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Deep cement mixing (DCM) is an environmentally friendly technique for offshore ground improvement without dredging or much disturbance to the marine ecological system. Several field construction factors can influence the unconfined compressive strength (UCS) of cement-stabilized soil. In this study, key construction factors are evaluated referring to site investigation records, construction records, and quality test results of a large offshore DCM construction project. The key factors were attributed to geological conditions, construction procedures, and curing conditions. Specific field construction factors include fluctuations of tidal level, original soil type, volume fraction of injected water, volume fraction of injected cement slurry, injection rate of cement slurry, penetrating and mixing time per meter, curing age, and moisture content. The importance of these construction factors on the DCM strength has been quantified using a statistical method based on construction records. The injected water volume and original soil type are noted to be the two most dominant factors on the UCS of the treated soils. Longer mixing time improved the strength of the treated soils. © 2022 American Society of Civil Engineers.

Nowadays, since, subsea equipment becoming larger and heavier, risks of the lifting/lowering operations by offshore support vessel (OSV) are also increasing and thereby predicting coupling motions of the vessel and whirling equipment and hydrodynamic forces acting on the suspended load in splash zone are important to prevent clash accidents and damage on lifted subsea equipment etc. Model experiment using OSV with heavy suspended load was conducted in Deepsea Basin in National Maritime Research Institute (NMRI). The experiments were carried out with several regular and irregular waves and heading angles. The OSV was moored with 4 lined spring system. During the experiments, the motions of moored OSV and suspended load were measured with visual 3D motion capture system. Computation fluid dynamics (CFD) simulations were conducted using CFD Solver NAGISA which was developed in NMRI to investigate the dynamics and hydrodynamical aspects. Dynamic overset grid method and multibody dynamics model were applied to reproduce the coupling motions of the vessel and suspended load. The CFD simulations were carried out when the suspended load is in the splash zone to predict hydrodynamic forces acting on the lifted equipment. The CFD simulation results were validated with the model experiment comparing with motions of OSV and suspended load.