Reliable deepwater anchor performance is critical for mooring floating Mobile Offshore Drilling Units (MODUs). Anchor failure can result in MODU’s going adrift and colliding with production structures and/or dragging anchors and damaging oil and gas pipelines or subsea production systems.
Drag embedment anchors (DEAs) are the most utilized anchor for mooring floating MODUs in the Gulf of Mexico. There have been a number of anchor failures in recent hurricanes (Ivan, Katrina, Rita, and Ike). During hurricanes Ivan, Katrina, and Rita, 24 MODUs experienced mooring system failures. Anchors were dragged during some of these MODU mooring failures and are suspected to have caused several instances of pipeline damage that in turn led to delays in restoring oil and gas production after the hurricanes. During hurricane Ike there were at least four failures in MODU mooring systems that caused MODUS to leave station.
The goal of this project is to increase the understanding of DEA performance and improve the design and application practices so as to increase the overall reliability of their application for moored MODUs.
This goal will be reached through experimental investigations, data analysis, and engineering interpretations that can be used to develop recommendations for regulatory assessment and engineering design practices.
The overall project is structured in two Phases. Phase 1 will focus on establishing the experimental protocol, analysis techniques, and engineering interpretations to better understand DEA performance and failures. A generic DEA model will be used. Results will include useful information on the general performance of DEAs as well as an overall protocol that will be used in Phase 2 to study actual anchors. It is envisioned that the results from Phases 1 will be immediately useable to develop useful assessment and design guidelines.
Phase 2 will use the techniques and tools developed in Phase 1 to study the performance of DEAs being used in the Gulf of Mexico. Phase 2 will be proposed as a Joint Industry Project, and participation would be solicited from operators, drilling contractors, and anchor manufacturers.
Phase 2 will provide similar information on more specific designs that can be used to refine assessment and design tools. It is anticipated that the information developed in this project will be used by the API to develop improved guidance and recommended practices for the reliable application of DEAs.
Experimental Testing Both in-plane and out-of- plane tests will be conducted. In-plane loading is the typical design loading situation in which the mooring line pull is perpendicular to the anchor’s flukes. Out-of-plane loading can occur when an anchor pattern begins to fail and the line pull is not parallel to the anchor’s shank causing the anchor to rotate as it is loaded. Tests will measure the force, drag distance, and fluke penetration during loading and anchor capacity of the installed anchor. Methods to measure depth, trajectory, and orientation of the anchor during loading will be investigated and used as feasible. Tests will be conducted for a variety of uplift angles. Multiple tests will be conducted for the same conditions to characterize the variability in anchor performance. The properties of the test soil will be measured prior to each test.
Small scale tests (~1:30) will be conducted in small tanks using both kaolinite and Laponite test soils. Small scale tests can be completed more quickly whicj=h permits more different conditions and replicates to be completed. The kaolinite test soil allows testing over a range of undrained shear strength profiles. Tests on both anchors that are initially embedded in the soil (“wished in place”) and anchors that are dragged will be conducted. The Laponite tests will complement the kaolinite tests in that the clear test media allows direct observation of the anchor trajectory and behavior during loading.
Large scale tests (~1:10) will be conducted in a large dredge/tow flume using an artificial test soil. The tests will be similarly designed, but the number of large-scale tests will be necessarily limited by time and costs. The large and small scale tests will be complementary. Comparisons will allow the correlation of tests at different scales, and the relative ease and quickness of the small scale tests can be exploited to explore a wider range of test variables.
Analysis The analyses of the experimental data will be based on analytical models that have been developed in other recent OTRC studies. These include finite element analyses as well as simpler models and spreadsheet-based solutions. The analyses will relate the performance of the anchors to the loading conditions and the soil properties, and to provide a simple means to predict the capacity, drag distance and trajectory, failure mode and behavior of DEAs.
Results The results of the model test experiments and data analyses will be interpreted and presented in a format that would be directly useful to engineers and regulators engaged in the design or verification of anchors in mooring systems designed for MODUs. While these Phase 1 results will be for a generic anchor, it is expected that these results will provide insight and guidance that will be useful in at least a qualitative sense for understanding the general performance of DEAs.
Aubeny, C.P. and Chi, C.-M. (2010) “Trajectory prediction for DEA’s under out of plane loading,” in review for International Symposium on Frontiers in Offshore Geotechnics, University of Western Australia, Perth.