Abstract: In the bay of Bengal, there are 4 nearly paralleled subsea oil pipelines to go across 2 paralleled channels with the breadth approximate to 1.2 km. By using the method of pipeline laying and post-trenching, trenching is carried out for an average of 20 times, and 4 submarine oil pipelines are buried below the elevation required by waterway dredging, where the maximum trenching depth could achieve 11.9 m. With respect to difficulties in the plan process and challenges in each stage, by various of technical evaluation, narrow deep trenching machine with air lift eduction is utilized, combined use of SBP and 3D Sonar Survey during trenching operation, pipeline safety and integrity is analyzed after trenching. The final investigation and evaluation results show that the post-trenching operation meets the design requirements and ensures the integrity of the pipeline. Practice has proved that water jetting trencher can be utilized for post-deep trenching combined with comprehensive technical measures.