Prospectus excerpt:  We have granted the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to 1,080,000 additional Series A Preferred units from us at the public offering price set forth on the cover page of this prospectus supplement, less the underwriting discount, solely to cover over-allotments. The Series A Preferred units do not have a maturity date, and we are not required to redeem or repurchase the Series A Preferred units. Accordingly, the Series A Preferred units will remain outstanding indefinitely unless we decide to redeem or repurchase them. When, as, and if declared by our board of directors out of funds legally available, distributions on the Series A Preferred units will be payable quarterly on March 15, June 15, September 15 and December 15 of each year, beginning September 15, 2018, at a rate per annum equal to 6.625%. Distributions on the Series A Preferred units are non-cumulative. Accordingly, if our board of directors does not declare a distribution before the scheduled record date for any distribution period, we will not make a distribution in that distribution period, whether or not distributions on the Series A Preferred units are declared or paid for any future distribution period.