This graph shows fairly clearly why offshore oil production is 30% of U.S. oil production.  It's because total U.S. oil production has been in decline since its peak in '69 (or the second peak in '84, take your pick).

The data for this graph is taken from the U.S. Energy Information Administration (www.eia.doe.gov).  In particular:

• http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRFPUS2&...
• http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRFP3FM2...
• http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRFP5F2&...
• http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=M_EPC0_FP...
• http://tonto.eia.doe.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRIMUS2&...

I'm using total U.S. oil production + imports as a proxy for total consumption, which I'm sure is wrong.  The real consumption/production balance is difficult to figure out since the imports and exports are in many different forms.

The main page there says that our "Dependence on Net Petroleum Imports" is 57%.  This means that more than half of our petroleum needs are met by foreign oil.

Roughly speaking, 33% (the share of our oil production from offshore drilling) of 43% (the percentage of our oil needs that we meet by our own production) is 14.2%.  

All our offshore oil production accounts for less than 15% of our oil consumption.

The price of oil is based on global production.  Global oil production is 85,472,000 barrels/day, while all the U.S. offshore oil production is 1,700,000 barrels/day.  This means that all our offshore production accounts for less than 2% of global oil production.