Offshore-focused McDermott International said it would buy onshore-based Chicago Bridge and Iron to create an integrated construction and engineering services provider amid a stabilizing global oil market.
McDermott's shares were down 9 percent, while CB&I rose 8 percent in after-market trading on Monday.
"Customers worldwide increasingly seek a single company that can offer end-to-end solutions, and the combination of McDermott and CB&I responds to these evolving customer needs," McDermott Chief Executive David Dickson said on conference call.
McDermott on Monday offered 2.47221 of its shares for every CB&I stock held, a 3 percent premium to the company's closing price. The offer translates to an equity value of $1.86 billion based on CB&I's outstanding shares, according to Reuters calculations.
The estimated enterprise value of the all-stock transaction is about $6 billion, based on McDermott's Friday close, the companies said.
CB&I, whose shares have lost about 44 percent of their value this year, has missed analysts' estimates on revenue and profit for the last four quarters. The company has been struggling since the crash in oil prices and has been offloading its assets.
The opportunity to combine with McDermott came when CB&I pursued the sale of its technology and former engineered products businesses, CB&I Chief Executive Patrick Mullen said in a statement on Monday.
McDermott, on the other hand, has benefited from a slew of contracts wins in the Middle East including from Saudi Aramco. The company's shares are up about 3 percent this year.
The deal with CB&I, which gets most of its business from the United States, could also help McDermott diversify its revenue streams.
McDermott's shareholders will own about 53 percent and CB&I investors the rest of the combined company, which will have pro forma annual revenue of about $10 billion and a backlog of about $14.5 billion, the companies said.
The companies said Dickson will lead the new company, which will be based in Houston. The deal is expected to be completed in the second quarter of 2018.
Goldman Sachs and Greenhill are the financial advisers to McDermott, while Centerview Partners LLC is the adviser to CB&I.