GAZA FSO (Slough Replacement)



PROJECT TITLE: GAZA FSO (Slough Replacement)

CLIENT: STX Offshore


YEAR: 2014

VESSEL: Subsea Construction Vessel Lewek Connector


  • Project Management and engineering
  • Procurement and fabrication of all temporary installation aids
  • All logistics, including receipt, storage and transport of permanent materials
  • Load-out, sea-fasten and transport all equipment to mobilisation location or to site
  • Perform all  survey  and  positioning,  including  pre-installation  and  post-installation  seabed surveys and associated works
  • Install nine (9) off 124mm diameter mooring anchor chains (3 x 3 cluster formation)
  • Install nine (9) off 84” diameter anchor piles
  • Hook-up of all mooring chains to FSO turret and final tensioning of chains
  • Install one (1) Mid Water Arch (MWA) with gravity base / anchor piles
  • Install one (1) Pipeline End Manifold (PLEM)
  • Install one (1) J-tube for the Subsea Power / Optical Cable on DP4 Platform
  • Install one (1) I-tube for the Flexible Flowline on DP4 Platform
  • Install one (1) I/J-Tube Protection Frame
  • Install two (2) 14.75” ID and One (1) 10” ID Dynamic risers from FSO turret to PLEM
  • Install one (1) Subsea Power / Optical Cable from DP4 Platform to FSO turret
  • Install one (1) 14.75” ID Flexible Flowline from DP4 Platform to PLEM
  • Perform all subsea tie-ins for risers and flowline to PLEM
  • Perform all topside tie-ins for risers / flowline / power and optical cable at FSO turret and DP4 platform
  • Trenching for the Flexible Flowline
  • Install pipeline crossing supports and concrete mattresses at DP4 Platform location for Flexible Flowline and Power Cable
  • Perform all flowline and risers’ pre-commissioning, including pigging/flooding, back seal testing, and post installation leak test of all flowline/risers
  • Perform Subsea Power / Optical Cable pre-commissioning post installation
  • Perform as-built ROV video surveys


I was only involve at the initial stage for this project. Mt scope of work was to come up and write up the mehtodlogy for the FSO Slough Installation and also to assist in coming up with the Pre-com methodology

I only stayed 2 month for this project before being ask to

From last what I heard, the project was successfully executed. Well Done Guys.


Mellitah Oil and Gas B.V. (hereafter referred to as COMPANY) has developed plans to replace the existing Floating Storage Offloading (FSO) “SLOUG” with a new built vessel at its offshore field “BOURI”. The field is located 120km North West of Tripoli, Libya and consist of a central processing platform DP4 and a satellite platform DP3 in water depths of 170m. STX Offshore & Shipbuilding (hereafter referred to as CONTRACTOR) and EMAS AMC (hereafter referred to as SUB-CONTRACTOR) have been awarded a contract to provide Engineering, Procurement, Construction, Installation and Commissioning (EPCIC) of the new offshore facilities which consist of the following:-

  • A Single Point Mooring (SPM) system;
  • A Floating Storage and Offloading Vessel;
  • New subsea pipeline including tie-in to DP4 platform and new FSO.
  • A separate power cable from DP4 to the new FSO

The installation and hookup of the new FSO is scheduled to take place in the first quarter of 2015.

Feasibility Study FSO Slough Installation

Mooring System Installation & Pre-tensioning

This section presents the proposed installation methodology for the mooring system and includes pre-tensioning of the system prior to hook-up. The mooring chains and piles shall be installed by the Boa Sub C and pre-tensioning shall be done later by the Lewek Falcon.

The mooring chains and piles shall be installed in two trips, with an interim load-out after the 1st trip. Six (6) mooring chains and piles shall be installed in the 1st trip, after which the MIV shall load the remaining chain and piles from the barge on site.

Vessel Preparations Works

  1.  Arrive in field. Perform field entry, DP trials and survey calibrations as required.
  2.  Perform pre-lay survey of mooring chain corridors.
  3. Perform pre-installation survey for PGF and pile positions.
  4. All subsea assets, design locations and target boxes of the piles and chain corridors are displayed on the vessel survey navigation screens.
  5. Position vessel in right location for overboarding the pile guide frame.

