Cyrus is a small oilfield located 10 kilometres to the north of Andrew in block 16/28 at a depth of 2,500 metres. The field was originally developed by BP’s tanker-based Single Well Oil Production System. The vessel, the OPV Seillean was used on the field from April 1990 to March 1992 recovering 4.4 million barrels of oil. An unusual feature of Cyrus is that due to the thinness of the reservoir, both original development wells were drilled horizontally, to maximise production.
- The first well, 16/28-11 was drilled in 1988 by the semi-submersible drilling rig Sedco 700, and was the first horizontal well to be drilled by a semi-sub in the North Sea.
- The second well, 16/28-13, which was successfully completed by BP’s semi-submersible rig, Sea Explorer at the end of 1989 set another precedent. At 821 metres, it was then the longest horizontal section ever drilled from a semi-submersible.
Two new horizontal wells tap the reservoir to access the remaining 18 million recoverable barrels of oil. These are tied-back via a specially fabricated flow line to the Andrew platform. The flow line bundle is:
- 6.5 kilometres long.
- 10-inch production pipeline.
- A 4-inch gas lift pipeline.
- A 2-inch services pipeline (methanol).
- Electrical controls cabling and hydraulic controls tubing.
The facilities are sheathed by a 28-inch diameter, 6.5 kilometres long carrier pipe with a structural towhead attached to each end of the bundle. The Andrew end contains Andrew subsea isolation valves and the Cyrus end contains manifold valves. The design and construction involved three companies: Rockwater -Bundle design, fabrication, installation and commissioning, Brown & Root - Towheads design, and BarMac - Towheads construction.