Addressing a Complex Seabed Depression Using Finite Element Modeling: Uncovering the Root Cause for Better Decision-Making in Offshore Restoration

Abstract Offshore geohazards refer to local or regional site and soil conditions that can trigger failure events, leading to loss of life, harm to health, environmental damage, or damage to field asset installations. However, industrial techniques for understanding geohazards and implementing engineering restoration remain insufficiently documented. This study highlights Finite Element Modelling (FEM) as a critical tool for identifying root causes, enabling precise and effective seabed restoration. As part of the methodology, a seabed depression scenario was constructed alongside a root cause analysis to determine the most likely cause of the depression. Six potential causes were evaluated, including scour, seepage, shallow gas, subsidence, jack-up rig deployment/removal, and drilling activities that disturb the seabed. The methodology encompassed both diagnostic and investigative processes. The diagnostic strategy involved three key steps: hypothesis formulation, screening, and testing. After screening, hydrofracturing was identified as the most likely root cause and subsequently tested for its potential to explain the observed depression. The investigation phase thoroughly reviewed available data, including bathymetry, geotechnical site investigations, ROV survey results, platform-level checks, drilling activities, well completion reports, and jack-up installations. The next step was to reconstruct the timeline of events and conduct analytical studies. Various hydrofracturing scenarios were simulated using this timeline, along with information on soil stratigraphy and properties, conductor geometry, and drilling fluid pressures. These simulations employed numerical modeling techniques, including coupled pore water flow and stress finite element analyses in PLAXIS 2D, as well as seepage modeling, to investigate the underlying mechanisms of the observed depression. The study and modeling revealed evidence of hydraulic fracturing in the soil surrounding the casing. A crack likely formed along the conductor or through the clay layer, allowing drilling fluids to flow and causing liquefaction in the sand layer above. This process may have exposed the sand layer around the conductor(s), and, in combination with scour, contributed to the observed seabed depression. Finite element modeling supported this hypothesis, indicating that hydraulic fracturing combined with scour provides a plausible explanation for the depression's formation. The analysis utilized an axisymmetric finite element model (FEM), with the axis of symmetry aligned along the conductor's centerline. Below the conductor, an open hole was modeled to represent the uncased drill hole. An excess mud pressure of 210 kPa was applied within the drill hole, assuming an open gap along the conductor had developed, allowing drilling fluids to escape into the surrounding sand layer.