IEC 61400-3:2009 specifies additional requirements for assessment of the external conditions at an offshore wind turbine site and specifies essential design requirements to ensure the engineering integrity of offshore wind turbines. Its purpose is to provide an appropriate level of protection against damage from all hazards during the planned lifetime. Focuses on the engineering integrity of the structural components of an offshore wind turbine but is also concerned with subsystems such as control and protection mechanisms, internal electrical systems and mechanical systems. It should be used together with the appropriate IEC and ISO standards, in particular with IEC 61400-1. This publication is to be read in conjunction with IEC 61400-1:2005.