DK201570785A1

DK201570785A1 - Subsea remotely operated chain tensioning and slacking system

Info

Publication number: DK201570785A1
Application number: DK201570785A
Authority: DK
Inventors: Geir Olav Hovde
Original assignee: Apl Technology As
Priority date: 2013-05-02
Filing date: 2015-12-01
Publication date: 2016-02-22
Also published as: GB2527721B; GB2527721A; NO20130615A1; NO337531B1; CA2911173A1; US20160176480A1; AU2014260519B2; CA2911173C; AU2014260519A1; AU2014260519A2; DK179299B1; 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 offshore mooring systems. More specifically it relates to a subsea, remotely operated tension adjusting system for mooring lines where the lines comprise chain sections.

Introduction and background

PeåMtions and Abb re via lions

The abbreviations psed in this document are: PFF Design Fatigue Factor EOF End of Life BSD Emergency Shut Down FPSO Floating Production Storage and Offloading FSO: .Floating Storage and Offloading fStl Floating.. Storage Unit

If Pi; High JPressare Unit: MS Hot Spot MBL Minimum Breaking Load MLSB Mooring Line Buoyancy Element MSL Mean Seawater Line MTF .Mechanical Transfer Function.

.ROY Remotely Operated Vehicle STL, Subtnerged Turret Loading, STP Submerged Turret Production SWL Safe Working Load.

TBD f o Be Determined

WRPV Work ROV

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

Buoy; Dompiete 8¾ / STP Buoy comprising:

Buoyancy Cone, Bearings Turret, ESD valves, Buoy part of Hyiihmlic· and Signal Connectors. Riser hang-offs and .ee^aøefioas,/Moøi:ing,CQBaecti€«s and Pick-up Assembly, Connecting device: Chain connecting device that is permatmaily/feed located in tire mooring line; also simply 'Connector*

Operating device: Tool to operate the connecting device during t ension adjusting operation; attachable to and detachable iroffi the connecting device.

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

Mooring System.: Complete mooring system, comprising: Anchors:, Chain, Wire, BOiyster Mope, Mooring Line Buoyancy Element- and Conneeikms:

Riser System: Plekible riser and umbilical system, from:, seabed to vessel

Riser*: Eexible riser lor transfer of liquids and gases.

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

Ri ser/umbi Meal System..

Umbilical; Idexibie umbilical for powerihydraulie and signal lines.

Mooring systems in deep and ultradeepwaters oftM require use of polyester ropes, because of weight issues and vessel offset limitationSi One drawback with polyester ropes is however that it creeps over tinte 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 riretchlng the rope to a high tension durrngthe offshore installation campaign, but for practical reasons (installation vessel capabilities and safety) there is an tipper 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 stic and. the; cohip!exlt:y: of the Buoy, He-iensioning as a combination ctf tensioning daring installation (to typically 20-30% Of the M.BL for the polyester rope), aid by opening the line, coding chain and closing the line again, if later re-tensioning is required. The latter operaion may however be expensive, as it typically tenures large vessel(sj with significant winch/erane capacity, ROV, weather limitations, long planning due to limited number of vessels that can do the operation, etc.

There exists thus a need h>r a new tensitming system in order ns simplify and reduce the cost/risk of the present tension ad|nsting methodology.

Summary

The present invention 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 tine. The system has the inventive feature that the operating device is remotely attachable to the connecting device 'before a tensioning operation and detachable from die connecting device after the tensioning operation.

One of the parts comprised in the system is ;a connecting; device, comprising:; a first cphheerion. arrangement for a first chain and a: second connection arrangement for a: second chain: wherein: the first connection tirrangement. provides a permanently fixed: connection of said firsichain and the second c<mnectii)n :.arra:ngetnent provides a: connection .^hich can be modified by an. operating device. The: connecting device comprises a first docking element - lot 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: clement being compatible with the first docking element, enabling tbe opemting device to firmly attach to the connecting device to perform a tension adjustment operation.

A farther aspect of the invention is a Method for tensioning the inoaring line with- the tension adjusting system disclosed above. The'-method comprises a. positioning a su.ri.aoe vessel above the tnooriog line, slightly to the Hide of a connecting device, where two sections of the mooring line are connected by the connecting device; h. attaching one end of a first chain or similar elongated element to one of two attachment points provided on a guide at an lower end of the operating device and attaching a first weight bar to the second end of#é:&®chiiin|: e. attaching a second chain or similar element, longerthan the fet chain to a second attachment print on the gnide at the lower end of the operatingdevice and attaching a second weight bar to the second end of the second chain; <1 attaching a lihihg/handiing equipment to a third aiiachtnent ^ end of the operating device; e, connecting' an am'bilical to a power system of the operating device, and to an 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, arid by moving the vespt alighdy passed hot: without, the second chairs 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 stop s against a first docking element, a second, docking element on the operating device connecting around the first docking element; k, ppmtkB- tiie operating device via the umbilical for pushing the mooring line in a tensioning direction; repeating this step until a requested tension is achieved; L lilting the operating dericeofpfc connecting device, unhooking from the first docking dement

Brief deserlpition of the drawings

Below the invention will he described with reference to the attached figures.

