Modelling the dynamic embedment of seabed pipelines

The as-laid embedment of a seabed pipeline is an important design parameter. As a pipe is laid on the seabed it oscillates, owing to vessel motion and hydrodynamic loading of the hanging pipe. This movement significantly increases the pipe embedment beyond the theoretical value related to the static pipe weight, even when corrected for any stress concentration caused by the hanging catenary. Dynamic lay effects are either ignored in practice, or are accounted for by scaling up the static embedment by an empirical factor, leading to significant uncertainty in this important design parameter. A series of centrifuge model tests has been conducted using two clays - kaolin and a high-plasticity natural clay - to simulate the dynamic embedment process. The results indicate that only a few cycles of small-amplitude oscillation (60.05D) are required to double or triple the pipe embedment, owing to the combined effect of lateral ploughing and soil softening. In these experiments the pipe embedment increased to up to eight times the static embedment after 100 cycles of motion, which represents a typical lay process. A model is proposed for the cycleby- cycle embedment of a pipeline under a given sequence of small-amplitude oscillations at a given applied vertical force. The trajectory of the pipe movement is assessed using a flow rule derived from plasticity-based yield envelopes. The effect of soil remoulding is explicitly captured by linking the accumulated disturbance to the decay in soil strength. Using input parameters derived from theoretical considerations and T-bar penetrometer tests, the model captures the essential features of the dynamic embedment process. With modest optimization of the model parameters, the mean discrepancy between the calculated and measured embedment is only 12% for both clays. The ultimate states predicted by this cycle-bycycle model also provide a rough estimate of the maximum pipe embedment for fully remoulded conditions, which include some degree of water entrainment caused by the lay process, evident in the optimised parameters. This ultimate embedment is governed by the remoulded soil strength and the pipe weight (augmented by any stress concentration). The amplitude of the cyclic motion affects the rate of softening, and hence the rate of settlement. This model provides a framework for assessing the as-laid embedment of seabed pipelines on a more rigorous basis than current practice.</p

Centrifuge modelling of pipe penetration due to dynamic lay effects

On-bottom pipelines are subjected to small-amplitude oscillations during installation due to dynamic motion of the pipe within the touchdown zone. This motion is driven by the movement of the lay vessel and hydrodynamic excitation of the suspended length of pipe. The increase in as-laid embedment caused by these dynamic lay effects has a significant influence on various aspects of the performance of the pipeline. A series of centrifuge model tests was conducted to assess the change in embedment of an unburied pipeline subjected to small-amplitude horizontal oscillations. Two clay samples were used - kaolin clay and a soft high plasticity clay recovered from offshore Angola. Dynamic lay effects were simulated by applying a constant vertical load to the model pipeline then imposing packets of cycles of horizontal motion, representing the pipe movement within the touchdown zone. The results demonstrate that a few (&lt;5) small amplitude cycles (±0.05D) can double or triple the pipe embedment, and a number of cycles that might represent the entire lay process (-100) could increase the pipe embedment by up to 8 times relative to the static embedment under the same vertical load. The amount of penetration is governed by the vertical load level relative to the bearing capacity, and the oscillation amplitude. Significant differences were observed between the two clays. The high plasticity clay was much more susceptible to softening and increased embedment under dynamic loading.</p

Modelling the soil resistance on seabed pipelines during large cycles of lateral movement

New oil and gas developments in remote offshore locations require the construction of long seabed pipelines that operate under high temperature and pressure. To accommodate the resulting thermal expansion, a novel design solution is to allow controlled lateral buckling. This design solution is reliant on accurate modelling of the pipe-soil interaction during large-amplitude lateral movements, during which large soil berms are created as the sweeping pipeline erodes the seabed. This paper describes a simple framework that allows existing pipe-soil interaction models to be extended to capture this large deformation behaviour using a kinematic hardening model. The hardening parameter, which governs the resistance created by the berm, is the area of berm currently being transported in front of the pipe. Cyclic behaviour is captured by the deposition and collection of berms when the pipe changes direction. This modelling framework mimics experimental observations, and reproduces the resulting load-displacement behaviour. Only four parameters are involved, each with physical meaning, and the framework is amenable to inclusion within the structural analysis of a pipeline. Limited experimental data on this behaviour is currently available and case-specific calibration of the model is necessary.</p

