Studi Numerik Dan Eksperimen Vortex-Induced Vibration Pada Riser Flng Akibat Kondisi Kerusakan Sistem Mooring

Arifin, Arifin (2026) Studi Numerik Dan Eksperimen Vortex-Induced Vibration Pada Riser Flng Akibat Kondisi Kerusakan Sistem Mooring. Doctoral thesis, Institut Teknologi Sepuluh Nopember.

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Abstract

Seiring berkembangnya eksplorasi minyak dan gas di perairan dalam, infrastruktur Floating Liquefied Natural Gas (FLNG) dengan sistem tambat turretmerupakan pilihan teknologi yang banyak diadopsi. Pada kondisi lingkungan yang ekstrem, kerusakan sistem tambat berpotensi terjadi yang berimplikasi pada kekakuan tali tambat, lebih jauh dapat meningkatkan gerakan lateral platform dan perubahan tension aksial yang signifikan pada riser. Riser merupakan salah satu komponen utama dalam sistem eksplorasi LNG ini yang rentan mengalami kerusakan akibat getaran berlebih yang ditimbulkan oleh aliran fluida turbulent disekitar struktur riser yang dikenal sebagai fenomena vortex-induced vibration (VIV). Penelitian ini bertujuan untuk mengkaji fenomena VIV pada riser FLNG dalam beberapa skenario kerusakan sistem tambat dengan metode komputasi dinamika fluida (CFD) yang dikombinasikan dengan metode element hingga (FEM) terintegrasi menggunakan software ANSYS. Pendekatan ini umum dikenal dengan two-way fluid–structure interaction (FSI) yang merepresentasikan interaksi fluida–struktur secara terintegrasi untuk menganalisis fenomena VIV. Beberapa skenario ditentukan dalam simulasi meliputi variasi kerusakan tali tambat, variasi kecepatan aliran fluida (0,3–1,0 m/s), variasi pretension ujung atas riser (0–400 N), serta variasi frekuensi dan amplitudo eksitasi gerakan ujung atas riser yang merepresentasikan gerakan platform terapung akibat kerusakan sistem tambat. Secara spesifik, studi simulasi menganalisis parameter yang terjadi pada fenomena VIV, yaitu perubahan amplitudo getaran, karakteristik frekuensi, dan fenomena lock-in. Lebih jauh upaya mereduksi parameter VIV dengan menambahkan triple helical strake pada struktur luar riser juga dievaluasi efektifitasnya. Semua scenario simulasi numerik dibandingkan dengan pengujian model di laboratorium untuk menguji keakurasiannya. Hasil penelitian menjelaskan bahwa kerusakan sistem tambat berpengaruh signifikan terhadap respons gerakan platform dan tension aksial, terbesar terjadi pada konfigurasi dua tali tambat mengalami kerusakan. Kecepatan aliran fluida menjadi faktor dominan yang memicu lock-in, dengan amplitudo getaran maksimum saat kecepatan aliran fluida antara 0,5 – 0,6 m/s dan frekuensi antara 3,0 – 6,6 Hz. Sementara itu, angka Strouhal meningkat dari sekitar 0,15 menjadi 0,22 seiring dengan bertambahnya kecepatan aliran fluida. Amplitudo getaran menurun sekitar 10% dengan meningkatkan pretension hingga 400 N, pada frekuensi dominan sekitar 3,33 Hz. Disamping itu, eksitasi lateral pada frekuensi 6,25 Hz menghasilkan respons getaran lebih besar dibandingkan kondisi tanpa eksitasi, menunjukkan bahwa gerakan platform FLNG dapat memperluas daerah lock-in yang terjadi ketika frekuensi eksitasi mendekati frekuensi alami riser. Metode two-way FSI berhasil merepresentasikan interaksi fluida–struktur dalam memprediksi fenomena VIV, namun pada skenario kecepatan aliran fluida 0.5 m/s menunjukkan perbedaan terbesar dengan hasil uji model di laboratorium sebesar 10 persen. Penambahan triple helical strake terbukti efektif menekan VIV dengan penurunan amplitude getaran maksimum 58 persen. Temuan ini memberikan dasar ilmiah bagi pengembangan desain dan evaluasi integritas system riser FLNG pada kondisi operasi yang melibatkan kerusakan sistem tambat dan beban lingkungan.
