Hyperbaric - Many bars - ie. at high pressure - ie. underwater
Pipeline - Like a garden hose, only bigger and most often made of steel
Repair - To fix what is broken

The worst nightmare of a oil company is that their newly laid pipeline has sprung a leak during final pressure testing. (The worst nightmare for the environmentalists is that an in-use pipeline has sprung a leak due to corrosion or trawl impact).

Unlike surface repairs, subsea repairs are inaccessible, difficult and costly. This is mostly due to the fact that a 2 man-day surface operation becomes a 100 men / two week operation at sea.

Most pipelines are laid by welding small segments of 12 m together. Even for short pipelines the number of welds are therefore substantial. Because the segments themselves have been pressure tested in a fabrication yard, the weak point of the pipeline is therefore its welds (and endfittings). This is the most probable location for a leak.

Because the finished pipeline is now a very long length of small segments, actual recovery of the entire line is most often unfeasible. Therefore a repair has to be done on-site, and subsea.
Though, if your leak is in the endfitting, you can bend the pipeline up to the surface, and repair it in the dry onboard a ship. This has been done on the ├ůsgard field in the North Sea.

When the pipeline starts to leak, it is often buried under the seabed. It is buried for protection, isolation and stability reasons.

                   ||||
                  |||
                 ||
B~~~~~~~~~~~~~~~||~~~~~~~~~~~~~~~~~~~B
                |
A===============X===================A
A==A is the Pipeline
B~~B is the Seabed
X marks the leak, and || is just harmless dye.

When the pipeline has been uncovered, and the leak verified, repairs can be done in four ways:

Clamps

If the pipeline carries non-hazardous fluids (eg. water injection pipelines), a clamp around the leak is a viable option. The pipeline is then uncovered, and the clamp is placed around the leak site. The clamp will vary in design, but a common look is to have two steel half-shells connected with long studbolts.

The clamp can also be used as a temporary solution to pipelines carrying product - but this is far more dodgy.

Coupling

The method of repair with coupling is to cut open the pipeline, and then insert both ends into a one-way locking device similar to the Chinese finger. This can be considered a permanent repair, and is the method of choice on deepsea pipelines where divers cannot go due to the pressure.

Welding

By far the most popular and fail-safe method of repair is welding.

Quite simply, the damaged section is cut out from the pipeline, and a new section is put in.

To be able to create strong durable welds subsea, one has to weld in an atmosphere. This is because the surrounding water would both pollute the weld, and cool it down much too quickly - giving a very brittle material. The atmospheric welding is facilitated by placing a divers habitat over the pipeline, creating a pocket of air around the pipeline ends. Sealing pigs are placed inside the pipeline to prevent the fluid inside it obstructing the welding process. The pipeline ends are then machined, brought together and welded. Depending on the material both pre and post heating can be done.

During the repair the seabed layout is similar to this:

              ______
B~\          |      |           /~~~~~~B
   \         |      |           /
A===\========|==X===|==========/========A
     ~~~~\   |      |    /~~~~~
          ~~~~~~~~~~~~~~~
The pipeline has been exposed, sealed and the habitat has been placed on top of the leak location.

A==A is the Pipeline
B~~B is the Seabed
X marks the leak, and the habitat is the square bit.

Flanges

If the pipeline is light and flexible (small diameters), it can be repaired by flanging. The pipeline is then cut in two at the repair point, and bent up to the surface. Once in air one can cut away the damage and weld on a flange. The pipeline is then lowered back down to the seabed, and the operation is done on the other end as well. Because the pipeline tends to be a bit shorter after cutting away the damage, the pipeline is often reconnected by using a spool piece. The limitation of light and flexible pipelines is because you end up lifting a good length of the pipeline to get it up to surface - and "most" ships doesn't handle dynamic loads in the order of a few hundred ton that well. (Well a rig could - but now it's starting to get expensive - why not weld it subsea ?).

Some hyperbaric links include :
http://www.fraserdiving.com/subsea.html
http://www.isotek.co.uk/images/PRS.jpg

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