Page 136 - Pipeline Pigging Technology

10 years of intelligent pigging

manifold compression platform MCP01. In 1982 the capacity of the pipelines
was further increased with the installation of compression facilities on
MCP01. In addition, the platform acts as a pig launching/receiving station and
allows other gas to join the system, which includes gas from the Tartan,
Ivanhoe and Rob Roy fields.
At Frigg a number of other fields are linked to the gas-transportation
system, namely Odin, East Frtgg, NE Frigg and Alwyn North. The line to
Alwyn North is 24in OD, and is operated by Total Oil Marine pic (ownership
is the same as for the UK Association). In addition, Total Oil Marine pic
operates a 12-in oil pipeline from Alwyn North to Ntnian Central, as well as
subsea flowlines around Alwyn North.

GAS QUALITY AND QUANTITY

Frigg field gas has historically made up over 90% of the gas transported to
StFergus, and is a sweet product. The levels of H 2S and CO 2 are extremely low,
and therefore the lines were fabricated for sweet service. In addition, the lines
have no corrosion allowance except due to using standard API wall thickness,
and any additional amount from the manufacturing process.
This is one of the reasons why a great deal of effort has been placed on
internal condition monitoring.
A second reason for employing a detailed monitoring programme is the
importance of the lines to the UK in general. The pipelines have recently
3 3
completed the delivery of 200 Billion Sm (7.02 trillion Sft ) of gas to British
3
3
Gas. The maximum flow on any one day was 80.4 MSm (2.82 Billion Sft ).
More importantly, the system has, on average, annually delivered between 30-
40% of all of UK gas supplies since operations commenced in 1978. Occasion-
ally, monthly deliveries have been up to 55% of the UK gas requirements.
Internal condition monitoring of the Frigg System is based on the following
methods:

product control analysis of the gas transported;
corrosion monitoring by means of corrosion probes and coupons; and
internal inspection.

The first two operations are carried out on most lines, but we believe they
are limited in application. Product control is not fool-proof; operational errors
do occur, and in particular the most important measurement (the water
dewpoint) is very problematical.

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