Tiebacks
TIE-BACKS
Transmission wire surrounded by a cross-linked polyethylene insulation material. The umbilical has a 186mm diameter.
additional seal. Hot shrink tube and armouring is installed over the lead soldering for protection.
A lead sheath will provide the primary water barrier with a polymer extruded over the lead layer for mechanical protection and against seawater invasion. Two layers of galvanised steel wires wound in the opposite directions give torsional stability and axial stiffness. They are embedded in bitumen to give corrosion protection. The deepwater section of the umbilical will have a greater steel cross-section.
“All testing and qualification work so far has been successfully concluded – we are exited and proud of being a part of this pioneer project” said Sørensen.
The heat generated by the transformer will heat the oil causing a volume change dependant on the load. The pressure/volume compensators have to accommodate for this volume change.
Penetrator
The pressure/volume compensator design is based on a thin corrugated stainless steel inner barrier and a nitrile rubber outer bellow, which is able to expand and retract to accommodate the volume change of around 800 litres of oil.
This pressure-compensated 145kV, 700A (nominal) penetrator system is designed with a true double barrier to ensure that seawater does not enter the subsea system. The assembly consists of two oil-filled housings. The inner housing has an electrical cable termination with a central conducting pin.
The transformer is to be integrated into a subsea power distribution unit (SPDU). This gives an anchor point for the power umbilical and provides protection and guiding for installation onto the subsea compression station. The SPDU will weigh around 130t. Landing this onto the compressor station will also require a vessel with substantial crane capacity.”
An alternative double shell, double barrier type has also been used offshore, in the Tyrihans, Topacio and Ceiba developments. This design features separate inner and outer shells that are pressure compensated. “StatoilHydro selected a design with single shell, but with double penetrator barriers for Ormen Pilot. The single shell improves the cooling performance of the transformer, while the double penetrator barriers ensure the best protection against ingress of water into the transformer” said Sørensen.
The temperature of the oil will be monitored using an embedded thermal coupling. Optical temperature sensors will also be set into the windings, although these will only be used at start-up to monitor the transformer efficiency.
The primary side of the transformer will be connected to the power umbilical by means of a high voltage dry-mate penetrator.
An outer seawater barrier is provided by the lead sheath soldered to the back of the flange with cold shrink tubes as
Installation Installing the LSPS is critical to the success of the system. The power umbilical itself will weigh around 6000t and its structure will require a specialised vessel with a large umbilical carousel.
The power line will be laid from Nyhamna to the subsea site, avoiding laying the line uphill on the steep Storegga slope, which would give less control with seabed touch down. This also allows testing from shore during laying and installation.
Artist’s impression of the Ormen Lange modules “One potential failure mode is if water penetrates into the transformer structure,” said Sørensen. “Even at 30-40ppm, the oil will start to lose its insulation properties – hence the water content will be measured by capacitive sensors monitoring the dielectric properties of the dielectric oil.”
Umbilical Design The second main area of investigation is the power cable. The group decided against using a simulator to test the whole 125 km power cable because of the size and complexity. Instead it used two separate software simulation packages. The mechanical tests will be performed on a 150m long section. The eventual 125km power conduit has a 90km shallow water and a 35km deepwater section. The electricity will pass through three 400mm2 high conductivity copper
UT2 AUGUST 2009
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