The capacity of the average turbine was down to 3.6MW last year from 3.8MW in 2012. BTM said that this was due to the "specific mix" of turbines installed that year, rather than an indication of the size of turbines offered on the market.
Considering the trend for manufacturers to develop ever larger turbines, with Vestas recently installing the first 8MW machine, these figures will surprise many industry observers.
When it came to manufacturers, Siemens dominated the offshore market, while the UK retained its status as the largest offshore installer with 47% of all new capacity in the country's waters.
Global cumulative offshore wind capacity passed the 6GW milestone in 2013 and now represents 2.1% of the total installed around the world, according to BTM's World Market Update.
In 2013, 1.7GW was installed offshore globally across 13 projects, 50% more capacity than added in 2012.
BTM predicts that offshore wind will make up about 15.6% of total annual global wind capacity installations by 2018.
While the UK and Germany are expected to remain the major players in the sector up to 2018, China will also contribute "significantly" from 2016 onward, according to BTM.