Pipeline laying and embedment in soft fine-grained soils: Field observations and numerical simulations

Westgate, Z.J., White, D.J., Randolph, M.F. and Brunning, P. (2010) Pipeline laying and embedment in soft fine-grained soils: Field observations and numerical simulations. In Offshore Technology Conference 2010, OTC 2010. vol. 1, Curran Associates, Inc. pp. 326-340 .

Abstract

The as-laid embedment of offshore pipelines governs several aspects of pipeline design and lay route architecture. The observed as-laid embedment in soft soils is greater than would be predicted based on the static penetration resistance of the seabed using the in situ soil strength. Empirical 'dynamic embedment factors' are used to scale up this calculated embedment to estimate the as-laid value. The source of this discrepancy is dynamic lay effects, including the form and duration of any dynamic vessel and pipeline movements, and the seabed soil conditions. This study presents data to support an improved methodology to estimate the likely range of as-laid pipeline embedment. Existing theoretical models are reviewed, using aslaid pipeline survey data from five pipelines across two soft fine-grained soil sites. It is shown how the dynamic embedment in the field can be estimated by accounting separately for (i) a reduction in soil strength due to pipeline motions in the touchdown zone and (ii) an increase in the pipeline catenary bearing pressure due to vessel and pipeline dynamics. This represents a more robust methodology than the common industry practice of applying an empirical dynamic embedment factor to the calculated static embedment. Guidance on the pipeline embedment that occurs when the usual lay process is interrupted is also provided, for example at sleeper crossings and during weather-related downtime, in-line tee connections, abandonment and recovery activities, and pipeline termination assembly connections.

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Contributors

Author: D.J. White

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