The Aberdeen-headquartered firm said that the acquisition from Lime Rock Partners by Nasdaq-listed Carlyle Group would support its continued global growth.
Capital for the investment will come from Carlyle International Energy Partners, a fund that invests in the global oil and gas sector outside North America.
EnerMech employs some 3,500 staff across 40 locations, in the UK, Norway, the Middle East, Caspian, Asia, Africa, Australia and Americas. It works on large scale projects across the oil and gas, liquefied natural gas, renewables, defence, power, infrastructure and petrochemicals sectors.
Last month, the firm revealed annual accounts for the year to December 2017 with revenues of £361m and underlying profits of £43.6m, and forecast that 2018 revenues were expected to reach £430m with earnings in the region of £59m.
The company was formed ten years ago by chief executive Doug Duguid and chief financial officer Michael Buchan, both of whom will remain with the business in their current positions.
Duguid said: “This transaction marks the beginning of a new chapter for EnerMech as we continue to develop our business, grow our global footprint and enter new markets.”
The value of the deal was not disclosed by either party but is believed to be for around £450m.