An insight is provided into the construction plan and risks associated with the deep trenching of a 30inch diameter natural gas pipeline by water jetting method.
Throughout the design and construction process, careful considerations have been given to the project's environmental and regulatory constraints, which include very limited trench top width defined by the government gazette boundary, maximum allowed jetting speed, construction during day time only etc.. A series of engineering assessments including slope stability assessment, detailed pipeline lowering analysis, both numerical and physical model tests etc. were carried out based on the site specific geotechnical survey results to confirm the engineering suitability of the selected construction plan and jetting machine. Risk assessment was carried out to identify the risks associated with the work and to work out the necessary mitigation measures as well. Recommendations and lessons learned for future similar projects are discussed based on the site actual conditions and challenges experienced during the deep trenching process.
The 30inch diameter pipeline crosses with a cable laying corridor, i.e. a very challenging and congested area within which 11 existing cables have been installed. However, two sterile corridors with nominal width of 1km and 0.5km respectively needs to be further established within the cable area, where the crossing and construction of the 30inch diameter pipeline should pose no constraint to the laying and burial of the future cables to min. 5m below seabed. Thus the 30inch pipeline is designed to be buried at least 8m below the seabed plus 1m thick rock armour placed on top of the pipeline against potential damage from the future cable construction. Following the approved EIA (Environmental Impact Assessment) report, the construction method for the pipeline deep burial is selected as combination of pre-dredging of the shallow layers and post-trenching after the pipeline is installed.
