NO346898B1

NO346898B1 - Linear Rope Tensioner

Info

Publication number: NO346898B1
Application number: NO20210300A
Authority: NO
Inventors: Helge Ingebretsen
Original assignee: Subseadesign As
Priority date: 2021-03-08
Filing date: 2021-03-08
Publication date: 2023-02-20
Also published as: WO2022191714A1; NO20210300A1

Description

Field of the Invention

The present invention relates to a device designed to preserve the structural integrity of subsea wellheads. More particularly the present invention relates to a device which reduces damaging loads from blowout preventers on subsea wellheads. More particularly the present invention relates to a device which provides tension on tethering lines affixed to blowout preventers connected to subsea wellheads.

Background

Subsea oil wells, or subsea installations, are an essential part of the extraction and exploration process of hydrocarbons embedded beneath the ocean floor. Subsea installations have been a staple for subsea oil reservoirs since the mid-20th century from which there have been significant technological advancements to this day. These subsea installations typically comprise of, but are not limited to, subsea wellheads, subsea manifold, subsea templates and Christmas trees.

These installations are often connected to subsea riser systems which comprise of, but are not limited to, blowout preventers (BOPs) and marine risers, which are lowered from a surface vessel, such as but not limited to drill ships/rigs and well intervention vessels. The aforementioned system connects to the subsea installation’s subsea wellhead for drilling and/or well intervention operations. A subsea wellhead is a rigid component, which generally comprise of flanges, valves and pipe spools affixed on top of an intermediate cemented casing.

A subsea wellhead, being a rigid structure embedded on the ocean floor, is exposed to structural fatigue due to cyclic loads and high structural loads from being regularly attached and detached to subsea riser systems. The interaction between a subsea installation and a subsea riser system is a generally straightforward interconnection during successful operations. But when structural loads from subsea-/drilling go awry, the subsea wellhead can become a point where the mentioned loads can act upon, creating a potentially damaging moment at the wellhead. Repairing and/or replacing a damaged subsea wellhead is a costly, resource intensive and time-consuming process, wherein the daily costs of an unplanned downtime of a subsea well in itself could be a six-digit number in US dollars. And not to mention the negative environmental impact of a damaged wellhead spewing out large amounts of hydrocarbon at the ocean floor, fi. due to a fatigue crack on the subsea wellhead.

Although such operations have over time become more efficient, safer, and arguably more sustainable, operating conditions have also correspondingly become more challenging, wherein newer subsea oil fields are located beneath rough seas and/or are installed at deepwater, ocean depths greater than 1000 ft, or ultradeepwater, ocean depths greater than 5000 ft. For the surface vessels it is imperative for them to be at a fixed position on the ocean surface during operation. Therefore, it is essential keep the cyclic and static forces from the subsea riser system to the subsea installation at a minimum, fi. wave movements, going adrift, etc., all of which affects the structural integrity of the subsea wellhead.

The components of subsea riser systems have also become both larger and heavier over time, wherein current BOPs could weigh up to 400 metric tons. BOPs are designed to withstand challenging deepwater conditions and meet operational requirements. And then there is the added weight of additional marine riser sections required for deepwater/ultra-deepwater operations, further adding to the total weight of a subsea riser system that is attached atop a subsea wellhead. Older subsea installations that are operational could risk damaging their wellheads to disrepair, not being designed to take on the high loads and weights from modern subsea riser systems.

There are current subsea safety solutions which can mitigate and evenly distribute cyclic loads and high structural loads affecting the wellhead to the rest of the subsea installation and/or surrounding environment. These systems are called Wellhead Load Relief (WLR) systems. Generally, a WLR configuration comprise of a tethering arrangement wherein there are a plurality of tensioned support lines where the upper ends of the support lines are affixed to the upper section the BOP and their lower ends are affixed to the subsea installation itself, more specifically to the subsea template. These support lines can be tensioned is such a manner to compensate for adverse moments affecting the subsea wellheads. Other embodiments of the WLR system have the lower ends of the support lines attached to adjacent suction pile anchors on the ocean floor.

The purpose of the WLRs tethering arrangement is to mitigate loads transferred through the subsea riser system and on to the subsea wellhead. By redistributing the loads from the subsea riser system, a WLR setup can reduce the cyclic bending moments on the wellhead by up to 60 to 80 %, resulting in increased fatigue life of the latter.

