The FSO conversion is done by modifying tanker in bow or in midship. External turret mooring system are employed by modifying tanker in the bow area. The FSO are mooring by even number of mooring chains i.e 6,8,12 depending on mooring requirements and commercial viability. Risers are attached from chain table to PLEM. The turret will be entirely above water level at all times.
Scope of work is summarized as follows:
- Receive and prepare the FSO for towing and installation
- Assemble marine fleet
- Load-out of chains, piles/anchors and riser
- Pre-installation of piles/anchors and chains including pretensioning
- Hook-up of chains to FSO and chain tensioning
- Install and hook-up of riser to FSO including leak testing
- Commissioning and start-up
- FSO with turret mooring system installed at the base.
- Embedment anchors or piles. The embedment anchors are likely to be of the Stevpris or Bruce high holding power type.
- Mooring chains. Each chain (as supplied) will consist of:
- Required length and diameter chain of studless grade R3 continuous chain, with enlarged end
- links for connection to anchors Grade R3 D-Shackles
- A riser system including end fittings and pulling heads, Buoyancy elements complete with their connections to the riser
- Temporary installation aids such as pulling heads
- Bend stiffener and bend restrictor
- Equipment on the FSO winches / windlasses on the bow of the FSO and sheaves and block shackles for chain tensioning and riser pull-in.
- Tandem mooring hawser
- Offloading hose.
External bow mounted turret mooring arrangement consist of even number of mooring chains i.e 6,8,10 etc with each lines separation of equal angles between lines, i.e. 6 mooring lines will be installed at 60 degree each and 12 mooring lines will be spaced at 30 degrees. Chain length and diameter shall be ascertained by mooring analysis for external turret mooring arrangement.
The anchors or piles are to be positioned at calculated radius from the turret centre. This radius will be lesser than chain length because of the chain catenary, inverse catenary in the soil, tension chain and anchor assisting tolerances.
Winches or windlasses are required for pull-in and tensioning of the mooring chains, and also pull-in of the riser. Ships existing winches and windlasses are to be checked for viability if they can be accommodated for this operation. If required new winches shall be installed. In case of ship’s existing winches, hydraulic winches are powered by the hydraulic power packs located in the engine room. Both the winches and power packs shall be inspected, refurbished and load tested as necessary during conversion of the tanker to FSO. If new winches are required, power packs can be installed at suitable locations.
The load during pre-tensioning of each chain shall be estimated. Winches can be used alone or in series connection for the operation. One winch will be used for direct pull and connection of risers, for initial connection of the mooring chains and for pulling of the riser. Other combination of winches shall be used for final tensioning.
Riser is hung off from the bend beneath the chain table. The bend is designed so that the bottom flange is at right angles to the riser in its final configuration.Bend is welded to a fixed pipe which passes through the chain table and is welded to it.
Above this pipe there is a vertical pipe spool and ESD Valve and the swivel stack are connected with that. Pipe finally leads onto the main deck where there is a pig receiver and the line leading to the FSO tanks. Spool will be assembled together with the rest of the piping.
After the pull-ins of the riser and chains, spool between the piping at the top of the chain table and the underside of the ESD Valve shall be refitted. A riser access platform shall be provided beneath the chain table. Manhole shall be provided in chain table so that personnel can descend to the platform in order to connect the riser to the bend.
Each anchor will be laid in turn, with the full length of chain attached to it. Single or double AHTS can be used for laying off this operation.
Anchors shall be laid in target box of 10x10 m square around nominal laying position. 2 transponders to the chain,at fixed distance buoys can be attached to determine location of these anchors.. These transponders remain above the seabed during anchor penetration. Thus, by locating them, the position of the anchor beneath the seabed may be determined. After laying each anchor the anchor chain is paid out from the AHT, with the AHT heading towards the final mooring centre.
After laying each mooring chain, the end of it shall be secured with a pennant to it with a surface marker buoy, with the number of the chain marked on the buoy. Another way is to secure a subsea beacon to the end of the chain which can be subsequently interrogated and the connection made to the end of the chain using an ROV. There is also an underwater buoy on a short pennant. When recovering the end of the chain, an ROV is used to connect a line to this
After laying all anchors and chains, the anchors must be pretension before the FSO is connected to the mooring system. The purpose is to make sure that when the FSO imposes loads on the mooring chains in service, the anchors do not drag.
