DK179299B1

DK179299B1 - Subsea remotely operated chain tensioning and slacking system

Info

Publication number: DK179299B1
Application number: DKPA201570785A
Authority: DK
Inventors: Geir Olav Hovde
Original assignee: Apl Tech As
Priority date: 2013-05-02
Filing date: 2015-12-01
Publication date: 2018-04-16
Also published as: GB2527721B; GB2527721A; NO20130615A1; NO337531B1; CA2911173A1; US20160176480A1; AU2014260519B2; CA2911173C; AU2014260519A1; AU2014260519A2; DK201570785A1; WO2014178721A3; WO2014178721A2; GB201519341D0

Abstract

A mooring line tension adjusting system comprises a connecting device, connecting and locking two adjacent chain sections of a mooring line, and an operating device, moving one of the chain sections inside the connecting device to change the tension of the mooring line. The system has the inventive feature that the operating device is remotely attachable to the connecting device before a tensioning operation and detachable from the connecting device after the tensioning operation. The invention discloses also a method how to use the tensioning system.

Description

Field of invention

The present invention relates to a system for subsea, remotely operated tensioning and relaxing of mooring lines according to the preamble of claim 1, to a method for subsea, remotely operated tensioning and relaxing of mooring lines by the system, and to a connecting device and an operating device for use in the system.

Thus the present invention relates to offshore mooring systems. More specifically it relates to a subsea, remotely operated tension adjusting system for mooring lines where the lines comprise chain sections.

Relevant prior art is disclosed in WO 2013/004749 A1 which discloses the features of the preamble of claim 1.

Introduction and background

Definitions and Abbreviations

The abbreviations used in this document are:

ESD Emergency Shut Down

MBL Minimum Breaking Load

MLBE Mooring Line Buoyancy Element

ROV Remotely Operated Vehicle

STL Submerged Turret Loading

STP Submerged Turret Production

WROV Work ROV

The definitions used in this document shall be understood as follows:

Buoy: Complete STL / STP Buoy comprising:

Buoyancy Cone, Bearings, Turret, ESD valves, Buoy part of Hydraulic and Signal Connectors, Riser hang-offs and connections, Mooring connections and Pick-up Assembly.

Connecting device: Chain connecting device that is permanently/fixed located in the mooring line; also simply 'Connector'

Operating device: Tool to operate the connecting device during tension adjusting operation; attachable to and detachable from the connecting device.

Tensioning system: System comprising connecting device and operating device; also called 'tensioner'.

Mooring System: Complete mooring system comprising: Anchors, Chain, Wire, Polyster Rope, Mooring Line Buoyancy Elements and Connections.

Riser System: Flexible riser and umbilical system from seabed to vessel.

Riser: Flexible riser for transfer of liquids and gases.

Subsea System: Field related system comprising: Mooring, Buoy and

Riser/umbilical System.

Umbilical: Flexible umbilical for power/hydraulic and signal lines.

Mooring systems in deep and ultradeep waters often require use of polyester ropes, because of weight issues and vessel offset limitations. One drawback with polyester ropes is however that it creeps over time when subject to continuous loading. It also creeps when it experiences loads higher than it has seen earlier. Part of this creep can be mitigated by stretching the rope to a high tension during the offshore installation campaign, but for practical reasons (installation vessel capabilities and safety) there is an upper limit on how much tension that can be applied. Hence, re-tensioning of polyester systems will most likely be required regularly over the design life of the field.

Buoys do often not have any re-tensioning possibility, because that feature would grow the size and the complexity of the Buoy. Re-tensioning has therefore been done as a combination of tensioning during installation (to typically 20-30% of the MBL for the polyester rope), and by opening the line, cutting chain and closing the line again, if later re-tensioning is required. The latter operation may however be expensive, as it typically requires large vessel(s) with significant winch/crane capacity, ROV, weather limitations, long planning due to limited number of vessels that can do the operation, etc.

There exists thus a need for a new tensioning system in order to simplify and reduce the cost/risk of the present tension adjusting methodology.

Summary

The present invention, as defined in claim 1, is a mooring line tension adjusting system comprises a connecting device, connecting and locking two adjacent chain sections of a mooring line, and an operating device, moving one of the chain sections inside the connecting device to change the tension of the mooring line. The system has the inventive feature that the operating device is remotely attachable to the connecting device before a tensioning operation and detachable from the connecting device after the tensioning operation.