To effectively mitigate undesired loads on the wellhead, the support lines of a WLR system must be tensioned in a proper manner. Should the tension in the support lines become too high, the connection points could break off from subsea installation and/or subsea riser system. On the other hand, if the tension in the support lines is too low, the adverse loads will affect directly on the subsea wellhead.

Prior art for WLR system with and/or without a support line mechanism are exemplified by the following documents:

- NO345282B1 is an embodiment of the aforementioned WLR system, wherein the lower ends of the support lines are attached to piles that are embedded in the subsea template.

- US9074447B1 is another embodiment of a WLR setup, wherein the support lines can be tensioned through affixed tensioning cylinder. The cylinder has an integrated ratchet rod that draws on the support line to a preferred pre-tension.

- US10724349B2 is another embodiment, wherein the support lines can also be tensioned with a mechanical rope tensioner with an integrated ratchet mechanism.

- WO 2016/118019 A1 describes a subsea wellhead assembly comprising a subsea wellhead, a template associated with the wellhead, subsea riser system equipment connected to the wellhead and one or more connection members.

- US 2014/374115 A1 describes a system for tethering a subsea BOP including a plurality of anchors disposed about the subsea BOP and secured to the sea floor. The system includes a plurality of tensioning systems, and one tensioning system is coupled to an upper end of each anchor.

- WO 2019/068130 A1 describes a system for tethering a subsea BOP or well head. The system comprises an interface associable with the BOP, and more than one anchors disposed about the BOP.

Summary of the Invention

The object of the present invention is to provide a novel line tensioner device to achieve a specific pre-tension on tensioned lines, fi. for use in a subsea tethering arrangement for a subsea wellhead assembly, comprising, but not limited to, a subsea wellhead and a BOP, which provides tension to tethering lines affixed to a BOP that is connected to a subsea wellhead, in order provide a load distribution on a subsea wellhead assembly that is non-damaging on the subsea wellhead.

Said objects are achieved in a line tensioner device as defined in the appended claims.

Brief Description of the Drawings

Certain features and functions of the present invention will be described with reference to the following appended figures in which:

Fig. 1 is a perspective view of the inventive line tensioner.

Fig. 2 is a cross section view of the inventive line tensioner from the side.

Fig. 3 is a perspective view illustrating an embodiment of a subsea wellhead assembly with a WLR tethering arrangement including the inventive line tensioner.

Fig. 4 is a perspective view illustrating an alternative embodiment of subsea wellhead assembly with a WLR tethering arrangement including the inventive line tensioner.

Detailed Description of the Invention

A line tensioner device 1 according to the invention is shown in Fig. 1 and Fig. 2 which illustrates a device for tensioning support lines 5 of a WLR tethering arrangement intended to safeguard the structural integrity of subsea wellhead. Said device 1 consists of three main mechanisms for tensioning a line 5, a clamping mechanism 2, a first tensioning mechanism 3 and a second tensioning mechanism 4. A line 5 is first entered through the clamping mechanism 2 and the first tensioning mechanism 3 from which the line 5 exits the latter mechanism 3. A plurality of rollers is in place between the clamping mechanism 2 and first tensioning mechanism 3 to guide the line 5 between the said mechanisms 2, 3.

The purpose of the clamping mechanism 2 is to hinder bidirectional movement of the support line 5. The clamping mechanism 2 consists of clamping jaw 2a and a hydraulic cylinder 2b, wherein said components 2a, b are pivotally connected. The clamping jaw 2a is activated and locks the line 5 to hinder bidirectional movement, when the hydraulic cylinder 2b pushes out its piston. To deactivate the clamping jaw 2a and release the line 5, when the piston returns back inside the cylinder 2b and the other mechanism 3, 4 are deactivated.

The purpose of the first tensioning mechanism 3 is to provide an initial pre-tension on the support line 5, where the intention is to have a taut line 5 prior clamping action by the clamping mechanism 2 in order for the said mechanism 2 to have proper bidirectional lock of the line 5. The first tensioning mechanism 3 consists of a line gripper 3a and a second hydraulic cylinder 3b, wherein said components 3a, b are pivotally connected. The first tensioning mechanism 3 is activated and provide an initial pre-tension on the support line 5, when the line gripper 3a grips on to the said line 5 and then the second hydraulic cylinder 3b pushes out its piston towards the support line exit 3c.

