Equistone Partners has sold its stake in the company to Inflexion, for what it says is a 2.9 times return on its investment.
PD & MS was this week ranked second in the International Track 200 table of fastest-growing UK exporters, with growth last year of almost 250 per cent.
It employs more than 200 in Aberdeen, Dundee, Great Yarmouth, Brazil, and Azerbaijan, and has deals in place to upgrade BP's North Sea, Norwegian and Caspian Sea platforms.
Led by chief executive Simon Rio, it racked up a record £47million turnover last year including £12m of international sales.
It has customer relationships with major blue chip operators such as BP and CNR.
Neither the value of the deal nor the stake size were disclosed.