How does Beowulf persuade king Hrothgar to let him fight Grendel?

Once at Hrothgar's throne, Beowulf introduces himself as a hero who can crush water sprites, among other things. Therefore he is equipped to defeat Grendel, if Wyrd (or Fate) will have it so. Hrothgar welcomes Beowulf as the son of Ecgtheow, the man whom Hrothgar had helped in settling a feud with the Wylfingas long ago.