Jet Trenching Research Articles

Study on the mechanism of soil erosion by submerged water jet vertical scouring in cohesive soils

The water jet trenching technique is widely used in the burial of submarine pipelines. However, its application in cohesive soils often leads to complexity in trench morphology and challenges in predicting trench dimensions due to unclear soil erosion mechanisms. These issues significantly impact pipeline burials. To investigate the soil erosion mechanism of water jet trenching in cohesive soils, two-dimensional physical simulation experiments of submerged vertical water jet erosion were conducted. The influence of jet pressure, impingement height, and nozzle diameter on the shape of the scour hole was analyzed. The erosion damage patterns of water jets on cohesive soils were studied, and a theoretical model for the development of scour holes was established. The study revealed that when the jet velocity reaches 1000 m/s and the nozzle diameter reaches 1 mm, a contracted neck forms at the upper part of the scour hole. The appearance of the contracted neck is due to excessive jet impact energy causing impact shear failure in the soil. The effective height and width of the contracted neck increase with jet pressure and nozzle diameter and decrease with impingement height. Based on Prandtl's bearing capacity model, a model for predicting impact shear failure in cohesive soils was established, and a predictive formula for the effective height and diameter of the neck was proposed. Experimental validation confirmed the accuracy of the predictive formula. These findings provide theoretical support for the application of water jet trenching techniques in cohesive seabed soils.

Energy saving and environment protection solution for the submarine pipelines based on deep learning technology

Underwater flowlines (pipelines and cables) have become a popular option worldwide as communication lines. Correspondingly the demand for underwater trenching increased with the most desirable method internationally, jet trenching for its benefits in protecting the flowlines from external hazards. However, the trenching method is suffering from a main problem that can affect the construction and operation phases. This problem is the use of maximum jetting pressure needed for underwater trenching which leads to extra costs in the construction phase. Furthermore, the unstraightening of the trench can cause extra stress on the flowline requiring extra maintenance in the operation phase. The key cause of this problem is the classification of the underwater soil, as if the underwater soils cannot be classified as accurately as possible, the suitable jetting pressure required to trench these soils could not be applied correctly. Therefore, this research aims to develop a machine learning classifier for soil classification as a solution for enhancing the performance of the underwater trenching process. This classifier was constructed, trained, and evaluated by MATLAB R2020a to classify the underwater soil type by analyzing the soil image samples by an artificial neural network (ANN). Then a prototype that mimics exactly the real trenching mechanism was built to test the developed classifier based on three different types of soil (sand, clay & gravel). The results showed that the classifier successfully identify and classify the three different types of soil used. Finally, the results confirmed that the classifier will enhance the trenching processes. As it eliminates the need for soil classification by conventional methods while executing it in an easy fast process that will save energy, and cost, and decrease the need for future maintenance processes.

Perspective Review on Subsea Jet Trenching Technology and Modeling

This paper presents a comprehensive review on subsea jet trenching technology via a critical analysis of its principle, mechanism, devices, modeling approaches, as well as properties of subsea sediments. This review shows that the success of jet trenching operations is closely related to some key factors including the soil conditions, trencher specifications, and characteristics of pipelines or cables. Three case histories are presented to demonstrate the importance of these key factors and their interrelationships. This paper also points out a number of challenges pertaining to the implementation of the jet trenching method in carbonate sediments, as well as some limitations and gaps in the existing modeling approaches. Future perspective researches that are recommended to develop: (1) promising theories, like turbulent submerged flow, and (2) robust numerical approaches, such as the smoothed particle hydrodynamics (SPH) and material point method (MPM) to substantiate the experimental experience and reveal possible hidden mechanisms.

Flotation of pipes and cables in consolidating backfill

This paper discusses the flotation of cables and pipelines that have been trenched using jet trenching during reconsolidation of the seabed soil. Utilising previously published data on the variation of clay strength with liquidity index, it is demonstrated for the first time that, while current industry guidance is appropriate for the low-plasticity clays for which much previous research has been carried out, it may be overly conservative for high-plasticity clays such as those found in the Gulf of Guinea or the Gulf of Mexico, owing to the differing strength and density characteristics of these soils.

