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 Water Semi-submersible Platform Mooring Coupled Analysis and Model Test
Licheng Qin, Huailiang Li, Wentai Yu, Weiwei Xie, Jin Jiang
Offshore Oil Engineering Co., Ltd.
Tianjin, China

Their large submerged structures causes a low center of gravity, which
gives excellent hydrodynamic performance. These platforms’symmetrical structure makes them insensitive to the directions of sea
conditions. In summary, the semi-submersible platform concept has
been widely developed, but its fixed form is mainly a form of its
mooring system. It can also be combined with dynamic positioning.
Especially for static production platforms, the mooring method ismainly used, which is demanded to resist once-in-100-yearstorms, oreven once-in-a-millennium storms, so mooring design plays asignificant role.

ABSTRACTThis paper describes mooring analysis of deep-water semi-submersible
platforms with a fully coupled method. Mooring lines, production
pipelines, hulls, etc. are considered for this coupled dynamic analysis.
High-strength cables have strongly non-linear properties so deep-watermooring line uses the tightening method. The mooring analysis isperformed on the mooring cables using the static-dynamic method.
Considering that the second-order drift force has a large impact on
deep-water semi-submersible platforms, the full quadratic transfer
function(QTF) method is used to calculate the drift force. Themaximum sway is 51.79m and the model test is 48.61m, the overall
deviation is less than 2%. The maximum force on mooring line 4 is31598.6 KN and the model test is 31414 KN, The numericalmaximum force on mooring line 2 is 3.2% more than the tests. All theresearch provided a reference for deep water mooring analysis.

The mooring analysis of deep-water semi-submersible platforms iscarried out using a fully coupled method. Mooring lines, production
pipelines, hulls, etc. are considered for this coupled dynamic analysis.
Deep-water mooring line uses the tightening method. High-strength
cables have strongly non-linear properties. The mooring analysis isperformed on the mooring cables using the static-dynamic method.
Considering that the second-order drift force has a large impact on
deep-water semi-submersible platforms, in order to calculate accurately
the full QTF method is used for to calculate he drift force. which
provided a reference for deep water mooring analysis.

KEY WORDS: semi-submersible platform; full QTF; deep water;
non-linear;

NOMENCLATURE
RAO= Response Amplitude Operator
SEMI=Semi-submersible Platform
LS17-2 = LingShui17-2QTF= Quadratic transfer function

The LS17-2 SEMI is a production platform located in the northern partof the Qiongdongnan Basin in the western continental shelf of thenorthern South China Sea, where the water depth ranges from 1220m to1560m. Qualified natural gas is pressurized and transferred to subsea
pipelines from the YC13-1platform to HongKong through a subsea
export pipeline. Condensate and oil are sent to the storage tank on theLS17-2 SEMI platform and then exported by the DP Shuttle oil tanker.
The main dimension structural specifications of the LS17-2semisubmersible platform are given in Table 1.

INTRODUCTION
With the increase in energy demand in recent years, global marine oiland gas resources now account for about 34% of the total known oil
and gas reserves, of which deepwater and ultra-deepwater resources
account for 30% of total marine resources. Increased development of
deepwater reserves is ongoing. The offshore oil industry is developing
rapidly, and semi-submersible platforms are widely used for deepwater
development. Due to their small waterline surface area, these platforms
have a long natural heave period, which contributes to avoiding theoscillation caused by the main frequencies waves. Its large-spancolumns make up for the lack of water surface area and ensure stability.

The SEMI will be moored by a semi-taut chain-polyester rope-chain
mooring system, with Platform North aligned with True North. Themooring system will be composed of four bundles, one for each corner.
Each bundle is comprised of four mooring lines. The angular spacing
between bundles is 90°, with a 4° separation between adjacent lines
within the same group. Each mooring line will be made up of studless3835