Basics of Riser installation
The estimated maximum installation tension shall be derived as sum of Weight of the pipe (FOW) in the water column (maximum water level) and an additional water column pressure load.
Temporary Abandonment
During installation, if the flexible pipe needs to be abandoned on the seabed temporarily because of typhoons or other emergency scenarios, the flexible riser and flowline system will need to be filled with chemically treated seawater. Chemicals that should be added to the seawater will include corrosion inhibitor, oxygen scavenger.
The flexible line system is free flooded during installation. Flooding water change out is recommended to be carried out by injecting a volume of gel to act as an interface between the free flood water and the chemically inhibited water. The lines is recommended to be flooded to 30% over volume allowance to ensure that all untreated seawater is flushed from the system and displaced with chemically treated seawater.
Installation Sequence
The installation of the flexible system shall include but not be limited to the following steps:
The estimated maximum installation tension shall be derived as sum of Weight of the pipe (FOW) in the water column (maximum water level) and an additional water column pressure load.
- The minimum allowable bend radius should be followed to avoid any uncontrolled curvature of the pipe. This is highly recommended when handling the pipe near the bend stiffener and towards the extremities (end fittings) where high stiffness variation occurs.
- Maximum allowable twist and compression shall be followed from vendor specification.
- If a tensioner is required for installation, the maximum tightening force on the pipe depends on the tensioner parameters .The maximum allowable tightening force on the pipe is given as an utilisation factor of 0.90 times the flexible pipe’s ultimate crushing capacity.
- If damage to the riser external sheath is detected during the laying operation before the damaged area reaches the water surface, repair of the external sheath must be performed prior to the riser immersion in the water to prevent any water ingress.
- If damage to the external sheath is detected during the laying operation after the damaged area has reached the water level, then the riser must be retrieved and the external sheath repaired to prevent any further water ingress. The riser annulus must also be flushed to remove the water.
- After installation, a vacuum test of the riser annulus can be performed in order to confirm that the annulus is not flooded. If the annulus is found to be flooded, the riser must be retrieved and inspected.
- The riser may be installed either full of treated seawater or empty and filled with treated seawater once installed.
- The flowline should not be laid over large seabed obstructions (e.g. rocks) as this may cause loads that exceed the allowable crushing capacity of the pipe. If it is not possible to alter the flowline route to avoid an obstruction.
Temporary Abandonment
During installation, if the flexible pipe needs to be abandoned on the seabed temporarily because of typhoons or other emergency scenarios, the flexible riser and flowline system will need to be filled with chemically treated seawater. Chemicals that should be added to the seawater will include corrosion inhibitor, oxygen scavenger.
The flexible line system is free flooded during installation. Flooding water change out is recommended to be carried out by injecting a volume of gel to act as an interface between the free flood water and the chemically inhibited water. The lines is recommended to be flooded to 30% over volume allowance to ensure that all untreated seawater is flushed from the system and displaced with chemically treated seawater.
Installation Sequence
The installation of the flexible system shall include but not be limited to the following steps:
- Pay out on platform winch and come up on vessel winch to transfer J-tube messenger wire and platform winch wire to vessel deck.
- Connect flexible flowline 1st end end-fitting to platform winch wire and lift flowline 1st end end-fitting over layover arch.
- Come up on platform winch and pay out on reels to transfer flowline 1st end end-fitting to J-tube bellmouth.
- Adjust flexible catenary for entry into J-tube bellmouth.
- Pull in flexible through J-tube. ROV to monitor riser entry into J-tube bellmouth.
- Hang-off flexible on top of J-tube.
- Lay away on require route alignment at the required installation tolerances.
- Make up flowline-riser flange connection on deck, install anodes, insulation covers,etc and test connection.
- Overboard flowline-riser flange connection from vessel.
- Lay flexible system overlength.
- Pay out on reels such that the insert clamp (on riser) is approximately 5m above the MWA location.
- Continue pay-out on reel system such that riser insert clamp engages into MWA clamping system. ROV monitors and confirms correct engagement.
- Crosshaul the FSO pull-in winch wire to the assisting vessel and connect to the riser 2nd end end-fitting.
- Pay out pre-determined amount of riser.
- Come up on FSO pull-in winch with ROV monitoring the riser 2nd end end-fitting
- Transfer riser load onto the FSO pull-in winch whilst continuing to pay out riser in steps.
- Hang off the end fitting at the buoyant module “spider buoy” and secure.
- Leak test flexible system.
- Conduct as-built surveys and prepare as-built records of the flexible system installation.
