Installation of Flexible Riser 1

[JohnLL]

Guys!
Anybody who is kind enough to briefly explain to me (or recommend a good reference) about:
1-the procedure of installing flexible risers on the seabed and attaching them to let's say a subsea manifold/PLEM. (I know it should be very similar to pipeline initiation, laying and lay-down but how they manage to put it in right place while there will be no spool piece later to compensate distance to PLEM)

2- the procedure of taking it up to the surface for connection to sea-surface facility.

I believe no method leads to dragging the flexible pipe on seabed is allowed as it might damage the sheath, am I right?

 

[GRoStruc]

Have a look on websites of connection system manufacturers (FMC , Vetco, Cameron, Aker etc)

They usually have some good info on how their systems work and perhaps installation simulations.

Basically there are 2 main types of diverless connection systems (vertical and horizontal) and the installation methodology differs slightly for each.

For instance, for a horizontal system to ensure we have the correct length we usually calculate the length required (erring on the long side!) and then as we approach the structure a loss loop is used to loose the excess flexible (not really a loop, picture more of a ‘chicane’ on the lay route)

A second ‘tie-in’ loop is then laid in the line and the termination laid down a set distance short of the tie-in point. The connection system on the ROV is then used to pull the termination head up to the structure and the ROV / tie-in tool is used to make up the connection. The slack in the tie-in loop is pulled straight(er) during the tie-in.

No major problem in pulling product along the seabed, depending on soil conditions.

Alternatively for a vertical connection a loss loop will still be used to get the length correct but in this case the 2nd end may be landed out directly on a guide post on the structure and the tie-in can be completed.

Assuming the flexible riser is free hanging then the procedure for recovery is pretty straight forward, just connect the crane / winch etc and pick it up. Using a vessel you just have to ensure that you look after the layback to ensure you don’t exceed the flexible MBR.

At a suitable depth the first end would then be transferred to the FPSO / platform winch, the vessel crane disconnected and the FPSO / platform completes the pull-in.

Lots of variations on those themes depending if it’s first / second end pull-in, the type of riser configuration, the facility that’s doing the pull-in etc etc.

 

[JohnLL]

GRoStruc, That was quite a helpful reply.
I checked also the references you made.

Do you also have any comment on installing the buoyancy tanks? Considering a Lazy wave riser which the flexible riser first is laid on seabed and later will be retrieved for connection to sea surface facility.
When they usually attach buoyancy tanks (for lazy wave)to riser:
-PRIOR to laying the riser on seabed
(riser rests on seabed with buoyancy tanks attached)
-PRIOR retrieving riser for attachment to sea surface facility
(riser will be retrieved to sea surface with buoyancy tanks attached)
-After attachment to sea surface facility
(first shape is catenary and then by attaching buoyancy tanks it will change to lazy wave)

 

[GRoStruc]

The easiest way is to install the modules and either hang off or complete the pull-in at the same time. Initiation at the facility is easiest to get the right riser configuration but you have to be able to keep the product stable and sink the buoyancy before the sag bend has formed.

Laying into the facility can be easier as you have the product to sink the buoyant section but you may have a tight target box for the TDP which can cause you other headaches.

If I had to install before the FPSO for instance is on station and had the choice / option I would lay into the facility and install the modules before laying down the 2nd end (end to be pulled in). Product already in the water helps to counteract the buoyancy.

What needs to be checked is the riser profile on the seabed with the buoyancy attached and it’s stability with the currents in that area.

What I have done previously is attach shots of chain (around 0.5Te ea. I think) at each module location to counteract the buoyancy and help stabilise on the seabed. As we picked up the abandoned end the ROV then went through and cut the chain free.

You could lay down without buoyancy attached and then recover back onto the installation vessel and relay but that creates other problems in getting the bend stiffener back on board and onto a reel and the hassle of repacking the product. Can be done but not ideal.

 

[JohnLL]

GRoStruc,
Based on what you said, Now I am getting worried about another issue.
During operation, Our flexible risers (there are two of them connected to sea surface turret) are supposed to be clamped to each other by buoyancy modules. So, there should be connections between buoys used for Lazy wave shape (I am not sure about the reason anyhow).

My initial thought was to lay each riser on seabed separately; Retrieve and install them later separately( to connect to surface turret) and Then clamp them together in the middle of water.

But it seems it will be very difficult to deal with them subsea. Do you thing it will be preferable to first clamp them together on deck and then lay/install them simultaneously?

 

[GRoStruc]

That's not a configuration I've seen before.

To be honest there's too many questions about doing this either way to say definitively if one way is any better than the other.

It sounds difficult either way which means it will probably come down to specific details (i.e lay tension, how they are being laid [lay equipment], how they are being connected together, water depth, product size etc.)