Respon Dinamis Nonlinier Struktur Tension Leg Platform Akibat Gaya Gelombang

Rudi Walujo Prastianto, Gde Pradnyana Sutha

Abstract


Abstrak. Kecenderungan aktivitas pencarian sumber minyak dan gas bumi baru yang mulai diarahkan ke perairan yang lebih dalam menyebabkan kebutuhan terhadap jenis struktur anjungan lepas pantai yang sesuai untuk perairan dalam (lebih dari 1000 ft) meningkat. Salah satu jenis struktur yang bisa menjadi alternatif adalah Tension Leg Platform (TLP). Dalam penelitian ini telah dibuat model dinamis struktur TLP dengan Metode Elemen Hingga melalui program MSC/N4W yang meliputi pemodelan strukturnya (mencakup nonlinieritas akibat pretension, berat-sendiri dan deformasi besar tether) dan pemodelan gaya gelombang yang bekerja pada hull. Validasi model, dilakukan melalui komparasi dengan hasil analitis atas dua perhitungan. Pertama, perhitungan respon perpindahan hull akibat gaya statis arah surge dan kedua perhitungan frekuensi alami TLP model. Selanjutnya menghitung respon perpindahan dinamis TLP dalam bentuk riwayat waktu (time history) untuk gerak Surge, Heave dan Pitch. Sebagai studi kasus digunakan model TLP full-scale yang telah diuji di laboratorium dan akan dioperasikan di perairan Selat Makasar yang berkedalaman ± 975 m. Validasi model menunjukkan hasil yang baik. Perbedaan frekuensi alami TLP untuk gerak Surge, Heave dan Pitch antara hasil MEH dan analitis berturut-turut adalah 1.4%, 0.04% dan 2.3 %. Dalam respon dinamis TLP terlihat perpindahan kopel hull karena faktor kekakuan geometris akibat large displacement. Signifikansi adanya respon kopel ini harus diperhatikan terkait kemungkinan terjadinya fenomena ringing (frekuensi perpindahan kopelnya yang mendekati frekuensi alami modus gerak heave atau pitch) yang membahayakan struktur TLP.

Abstract. Demand of type of offshore structures for deep-water application increase caused by exploration activities of new oil and gas fields that tend to deeper water area. Tension Leg Platform (TLP) could become a suitable alternative for the demand. A finite element model of TLP has been made by using MSC/Nastran for Windows. The platform of TLP modeled as a rigid body element and the tethers as beam elements having stiffness in axial, lateral and bending directions respectively. The modeling of the structure includes some non-linearities caused by pretension, self-weight and large deformation of tethers. Wave loads on platform have been modeled as a nodal load at a lump mass node for each of threes degree of freedom (i.e. surge, heave and pitch). Verification of the model will be conducted through the calculation of a static load-deflection curve and mode shape analysis. The results will be compared with an analytical solution. The further steps are calculation of hull displacements in time domain analysis of the model through a nonlinear transient response analysis run capability of MSC/N4W. As a case study, used a full-scale TLP m odel has been tested experimentally. The model will be applied at the ± 975 m of water depth. The results of FEM approach indicate a good agreement with the analytical solution. The differences of natural frequency of TLP between the FEM results and analytical solution for three modes of motion (surge, heave and pitch) are 1.4%, 0.04%, and 2.3%, respectively. In the dynamic response analysis occurred displacements couple in all of the motions considered (surge, heave, and pitch directions respectively). This occurrence can lead to the phenomenon called as a ringing response that dangerous for safety of the structure.

Keywords


Tension leg platform; Pretension; Large displacement; Geometric non-linearities; Ringing

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DOI: http://dx.doi.org/10.5614%2Fjts.2003.10.3.1

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