Why replacing a warship's machinery becomes structural surgery

What are the major engineering problems in a large warship machinery refit?

Changing boilers and machinery also means replacing piping and cutting new routes through the ship's structure, so the work is extensive and must be executed precisely.

A major machinery refit changes nearly everything around the plant. New boilers require new piping, and that piping often needs fresh penetrations through structural members and compartments. Propulsion modernization is labor-intensive and expensive, with serious risks when structural and systems integration are mishandled.