It is understood lifting equipment overturned while a wind turbine tower section was being moved. According to reports, the tower section rolled onto a digger.
A spokesperson from Belfast Harbour said the Siemens turbine rolled rather than toppled and no one was seriously hurt or injured.
Belfast Harbour leases part of their site to Siemens and Dong Energy who are working on the 389MW West of Duddon Sands site in the Irish Sea.
The harbour spokesperson said the companies work together but have distinctive control of their parts of the site.
A Siemens spokesperson said: "Siemens confirms that an incident involving a vehicle moving a wind turbine tower section occurred today (Thursday 10 October) at the site at Belfast harbour.
"No one was injured. The HSE has been informed. Siemens is currently investigating the incident."
Asked for more details about the incident Siemens did not comment further other than adding it "involved a stacker vehicle during a routine moving operation of a wind turbine."
A Health and Safety Executive Northern Island spokesperson said: "HSENI is investigating an incident at Belfast Harbour involving the overturning of lifting equipment during the transit of a wind turbine part."