Contents of the figures as fellows:

Figure 0.. i Typical mooring line composition including both rope and chain

parts. The inventive tensioning system may replace QI, 0CS2 and SP

Figure 0.2 Location of the tensioning systernfeonoectpr in the mooring system

Figure 0.3 Assembly of the connecting device and the operating device of the tensioning system

Figure 0.4 Fixed, tensioner part with illustration of force direction from chain onto: locking elements:

Figure 0,5 Connecting device of tensioning system

Figure 0.6 General arrangement of the connecting device of the tensioning system

Figure Of? Operating dev ice of the tensioning system·.

Figure 0.S General aixangement of operating device of tensioning sy stem

Figure 0.9 Docking of operating device onto connecting device

Figurs 0.10 Illustration of fixed and operating device including tensioned chain and free end

Figure 0,11 Illustration of docking of operating device onto connecting des Ice

Figure 0,12 Tensioning operation

Figure 0.13 Slackening operation

Figure 4.1 Mid f.ine2fensioning systetn:

Figure 4,2 Mid Line Tensioner Connecting device

Figure 4.3 Mid Line Removable Tool

Figure·4,4 Locking Element in Connecting device;

Figure: 4,5 Pushing, Element ..in Operating device

Pdaied deseriptloo

Main goal of ibis .inventive concept is to manage regular tensioning adjustments of mooring lines - both tensiuning Mel mlaxingyfh order to stay within the design envelope of the mooring system.: The tensioning system shall not requite a huge: offshore campaign, and the operation shall he alone with a relatively small vessel in combination with an ROV, without opening the mooring line. The re-tensioning Operaticm is planned conducted with a vessel with minimum crane or A-frame capacity, hut equipped with an ROV/WM3V that can observe the operation ps well as operate ; the power (normally hydraulics) of the tensioningsystem. Alternatively,: the power can be operated via an umbilical between the tensioner and the vessel.

The inventive tensioning system comprises two main parts; confer Figure 2.1 for a general illustration and figures 4.1,4,2, 4.3,4.4,4.5 for details: * Couneeting device; this component becomes a permanem/rixed part in the mooring line y Operating device; this is a tool, used (only temporary) for the tesisioningAlacking process, also called 'removable part’

The operating device of the tensioner can he mounted/docked on top of the connecting device through a hook or similar arrangement at the: lower end. During the tensioning process the pushing dement of the operating device pushes the upper chain towards: the lower end of the connector. The movement of the chain unlocks the locking element of the connecting device, which remains unlocked until the chain: has moved far enough for the locking element to drop down by gravity and thereby lock the chain again. The pushing element can. then he retested; such that it can take a new grip and repeat the sequence until the;hxidring line has been shortened to the desired length. Each cycle will typically inpve two chain links.

A corre:si5øadirig operation can also be used to lengthen the mooring line tbus reducing the tensiOE, but this requires thafthe locking elements of the connecting device are lifted by the ROY when the pushing element has off-loaded the contact between the chain anti the locking elements. Otherwise it will lock the chain frøn being moved backwards.

The tensioner will most likely be located above or below the Mooring Line Buoyancy Element < Ml .Eli, as shown Jn figure 1.1 and ..Figure I -2.

The reason for dividing,the tensioner into a fixed and a removable-part is to minimize: the permanent weight in the mooring system and thereby minimize the required bpoyaney of the MLBE and the STL/STF Buoy, it: will also reduce the o verall cost since the same removable part can be used fin all mooring lines, and maintenance of the hydraulic parts and the mechanical components will be easier, A fixed weight for instance a hinged rod is attached to the connecting device in order to lower the overall centre of gravity and thus ensure that the connecting device is always upright.. This weight limy not be required, :Th.é\lbcking'ileihiéti;iå;f0r the chain can be kept; in place by plates on each side aa well as a one-sided bolt with threads on one side, and threadless and headless on the other side;. This bolt is entered through tie outer tensioner; wall into the: locking clement, and locked inside the element preferably with aiXipNud see Figure 2.6,

When the bolt is felly fixed it is flush with the outer tensioner wall and free to rotate inside the hole in the walk There is clearance between the wall opening and the bolt such that the load from the chain into the locking element and farther into the support structure does «or atress the bolt. Main reason for using the Zipblut technology Is to he able lD^l!daUvelye8sily^la^il^eJockiI^gt^bιϊϊerίtwiίha,WR'OV'l

Hydraulically operated cyB'nders f, ppteifr W'røaatpd^a the operating device, preferably one oa each side of the tool in order-to· provide the-ieqaired force to push theehaim Total evaifeble-loree>lrom: two feyd råolie cylinders could be 130 tonnes, which means that each unit would have to provide ttdnintum 75 tonnes, The cylinder uni is will be connected together sueh that they provide the same posh at the same time, Hydraulic pressure could be provided via a WR.OV or directly via an umbilical from the surface.

