(Portland, OR)  --  Portland police say they've arrested a sex offender who's accused of kidnapping and sexually assaulting a woman in the city's Portsmouth neighborhood.  Authorities say 48-year-old Robert Shelby, who failed to register as a sex offender, is facing multiple charges after being accused of forcing his way into the victim's home in late March, holding a garden trowel to her ribs, and assaulting her.  Shelby is slated for his next court appearance on April 19th.