Pile Guide Frame (PGF) Over-boarding

Perform pre-deployment checks of the PGF:-

  • Ensure transponders are fitted to the PGF and fully charged.
  • Check the bullseye and confirm steel ball is visible
  • Ensure PGF lift rigging is connected and in good condition for lifting
  • Ensure ROV docking station is in place for orientation of PGF
  • Ensure tag lines in place for overboarding
  1. Remove seafastening for the PGF.
  2. Connect PGF lift rigging to MIV main crane block
  3. Confirm vessel is in position for overboarding the PGF
  4. Lift  PGF  off  the  deck  of  the  MIV  and  slew  crane  as  required  to overboard PGF.
  5. Lower PGF through splash zone up to 50m below surface.
  6. Lower the PGF to 20m off seabed and all stop on crane.
  7. Activate heave compensation on crane.
  8. Step vessel into PGF target box location with ROV monitoring.
  9. Once PGF is in target box, ROV to dock into docking frame on the PGF and orientate the PGF as required to the right orientation.
  10. Once the PGF is within the required target box and within the specified heading envelope, come down on the crane gradually and land the PGF on the seabed.
  11. Survey to confirm the PGF installation position and orientation. ROV to check the bullseye on the PGF as a measure of verticality.
  12. Record the position, orientation and verticality of the PGF.
  13. Disconnect crane from PGF rigging and recover crane to surface.

Mooring Pile Overboarding and Installation

  1. Perform pre-deployment checks for the pile and chain:-
    · Ensure mooring chain is connected to pile padeye via special anchor shackle.
    · One side of alternate chain links have been painted white for twist monitoring during lay (This shall be completed during load out)
    · Confirm right chain (correct length) to be laid at pile position
  2. Record  mooring  components  identification  and  take  photos  prior deployment.
  3. Assemble pile hang-off pennant rigging arrangement.
  4. Install pile hang-off pennant onto hang-off platform on STBD side.
  5. Recover 1st chain end from the chain locker compartment.
  6. Engage chain in gypsy and ensure chain is correctly seated in gypsy chain pockets.
  7. Pay  out  chain  using  the  gypsy  and  with  a  suitable  deck  sheaving arrangement, use a tugger winch to pull the chain from the chain locker exit to the main deck within the reach of the aft crane.
  8. Connect suitable lift rigging attached to the end link of the chain to the aft crane.
  9. Lift the end of the chain with the aft crane, slew and overboard the chain over the stern roller and pay out the chain as necessary by the gypsy.
  10. Continue paying out on the chain gypsy and have the chain over the stern roller, creating a belly underneath the MIV.
  11. Once  the  belly  is  ~20m  below  water  surface,  slew  the  aft  crane outboard and towards the STBD side of the vessel.
  12. Bring the end of the chain inboard and connect to the pile padeye of the pile to be overboarded. Note: Belly of chain is still intact.
  13. Remove seafastening for pile to be overboarded.
  14. Stab   ILT/PUT   and   rig   up   pile   for   overboarding   as   per   lifting arrangement.
  15. Ensure tag lines are attached to the piles for overboarding.
  16. Lift  pile  using  main  crane,  slew  outboard  towards  STBD  side  and deploy pile through splash zone. Note: Ensure sufficient clearance for pile overboarding between main crane and aft crane during slewing and overboarding operation. Ensure vessel position / pile does not induce a twist in the chain.
  17. Lower pile to approximately 25m water depth.
  18. Deploy ROV to pile location.
  19. Orientate pile such that up-ending sling is adjacent to the pile hang-off pennant ROV hook previously deployed.
  20. ROV to connect up-ending sling into ROV hook attached to end of hang-off pennant
  21. Commence lowering main crane. Such that up-ending sling starts to take weight and pile starts to upend. ROV to monitor up-ending operation.
  22. Continue lowering main crane until pile is fully upended and crane lift rigging is slack.
  23. Disconnect pile overboarding rigging by ROV and recover crane to surface.
  24. Install pile installation rigging onto main crane and re-deploy crane to hang-off position.
  25. ROV to connect pile lift rigging to 2nd  transfer sling at pile and take up weight on crane.
  26. Come  up  on  crane to take  weight  of  pile  and  disconnect  hang-off pennant by ROV.
  27. Start  lowering  pile  towards  the  seabed  while  stepping  the  vessel towards the PGF position.
  28. Continue lowering until pile is approximately 5m above top of PGF.
  29. Activate AHC on main crane.
  30. Complete final vessel positioning above PGF and continue lowering pile towards PGF.
  31. Lower pile until the tip engages with the guide frame, continue lowering until pile guide aides engage with pile guide vanes to finally orientate pile, maintain verticality with fine positioning of crane/vessel as required.
  32. Lower pile into seabed until full self-penetration is achieved. Estimated self-penetration of pile is 7 m.
  33. Confirm pile is within installation critieria. Check bullseye reading on PGF to confirm pile landing has not upset level of PGF.
  34. Disconnect ILT/PUT from top of tile and recover back to surface.

