Characterization of Gulf of Mexico clay at low effective normal stresses for offshore pipeline applications
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Pipelines and flowlines represent a major cost in offshore oil and gas operations. The initial embedment of the pipeline during laying influences the pipeline service resistance to axial and lateral motion and is dependent on the strength of the supporting soil at low effective normal stresses. This study aims to characterize Gulf of Mexico clay at low effective normal stresses in different scenarios. T-bar tests are performed to estimate the undrained shear strength of the clay. Embedment tests are conducted to estimate the initial penetration of pipes and consolidation after laying. Tilt-table tests are carried out to measure the drained residual interface strength between pipeline coating and Gulf of Mexico clay from three different offshore sites. T-bar results indicate that thixotropy of the clay has an impact on the undrained shear strength shortly after the clay is remolded and, after approximately 72 hours, a large portion of the increase in the undrained shear strength of the clay due to thixotropy occurs, with the sensitivity of the clay being about 2. For values greater than 1.5, the sensitivity seems to slightly decrease with depth. Embedment test results indicate that methods commonly used in practice to predict the initial penetration of pipelines agree well with model tests performed with a 3-ft long pipe section and provide a reasonable starting point for larger sections (i.e. 10-ft long). Additionally, consolidation following installation plays an important role in the final embedment of pipes. The c [subscript v] and C [subscript c] of the clay are estimated to be approximately 0.009 ft²/day (0.3 m²/year) and 0.71 to 1.26 (average value of 0.78), respectively. Results from tilt-table tests using stresses lower than 150 psf (7.2 kPa) indicate that 1) the interface residual strength is mobilized at about 1 to 2 in (25 to 50 mm) of shear displacement; 2) the Mohr-Coulomb failure envelope is non-linear for very low effective normal stresses and the curvature becomes flatter as the stress increases; 3) the drained residual strength at the interface depends on the composition of the clay and; 4) similar coatings obtained from different manufacturers may present minor differences in performance under similar stress conditions.