(S (S (NP (BASENP (DT An) (NN oil) (NN rig)) (PP (IN in) (BASENP (DT the) (NNP North) (NNP Sea)))) (VP (VBZ has) (VP (VBN been) (VP (VBN evacuated) (PP (IN after) (BASENP (DT a) (NN security) (NN alert))))))) (PERIOD .))