One of the parts comprised in the system is a connecting device, comprising a first connection arrangement for a first chain and a second connection arrangement for a second chain wherein the first connection arrangement provides a permanently fixed connection of said first chain and the second connection arrangement provides a connection which can be modified by an operating device. The connecting device comprises a first docking element for instance a pin - enabling the operating device to firmly attach to the connecting device prior to performing a tension adjusting operation.

The second part of the system is an operating device with a second docking element being compatible with the first docking element, enabling the operating device to firmly attach to the connecting device to perform a tension adjustment operation.

A further aspect of the invention is a method for tensioning the mooring line with the tension adjusting system disclosed above. The method comprises

a. positioning a surface vessel above the mooring line, slightly to the side of a connecting device, where two sections of the mooring line are connected by the connecting device;

b. attaching one end of a first chain or similar elongated element to one of two attachment points provided on a guide at a lower end of the operating device and attaching a first weight bar to the second end of the first chain;

c. attaching a second chain or similar element, longer than the first chain to a second attachment point on the guide at the lower end of the operating device and attaching a second weight bar to the second end of the second chain;

d. attaching a lifting/handling equipment to a third attachment point at an upper end of the operating device;

e. connecting an umbilical to a power system of the operating device, and to a power supply system on board of the surface vessel;

f. overboarding the operating device with the umbilical and accessories and lower it slightly above the same depth as the connecting device mounted into the mooring line;

g. moving the vessel such that the second chain hits the connecting device, and by moving the vessel slightly passed but without the second chain jumping over the operating device should rotate such that it orients correctly relative to the connecting device;

h. lowering the operating device such that the connecting device is between the first and the second chain;

i. continuing to lower the operating device until it sits on top of the connecting device;

j. after the lower end of the operating device has docked onto the connecting device sliding it down along the connecting device until it stops against a first docking element, a second docking element on the operating device connecting around the first docking element;

k. operating the operating device via the umbilical for pushing the mooring line in a tensioning direction; repeating this step until a requested tension is achieved;

l. lifting the operating device off the connecting device, unhooking from the first docking element.

Brief description of the drawings

Below the invention will be described in detail with reference to the attached figures. Contents of the figures as follows:

Figure 1	Typical mooring line composition including both rope (25) and chain parts (26). The inventive tensioning system may replace koblingselementerne (27, 28)
Figure 2	Location of the tensioning system/connector (1) in the mooring system
Figure 3	Assembly of the connecting device (1) and the operating device (2) of the tensioning system
Figure 4	Fixed tensioner part (1) and operating device (2) with illustration of force direction (18) from chain onto locking elements
Figure 5 Figure 6 Figure 7	Connecting device (1) of tensioning system Operating device (2) of the tensioning system Docking of operating device (2) onto connecting device (1)
Figure 8	Illustration of connecting device (1) and operating device (2) including tensioned chain (4) and free end (19)
Figure 9	Illustration of docking of operating device (2) onto connecting device (1)
Figure 10 Figure 11 Figure 12 Figure 13 Figure 14	Tensioning operation Slackening operation Mid Line Tensioning system (1, 2) Mid Line Tensioner Connecting device (1) Mid Line operating device (2); Removable Tool

Figure 15 Figure 16

Locking Element (5) in Connecting device (1) Pushing Element (6) in Operating device (2)

Detailed description

Main goal of this inventive concept is to manage regular tensioning adjustments of mooring lines (30) - both tensioning and relaxing, in order to stay within the design envelope of the mooring system. The tensioning system shall not require a huge offshore campaign, and the operation shall be done with a relatively small vessel (11) in combination with an ROV, without opening the mooring line (30). The re-tensioning operation is planned conducted with a vessel with minimum crane or A-frame capacity, but equipped with an ROV/WROV that can observe the operation as well as operate the power (normally hydraulics) of the tensioning system. Alternatively, the power can be operated via an umbilical between the tensioner (2) and the vessel (11).

The inventive tensioning system comprises two main parts; confer Figure 4 for a general illustration and figures 3, 5, 6, 12 to 16 for further details:

• Connecting device (1); this component becomes a permanent/fixed part in the mooring line • Operating device (2); this is a tool, used (only temporary) for the tensioning/slacking process, also called 'removable part'

The operating device (2) of the system can be mounted/docked on top of the connecting device through a hook (21) or similar arrangement at the lower end. During the tensioning process the pushing element (6) of the operating device (2) pushes the upper chain (4) towards the lower end of the connector (1). The movement of the chain unlocks the locking element (5) of the connecting device (1), which remains unlocked until the chain has moved far enough for the locking element (5) to drop down by gravity and thereby lock the chain (4) again. The pushing element can then be retreated such that it can take a new grip and repeat the sequence until the mooring line has been shortened to the desired length. Each cycle will typically move two chain links.

