This paper describes temporal variations in embedment of several existing pipelines on the North-West Shelf (NWS) of Australia, and the sediment mobility processes that cause them. Distinct and explainable patterns in the extent, distribution and rate of the development of pipeline embedment have been revealed through systematic detailed examination of repeated annual integrity surveys by ROV. This represents a unique data-set that has been used to optimize the reliability of a newly designed pipeline. This paper explains why these clear findings should not be overlooked in both the buckling and stability design of initially unburied pipelines, which is in contrast to currently established industry practice. This new information supports the presumption that conventional approaches for calculating the hydrodynamic stability of unburied pipelines may be more conservative than necessary. Conversely, and arguably more importantly, it is shown that conventionally accepted methods for calculating pipe-seabed resistance forces when planning buckling schemes should be considered unsafe if embedment due to sediment mobility is possible. Consequently, this paper proposes an innovative calculation methodology that statistically captures these sediment mobility effects, and which facilitates a more justifiable geotechnical input to pipeline engineering than what is conventionally adopted. This methodology is currently being used by the authors as a state-of-the-art design practice for unburied offshore pipelines in regions of sediment mobility.
In order to design unburied pipelines, pipeline engineers require information about lateral and axial seabed resistances (normally known as ‘friction factors’) prior to and during operation. Geotechnical engineers must provide this information based on knowledge of pipeline properties, seabed conditions and metocean conditions. Accurate prediction of the seabed resistances requires accurate prediction of pipeline embedments. This paper discusses how pipeline embedments and geometries can change from the as-laid state as the result of sediment mobility around pipelines and demonstrates how these embedment changes will change the pipe-soil friction factors using a series of numerical analyses. Sediment mobility tends to cause a general trend of increasing pipe embedment, which is beneficial for hydrodynamic stability but can be onerous for thermal buckling. Recommendations are provided to accommodate these effects in design, without unnecessary levels of over-conservatism.
Rock berms installed over discrete sections of high temperature, high pressure (HTHP) subsea pipelines may be used to provide axial restraint with a view to minimising the end expansions or to improve the later-life buckle performance. For such applications, the paramount consideration for an efficient rock berm design is the interaction of the rock berms with the pipeline and simultaneously, of the pipeline with the seabed during operation. This paper re-introduces the mechanism of stress re-distribution in the berm or ‘arching’ occurring during pipeline operation-driven cyclic embedment. The development of cyclic pipeline embedment due to shearing and re-consolidation of the soils around the pipeline is quantified at different times during the pipeline operational life. This is assessed analytically by consideration of the generated pore pressures during episodic shearing and intervening consolidation in a framework described herein. Further, the results from Finite Element (FE) analyses that correlate the pipeline embedment and the effective rock berm-pipeline contact stress, are combined with predictions of the cyclic pipeline embedment from the analytical framework presented here. This allows to form-find the rock berm cross-profile and screen the rock volume requirements in a reliable manner.
Knowledge of how an unburied pipeline interacts with the seabed during operational buckling is critical both to check the fatigue life of a pipeline in the buckling zone and to make accurate predictions of the pipeline end-expansions and walking. This paper explains how site-specific seabed conditions affect pipe-soil interaction (PSI) for buckling pipelines during their operational life. The paper first demonstrates that for many soil conditions, cyclic PSI models that only consider the ‘berm push’ resistance fail to capture all the key elements of the pipeline-seabed interaction during cyclic buckling. An additional mechanism associated with the changing pipeline trajectory which develops after many pipeline lateral movements (‘sweeps’) is identified. For some soil and pipeline conditions this second mechanism provides the main component of the seabed resistance after many operational cycles. Appropriate analysis methodologies are discussed which allow the ‘berm-push’ and trajectory mechanisms to be considered in light of soil conditions and pipeline input conditions. Secondly, published design approaches to assess cyclic PSI during buckling have typically assumed that the seabed around pipelines is not mobile (i.e. unscourable). However, in some situations, the combination of pipeline geometry, seabed properties and metocean conditions may lead to seabed scour around the pipeline and surrounding soil berms. This may therefore affect how the lateral soil resistance develops during the operational lifetime of a pipeline and should be considered in design. The paper discusses this issue, presenting results from CFD scour analyses of a particular berm geometry for different metocean conditions. The implications of berm scour on pipe-soil interaction for this geometry is explained.
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