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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

Time Domain Dynamic Response of Coupled Platform-mooring-riser System under Mooring Failure
Yang Yu, Mingren Zhao, Jianxing Yu, Cheng Liu, Zhenmian Li
State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin, China
Tianjin Key Laboratory of Port and Ocean Engineering, Tianjin University, Tianjin, China

analysis based on SESAM and compared the results between the coupledand de-coupled method. Hao, et al. (2020) mainly studied the hydraulicpneumatic tensioner in TLP-TTR system under its local failure based on
AQWA. Cheng, et al. (2021) conducted the detailed response of TLP, i.e.,time series, spectral analysis, phase diagrams and tendon stiffness, under
the condition of tendon broken. Chuang, et al. (2020; 2021) investigatedthe mooring disconnection in the SEMI-mooring-riser system, aiming at
the response of platform, tension of remaining lines and risk of riser
clashing. More broadly, Bae, et al. (2017) carried out a complex simulationof floating wind turbine with broken mooring line considering the aerohydro-mooring coupled effect in the accident.
It could be intuitively found that previous studies have focused on theresponse of platform or mooring in conditions of mooring failure but little
attention was particularly paid to the dynamics of riser system. In thisstudy, a self-developed MATLAB program has been established toacquire the time domain dynamic response of the coupled system withthe disconnection of one or several mooring lines. The dynamics ofSEMI platform, positioning mooring lines and production risers aretotally included in the analysis. The paper is organized as follows. Firstly,
the model of subsystems and coupled method have been describedbriefly in analytical method. Then, the results obtained from the program
have been validated and proved reliable. Finally, different numbers ofbroken mooring lines have been addresses in results and discussion. The
spectrum analysis of platform, comparison of mooring tension before
and after mooring failure, riser motion or tension along its length intransient stage and healthy stage have been discussed in order.

ABSTRACTThis paper has investigated the effect of mooring failure on the global
performance of a typical offshore structure under regular surface wave.
A self-developed time domain program has been set up to capture thetransient response of the system when one or several mooring lines are
simultaneously disconnected from fairlead. The results indicate that thewhole response could be divided into healthy stage, transient stage andsteady stage accordingly. The low frequency of platform is almost
decreased with increasing number of broken cables due to the reduced
stiffness. The remaining mooring lines on the up-wave side and close to
the broken position would exhibit profoundly increasing tension range.
The riser tension is less affected by mooring failure due to its verticalconfiguration and low tension level compared to mooring tension.

KEY WORDS: Coupled offshore system; mooring failure; timedomain dynamics; risers; regular wave.

INTRODUCTION
A typical floating offshore system usually concerns three subsystems:
the top platform, the mooring system and riser system. They influenceeach other and respond to environmental loading in a complex way. The
positioning mooring lines are likely to be broken when subject to harshconditions in deeper water, which could result in the instability of
production system or even offshore accident. Therefore, an accurate
description of the system dynamics under the failure of mooring lines
could be a challenging but essential problem.
A majority of researchers have demonstrated the theory of the coupled
floating system and conducted its performance under broken moorings /risers. Veritas (2010) introduced 2 methods for the dynamics of coupledsystem: fully coupled analysis, that is the mass/damping/stiffness matrixesof riser and platform are assembled together to realize an overall solution,and efficient analysis, which means that the forces and moments inducedby the moorings or risers will act on the platform. Meanwhile, themotions of upper nodes from these slender structures will keep consistent
with those of connection points on the platform hull. Liu, et al (2016)combined the ABAQUS modular for riser and self-developed
FORTRAN modular for drilling platform and exchanged data with eachother to consider the coupled effect. Caire (2012) employed the coupled

ANALYTICAL METHOD
Platform model
The platform model is developed based on the rigid body dynamics
theory from Li (2020) and Yu (2020) considering large amplitude,
instantaneous position and immersion depth et al. In this approach, theexternal loading mainly includes wave loading, gravity loading,buoyancy loading and mooring loading.
As is shown in Fig. 1, the columns and pontoons are indexed from #1 to#8 in order. The wave loading can be obtained by integrating along theimmersion length of each column or pontoon li.
Fwi =  fni dli , Lwi =  ( rGi  fni ) dli , i =1, 2, ,8li

771li(1)