7 Advantages & Disadvantages
7.1 Thermoplastic umbilical
The main advantages of thermoplastic hoses compared to steel tubes are as follows:
Less expensive solution (typical manufacturing equipment, etc.);
Ease of handling, storage and installation due to low minimum bending radius.
The disadvantages of the thermoplastic umbilical with reference to the steel tube umbilical and ISU are further explained in the following sections.
7.2 Steel tube umbilical
The following summarises what are considered to be the main advantages of metal tubes as service and hydraulic lines compared to thermoplastic hoses:
Fluid (methanol) permeation has been eliminated;
Fluid compatibility problems have been reduced;
Collapse resistance is drastically increased;
Favourable response characteristics, especially for long length umbilical;
No mechanical effect on other umbilical elements when pressurising;
Higher weight/diameter ratio gives improved dynamic and seabed stability;
Maximum umbilical manufacturing length without factory jointing is increased, as tube strings are jointed by welding in the laying up process;
Cost effective solution (life of field) for better dynamic umbilical for ultra Deepwater field developments.
The main disadvantages of steel tube umbilical are:
Increased minimum bending radius, limitation in storage lengths, and limit the dynamic application to certain configurations;
Limitation in recovery cycle (e.g. for repair) as this will involve plastic strains;
Limit to super-duplex tube length, resulting in many butt welds and associated radiography/corrosion testing;
Attack by sulphide reducing bacteria on super duplex butt welded joints (e.g. BP ETAP, Shell ETAP, BP Troika);
Possible carbon steel longitudinal seam failure (e.g. Shell Mensa);
Risks of cathodic delamination of sheathing from carbon steel tubes leading to localised corrosion cells;
More onerous flushing requirements (steel tubes are difficult to clean). Loosen particles or welding scale (e.g. by bending during installation) are potential risks for control valves;
Increased weight during deep water installation;
Super duplex corrodes in seawater above 55°C, limiting use for hot water flushing and gas lift. However, 3LPP anti-corrosion coating can be applied as mitigation (Dalia IPB, Moho).
Three metal materials are thought to be candidates for umbilical applications: carbon steel, stainless steel and titanium. Each of these has advantages and disadvantages. Carbon steel is the less expensive material, but corrosion and fluid compatibility must be addressed to assure adequate service life. Carbon steel can be made in continuous lengths, which tends to lower fabrication costs. Stainless steel, usually "duplex" or "super duplex" alloys, are more expensive, are not currently made in continuous lengths, but have better corrosion properties. Titanium is compatible with most fluids, can be made in continuous lengths, but is also expensive. Further, some titanium alloys must be used with care since they are affected adversely by anhydrous methanol and form brittle titanium hybrids under certain conditions of cathodic protection.