The purpose of the second tensioning mechanism 4 is to provide final a pre-tension on the support line 5, provided that the initial pre-tension on the line 5 is done by the first tensioning mechanism 3, thereafter the bidirectional locking of the line 5 done by the clamping mechanism 2 have both been successfully completed. The second tensioning mechanism 4 consists of a jacking screw 4a, ratchet gear 4b, integrated load cell 4c, a torque tool interface 4d and a pivoting part 4e, wherein the ratchet gear 4b controls the bidirectional movement of the jacking screw 4a, the integrated load cell measures the tension on the jacking screw 4a thereby the actual tension on the line 5, the torque tool interface 4d controls the ratchet gear which can by operated by an ROV and the pivoting part 4e is a pivotable affixation point which can connect, but not limited to, a subsea template or an ocean floor embedded anchor. The clamping mechanism 2 and the first tensioning mechanism 3 can together be considered as an entity that is slidably connected to the second tensioning mechanism 4 through the jacking screw 4a.

A final pre-tension on the support line 5 is provided, given that the initial pretension on the line 5 has been done by the first tensioning mechanism 3, thereafter the bidirectional locking of the line 5 done by the clamping mechanism 2 have both been completed, by activating the ratchet gear 4b in a manner where the jacking screw 4a pulls towards itself the said entity, comprising of the clamping and first tensioning mechanisms 2, 3. The load cell 4c measures the increasing tension on the jacking screw 4a thereby the actual tension on the line 5, wherein the ratchet gear 4b stops when a desired pre-tension on the line 5 is achieved. To reduce the final pre-tension on the line 5 the movement of the jacking screw 4a is reversed, moving away from itself until a desired reduced pre-tension on the line 5 is achieved.

Fig. 3 shows an embodiment of the invention which illustrates the line tensioner device 1 being used as an integral part of a WLR setup for a subsea installation and subsea riser system, comprising of, but not limited to, a subsea wellhead, a subsea template, a BOP and a plurality of support lines 5 where a line tensioner device 1 is connected to each line 5 and acts as a tensioning interface between the support lines 5 and the subsea template. A further alternative embodiment of this setup could be where the line tensioner device 1 acts as a tensioning interface between the support lines 5 and the BOP.

Fig. 4 shows a further alternative embodiment of the invention similar to the embodiment as shown in Fig. 3. However, in this embodiment a subsea template is not included, which could be the available setup in some subsea oil wells. Instead, embedded anchors on the ocean floor adjacent to the subsea wellhead, acts as affixation points for the line tensioner device 1 and/or support lines 5.

Claims

1. A line tensioner device (1) for preserving the structural integrity of subsea wellheads, characterized in that the device (1) includes a clamping mechanism (2), a first tensioning mechanism (3) and a second tensioning mechanism (4), wherein the clamping mechanism (2) comprise of a clamping jaw (2a) pivotably connected to a first hydraulic cylinder (2b),

wherein the first tensioning mechanism (3) comprise of a line gripper (3a) pivotably connected to a second hydraulic cylinder (3b), wherein the second tensioning mechanism (4) comprise of a jacking screw (4a) and ratchet gear (4b),

wherein said clamping mechanism (2) and first tensioning mechanism (3) are slidably connected to said second tensioning mechanism (4), wherein the clamping mechanism (2) and first tensioning mechanism (3) are adapted to guide a line (5) through the device (1).

2. Line tensioner device (1) according to claim 1, wherein second tensioning mechanism (4) is pivotably connected to a pin.

3. Line tensioner device (1) according any of the previous claims, wherein the second tensioning mechanism (4) includes a load cell adapted to measure the tension on the line (5).

4. Line tensioner device (1) according to any of the previous claims, wherein the second tensioning mechanism (4) has a torque tool interface (6) which can be operated by a remote offshore vehicle (ROV).

5. Line tensioner device (1) according any of the previous claims, wherein a plurality of rollers (6) guide the line (5) through the clamp mechanism (2) and first tensioning mechanism (3).