Transocean Ltd , the world's biggest offshore rig contractor, aims to stop a seventh U.S. body from investigating the accident that sank one of its rigs while causing the largest U.S. offshore oil spill.

In a letter to the Chemicals Safety Board (CSB) dated December 30, Transocean lawyer Rachel Clingman repeated a request for documents as the company tries to demonstrate that the CSB lacks jurisdiction over April's Deepwater Horizon disaster.

A ruptured well led to an explosion on the Horizon, which was drilling in the Gulf of Mexico for BP Plc , and that accident ultimately sank the deepwater rig, and caused more than 4 million barrels of oil to spew into the sea.

Clingman had argued in a December 2 letter, which was released to reporters along with the latest correspondence, that the rig was not subject to CSB scrutiny because it was not a fixed source.

Clingman went on to quote Michael Bromwich, director of the Bureau of Ocean Energy Management, Regulation and Enforcement, as saying last month that CSB officials had acknowledged they weren't sure that they had jurisdiction to do this investigation.

A CSB representative was unavailable for comment on Friday.

According to publications on its website, CSB investigations look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems.

Probes of the Deepwater Horizon disaster have been opened by the Presidental Oil Spill Commission, the U.S. Coast Guard, the Department of Justice, and the Department of Interior's inspector general, as well as four Senate committees and five committees in the House of Representatives, Transocean said.

(Reporting by Braden Reddall, editing by Dave Zimmerman)