Rose Rock Midstream, a limited partnership recently formed by SemGroup to own and acquire midstream energy assets in the western US, filed on Friday with the SEC to raise up to $181 million in an initial public offering. The Tulsa, OK-based company, which was founded in 2011 and booked $239 million in sales for the 12 months ended March 31, 2011, plans to list on the NYSE under the symbol RRMS but has yet to disclose the terms of its offering. Barclays Capital is the lead underwriter on the deal.