Earthquake Soil-Structure Interaction Assessments of Jack-Up MODU for Tangguh Development, Offshore Papua Barat, Indonesia â€“ Part 1-Paper

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The study assessed whether a three-legged jack-up mobile offshore drilling unit could be installed and operated safely at three sites in Indonesia’s Tangguh development under earthquake loading. The area combines moderate-to-high seismic hazard with strong tidal currents and highly variable, actively transported seabed sediments, making foundation behaviour difficult to predict.<br /><br />Site-specific investigations included soil borings, in-situ testing, shear-wave velocity measurements and advanced dynamic laboratory tests. A probabilistic seismic hazard analysis provided earthquake records for nonlinear time-history analyses. Initial assessments using simplified, equivalent-linear foundation models predicted excessive differential settlement and possible structural collapse. Because these models could not adequately represent the coupled, nonlinear response of the soil and foundations, the team progressed to two-dimensional sensitivity studies and then three-dimensional continuum finite-element models incorporating the jack-up structure, spudcans and surrounding soil. The large models were run using BP’s high-performance computing facility.<br /><br />The advanced analyses demonstrated acceptable structural performance at Sites A and B under the assessed design earthquakes. At Site A, the jack-up remained stable, although it tilted slightly; some leg-chord stresses marginally exceeded minimum yield strength, but no buckling or global collapse was predicted. At Site B, settlements were broadly similar across the three legs, and structural components remained within acceptable limits.<br /><br />The initial assessment at Site C could not demonstrate stability: differential settlement increased, and rising load on a deeper leg indicated a destabilising P-delta effect. Analyses suggested that softening of infill sand contributed to this response. A scenario involving removal of the infill sands produced differential settlements considered acceptable against static overturning stability criteria, but Site C remained subject to further review.<br /><br />Overall, the work showed that rigorous site characterisation and nonlinear soil–structure modelling were essential to assess jack-up performance reliably in complex seismic conditions. The study was among the first applications of three-dimensional nonlinear continuum analysis for an independent-leg jack-up unit.

Keywords

jack-up drilling unit

earthquake loading

seismic hazard

Tangguh development

Indonesia offshore

soil-structure interaction

finite element analysis

differential settlement

spudcans

nonlinear dynamic modeling