The normal Class requirement is to apply a load of at least 80% of the highest calculated intact loads to which the line may be subjected in service. For example if the highest calculated load on the line in service is 100 tonnes, applied pretension load must be 80 tonnes.
Mooring lines configured on opposite sides shall be pretensioned together.
Potentially ways of doing pretension are
- To secure one of the two lines to a Stev-tensioner or Bruce tensioner and to pass the opposing line through the tensioner. Then, by pulling upwards using an AHT or crane, the pretension may be achieved.
- One of the chains is secured to a tensioning barge or vessel. The other chain is brought onboard the same barge or vessel and a winch is used to tension one against the other.
- Two large AHTs are connected to one of the chains and they both pull so as to apply the 80% of max operational load. Combined effective Bollard Pull must exceed this load to achieve this pretension.
Each of the piles has a welded padeye part way down the pile. Thus, the mooring chain must be shackled to this padeye before driving the pile and each pile will have all or part of the chain shackled to it during driving. The pile may be driven using a conventional or underwater pile hammer.
If an underwater hammer is used, Contractor will need to provide a guide frame which can be positioned on the seabed at the correct location and use it to maintain verticality and correct azimuth of the pile during driving. If a conventional pile hammer is used, Contractor will need to provide followers and a hang-off frame on his vessel. The tolerances are:
- The piles must be installed within +/ - 1 metre of the nominal location.
- Orientation (Azimuth) must be correct within + / - 2 degrees
- Verticality must be correct within + / - 2 degrees
After installing each pile it must be preloaded to a sufficient degree to develop the ‘inverse catenary’ of the chain at the pile. This means that the chain cuts into the seabed, between the padeye and the point where the chain emerges from the seabed. The estimated load to be applied, to achieve adequate inverse catenary .
Prior to connecting the mooring chains:
The chains and anchors will have been prelaid, pretensioned and buoyed off, as above.On the FSO the wire from the pull-in winch will be led over the guide at the main deck and through one of the six fairleads on the chain table.
The FSO is assisted in maintaining station by anchor handling tugs. The number and type is to be decided, but there are likely to be 3 AHTs, initially with 2 on the bow and one on the stern. Note that these vessels have to able to maintain the correct heading of the FSO for pull-in of each mooring chain, so that the winch wire is in line with the mooring line being connected.
The FSO winch wire will now be lowered away to sea level. Another Contractor’s AHT will recover from the seabed the upper end of the selected mooring chain such that the chain is secured on her deck using her sharks jaws (or equivalent). The AHT will retain the recovery buoy and the pennant onboard. The end of the winch wire from the FSO is then passed to the AHT. This is normally done using a work boat. The winch wire is slackened at the FSO.
The AHT receives the end of the wire onboard and shackles it to the end of the mooring chain. The winch on the FSO will then pull in the wire until the top of the chain is pulled through the chain fairlead on the chain table. The brake on the FSO winch will be applied and the chain stopper will be placed around one of the top links of the chain.
This stopper is suspended from the gantry on chain blocks by the Contractor. The winch wire then slackens until the stopper engages in its housing above the fairleader. The winch wire is then disconnected from this chain and led down to the fairleader for the next chain to be connected.
The above procedure is followed for connection of all chains, with the vessel being rotated using the AHTs for connection of each mooring line, and held on the correct heading.
Decide the order in which to connect the mooring chains. This to some extent depends on the weather conditions that prevail on the day. The preferred option is to first secure three mooring chains, for example nos 2, 4 and 6.
With these secured the position of the head of the FSO is approximately fixed. Following this, it is only necessary for tug(s) at the stern of the FSO to move the stern around to achieve the correct heading.
When these three chains are connected, all operations could in principle be postponed if the weather deteriorates since the turret is then restrained in all directions. However, this should not be necessary because of the estimated short duration of these operations which can be completed within the period of a reliable weather forecast. It is estimated that all six (6) chains can be connected, but not pre-tensioned, within a twenty four (24) hour period.
The chains will now be pre-tensioned one at a time using the FSO winch. For this operation two tugs are connected at the stern in order to maintain the correct heading of the vessel.
Each chain is pulled up until the required number of links have been pulled in, in accordance with instructions from the LFPL attending engineer. After each pull-in, the chain stopper is positioned around the chain and the winch then lowers the chain until the stopper is in its seating at the top of the chain hawse, as previously.
