Salaries vary by a considerable margin. Check out the Career Mapper Job Roles as well as the position, salaries can vary by experience. Starting out in the offshore industry you could be earning £15,000 - £25,000, whilst this isn’t a bad salary, with experience this could rise to £50,000 / £75,000 or even more than £100,000.
Working for a company provides a certain level of job security, however you can become self-employed as a contractor and earn very high day rates. Although this is tempting, remember that you could go for months without any work at all.