Characterization of Near Seabed Surface Sediment

Low, Han Eng; Randolph, Mark; Rutherford, Cassandra J.; Bernard, Bernie B.; Brooks, James M.

doi:10.4043/19149-ms

Cited by 17 publications

(6 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It is clear that repeated separation of the T-bar from the soil surface increases the rate and magnitude of strength degradation, which is attributed to entrainment of water from the free surface. Similar observations have been made from offshore cyclic T-bar tests conducted in the upper 0.5 m of the seabed (Low et al 2008). …”

Section: Observed Field Embedmentsupporting

confidence: 79%

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

Westgate¹,

White²,

Randolph

et al. 2010

Proceedings of Offshore Technology Conference

View full text Add to dashboard Cite

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.

show abstract

Section: Observed Field Embedmentsupporting

confidence: 79%

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

Westgate¹,

White²,

Randolph

et al. 2010

Proceedings of Offshore Technology Conference

View full text Add to dashboard Cite

show abstract

“…To assess embedment during installation, the touchdown dynamic motion in clay soils is accounted for with the fully remolded strength of the soil, which is now measured routinely by cyclic penetrometer tests (typically a T-bar or ball penetrometer), as described by Low et al (2008). There are counteracting influences on the expected level of remolding because of water entrainment that would increase the sensitivity (further reducing the remolded strength) and the limited number of load cycles that occur in the touchdown zone, which may not be sufficient to fully remold the soil.…”

Section: Operative Soil Strength During Installationmentioning

confidence: 99%

An Improved Model for the Prediction of Pipeline Embedment on the Basis of Assessment of Field Data

Bruton¹

2015

Oil and Gas Facilities

View full text Add to dashboard Cite

ences both axial and lateral resistance, and these PSI responses are usually the most-significant uncertainty in the design of pipelines laid on the seabed. The case history presented in this paper shows that this approach is invaluable and provides a significant contribution to good design practice.Two existing export pipelines of significantly different overall pipeline diameter and submerged weight were laid along the same route, in the same soils. These pipelines might have been expected to reach quite different levels of embedment following installation and flooding, and current models for predicting embedment (described in the following) confirmed this; yet, the final levels of embedment were relatively similar and deeper than those that would be predicted with current practice. This is clearly a concern because higher levels of embedment generally lead to higher levels of resistance from the soil, which is often the most-challenging design case in the assessment of lateral buckling (Bruton et al. 2007). This finding has therefore provided an excellent opportunity to modify and calibrate embedment models for use in defining PSI responses on current projects.An assessment of the embedment mechanisms during installation and post-installation flooding has led to a modified methodology supported by geotechnical principles that provide a much-improved correlation between predicted and measured embedment levels for these pipelines. This new approach is recommended for prediction of pipeline embedment levels on current projects. This paper addresses some important revisions to current embedment models:• Improved modeling of penetration resistance because of buoyancy, heave mounds, and bearing capacity at embedment levels greater than one-half diameter, which is a concern in weaker soils. • Improved modeling of the likely operative shear strength at the time of pipeline flooding, to account for the level of strength regained because of reconsolidation of the soil under the weight of the empty pipe. In this assessment, one can assume that sufficient time (2 to 4 months) has passed to achieve a relatively high level of reconsolidation. Further work is required to quantify the likely increase in operative strength with time because the duration between installation and flooding is potentially an important input to the final pipeline embedment. Indeed, this methodology confirms that insufficient time between installation and flooding can result in excessively deep pipeline embedment. Pipeline-Embedment ModelDefinition of Pipeline Embedment. Nominal pipe embedment (w) is defined as the depth of penetration of the invert (bottom of the pipe) relative to the undisturbed seabed (usually termed "far" embedment), as shown in Fig. 1. Penetration of the pipe increases the local embedment of the pipe (usually termed "near" embedment) by raising the soil surface against the shoulders of the pipe. While heave mounds are observed frequently in pipeline surveys SummaryThis paper reviews the current methodology for assessing pipelin...

show abstract

“…To assess embedment during installation, the touchdown dynamic motion in clay soils is accounted for by using the fully-remoulded strength of the soil, which is now routinely measured by cyclic penetrometer tests (typically a T-bar or Ball penetrometer) [16] . There are counteracting influences on the expected level of remoulding due to: (a) water entrainment that would increase the sensitivity (further reducing the remoulded strength), and (b) the limited number of load cycles that occur in the touchdown zone, which may not be sufficient to fully-remould the soil.…”

Section: Operative Soil Strength During Installationmentioning

confidence: 99%

Advances in Predicting Pipeline Embedment Based on Assessment of Field Data

Bruton¹

2014

Day 3 Wed, May 07, 2014

View full text Add to dashboard Cite

This paper reviews the current methodology for assessing pipeline embedment against field data, where embedment levels were under-predicted leading to an unconservative design. An updated model is provided for predicting pipeline embedment. One important principle behind this new approach is to define the operative soil strength, based on reconsolidation of the remoulded soil beneath the weight of the empty pipe during the intervening period between laying and flooding. This approach is based on pipeline embedment data from the field, which has demonstrated that soil around the pipe can become fully remoulded during installation. Therefore, embedment in the flooded condition should be assessed using an operative strength from reconsolidation, rather than the intact strength, which is current practice. This paper also provides current equations that improve the prediction of embedments over one-half diameter, by proposing methods to take account of the increasing buoyancy with depth, the reducing influence of heave mounds and modified penetration resistance at deeper embedments.Pipeline embedment is a fundamental input to the assessment of pipe-soil interaction, which is the largest uncertainty faced in the design of pipelines subject to lateral buckling and walking phenomena. However, pipeline embedment is notoriously difficult to predict due to the inherent uncertainty of the installation process. Improving the prediction of pipeline embedment is critical to reducing the range of predicted pipe-soil responses in design.A current project has compared high quality observations of pipeline embedment from an earlier phase of the same field development, against predicted embedments using more recent, high quality soils data from parallel pipeline routes, including fully remoulded soil strength from cyclic penetrometer testing. While current design approaches were under-predicting embedment levels, leading to a potentially unconservative design; this updated methodology provided a much-improved match to actual embedment data. The revised model was successfully verified against measured levels of pipeline embedment and is now being used for the design of future pipelines in the area, with improved certainty and easing of the design challenge.Under-predicting pipeline embedment could be critical to design integrity. The methodology presented in this paper has the potential to improve assessments of embedment and avoid unconservative pipe-soil responses on future developments.

show abstract

Characterization of Near Seabed Surface Sediment

Cited by 17 publications

References 0 publications

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

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

An Improved Model for the Prediction of Pipeline Embedment on the Basis of Assessment of Field Data

Advances in Predicting Pipeline Embedment Based on Assessment of Field Data

Contact Info

Product

Resources

About