Analisis Pengaruh Initial Imperfection Terhadap Kekuatan Maksimum Tangki Kapal LNG Akibat Internal Pressure Menggunakan Metode Elemen Hingga

Setiawan, Tolopan Daniel (2025) Analisis Pengaruh Initial Imperfection Terhadap Kekuatan Maksimum Tangki Kapal LNG Akibat Internal Pressure Menggunakan Metode Elemen Hingga. Other thesis, Institut Teknologi Sepuluh Nopember.

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Abstract

angki LNG tipe Self-supporting Prismatic Tipe B (SPB) merupakan salah satu sistem penyimpanan kriogenik yang digunakan pada kapal pengangkut LNG, seperti Polar Eagle. Tangki ini dirancang untuk menahan beban akibat internal pressure dari boil-off gas (BOG) dan tekanan hidrostatik cairan LNG selama pelayaran. Namun, dalam praktik fabrikasi, geometri struktur tangki tidak pernah benar-benar sempurna akibat adanya initial imperfection, yang dapat memengaruhi kekuatan maksimum struktur dalam menahan internal pressure. Oleh karena itu, penelitian ini bertujuan untuk menganalisis pengaruh initial imperfection terhadap kekuatan maksimum tangki LNG tipe SPB akibat internal pressure menggunakan metode elemen hingga. Model yang digunakan adalah setengah tangki LNG tipe SPB kapal Polar Eagle. Material tangki berupa Aluminium Alloy 5083-O. Simulasi dilakukan dengan dua tahap, yaitu tahap pertama, analisis linear buckling untuk memperoleh mode buckling pertama sebagai dasar pembentukan initial imperfection; kedua, analisis nonlinear menggunakan step Static, General dengan kombinasi pembebanan tekanan hidrostatik dan tekanan BOG sebesar 0.25 bar. Initial imperfection dimasukkan dalam bentuk hasil buckling pertama dengan tiga tingkat amplitudo, yaitu slight, average, dan severe. Variasi dilakukan terhadap tiga ketebalan pelat top transverse, yaitu 10 mm, 12 mm, dan 14 mm. Evaluasi dilakukan terhadap tegangan maksimum (von Mises) dan tekanan saat tegangan mencapai Ultimate Tensile Strength (275 MPa). Hasil penelitian menunjukkan bahwa initial imperfection secara signifikan menurunkan kekuatan maksimum struktur terhadap internal pressure, terutama pada pelat tipis. Semakin besar initial imperfection dan semakin kecil ketebalan pelat, semakin rendah internal pressure maksimum yang dapat ditahan sebelum mencapai kegagalan. Pada pelat 10 mm, penurunan kekuatan akibat initial imperfection severe mencapai 0.0174%. Sebaliknya, pengaruh initial imperfection pada pelat 14 mm sangat kecil, dengan penurunan kekuatan maksimum kurang dari 0.002%. Seluruh model dengan tebal 12 mm dan 14 mm menunjukkan hasil yang masih jauh di bawah tegangan izin ABS sebesar 206.250 MPa, sedangkan model pelat 10 mm dengan initial imperfection severe memiliki margin keamanan yang sangat kecil. Temuan ini menegaskan bahwa ketebalan pelat dan initial imperfection adalah dua parameter krusial yang harus diperhitungkan secara eksplisit dalam proses desain dan evaluasi kekuatan tangki LNG, khususnya pada desain tangki SPB dengan pelat berpenegar.
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The Self-supporting Prismatic Type B (SPB) LNG tank is a cryogenic containment system used on LNG carriers such as the Polar Eagle. This tank is designed to withstand internal pressure from boil-off gas (BOG) and the hydrostatic pressure of LNG during voyages. However, in fabrication practice, the tank structure geometry is never perfectly ideal due to the presence of initial imperfection, which can affect the maximum structural strength against internal pressure. Therefore, this study aims to analyze the effect of initial imperfection on the maximum strength of SPB-type LNG tanks under internal pressure using the finite element method. The model used in this study is a half-section of the SPB LNG tank installed on the Polar Eagle, made of Aluminum Alloy 5083-O. The simulation consists of two stages: first, a linear buckling analysis to obtain the first buckling mode as the basis for defining the initial imperfection; second, a nonlinear analysis using the Static, General step with a combination of hydrostatic pressure and BOG pressure of 0.25 bar. The initial imperfection is applied based on the first buckling mode with three amplitude levels: slight, average, and severe. Variations are also applied to three different top transverse plate thicknesses: 10 mm, 12 mm, and 14 mm. Evaluation is carried out based on the maximum von Mises stress and the pressure at which the stress reaches the Ultimate Tensile Strength (275 MPa). The results indicate that initial imperfection significantly reduces the maximum structural strength against internal pressure, especially for thinner plates. The greater the imperfection and the thinner the plate, the lower the internal pressure that can be sustained before failure occurs. For the 10 mm plate, the strength reduction due to severe initial imperfection reaches 0.0174%. In contrast, the influence of imperfection on the 14 mm plate is minimal, with a strength reduction of less than 0.002%. All models with 12 mm and 14 mm plates remain well below the ABS allowable stress of 206.250 MPa, while the 10 mm plate with severe imperfection shows a very narrow safety margin. These findings emphasize that plate thickness and initial imperfection are critical parameters that must be explicitly considered in the design and strength evaluation of LNG tanks, particularly for SPB-type tanks with stiffened plates.

Item Type:	Thesis (Other)
Uncontrolled Keywords:	initial imperfection, internal pressure, kekuatan maksimum, metode elemen hingga, tangki LNG SPB, finite element method, initial imperfection, internal pressure, SPB LNG tank, ultimate strength.
Subjects:	V Naval Science > VM Naval architecture. Shipbuilding. Marine engineering
Divisions:	Faculty of Marine Technology (MARTECH) > Naval Architecture and Shipbuilding Engineering > 36201-(S1) Undergraduate Thesis
Depositing User:	Tolopan Daniel Setiawan
Date Deposited:	05 Aug 2025 03:19
Last Modified:	05 Aug 2025 03:19
URI:	http://repository.its.ac.id/id/eprint/127134

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