Subsea wellhead spud-in marking and as-built position estimation method based on ultra-short baseline acoustic positioning

•The use of ultra-short baseline (USBL) acoustic technique in subsea is explored.•Proposed method is built upon box-in estimation, which uses only the distance.•Redundant measurements and avoidance of angular errors aid in improving accuracy.•Proposed method involves, among others, offset transformation and M−estimation.•As a result, a marker buoy was positioned well within the desired accuracy of 10 m.