** Royal Dutch Shell affiliate's shares up 4.2 pct at $43.20
** Buys additional stakes in Zydeco Pipeline Co LLC andColonial Pipeline Co for $448 mln from Shell Pipeline Co LP, awholly owned subsidiary of Royal Dutch Shell
** Shell Midstream's first acquisition since its IPO inNovember will increase its ownership interest in Zydeco to 62.5pct and in Colonial to 3 pct
** Says expects to raise gross proceeds of $300 mln throughoffering of 7.7 mln units, to be used to partially fund deal
** Reports better-than-expected Q1 profit, helped by strongvolumes
** Up to Tuesday's close, stock had risen 5.4 pct this year