Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Deep Trenching of 30-inch LNG Pipelines to 8m below Seabed by Water Jetting – Engineering Case Study
Le Qin Wang, Zhi Gang Liu, Guo Min Sun, Lin Wang, Wei Liu, Shao Min Cui
Offshore Oil Engineering Co. Ltd.
Tianjin Chinalayers and post-trenching after the pipeline is installed.
Site specific geotechnical survey was conducted prior to construction inorder to determine the soil’s both chemical and geotechnical properties.
Engineering assessments including slope stability assessment, detailed
pipeline lowering analysis, physical model tests etc. were carried out
based on the site specific geotechnical survey results to confirm theengineering 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 andchallenges experienced during the deep trenching process. This paper
demonstrates how good engineering practices can be successfully
applied to assist to work out a safe, cost effective and robust
construction plan in order to achieve the target offshore pipeline trench
depth in a challenging area under various construction limitations.

ABSTRACTAn insight is provided into the construction plan and risks associatedwith the deep trenching of a 30inch diameter natural gas pipeline bywater 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 thegovernment 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. werecarried out based on the site specific geotechnical survey results toconfirm the engineering suitability of the selected construction plan andjetting 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 andchallenges experienced during the deep trenching process.

ENGINEERING STUDY
Project Information

KEY WORDS: Offshore pipeline, Deep trenching, Site specificgeotechnical survey, Water jetting, Back silting, Engineering
feasibility, Risk assessment, Slope stability

The proposed pipeline is 30-inches in diameter, 45 km in length with adesign life of 50 years and shall be trenched for protection along thewhole route, and also crosses with the congested cable laying area fromKP0 to KP5, within which 11 existing cables have been installed. Two(2) cable sterile corridors (see Fig. 1 and Fig. 2) from KP1.497 toKP2.85 and from KP3.558 to KP4.424, where the pipeline BOP(Bottom of Pipe) shall achieve 8.0 m below the seabed thus theproposed 30inch pipeline including any additional protection measures
will not pose any constraint to the laying and burial of the future cables
to min. 5m below seabed, based on agreement with relevant stakeholders.

INTRODUCTION
The 30inch diameter pipeline crosses with a cable laying corridor, i.e. avery challenging and congested area within which 11 existing cables
have been installed. However, two sterile corridors with nominal widthof 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 isdesigned to be buried at least 8m below the seabed plus 1m thick rockarmour placed on top of the pipeline against potential damage from thefuture cable construction. Following the approved EIA (EnvironmentalImpact Assessment) report, the construction method for the pipeline
deep burial is selected as combination of pre-dredging of the shallow

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