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With the continued expansion of deep-water oil and gas exploration, the use of Floating Liquefied Natural Gas (FLNG) facilities equipped with turret mooring systems has become increasingly widespread. The riser is a critical component of the LNG production system and is highly susceptible to VortexInduced Vibration (VIV) caused by its interaction with ocean currents. Under extreme environmental conditions, partial failure of the mooring system may increase the platform's lateral motion and alter the riser's axial tension, thereby affecting its VIV characteristics. This study aims to investigate the vortex-induced vibration (VIV) phenomenon of an FLNG riser under several mooring system damage scenarios using a Computational Fluid Dynamics (CFD) approach coupled with the Finite Element Method (FEM) within the integrated ANSYS software environment. This approach, commonly referred to as two-way Fluid–Structure Interaction (FSI), enables fully coupled fluid–structure interaction to accurately simulate and analyze the VIV phenomenon. Several numerical simulation scenarios were considered, including variations in mooring line failure, flow velocity (0.3–1.0 m/s), riser topend pretension (0–400 N), and the frequency and amplitude of top-end excitation representing the motion of a floating platform resulting from mooring system damage. Specifically, the simulations evaluated the key parameters governing the VIV response, including vibration amplitude, frequency characteristics, and the lock-in phenomenon. Furthermore, the effectiveness of triple helical strakes installed on the outer surface of the riser as a passive VIV suppression device was also investigated. The results obtained from all numerical simulation scenarios were validated against laboratory-scale experimental tests to assess the accuracy and reliability of the proposed numerical model. The results demonstrate that mooring system damage significantly affects platform motion response and axial tension, with the greatest changes occurring when two mooring lines fail. Flow velocity was identified as the dominant parameter governing the onset of the lock-in phenomenon, with the maximum vibration amplitude occurring at flow velocities between 0.5 and 0.6 m/s and dominant frequencies ranging from 3.0 to 6.6 Hz. Meanwhile, the Strouhal number increased from approximately 0.15 to 0.22 as the flow velocity increased. Increasing the riser top-end pretension to 400 N reduced the vibration amplitude by approximately 10%, while the dominant frequency remained close to 3.33 Hz. Furthermore, lateral excitation at a frequency of 6.25 Hz produced a greater vibration response than the unexcited condition, indicating that FLNG platform motion can broaden the lock-in region when the excitation frequency approaches the riser's natural frequency. The two-way Fluid–Structure Interaction (FSI) approach successfully captured the coupled fluid–structure interaction in predicting the VIV phenomenon. However, under the flow velocity scenario of 0.5 m/s, the numerical results exhibited the largest deviation from the laboratory-scale experimental results, with a maximum difference of approximately 10%. The implementation of triple helical strakes proved effective in suppressing VIV, reducing the maximum vibration amplitude by up to 58%. These findings provide a scientific basis for the design and integrity assessment of FLNG riser systems operating under mooring system damage and environmental loading conditions.

Item Type:	Thesis (Doctoral)
Uncontrolled Keywords:	Floating Liquefied Natural Gas (FLNG); Kerusakan Sistem Mooring; Lock-in; Respons Dinamis; Riser; Two-Way Fluid–Structure Interaction(FSI); Vortex-Induced Vibration (VIV) Dynamic Response; Floating Liquefied Natural Gas (FLNG); Lockin; Mooring Failure; Riser; Two-Way Fluid–Structure Interaction (FSI); VortexInduced Vibration (VIV)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) > TA355 Vibration.
Divisions:	Faculty of Marine Technology (MARTECH) > Ocean Engineering > 38001-(S3) PhD Thesis
Depositing User:	Arifin
Date Deposited:	30 Jul 2026 20:49
Last Modified:	30 Jul 2026 20:49
URI:	http://repository.its.ac.id/id/eprint/142140

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