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

Deepwater Pipeline Route Optimization by DTM, Advanced PSI and Bottom Roughness Assessment
Le Qin Wang1, Yang He1, Xian Hong Feng2, Gang Liu1, Han Eng Low3, Chun Hong Hu1, Zhi Bing Yu1, Bo Wang1
1. Offshore Oil Engineering Co. Ltd., Tianjin, China
2. CNOOC Research Institute Co., Ltd., Beijing, China
3. Fugro, Perth, Australia

ABSTRACT

depth varying from 520m to 1120m. During the basic design, the freespan assessment was performed considering the maximum allowable
free span length determined by the screening method recommended in
DNVGL-RP-F105 [2] and pipeline bottom roughness analysis wasperformed using the pipe vertical load penetration response model
recommended in DNVGL-RP-F105 [2]. This approach resulted in alarge number of predicted free span requiring corrections, leading to ahigher capital cost estimate for the project.
In this sense, a basic design optimization was further done, in which apipeline route optimization was carried out using DTM tools in
Fledermaus software firstly to avoid the pipeline being routed throughrough and undulating areas as much as possible; then pipeline bottomroughness was carried out using a modified pipe penetration model incohesive soil. Through engineering optimization in this stage, thepredicted number of spans decreased by two thirds although there werestill around 10 spans that require correction. Through this campaign, itwas proved that the pipe vertical load penetration response affects thefree span prediction results significantly.
In the EPC stage, it was decided to carry out a proper seabedcharacterization and a refined pipe soil interaction (PSI) assessment topredict the pipeline as-laid embedment with site specific soil
parameters and project specific pipelay parameters (such as bottom laytension, lay method, lay rate etc.). The bottom roughness assessment
was then re-run using vertical seabed stiffness estimated from therefined PSI assessment and showed that no more span correction isrequired for both pipelines and the lengths of all predicted spans arewithin acceptable range.
Based on as-built survey results upon completion of pipeline
installation, there were no excessive spans identified along both 12”
production flowline and 6” MEG supply flowline. Comparisons
between the pipeline as-laid embedment estimated by the refined PSI
assessment and the actual as-built survey results were also made. It was
found that along most pipeline segments, the range of predicted as-laidembedment agrees with the actual as-built survey results reasonably
well, except a few locations where changes were made to the pipeline
laying arrangement at site based on review of the pipeline laying
tension records.

A 12” rigid production flowline of approximately 27 km in length and a6” rigid MEG supply flowline of approximately 38.5 km in length wereinstalled in the Liu Hua 29-1 field in South China Sea on a fairly roughseabed in water depth varying from 520m to 1120m. The preliminary
bottom roughness assessment in basic design gives an indication that alarge number of the excessive spans may occur based on the seabedprofile along the selected route. Following that, a pipeline route
optimization was carried out using DTM tools in Fledermaus software
firstly to avoid the pipeline being routed through rough & undulatingareas as much as possible. During the EPC stage, a proper seabedcharacterization and a refined pipe soil interaction (PSI) assessment
were carried out to predict the pipeline as-laid embedment using site
specific soil parameters and project specific pipelay parameters. The
bottom roughness assessment was then re-run using the pipeline as-laidembedment predicted by the refined PSI assessment. The predicted
number of spans finally dropped to less than 10, all of which require nocorrection measures and was confirmed by the pipeline as-built survey
results. The aim of this paper is to showcase the design optimization
process used in the design of deepwater pipelines installed on a roughand undulating seabed terrain to minimize the pipeline post-lay spancorrection work and how the advanced PSI assessment can benefit theprediction of pipeline frees spans. As the results of this design
optimization process, an optimized result in terms of most technoeconomically feasible route was achieved in the end.

KEY WORDS: Deepwater pipeline; Route optimization; DTM(Digital Terrain Model); Pipe soil interaction; As-laid embedment;
Bottom Roughness; Span correction

INTRODUCTION
The subsea gas production system of Liu Hua 29-1 field in South China
Sea is composed basically of a rigid 12” production flowline
approximately 27 km long and a 6” rigid MEG supply flowline
approximately 38.5km long installed on a fairly rough seabed in water

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