Upper bound plasticity analysis of a partially-embedded pipe under combined vertical and horizontal loading

Seabed pipelines undergo temperature cycles that create axial load which can be relieved through controlled lateral buckling. The prediction of lateral buckling in design requires accurate assessment of the lateral breakout resistance. This Technical Note describes upper bound plasticity analysis of a partially-embedded pipe on undrained soil. The purpose is to generate failure envelopes for vertical and horizontal loading to provide a theoretical basis for estimating breakout resistance. The following cases have been considered: smooth and rough pipes, with and without separation at the rear face of the pipe. The envelopes are similar to those developed previously for surface foundations, but capture additional effects that are due to the curved geometry of the pipe surface. The breakout resistance and the movement of the pipe at failure are strongly influenced by the separation condition. Pipe roughness and soil self-weight have a relatively minor effect on breakout resistance. Existing empirical expressions usually assume a linear variation in breakout resistance with embedment and vertical load. This theoretical analysis demonstrates that these relationships are non-linear. The resulting envelopes provide a more rigorous basis for predicting the breakout resistance of partially-embedded pipelines.</p

Deformation mechanisms during uplift of buried pipes in sand

To investigate the deformation mechanisms during uplift of a pipe buried in sand, a series of tests was conducted in a plane-strain calibration chamber. Image analysis was used to track the soil movement through a window. From an initial embedment of 3 diameters, a model pipe was extracted vertically whilst digital cameras captured the soil movement. Tests were conducted in uniform silica sands with grain sizes varying over one order of magnitude. During uplift, wide zones of distributed shear developed between the pipe shoulders and the ground surface. Beyond peak resistance, the deformation localised into thin shear bands, leading to strain softening behaviour. Particle size did not affect peak uplift resistance or mobilisation distance for the chosen grain size range and cover depth.</p

Uplift mechanisms of pipes buried in sand

Reliable design against upheaval buckling of offshore pipelines requires the uplift response to be predicted. This paper describes a model-scale investigation into the mechanisms by which uplift resistance is mobilized in silica sand, and illustrates how the observed mechanisms are captured in prediction models. A novel image-based deformation measurement technique has been used. The results show that peak uplift resistance is mobilized through the formation of an inverted trapezoidal block, bounded by a pair of distributed shear zones. The inclination of the shear zone is dependent on the soil density, and therefore dilatancy. After peak resistance, shear bands form and softening behavior is observed. At large pipe displacements, either a combination of a vertical sliding block mechanism and a flow-around mechanism near the pipe or a localized flow-around mechanism without surface heave is observed, depending on the soil density and particle size.</p

Reply to the discussion by McCarron on "large-scale modelling of soil-pipe interaction during large amplitude cyclic movements of partially embedded pipelines"

The authors are grateful for the discusser's interest in our work. The discussion raises a number of important points that we are able to clarify and expand on and highlights certain areas of pipe–soil interaction that we agree remain challenging sources of uncertainty. Some of the points raised in the discussion have been the focus of the authors' more recent work. These recent findings have been published elsewhere, and references are given in this reply.published_or_final_versio

The uplift resistance of pipes and plate anchors buried in sand

The design of buried anchors and pipelines requires assessment of the peak uplift resistance. This paper describes a limit equilibrium solution for the uplift resistance of pipes and plate anchors buried in sand. The geometry of this solution reflects observations from model tests. Peak angles of friction and dilation are found using established correlations that capture the influence of stress level and density. These angles govern the geometry of the failure mechanism and the mobilised resistance. The solution is validated using a database of 115 model tests on pipes and strip anchors assembled from the published literature. Good agreement with the overall database is shown, without optimisation of any input parameters. The method overpredicts the uplift resistance of smooth model pipes by ∼10%, highlighting the influence of pipe roughness. In contrast, it is shown that the solution for uplift resistance based on the limit theorems of plasticity is generally unconservative. The assumption of normality, which is required by the limit theorems, leads to an unrealistic failure mechanism involving uplift of a far wider zone of soil than is seen in model tests. Plasticity theory, with normality, is inappropriate for modelling this class of kinematically restrained problem in drained conditions, as normality is not observed. As finite element analysis is not routinely used in practice - partly owing to the difficulty in selecting appropriate input parameters to describe dilatancy and plastic flow -the simple analytical idealisation described in this paper provides a useful tool for uplift resistance prediction. Simple charts for the prediction of peak uplift resistance from critical state friction angle, relative density and normalised burial depth are presented, to aid the design of buried pipes and anchors.</p

