You can’t miss the announcement that Microsoft will stop selling Windows XP as a retail product after today (30th June, 2008). It will still be supported till 2014. Considering it was introduced in October 2001, it has had a long streak.
Now, how long do you expect offshore hardware to be supported? For field instrumentation, you really don’t care. Discontinued? As long as you have the original specs and current requirements for the item, you are covered. You usually replace the whole item, not individual components like displays, power board and logic circuits.
Machinery is another matter. You have sunk, oh say USD8M in a compressor/turbogenerator package which is meant to last the life of the platform (20 years). You need support for that whole period, either with the vendor maintaining a warehouse of replacement parts, or a part-by-part or bundle-by-bundle upgrade program. I would say that this program is similar to aircraft engine upgrade schemes, seeing that offshore generators are generally aero-derivatives to save weight, i.e. having a large power to weight ratio (efficiency is another matter).
The one that would trouble me are the brains of the asset. This would cover flow computers, DCS, PCS, SCADA, MMI, SDS, F&G. The vendor has to provide hardware and software support for yonks (or the term stipulated in the PO). And us consumers know how fast electronics evolve. That cool Zip drive of 1998 is no longer in sight. When was the last time you saw a floppy? 50MB hard drives are now hard to come by. Electronics are proprietary (sorry, Low Yat plaza). Replacing or upgrading these systems are not as easy as picking out the successor from a vendor catalog. Months and years of planning are in place, usually requiring an asset shutdown (USD80/bbl x 100 kbl/d x 4 days…). There are redundancies in place, but it still requires a something akin to a heart-lung transplant. kudos to the I&E surgeons for performing the surgery under pressure.
I don’t worry about PLCs not so much, as I suspect they are easier to replace, similar to field instruments.
Me, I’m a process guy. I deal with vessels and stuff, no electronics to worry about. Though, a pressure surge can really damage orifice plates, vane mist pads… buy a replacement in Balakong, that’s the ticket.
Wata - Malaysian Oil and Gas Engineer 
offshore system, keep it simple and nice.
Have you heard of a compressor controls system crashing. My PC crashes even when i try to type a word document (especially with the new ofiice2007).
They are trying to make things look fancy, but useless. next, microsoft windows will be able to make you a coffee while you work in the morning!