Engineering company Weir Group expects its annual revenue, margins and profit to be weighted more towards the second half than in previous years.The FTSE 250 warned that the second six months of 2015 were proving to be very challenging for its oil and gas division, as the US rig count continues to fall, although at a somewhat more subdued rate in May."Minerals, the group's largest division, continued to demonstrate resilience in subdued markets and will today outline details of its strategic progress and plans for future growth," said group chief executive Kevin Cochrane.Analysts at Panmure Gordon & Co downgraded their forecast for oil and gas revenues by 10% to £695m, reducing their estimate on earnings per share by 5% to 93p.Weir shares were down 2.65% to 1,871.00p at 09:36 on Wednesday.