A corresponding operation can also be used to lengthen the mooring line thus reducing the tension, but this requires that the locking elements (5) of the connecting device (1) are lifted by the ROV when the pushing element (6) has off-loaded the contact between the chain (4) and the locking elements (5). Otherwise it will lock the chain from being moved backwards.

The system will most likely be located above or below the Mooring Line Buoyancy Element (MLBE, 29), as shown in Figure 1 and Figure 2.

The reason for dividing the system into a fixed (1) and a removable part (2) is to minimize the permanent weight in the mooring system and thereby minimize the required buoyancy of the MLBE and the STL/STP Buoy. It will also reduce the overall cost since the same removable part can be used for all mooring lines, and maintenance of the hydraulic parts and the mechanical components may be easier.

A fixed weight for instance a hinged rod (8) is attached to the connecting device in order to lower the overall centre of gravity and thus ensure that the connecting device (1) is always upright. This weight may not be required.

The locking elements (5) for the chain can be kept in place by plates on each side as well as a one-sided bolt with threads on one side and threadless and headless on the other side. This bolt is entered through the outer tensioner wall into the locking element and locked inside the element.

When the bolt is fully fixed it is flush with the outer tensioner wall and free to rotate inside the hole in the wall. There is clearance between the wall opening and the bolt such that the load from the chain into the locking element and further into the support structure does not stress the bolt.

Hydraulically operated cylinders (7) f. inst can be mounted on the operating device (2), preferably one on each side of the tool, in order to provide the required force to push the chain (4). Total available force from two hydraulic cylinders could be 150 tonnes, which means that each unit would have to provide minimum 75 tonnes. The cylinder units will be connected together such that they provide the same push at the same time. Hydraulic pressure could be provided via a WROV or directly via an umbilical (14) from the surface.

The tensionadjusting system can be protected against corrosion in order to avoid any degradation of the functionality due to the marine environment. However, since the operating device (2) typical is only used temporarily, this protection needs only be considered to be applied to the connecting device (1)·

Operation procedure

The following provides a possible high level description of the tensioning operation. The main steps in the tensioning operation would be, see also Figures 7, 8, 9, 10 and 11:

1) Position a surface vessel (11) above the mooring line, slightly to the side of the connecting device (1).

2) Attach a chain (23) or similar to one of the padeyes on the V-shaped guide (10) at the end of the operating device (2). Attach a weight bar (22) to the end of the chain (23).

3) Attach a longer chain (16) or similar to the other padeye on the Vshaped guide (10) at the end of the operating device (2). Attach a weight bar (15) to the end of the chain (16).

4) Attach a lifting/handling wire (13) to the padeye at the upper end of the operating device (2).

5) Connect the (typically hydraulic) umbilical (14) to the power system (7) of the operating device (2), and to power providing unit on board the surface vessel (11).

6) Overboard the operating device (2) with the umbilical (14) and the chain accessories (16, 23) and weight bars (15, 22) and lower it to almost the same depth as the connecting device (1) mounted on the mooring line. Pay out the umbilical (14) accordingly. The operating device (2) should now be positioned slightly to the side (the side depends on which side of the operating device (2) the longer chain (16) is connected) of the connecting device (1), with the lower end of the weight bar (22) connected to the short chain (23) above the connecting device (1) and the other bar (15) below the connecting device (1).

7) Move the vessel (11) such that the longer chain (16) hits the connecting device (1). By moving the vessel slightly passed but without the long chain (16) jumping over the operating device (2) should rotate such that it orients correctly relative the connecting device (1).

8) Lower the operating device (2) such that the connecting device (1) is between the two chain segments (3 and 4); confer Figure 7 and Figure

9. Continue to lower it until the operating device (2) sits on top of the connecting device (1). After the lower end of the operating device (2) has docked onto the connecting device (1) it will slide down the connecting device (1) until it stops against the pin (20), which the hook (21) shall connect to. Continue to pay out until the operating device (2) has fully docked, and the hook (21) is fully engaged around the pin (20). The hook (21) will connect to the pin (20) at a relative angle of 30 to 45 degrees between the fixed device (1) and operating device (2); 0 degrees is when they are fully latched together.