Pile Hammer Deployment and Piling operations

  1. Hammer technicians to perform all pre-deployment checks according to vendor’s operation manual.
  2. Connect hammer lift rigging to main crane block and ensure tag lines are attached to hammer.
  3. Remove all seafastenings.
  4. Upend hammer with sleeve on its storage frame. Note: Ensure there is sufficient slack on hammer umbilical and control lines during upending.
  5. Lift hammer out of the upending frame, and slew hammer over the STBD side of the vessel. Payout umbilical and control cable as required to maintain slack.
  6. Lower hammer through the splash zone, stop lowering and function test the water level sensors.
  7. Continue lowering the hammer through the water column towards the PGF.
  8. ROV to monitor catenary of hammer umbilical and control cable during deployment.
  9. Continue lowering until hammer is approximately 5m above pile.
  10. Activate AHC for crane.
  11. Stab pile hammer sleeve over the top of the pile.
  12. Adjust  hammer  umbilical  and  control  cable  catenaries  to  provide sufficient slack during driving.
  13. Position ROV to monitor pile penetration.
  14. Start  pile  driving  with  a  low  blow  rate  and  energy  level;  continue monitoring pile verticality while gradually increasing the blow rate and energy level until hammer and pile are stable.
  15. Set energy level according to driving conditions and continue driving in automatic position.
  16. ROV to maintain visual at all times to estimate pile verticality.
  17. Record a running log of all pile driving operations.
  18. Slow down the driving speed as the release ring approaches 2m from top of frame sleeve.
  19. When the release ring reaches 2m, stop driving and disengage the frame safety lock bars by ROV.
  20. Re-start driving steadily. Note: Once the release ring passes the latch kick plates in the sleeve, the PGF will open up clear from pile – ROV to remain clear.
  21. Record new penetration depth for pile once PGF has opened.
  22. Continue driving operations at normal speed until target penetration depth is reached.
  23. Record final penetration depth on chart and complete pile driving log.
  24. ROV to carry out survey and record as-installed position of pile.
  25. Come up on crane to lift hammer clear of pile top.
  26. Continue lifting hammer while paying in umbilical hoses, control cable until 50m water depth.
  27. Recover hammer back on deck.

Mooring Leg Installation

  1. On completion of pile installation activities, the mooring chain can be laid along the mooring route corridor.
  2. Set-up vessel on best heading for laying the mooring leg.
  3. Commence paying out the chain gypsy while stepping the vessel along the lay corridor.
  4. Maintain the minimum layback distance as per the chain lay tables.
  5. ROV to monitor the TDP during lay and record any twists visible in the mooring chain.
  6. Continue laying the remainder of the chain along the corridor.
  7. Once the end link of the chain exits the chain locker, suspend all lay operations.
  8. Lock off chain using hang-off plate below gypsy.
  9. Connect chain A&R rigging to the 6th link from the end of the chain.
  10. Connect work winch wire to chain using suitable rigging.
  11. Lift chain end, take catenary weight with winch and withdraw chain gypsy hang-off plate.
  12. Abandon chain towards seabed while stepping vessel along mooring corridor. ROV to monitor whole operation. Note: End of chain to be attached with subsea recovery rigging / Clearly mark subsea buoy with the current chain leg.
  13. Once the chain end lands on seabed, ROV to disconnect work wire from A&R rigging.
  14. Recover work wire back to surface.
  15. ROV to take a fix at the A&R connection chain link and mark chain position on the survey screen.

