According to the Ministry of Foreign Affairs of Ukraine, “[o]n 14 September 2016, the Ministry of Foreign Affairs of Ukraine officially served the Ministry of Foreign Affairs of the Russian Federation with a notification of arbitration and statement of claim instituting ad hoc arbitral proceedings under Annex VII of UNCLOS”.
For more information, see here.
Russian Foreign Ministry spokeswoman Maria Zakharova has told a news briefing that the Russian Federation had not, by then, received any notification from Kiev on that matter. That statement can be found here.
De Maribus 
While Russia rejects that it received such a notification =)