Subsea umbilicals
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A subsea umbilical is an assembly of hydraulic hoses which can also include electrical cables or optic fibres, used to controlsubseastructures 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.
Functions
Its main functions are:
- 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
What are typically included in an umbilical?
To delivery these function, a deepwater 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 deepwater; 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 – hangoff 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 suceptible
Super Duplex Stainless Steel (Pitting Resistance Equivalent - PRE >40)
- Pros:
- High strength means small diameter, light weight for installation and hangoff.
- 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 commisioning
Newly installed umbilcials 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 incompability is expected. For example, to commision an asphaltene inhibitor line, a mutul solvent like EGMBE is needed to provide buffer between the asphaltene inhibitor and storage fluid since they are typically incompatible.
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