Getting Error while connecting to Oracle Database using Groovy Script in SoapUI 5.1.3

Hi - Getting below error while connecting to the Oracle Database using Groovy Script in SoapUI 5.1.3 , please help me how to resolve the issue.

java.lang.NoSuchMethodError: oracle.i18n.text.converter.CharacterConverterOGS.getInstance(I)Loracle/sql/converter/CharacterConverters; error at line: 80

I have Java Version - 1.8.0_91 and Oracle Client : 12.1.0.1 so I have downloaded orai18n.jar file and copied SoapUI 5.1.3\ lib folder and also downloaded ojdbc6.jar and copied into SoapUI\bin\ext folder..