9) With the supervision of an ROV, operate the hydraulic units (7) via the umbilical (14) to the surface vessel (11) for the pushing the chain. Number of cycles of 2-link pushes depends on how much total length adjustment that is required, confer the pushing sequence shown in Figure 10.

10) When the tensioning has been completed the operating device (2) is lifted off the connecting device (1), and either lifted onboard the surface vessel (11) or moved to the next mooring line.

For slackening the system, the same procedure can be used, but the cylinder pistons (7) are operated in the opposite direction, confer Figure 11. Another difference is that when the pushing segment release the stresses on the locking elements (5) by pushing at the far end of the 2-link grip the locking elements (5) on the connecting device (1) must be lifted/opened by the ROV; otherwise the locking elements (5) will prevent the chain (4) from being moved backwards. The ROV will let the locking elements engage with the mooring chain as soon as the first link has passed underneath the element (5).

Claims

patent claims

A system for submarine, remote controlled tightening and mooring of mooring lines comprising:

a connecting device (1) for permanently connecting two adjacent chain sections (3,4) of a mooring line (30) of a desired length, wherein an adjacent chain section is a chain held in a joint by a locking device (6), characterized by a control device (2) which can be remotely connected to the connecting device, to tighten or relax the tension in the mooring line by adjusting the length of the mooring line, by moving the chain and changing the connection within the connecting device, the system comprising a pin (20) of the connecting device and a a hook (21) of the operating device at a lower end where the hook is arranged to engage the pin to connect the operating device to the connecting device.

Method for submarine, remote controlled tightening and quenching of mooring lines by the system of claim 1, characterized by the following steps:

a. placing an operating device (2) on top of a connecting device (1);

b. locking the operating device to the connecting device by means of a pin and a hook engaging around the pin;

c. controlling the connection device by means of the control device to tighten or loosen the mooring line, by means of the chain section, until the desired tension is obtained;

d. releasing and removing the control device from the connection device.

The method of claim 2, wherein placing the operating device on top of the connecting device comprises the following steps:

a. securing one end of a short elongate member (23) to one of two attachment points provided on a guide at a lower end of the operating device (2);

b. securing one end of a longer elongate member (16) which is longer than the short elongate member to another point of attachment to the guide at the lower end of the operating device;

c. securing the lifting / handling equipment (13) to a third point of attachment at an upper end of the operating device (2), and lowering the operating device down to the short elongated member and the longer elongated member substantially in in relation to depth, the connecting device (1) overlaps in the mooring line;

d. moving the control device sideways until the longer elongate member (16) strikes the connecting device and the control device then rotates the short elongate member on the opposite side of the connecting device as viewed from the longer elongated member (16);

e. lowering the operating device down on the connecting device, with the short and the longer elongated members on either side of the connecting device.

The method of claim 3, wherein at least one of the short and longer elongate members (23, 16) is a chain.

The method of claim 2, wherein locking the operating device (2) to the connecting device (1) comprises

a. lowering the operating device in vertical orientation, down to the non-vertical, in a mooring line connecting device;

b. moving the operating device along the connecting device until the hook (21) engages around the pin (20); and

c. changing the orientation of the operating device from vertical to parallel to the connecting device to lock the hook around the pin.

A method according to claim 2, wherein the release of the operating device (2) from the connecting device comprises

a. changing the orientation of the operating device (2) from being parallel to the connecting device to being vertical, to release the locking of the hook around the pin;

b. moving the control device along the connecting device until the hook is no longer engaged around the pin; and

c. removing the control device from the connector.

The method of claim 2, wherein the length of the mooring line is adjusted by

a. to take on a tensioning force in the chain section with the control device;

b. releasing the locking device (6) from the chain section;

c. moving the chain section in the tensioning or quenching direction with the control device;

d. transferring the tensioning force in the chain section back to the locking device (6).

The method of claim 2, wherein the operating device is operated via a supply cable.

The method of claim 8, wherein the operating device is connected to the supply cable of a vessel or a remote-controlled submarine.

The method of claim 9, wherein power and control signals are transmitted through the supply cable to the operating device.

The method of claim 10, wherein power comprises hydraulic power.

Connecting device for use in the system of claim 1, characterized by a pin (20) of the connecting device, the pin being adapted to engage a hook (21) on an operating device (2) around it to connect the control device for the connection device.

Controller for use in the system of claim 1, characterized by a hook (21) of the operating device at the lower end, wherein the hook is arranged to engage a pin (20) on a connecting device (1), to connect the operating device to the connecting device.