The tenstonadiustiiig system ean-betprotected: .afaib^orrosioa in older to avoid any degradation (ti the functionality due to the marine environment. However, since the Operating device typical is only used terø|^i^ly»..'ibl:S· protection'm«da'..øpily fee considered to be applied to the connecting device.

Operation procedure

The following provides a possible high level description of the tensioning operation. The main steps in the tensioning' operation would, bet i) Position I surface; vessel above the;mooring line, slightly to the side of the connecting; device.

2} Attach a chain or similar to one of the padeyes on. the V-shaped guide as the end of the operating device. Attach a weight, bar to the end of the chain.

3) Attach a longer chain or similar to the other padeye on the. V-shaped guide at tie end of the operating device. Attach a weight, bar to the end of the chain, 4) Attach a iifiing/ltandlittg wire; to; the padeye at the upper end of the operating device.

5} Connect the (typically hydraulic) umbilical to the power system of the operating device, and. to power providing; unit on board the surface vessel.

6) Overboard the operating; device with, the umbilical and the chain accessories and weight bars and lower it to almost the same depth as the connecting device mounted on the mooring line. Pay out the umbilical accordingly. The operating device should now fee positioned slightly to the side (the side depends on which side of ihefrperøfirig device else longer chain is certified) of the cooneeiing device, with the lower end of the weight bar connected to the short end above the connecting device and the other bar below the fixed bar.

7) Move the vessel snch that the longer chain hits the connecting device. By moving the vessel slightly passed but without the long chain jumping over (he operating device should rotate such that it orients cormctly Motive the connecting device, 8) Lower the operating device such that the connecting device is between the two chain ;seghl©nts; confer Figure 3.1 and Figure 3.3, Continue to lower it until: the operating device sits on top of the connecting device. After the lower end of the operating device has docked onto the eonneeting device it will slide down the eonneeting device until it; stops against the pin, which the hook shall connect to. Continue to pay out until the iterating device has fully docked, and the hook is felly engaged around the pin- The hook will connect to the pin at a relative angle of 30 to 45 degt^ss between she fixed and operating device; 0 degrees is when they are felly latched together.

9} With the supervision of an RQV> operate the hydraulic units via die umbilical to the surface vessel, for the pushing:.the chain, Humber of eyeles of 23infc pushes depends on how much total length adjustment that is required, confer the pushing sequence shown ::io Figure 3.4 i 0) When the tensioning has;been completed the operating device is lilted off the connecting device, and either lifted onboard the surfree vessel or moved to the next mooring ho©..

For slackening the sy stem the same procedure -can he us^..hut.siii;dyl|nderpist0ni are operated in the opposite direction, confer Figure 3.5. Another fefference is that when the pushing segment release the stresses On the locking elements by pushing at the fer end of the 2-link grip the locking elements on the connecting device must he lified/opened by the ROV; otherwise the locking elements will prevent the chain from being moved back wards. The ROV will let the looking elements engage with the mooring chain as soon as the first link has passed underneath the element.

Claims

2 Connecting device ior use in a mooring line tension adjusting system according to: claim 1, cpniprising a first cdnneetion arrangement for a first chain and a second connection ..arrangement for a. second Cham wherein the first connection arrangement .provides a pemtanenily fixed connection of said first chain and the second connection arrangement provides a connection which can be. modified by an operating device^ eharaeferismd by the connecting device comprising a first docking element enabling the operating device to firmly attach to the connecting device prior to performing a tension adjusting operation,
3 Operating device for use in a mooring line tension adjusting system according to claim 1, «itaraeierixcdi by a second docking element being compatible with a SirntdobkiBg'^lJ^it on a connecting device according to claim 2, enabling the operating device to firmly attach to the connecting device to perform a tension adjustment operation.
4 Method for tensioning a mooring ii.no with a tension adjusting system according to claim 1, said system comprising a connecting device connecting two chains and :an operating device, the method comprising the steps of a. positioning a stmiueevessel above the mooring line, slightly to the side of a connecting device, where two sections of the mooring line are connected by the coonectingdeyiee; h. attaching: one end of a first chain to one of two attachment points provided on a guide at an 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, longer than theirst 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 4 hfting/handlmg equipment to a third attachment point at an upper end of the; opemting device; e, connecting att umhilicai to a power system of the operating device, and to art power supply system on board of the surface vessel; % overboard:the operating device with the umbilical and accessories-and lower it. to slightly above the same depth as the Connecting device mounted into the mooring: line;:: g, mo vi n g the vessel such that the second:: chain hits the connecting device^ and: by rarwing the vessel:.slightly passed hot without the second chain jumping: over the operating/devlce should rotate such: that it orients correctly .relative to the connecting device; h, lowering the: operatingrleviee such that,the connecting device is between the first and the second, chain: i continuing to lower the opensting device until it sits on top of die cormecting device; ]. after the iower end of the operas ing 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 conneciihg to the first doeking elemeht;: 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; and L lilting the operating device off the connecting device., unhooking from the first docking element.