On 20 April 2009, "Sydney" and the "Anzac"-class frigate departed from Sydney as part of Operation Northern Trident, a six-month round-the-world voyage by the two vessels, with numerous diplomatic visits and joint exercises with foreign navies. On 17 May, "Sydney" and "Ballarat" provided aid to two merchant vessels in the Gulf of Aden, driving off two separate groups of Somali pirates attacking the ships. "Sydney" remained in the area to report the incidents to Combined Task Force 151, while "Ballarat" escorted an impromptu convoy of eight ships, including the two that were attacked, to safety. The two warships visited ports in Western Europe, North America, the Pacific and northern Asia, with "Sydney" arriving back in her namesake city on 19 September.