the tethering member.
21. The method according to any of claims 19 or 20, wherein the tensioning apparatus comprises any of the features of claims 1 to 18.
22. A subsea tensioning system for tethering under tension a subsea well control equipment to a subsea mount, the system comprising: a tethering member arrangeable to tether the subsea well control equipment to the subsea mount; a subsea tensioning apparatus arrangeable to apply tension to the tethering member, wherein the tensioning apparatus comprises: a load chain; a first chain block having a first gear ratio, wherein the first chain block is configured to convey the load chain to apply a first tension to the tethering member; and a second chain block having a second gear ratio, wherein the second chain block is configured to subsequently convey the load chain to apply a second tension to the tethering member; wherein the second gear ratio is higher than the first gear ratio.
23. The subsea tensioning a system according to claim 22, further comprising the subsea mount arrangeable on a sea bed in spaced relationship to the subsea well control equipment.
24. The subsea tensioning system according to claim 23, wherein: the subsea tensioning apparatus comprises a first coupling and a second coupling, wherein the first coupling or the second coupling is arrangeable to engage an end of the tethering member; and the subsea mount comprises a third coupling, wherein the third coupling is arrangeable to engage a respective other end of the tethering member.
25. The subsea tensioning system, wherein the subsea tensioning apparatus comprises any of the features defined in any of claims 1 to 18. 15
26. A subsea method for tethering a subsea well control equipment under tension, the method comprising: providing a subsea mount; providing a tethering member, providing a tensioning apparatus, wherein the subsea tensioning apparatus comprises a first chain block having a first gear ratio, a second chain block having a second gear ratio, a load chain configured to be separately conveyed by the first chain block and the second chain block, wherein the second gear ratio is higher than the first gear ratio; arranging the subsea mount on the sea bed in spaced relationship to the subsea well control equipment; coupling the tensioning apparatus to the subsea well control equipment or the subsea mount; coupling an end of the tethering member to the tensioning apparatus; coupling a respective other end of the tethering member to the respective other of the subsea mount or the subsea well control equipment; actuating the first chain block to convey the load chain to apply a first tension to the tethering member; after applying the first tension, actuating the second chain block to convey the load chain to apply a second tension to the tethering member, wherein the second tension is greater than the first tension.
27. The method according to claims 26, wherein the tensioning apparatus comprises any of the features of claims 1 to 18.