The chain is then cut by the Contractor at the link above the one that is secured in the stopper. Following this the cut length of chain is pulled up onto the main deck using the winch and manoeuvred to one side on the main deck and secured for subsequent removal to shore by the Contractor.
This operation will be carried out on each of the six chains in turn. The position of the turret using transducer/transponder reference shall be checked and compared with predicted values and angles of the chain will then be checked. Following this there will be a final pull-in of each chain, which follows the same procedure as above, except that the cut length of chain will be much shorter (approximately 3m).
After final cutting of chains the position of the turret centre, relative to the PLEM, will again be monitored using transponder/transducer references. It is important that the distance and azimuth are correct within the required tolerances which will be approximately:
Distance +/- 2m
Azimuth +/- 5 degrees
There is flexible riser to be installed between the PLEM and the turret. The riser configuration is lazy wave. The top of the riser will be fitted with end fittings that will enable it to be pulled in and hung beneath the chain table. The flexible riser is a delicate piece of equipment that must be handled carefully, as most accidents are caused or initiated during installation. In particular, care must be taken not to overbend the riser during installation.
There are two options:
a) The riser is reeled off from the installation vessel and the head of it is connected at the chain table. The riser is then reeled off until the lower end can be lowered to the seabed and connected to the flange at the PLEM.
b) His installation vessel may lower away the riser so that he can connect it to the PLEM. The vessel then pays out the riser, the head of which is cross-hauled to the chain table for connection.
The procedure of cross-hauling the riser to the chain table is similar in all cases. The winch wire will have been passed down through the piping to the waterline. The installation vessel will recover the end of the upper end of the riser to her main deck. If procedure a) above is followed, then the upper end of the riser will already be on the reel on the installation vessel.
The end of the winch wire, suspended from the turret is passed across to the installation vessel. This is normally done using a work boat, although, depending on the size of the installation vessel, it may be safe for this vessel itself to manoeuvre
beneath the turret to recover the end of the winch wire, without risk of the vessel colliding with the FSO and chains.
After recovering the winch wire the installation vessel moves clear of the turret whilst the winch wire is paid out from the turret so that it remains slack. The installation vessel then shackles the end of the winch wire to the riser pull-in head. The winch wire on the turret gently hauls in the riser whilst the installation vessel pays it out, keeping slack in the riser.
it will essentially be as follows:
The winch wire will recover the riser to a point where the pull-in head is approximately 1 to 1.5m below the pipe flange.
The weight of the riser will then be transferred so that it is temporarily taken between padeyes on the underside of the chain table and padeyes at the riser pull-in head. This may be done using chain blocks, Tirfors or threaded bolts.
When the load has been completely and securely transferred as above, then the winch wire on the turret can be slackened, the shackle disconnected from the riser pull-in head, and the pull-in head removed and set aside. The chain blocks (or similar) are then used to pull the head of the riser carefully up to mate with the flange. The gasket and bolts are carefully inserted insuring there is no damage to either of them. Depending on the type of riser that is used, there may be
insulating sleeves around these bolts in order to ensure electrical discontinuity between the riser and the chain table. In this case there will also be insulating washers beneath the heads of the bolts as well as steel washers. If these sleeves and washers are required it is essential to double checks that they are available on board, with spares. The bolts are then tensioned to the required torque using suitable bolt tensioning equipment.
Simply pay out the riser, after connecting it to the FSO chain table and then fit the buoyancy elements around the riser, with the riser laid out on the deck of the installation vessel. For this to be acceptable the it shall demonstrate that the installation vessel can safely maintain station during attachment of the buoyancy elements, with provision for safe temporary abandonment if the weather deteriorates.
Normally the riser is guided down to the PLEM using two steel guide wires which are rigged from the installation vessel to padeyes on the PLEM passing through padeyes welded to the pull-in head of the riser. When the riser head is close to the PLEM it can be removed and the riser pulled in to flange up with the PLEM , except that there is no need for insulating sleeves and washers at this connection.
When the riser has been installed and connected to the chain table, check the profile of the riser, by divers or otherwise, and adjust if necessary. Conduct the leak test of the riser. it shall stand by until this is satisfactorily complete and should be prepared to act, for example to adjust the connections to the PLEM, if required. In principle the mooring should not require to be adjusted or commissioned. However, engineers should be prepared to, for example, rotate the FSO, to test that the turret centre remains within the allowable envelope and that the riser has the correct clearance to the hull.