Caracterização do desempenho de vacas primíparas Holstein Frísia em Portugal

O objetivo deste estudo foi avaliar associações entre os possíveis efeitos do maneio das novilhas em explorações Portuguesas nos parâmetros reprodutivos e produtivos, nomeadamente- idade ao primeiro parto, dias em leite (DEL), produção na primeira lactação, bem como na longevidade. A totalidade dos animais cuja primeira lactação ocorreu em 329 explorações, nascidos entre Janeiro de 2000 e Fevereiro de 2009 foram identificados (n=13153). Foi posteriormente identificada a exploração de nascimento (n=629) desses animais e obtidos os respetivos dados produtivos a partir do contraste leiteiro nacional. Foram recolhidas até Fevereiro de 2013 as datas de morte ou saída para abate de todos os animais até referida data. Apenas foram considerados para análise animais da raça Holstein Frísia com mais de 30 meses ou que já tinham iniciado a lactação. Do total de 13153 animais apenas 70% (n=9186) morreram ou foram enviados para abate no final do período em estudo. Foi realizada a análise descritiva dos dados e posteriormente investigada a possível relação entre a idade ao primeiro parto e a longevidade das novilhas, mediante análise de covariância, onde foram incluídos os fatores exploração de nascimento (EN), produção na primeira lactação corrigida aos 305 dias (PLact305) e trimestre do primeiro parto (TParto) A média de idade observada ao primeiro parto foi de 27,38 (sd±3,73) meses; a PLact305 correspondente das primeiras lactações foi de 8376 (sd±2443,34), a longevidade média dos animais com registo de fim de vida foi de 70,03 (sd ±20,23). A idade ao primeiro parto e a longevidade foram significativamente afetadas - pela exploração de nascimento e ano de nascimento e respetivamente (p<0,01) - o trimestre do primeiro parto apenas influenciou a longevidade (p<0,05), observando-se que animais com primeiro parto durante o Verão e o Outono apresentaram longevidade superior relativamente aos que tiveram o 1º parto no Inverno e Primavera (p<0,05). Observou-se um decréscimo na longevidade entre 2001 e 2009, com uma média de 48,01 (ep±0,5) meses em 2009 e de 120,05 (ep±1,35) em 2001 (p<0,05).The primary objective of this study was to evaluate possible associations between management, reproductive and productive standards, average production of first lactation and longevity in Portuguese heifers. Data from Holstein Frisians was collected since January of 2009 until February of 2013. Only heifers with more than 30 months or in production where considered. From a total of 13153 animal, born in 629 farms between 2000 until 2009, only 9186 animals were analysed, because 30% of the animals were still alive on the date in which the data collection was stopped. First Lactations of these cows occurred in 190 herds, and production data, calving date and days in milk was obtained from national official records. Descriptive statistic of the data was followed by an analysis of covariance of age at first calving and longevity and days in milk as a response variables to factors like farm of birth, average production of first lactation and trimester of calving. Average age at first calving was 27,38 (sd±3,73) months with an average production of 8376 (sd±2443,34) litters on first lactation, and average longevity was 70,03 (sd ±20,23). The effects of farm of birth and year of birth, were significantly associated with differences in age at first calving, days in milk and longevity (p<0,01). The trimester of the first calving only influenced longevity (p<0,05); heifers with first calving during summer and fall had superior longevity when compared to the winter and spring calving heifers (p<0,05). A decreased longevity between 2001 and 2009 was observed, with an average of 48,01(ep±0,5) months in 2009 (p<0,05) versus 120,05 (ep±1,35) in 2001