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).

Description ofproposed paper: Burial ofsubmarine pipelines and cables is standard practice in offshore oil and gas regionswhere potentially damaging threats such as interaction with fishing gear anddragged anchors are high. The requirement to bury pipelines in Asia-Pacific hasnot had the same imperative as the North Sea for example but this region is nowseeing a growing demand for pipeline burial to increase mechanical protectionand stability. Performance benchmarks for most trenching systems are based onthe seabed soils typically found in the North Sea and consequently do notinclude the unusual carbonate rich seabed sediments that are prevalent in theAsia-Pacific region which are often cemented and pose a significant challenge to trenching. Application: Trenching isconsidered an activity with significant risk so it is particularly important toselect the correct tool for the field conditions and to establish realisticperformance criteria based on regional experience in the prevailing seabedsoils, including carbonate rich sediments. This paper addressesthe primary differences that exist in seabed sediments and trenching conditionsbetween Asia-Pacific and other regions and show how these dissimilaritiesimpact on the choice of burial equipmentand tool performance. Results, Observations, and Conclusions:A state of theart review of trenching methods and technologies is presented. Trenching casestudies from Australia and Asia are presented which highlight the challenges ofdesigning an appropriate protection and burial strategy for this region and providesindications of actual performance that can be expected across the range ofseabed conditions typically found in the Asia-Pacific region. This includesdata for trenching in harder cemented soils to help fill the gaps associatedwith performance predictions in carbonate sediments. Significance of Subject Matter: The significanceof this proposal paper can be summarised as follows:It provide a reference point onwhich to plan trenching work in Asia-Pacific region,It addresses the knowledge gapsassociated with the relative inexperience of trenching in Asia-Pacific regionPerformance benchmarking is extendedto include the carbonate rich and cemented sediments.

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Driving piles into carbonate strata, particularly weak rock (calcarenite) and cemented sands is traditionally thought to carry high risk in terms of premature refusal, exceedance of fatigue criteria or pile tip buckling. Although soil resistance to driving (SRD) algorithms have been developed for carbonate deposits they are based on limited experience (primarily Arabian Gulf) and there is a need to expand this database to include installation experience elsewhere in carbonate conditions such as the Australian north west shelf where significant oil and gas developments are planned.Much of the uncertainty surrounding the prediction of pile driving in carbonate strata can be attributed to the difficulty in establishing a reliable and continuous soil resistance profile which is primarily due to i) the difficulty obtaining suitable samples of rocks such as calcarenite for compressive strength testing, and ii) the equipment limitations for pushing cone penetrometers into weak rock.It is standard industry practice to base hammer selection, fatigue damage analyses and driveability assessments on upper bound plugged conditions which, for weak rocks such as calcarenite, gives significantly higher resistances than for the unplugged case and this can lead to the conclusion that driving is either completely unfeasible or not possible beyond a certain depth.This study presents data from a pile installation in the Timor Sea to support modifications that may be made to existing SRD algorithms to improve reliability of the predictions, at least for this particular site. Back-analyses of the installation data show that unplugged conditions are far more likely to prevail when driving a large diameter pile which suggests that automatically assuming plugged conditions is over-conservative. Possible correlations between cone tip resistance and unconfined compressive strength (UCS) of calcarenite are made by fitting SRD algorithms that require UCS as an input to those that are based on cone tip data.

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