PGF relocation 

  1. Set-up  vessel  on  best  heading  for  lifting  operations  above  PGF position. 
  2. Deploy ROV to PGF. 
  3. Deploy main crane with stinger rigging with ROV hook on the end.
  4. ROV to connect PGF lift rigging to ROV hook. Note: The PGF will be in its “open” configuration, and will self- close as the lift progresses.
  5. Take  tension  in  the  PGF  lift  rigging,  maintain  tension  and  use vessel/crane motion to break seabed suction until frame lifts off seabed. Note: Monitor crane hook load at all times during breakout of PGF.
  6. Continue lifting PGF until sufficient clearance off seabed is achieved (minimum 25m). 
  7. ROV to check PGF is fully closed and all safety lock bars are engaged. 
  8. Step vessel to next pile location at low speed and ensure clear of any subsea assets. 
  9. ROV to monitor PGF during vessel move.
  10. Repeat the above steps until all chains 1-9 are installed and abandoned on the seabed. This includes the interim mobilization of the remaining chains and piles after the 1st trip.
  11. Recover the PGF back to deck of the MIV/barge. 

Pre-tensioning of Mooring Chains 

  1. Prepare chain tensioning rigging on deck of Lewek Falcon.  
  2. Based  on  the  survey  system  which  indicates  the  location  of  each subsea buoy, Lewek Falcon shall approach subsea buoy for first chain to be recovered.
  3. Deploy work wire with ROV hook to buoy location and ROV to connect ROV hook to recovery rigging.
  4. Recover subsea buoy to deck of Lewek Falcon and continue recovering until the chain can be locked off in the karm forks.
  5. Lock the chain link using the karm forks. 
  6. Release tension on the work wire and disconnect subsea buoy rigging from chain. 
  7. Continue paying in on the work wire and recovering the chain once buoy rigging is disconnected. 
  8. Recover the end of the chain across the main deck to entrance of the chain locker. 
  9. Using the chain gypsy, start recovering the chain and storing them into the chain locker
  10. Continue  doing  this  until  approximately  500m  of  chain  have  been recovered on board the vessel. Note: The length of chain to be recovered shall depend on the chain   catenary   length   on   seabed   required   to   achieve   the pretension value.
  11. Lock chain link in karm fork.
  12. Connect chain pre-tensioning rigging with one end to chain link after the karm fork and the other end to the work wire.
  13. Check and confirm load cell is working condition. 
  14. Position vessel for chain tensioning operations. 
  15. Take up tension  on the  work  wire  and  transfer  full  chain  catenary weight on karm fork to work wire. 
  16. Release and lower down karm fork.
  17. Start pre-tensioning of the mooring chain, gradually increasing vessel thrust until the required pre-tension value is achieved. (Current estimate is 275te)
  18. Hold the intended tension value for 15 minutes. 
  19. Obtain approval and sign-off from the Client 
  20. Gradually reduce the vessel thrust and step the vessel back to slack off the chain catenary.
  21. Lock chain link off in karm fork and disconnect chain tensioning rigging from work wire.  
  22. Take up tension with the chain gypsy and lower karm fork. 
  23. Relay chain along mooring corridor towards turret center.
  24. Once the end link of the chain exits the chain locker, suspend all lay operations.
  25. Lock off chain using hang-off plate below gypsy. 
  26. Connect chain A&R rigging to the 6th link from the end of the chain. 
  27. Connect work winch wire to chain using suitable rigging. 
  28. Lift chain end, take catenary weight with winch and withdraw chain gypsy hang-off plate. 
  29. Abandon chain towards seabed while stepping vessel along mooring corridor. ROV to monitor whole operation. 
  30. Once the chain end lands on seabed, ROV to disconnect work wire from A&R rigging. 
  31. Recover work wire back to surface.
  32. ROV to take a fix at the A&R connection chain link and mark chain position on the survey screen. Note: Chain should be peeled back and end of chain attached with recovery rigging for hook-up to FSO.

