Complex sentence: In offshore however, there is a lighter column of sea water and column of rock do not go all the way to the surface therefore a lower overburden pressure is usually generated at an offshore depth than would be found at the same depth on land.
Split into simpler sentences: In offshore however, there is a lighter column of sea water, and the column of underwater rock does not go all the way to the surface. Therefore, a lower overburden pressure is usually generated at an offshore depth, than would be found at the same depth on land.