Pipeline Laying and Embedment in Soft Fine-grained Soils: Field Observations and Numerical Simulations

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 as-laid 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. Introduction The as-laid embedment of an offshore pipeline is an important design parameter, influencing pipe-soil resistance, exposure to external loading, and thermal insulation. The magnitude of the as-laid embedment in soft soils is greater than the penetration of the pipe into the seabed under its submerged weight alone. This additional embedment is due to (i) the additional vertical pipe-soil contact force from the pipeline catenary and (ii) dynamic lay effects. The additional contact force multiplier, termed the touchdown lay factor flay, can raise the contact force by a factor of four compared to the submerged pipe weight for typical pipe lay conditions (Cathie et al. 2005, Bruton et al. 2006) although factors between 1.5 and 2 are more relevant for deep water developments. In practice, a dynamic embedment factor fdyn is then applied to this calculated pipe embedment to account for dynamic lay effects. Values of fdyn between 1 and 10 have been reported (Lund 2000, Bruton et al. 2006), but there is no established basis for adopting specific values. As a result, predictions of as-laid pipe embedment can span an order of magnitude. If the length of pipe that has just reached the seabed in the lay process - in the area referred to as the touchdown zone - is exposed to an extended duration of dynamic motions, a trench can form around the pipeline, increasing the embedment further. This is effectively the earliest stage of trench formation that is observed in the touchdown zone of a steel catenary riser (SCR). These trenches can reach several diameters in depth within the first year or two of SCR operation (Bridge and Howells 2007).

Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design

“…This program includes a nonlinear seabed model to capture changing vertical seabed stiffness during cyclic loading (Randolph and Quiggin 2009). The dynamic amplification factors (V dyn /V max , where V dyn is the dynamic vertical pipe-soil contact force and V max is the maximum static vertical pipe-soil contact force), averaged over the touchdown zone length, ranged from just over 1 up to values of 2, with peak values over a shorter length scale being significantly greater than 2 (Westgate et al 2010b).…”

“…A small hybrid S-lay/J-lay vessel with a sternmounted lay ramp was used at site A, while an intermediate size J-lay vessel with a gimbaled mid-ships lay ramp was used at site B, both of which are detailed in Perinet and Frazer (2007). The different lay ramp geometry between the two vessels strongly affected the transfer of vessel motions to pipeline motions at the touchdown zone (Westgate et al 2010a(Westgate et al , 2010b.…”

“…The two cases comprise sites with different geographic locations, water depths, soil conditions, lay vessel characteristics, and metocean conditions. The global statistics of the as-laid embedment of each pipeline have been backanalysed in detail previously (Westgate et al 2010a(Westgate et al , 2010b(Westgate et al , 2013. The survey data are used to illustrate different approaches for narrowing the range of pipe-soil resistance for a particular design check, which comprise (i) averaging over a length scale or (ii) filtering across multiple length scales.…”

“…Research by the authors and others has shown that use of the fully remoulded undrained shear strength to account for dynamic lay effects in a static vertical pipe bearing capacity analysis provides a satisfactory approach to assessing the as-laid embedment of pipelines in fine-grained soils (Westgate et al 2010b). This approach is based on existing plasticity solutions for pipe penetration in fine-grained (undrained) soils (Randolph and White 2008a;Merifield et al 2009), and incorporates catenary overstress effects (Randolph and White 2008b).…”

“…This approach is based on existing plasticity solutions for pipe penetration in fine-grained (undrained) soils (Randolph and White 2008a;Merifield et al 2009), and incorporates catenary overstress effects (Randolph and White 2008b). This process has helped to validate simple models to predict as-laid pipeline using field measurements of remoulded soil strength (Westgate et al 2010b). …”