Page 318 - Pipeline Pigging Technology

Coble-operated and self-contained ultrasonic pigs

Fig. 15. Layout of the riser-pipe inspection tool (RPlT).
The J-tubes are partially embedded in the concrete platform and crude oil
storage cells (Fig. 18), and are thus inaccessible from the outside for checking
the integrity of critical areas. It was established that a slightly-modified RPIT
could be used to verify the presence or absence of internal or external
corrosion. Since fluid propulsion was excluded, a wire-line operation was the
only means available for traversing the RPIT down and up the J-tube. This
required a pulling wire through the J-tube, operated from a winch on a vessel,
with a second winch and wire operated from the platform; both wires were
connected to the RPIT.
By careful synchronous operation of winches, the tool was traversed with
an almost constant speed through the J-tubes. Divers stationed at the bottom
end of each tube, at 150m below sea level, monitored the entire operation.
In November, 1987, the 20-in RPIT was tested on the Cormorant pipeline
in the North Sea. This 17-km long oil pipeline connects the Cormorant North
platform with Cormorant Alpha. As shown in Fig. 19, Cormorant North is a
steel platform, while Alpha is made of concrete, and it was the intention to
inspect the riser of the downstream platform. The oil at North has a tempera-
ture of 38°C; at Alpha, it has dropped to 10°C. For the Alpha riser, the RPIT
was launched at North and propelled by the oil flow with a speed of Im/sec
to Alpha. During the travel time of roughly 4 hours, the RPIT was in a dormant
condition to save energy and memory. At the correct location, the RPIT was
switched on by an external radioactive source which had been placed on the
pipe by divers.
299