The world’s biggest oil companies have vowed to bring down the costs of big projects in the face of slumping oil prices, but the unrelenting price weakness—with crude below $50 a barrel—suggests they could have to dig deeper still. In the past year, as oil prices plunged 60% from highs of $114 in 2014, U.K. energy giant BP BP 0.16 % PLC began testing new projects for profitability around $60 a barrel, down from $80 a barrel last year. Its Anglo-Dutch rival Royal Dutch Shell RDS.A -0.17 % is testing projects at prices as low as $50 a barrel, though its overall price outlook is between $70 and $110 a barrel. Under other circumstances, those would seem like draconian cuts. Billions of dollars less is being spent on everything from exploration and project engineering to construction equipment and drill rigs. But last week, when all of the world’s biggest […]