The prediction of traveling jet trenching in stiff clay based on the erosion failure mechanism

ABSTRACTThe performance of traveling jet trenching in stiff clay was tested. All the key parameters varied in the experiment, including the jet velocity, the shear strength of clay, the diameter of the nozzle, and the translational velocity of the nozzle. The jet–soil interface was found to be an arc. An equation was given to describe the interface. On the basis of the erosion failure mechanism, a new method is presented for predicting jet trenching depth. We first calculated the trajectory of the jet–soil interface and then predicted the distribution of the boundary layer shear stress along the jet–soil interface. The point where the wall shear stress was equal to the critical shear strength of clay was the ultimate trenching depth. The predictive results were found to fit the experimental results well.

MPM Simulations of the Interaction Between Water Jet and Soil Bed

In offshore engineering, pipelines are often buried in the seabed to avoid the damage caused by ocean waves, currents, fishing activities, etc. For its reliability, the high-speed water jet has been increasingly used for constructing the pipe trenches. The jet-soil interaction is a highly complicated two-phase problem. A three-dimensional Material Point Method (MPM) model, Anura3D (www.anura3d.com), is used to simulate the jet trenching processes. In this preliminary study, the effect of the water jet speed on the trenching process is simulated. The research demonstrates the advantages of the MPM model in handling the free surface and soil-water interaction problems, and the results are useful for the offshore oil and gas industry. In future research, the different controlling parameters of the trenching operation, including the jet size, water pressure, translational speed and soil strength, will be tested to optimize the trenching operation to achieve high efficiency.

Model uncertainty in uplift resistance calculations for sandy backfills

Trenching and backfilling is one of the most practical and cost-effective methods for protection and stabilization of offshore pipelines. Defining the geotechnical properties of backfill material resulting from mechanical backfilling or jet trenching is an area of substantial uncertainty and the resistance against pipeline uplift provided by these backfills needs to be characterized accounting for these uncertainties. This paper compares the properties of sandy backfill material and the available calculation models for uplift resistance against a database of more than 300 controlled pipeline uplift tests. The model uncertainty for uplift resistance calculations is derived from a back-analysis of the uplift tests. The uncertainties on uplift resistance and mobilization distance are correlated and characterized using a joint probability distribution. The selected distributions are applied to an example uplift resistance problem. When compared against this probabilistic formulation, the factors applied to uplift resistance in pipeline analysis can be refined to lead to a more cost-effective solution.

Experimental Research on Trenching in Stiff Clay by Submerged Vertical Traveling Jets

ABSTRACT Zhang, S.; Zhao, M.; Ge, T., and Wang, C., 2016. Experimental research on trenching in stiff clay by submerged vertical traveling jets. This study investigated trenching by submerged circular traveling jets in stiff clay using an experimental method. The testing clay was prepared as mixtures of natural clay and cement with shear strength ranging from 60 kPa to 90 kPa. The jet pressure, the nozzle translational velocity, and the diameter of the nozzle all varied in the test. A cohesive sediment scouring theory was employed to reveal the mechanism of the jet trenching. It showed that the boundary layer shear force on the jet-soil interface might be the predominant mobilizing force for jet trenching. The trench depth was found to increase with the jet pressure, erodibility coefficient, and the diameter of the nozzle and decrease with the translational velocity of the nozzle and the threshold stress of the clay. The essence of the traveling jet trenching could be considered as jet erosion. The effici...

Experimental Investigation on Scour around Marine Pipelines in Silt Beds by Jet Trenching

Jetting power has been applied more and more frequently to create trench in the seabed to bury cables or pipelines under a protective layer of sand in offshore engineering. However, there are still several key problems to be understood further and solved both in their design and operating. This study investigates the topographic deformation due to the scour of silt beds which lie below marine pipelines and are impinged by moving two jets in a model experimental system. The response of silty sea beds was also examined during the experiment. The important parameter that influences the trenching performance, the jet velocity, was studied. The speed of the jet horizontal motion was varied to examine its effect on the scour profile. By presenting the variation of a scour profile with time after impingement, the change process was analyzed. The asymptotic scour profiles were found to be very similar when made non-dimensional, and a function was employed to describe the non-dimensional profile. The results of the above work will permit a better understanding of the mechanisms controlling the performances of underwater trenching by jet trenching machines.