F3 Maths--Deductive Geometry(10分!!!!)

**** A
*****/I\
****/*I*\
***-/* I* \
***/**I** \
**/***IO**\
B/***I*****\C
**--------------
D

(*is space ,and there are lines linking OB and OC in the figure)



In the figure,▼ABC is equilateral,AD is an altitude,O is the circumcentre of th triangle.If AB=18 cm,find OA+OB+OC,leaving your answer in surd form.