Depends on wher ethe flowlines are aboveground, buried, subsea and whether you have access to external surface. Are you sure they are truely unpiggable I find that nearly 50% of the time people tell me things are unpiggable and its not true they have not explored all the options available. Are they totally unpiggable or just no access for 'intelligent tools".
Why do you think you have SRB attack - are you monitoring chemistry and flowrates, have you been using effective inhibitor levels. Do you have sand production and deposits in the flowlines.
If you have external access and thin coatings you could consider a real time RT approach as mentioned by another poster. Another way may be to risk assess the line and identify potential SRB attack points and then AUT corrosion map 2 x 2 mm grid at those locations.
Of course if its a real concern - make the lines piggable - there is a lot more flexibility now in terms of bend radii that can be negotiated, flow rates and temporary launcher - receiver systems.