Hess Midstream (HESM) said Monday it commenced an underwritten public offering of 10 million class A shares by an investor.

The shares constitute limited partnership interests in Hess held by an affiliate of Global Infrastructure Partners, a BlackRock (BLK) subsidiary, the company said.

Hess said the selling shareholder intends to grant underwriters a 30-day option to purchase up to 1.5 million additional Class A shares.

Shares of Hess were down more than 2% in after-hours activity.