Mooring System Hook-up to FSO and Final tensioning

This section presents the method statement for hooking up the mooring chains to the FSO and performing the final tension required for the mooring chains. This activity shall only be performed upon the arrival of the FSO in field.

Vessel Preparations 

  1. Ensure all preparatory works on FSO for chain pull-in performed:
    · Survey system on FSO tested and functional
    · Pull-in routing arrangement from pull-in winch down to chain stopper in order
    · Verify chain stopper/chain table orientation for first chain hook-up based on FSO heading
    · Pull-in winch function tested
    · Inspect messenger lines moused through chain stopper
  2. FSO to arrive in field. Three (3) heading control and positioning tugs shall be connected to the FSO for heading control of the FSO during the mooring hook-up.

Recovery of Mooring Chain 

  1. Lewek Falcon to position over the end of the first mooring chain to be recovered as indicated on survey navigation screen. 
  2. Deploy ROV and locate subsea recovery rigging attached to end of chain. 
  3. Deploy crane whip line with ROV hook attached to end of crane stinger.
  4. ROV to connect ROV hook to subsea recovery rigging. 
  5. Recover chain end over stern roller of Lewek Falcon, stepping vessel back along lay path during recovery. 
  6. Lock off chain link in karm fork and disconnect crane wire and recovery rigging. 

Initial Pull-in of Mooring Chain 

  1. Stabilise FSO with the assistance of the heading control tugs and survey.
  2. Step Lewek Falcon to within 20m of turret centre.
  3. Pay out and deploy FSO pull-in winch wire and messenger line until end is ~ 50m below water line. Note: FSO winch wire has been previously routed from pull- in winch through chain stoppers.
  4. Recover messenger line and pull-in winch wire to deck of Lewek Falcon with the assistance of crane and ROV. 
  5. Attach and connect end of pull-in wire to mooring chain end link.
  6. Connect crane/work winch to sacrificial sling connected to chain link 5m away from end link and take up tension on crane.
  7. Lower  karm fork  and overboard chain over  stern roller  using main crane.
  8. Continue lowering chain with crane and slowly cross haul to FSO with ROV monitoring chain catenary.
  9. Commence hauling in on FSO pull-in winch wire and “sucking” in excess chain length while Falcon’s crane is still taking on the chain catenary’s weight.
  10. ROV to cut sacrificial sling at chain link once chain is almost directly below FSO chain table. Recover work wire to deck. 
  11. Continue  pulling  in  chain  until  2nd   link  of  mooring  chain  is engaged in chain stopper. 
  12. Deploy air divers to disconnect pull-in winch wire from chain at chain table.
  13. Reposition FSO and Lewek Falcon as required and re-configure chain pull-in arrangement for subsequent chain pull-in for remaining chains. Note: Centre chain for each cluster to be pull-in first. Once the 3 chains have been pulled in, the two bow tugs can be disconnected and the FSO heading maintained by the stern tug.
    · Scuba Replacement needs to be evaluated to complete this activity.
  14. Repeat Steps 1-13 for remaining chain legs in sequence. 

Chain Tensioning 

  1. Once  all  9  chains  have  been  pulled  in  and  locked  in  chain stoppers, system is ready for final tensioning. 
  2. Route pull-in winch wire through upper cantilever penetration and lower to chain table through the mooring wheel.
  3. Deploy air divers to work on chain table top. 
  4. Connect pull-in winch wire to chain end link.
  5. Commence hauling in on winch and hoist chain out of chain stopper allowing chain clacker to open.
  6.  Continue hauling in on winch and pulling in chain until maximum stroke is achieved or target link is reached. Note: This operation may be performed over several strokes of pull-in. Final link shall be identified and confirmed by attending engineer from LMC.
  7. Once target link is pulled through and locked in chain stopper, slack pull-in winch wire and lower chain onto chain table. 
  8. Air divers to cut link above locking link using broco torch.
  9. Once the chain link is cut, the cut length of chain shall be cross hauled to Lewek Falcon for disposal.
  10. Repeat Steps 2-9 for remaining chain legs until all 9 chains have been tensioned to desired value.
  11. The position of the turret shall be checked and compared with predicted values using transponder/transducer references. Angles of each chain shall also be checked against design values. In addition, diver to check using chain angle measurement tool.


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