Subsea Umbilicals

Subsea Umbilicals


Subsea Umbilicals

 

A subsea umbilical is an assembly of hydraulic hoses which can also include electrical cables or optic fibres, used to control subsea structures from an offshore platform or a floating vessel. It is an essential part of subsea production system, without which sustained economical subsea petroleum production is not possible.

 

Main Functions:

 

Provide hydraulic power to subsea control systems, such as to open/close valves

Provide electric power and control signals to subsea control systems

Deliver production chemicals for subsea injection at tree or downhole

Deliver gas for gas lift operation

 

To delivery these function, a deep water umbilical can include:

 

Chemical injection tubes

Hydraulic supply tubes

Electrical control signal cables

Electrical Power cables

Fiber optic signal

Large tubes for gas lift

 

Key components

 

Topside Umbilical Termination Assembly (TUTA)

The Topside Umbilical Termination Assembly (TUTA) provides the interface between the main umbilical and the topside control equipment. The unit is a free standing enclosure that can be bolted or welded in a location adjacent to the umbilical hang-off in a hazardous exposed environment onboard the topside facility. These units are usually tailor-made to customer requirements with a view to hydraulic, pneumatic, power, signal, fiber optic, and material selection.

 

The TUTA usually incorporates electrical junction boxes for the electrical power and communication cables, as well as tube work, gauges, and block and bleed valves for the appropriate hydraulic and chemical supplies.

 

(Subsea) Umbilical Termination Assembly (UTA)

UTA, sitting on top of a mud pad, is a multi-plexed electro-hydraulic system allows many subsea control modules to be connected to the same communications, electrical and hydraulic supply lines. The result is that many wells can be controlled via one umbilical. From the UTA, the connections to the individual wells and SCMs are made with jumper assemblies.

 

Steel Flying Leads (SFL)

Flying leads provide electrical/hydraulic/chemical connections from the UTA to individual trees/control pods. They are part of the subsea distribution system that distributes umbilical functionalities to their intended service targets. They are typically installed after umbilical and connected by ROV.

 

Umbilical materials

Depending on the types of application, the following materials are typically available:

 

Thermoplastic

Pros: It is cheap, fast delivery, and fatigue resistant

Cons: Not suitable for deep water; chemical compatibility problem; aging, etc.

 

Zinc coated Nitronic 19D duplex stainless steel

 

Pros:

Lower cost compared with super duplex stainless steel (SDSS)

Higher yield strength compared to 316L

Internal corrosion resistance

Compatible for hydraulic and most chemical injection service

Qualified for dynamic service

 

Cons:

External corrosion protection required – extruded zinc

Concerns about the reliability of seam welds in some sizes

Tubes are heavier and larger than equivalent SDSS – hang off and installation concerns

 

Stainless Steel 316L

Pros:

Low cost

Needs little or no cathodic protection for short duration

Low yield strength

Competitive with thermoplastic for low pressure, shallow water tiebacks –cheaper for short field life

Cons:

Not qualified for dynamic service

chloride pitting susceptible

 

Super Duplex Stainless Steel (Pitting Resistance Equivalent - PRE >40)

Pros:

High strength means small diameter, light weight for installation and hang off.

High resistance to stress corrosion cracking in chloride environments (pitting resistance equivalent > 40) means no coating or CP required.

Extrusion process means no difficult-to-inspect seam welds.

Cons:

Inter-metallic phase (sigma) formation during manufacture and welding must be controlled.

Highest cost, longest lead times of steels used for umbilical tubes

 

Zinc coated carbon steel (ZCCS)

Pros:

Low cost relative to SDSS

Qualified for dynamic service

Cons:

Seam welded

Less Internal corrosion resistance than 19D

Heavy and large diameter compared to SDSS

 

Umbilical commissioning

Newly installed umbilicals typically have storage fluids in them. The storage fluids need to be displaced out by the intended products before they are utilized for production. Cares must be taken to look out for potential incompatibility problems that can result in precipitates and cause umbilical tubes to get plugged up. A proper buffer fluid is required if incompatibility is expected. For example, to commission an asphaltene inhibitor line, a mutual solvent like EGMBE is needed to provide buffer between the asphaltene inhibitor and storage fluid since they are typically incompatible.