Earthquake Soil-Structure Interaction Assessments of Jack-Up MODU for Tangguh Development â€“ Part 3-Paper

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The paper describes three-dimensional dynamic soil–structure interaction analyses used to assess a jack-up rig at Site C of the Tangguh development, offshore Papua Barat, Indonesia. It focuses on how sand behaves differently during installation and an earthquake: drainage is expected during static spudcan penetration and preloading, but seismic shaking produces undrained conditions. The authors calibrated site-specific relationships to assign consolidated undrained strengths to sand under the stresses imposed by the rig.<br /><br />The initial analysis, which retained a sand plug beneath the spudcans, predicted severe settlement and differential movement. The model became non-convergent after 42 seconds of the 100-second earthquake record, with large leg-to-leg differences. The sand plug was identified as a major contributor, leading to assessment of a scenario in which it was removed before the rig was installed.<br /><br />Because available commercial 3D software lacked suitable effective-stress models for directly simulating pore-pressure accumulation, the authors used a total-stress approach. Their procedure extracts element-level shear-stress histories, estimates cyclic loading and equivalent cycles using laboratory-derived accumulation curves, and develops softened cyclic stress–strain curves for a subsequent seismic analysis. A preliminary analysis used uniform assumptions for cyclic load ratio and equivalent cycles rather than fully iterated element-specific values.<br /><br />With the sand plug removed, predicted settlements were more moderate and generally stabilized toward the end of shaking. Final leg settlements ranged from about 0.8 to 2.2 m, with differential settlements of 0.56–1.41 m. Structural checks found the response generally acceptable, though some utilization ratios were marginal. Post-analysis assessment suggested that refined cyclic-softening inputs could further reduce differential settlement, although overall mean settlement might increase. The site’s final operational acceptability remained under review.<br /><br />The study demonstrates that drainage transitions and cyclic softening in sand can materially affect jack-up seismic performance and should be considered in design assessments.

Keywords

jack-up rig

soil-structure interaction

seismic analysis

sand plug

cyclic softening

drainage transition

spudcan penetration

settlement

differential movement

undrained conditions