Hey I like these petroleum engineering discussions, now I can almost justify using the company's computer to read supercub.org.
The TransAlaska pipeline system rates "came off of pipeline max" in December of 1988.
Why? Decline of the reservoirs which produce into the line. Without doing any sort of work on an oil well, they typically decline in production about 10 to 20% per year. Stimulations can reduce that decline, but they are expensive ($250-750,000) and usually only the first one on a well has a big effect.
Early in field life we drilled enough new "infill" wells to offset the decline, but now that nearly all of the little pockets of rock have a well draining them effectively, there aren't many drilling targets left in the two big fields.
So, for the last 5 or 6 years, most of the "new" oil coming on line has been coming from much smaller reservoirs like Tarn, Alpine, Northstar, Aurora, Borealis etc. The biggest of these is probably Alpine brought on line in 1999-2000 which I think is making a bit over 100,000 BOPD.
With the existing wells in the main field at Prudhoe and the main field at Kuparuk declining by perhaps 100,000 BOPD/year combined (I'm really not sure what this number is, but it's probably +/- 50% of that), you need a lot of little fields and infill drilling to keep up, or a new medium size one every year or so to keep the pipeline rates up.
If you want to know more, let me know
KL