OIL services magnate Sir Ian Wood questioned official estimates of how fast the country s offshore renewable energy capacity could be developed, saying it could take twice as long as the Scottish Government expects.  The chairman of the Wood Group services business highlighted the scale of investment required to develop offshore wind energy capacity in a speech that underlined the challenges that the Scottish Government will face in meeting its ambitious targets.  With the First Minister alex Salmond an enthusiastic champion of the potential of renewables, the Scottish Government wants 80% of Scotland s gross annual electricity consumption to come from such sources by 2020. Offshore renewables will play a key role.  However, Sir Ian told industry leaders:  The Scottish Government are estimating two gigawatts per annum coming on stream for something like 20 years. This would require  ?6 billion per annum capex (capital expenditure) and ?2bn per annum opex (operating expenditure). It s my view this investment will take place over probably double that period.  Sir Ian said even if investment does proceed at the slower pace there will still be huge sums for the industry to play for off Scotland in coming years:  That s still ?3bn capex and ?1bn opex pa beginning to build up from 2015 onwards,  he told the all Energy conference in aberdeen.  Sir Ian said oil and gas services firms could provide huge amounts of expertise that could help in t