Subsea pipelines in Arctic environments face the risk of damage from ice gouging, where drifting ice keels scour the seabed. To ensure pipeline integrity, burial using methods like ploughs, mechanical trenchers, jetting, or hydraulic dredging is the conventional protection method. Each method has capabilities and limitations, resulting in different trench profiles and backfill characteristics. This study investigates the influence of these trenching methods and their associated trench geometries on pipeline response and seabed failure mechanisms during ice gouging events. Using advanced large deformation finite element (LDFE) analyses with a Coupled Eulerian-Lagrangian (CEL) algorithm, the complex soil behavior, including strain-rate dependency and strainsoftening effects, is modeled. The simulations explicitly incorporate the pipeline, enabling a detailed analysis of its behavior under ice gouging loads. The simulations analyze subgouge soil displacement, pipeline displacement, strains, and ovalization. The findings reveal a direct correlation between increasing trench wall angle and width and the intensification of the backfill removal mechanism. Trench geometry significantly influences the pipeline's horizontal and vertical displacement, while axial displacement and ovalization are less affected. This study emphasizes the crucial role of trenching technique selection and trench shape design in mitigating the risks of ice gouging, highlighting the value of numerical modeling in optimizing pipeline protection strategies in these challenging environments.

The long-term settlement of subsea pipelines on a clayey seabed is crucial for the on-bottom stability of the pipelines, especially in deep waters. In this study, a poro-elasto-viscoplastic finite element analysis is performed for predicting long-term settlement of subsea pipelines by incorporating a rheological constitutive model. A method for identifying the creep-settlement (Sc) from the total-embedment (Sk) is proposed on the basis of the obtained linear relationship between the secondary consolidation coefficient (C alpha e) of the clayey soil and the total-embedment (Sk) of the pipe. The identifying method is validated with the existing theoretical solutions and experimental data. Parametric study is then performed to investigate the key influential parameters for long-term settlement of subsea pipeline. A non-dimensional parameter Gc is introduced to quantitatively characterize the soil rheology effect on pipeline settlement. The relationship between the proportion of creep-settlement in the total-embedment (Sc/Sk) and Gc is eventually established for identifying whether the proportion of creep-settlement in the total-embedment is remarkable.

Hundreds of millions of tons of dredged sludge are generated by waterway dredging worldwide every year. Traditional disposal of dredged sludge, such as in-situ stockpiling and offshore dumping, cannot avoid the waste of land resource and the pollution to marine environment. Sludge stabilization/solidification treatment currently used can achieve the reuse of drudged sludge but requires large investment and time. Therefore, how to turn waste into treasure in an effective, environmentally friendly and cheap way is a notable problem. In this study, the variation of strength of solidified sludge cured in air with water-cement ratio, water content and curing time by unconfined compression test was investigated, and the inner mechanism of strength influenced by watercement ratio and water content was revealed by XRD test, which offered an optimal working condition. Also, solidified sludge with the maximum strength in the optimal working condition was immersed into seawater at different times, which showed the 7d strength after mixing completion for 8 h immersed into seawater could reach 20.60 MPa (1.37 times of the strength in air), and the prediction formulas considering all the parameters mentioned above were established. At last, a field test of solidified dredged sludge for protection of submarine pipelines was carried out in Bohai Bay, China, which demonstrated the feasibility of mixing dredged sludge with cement on board and solidifying in seawater environment. Compared to the traditional subsea pipeline protection solutions, the cost of using solidified sludge to protect subsea pipelines is 25 % and 39 % less than the cost of using sandbags and concrete mats, respectively. This study provides a more economic and environmentally friendly idea for dredged sludge treatment and subsea pipeline protection than the conventional methods, which provides a new source of green ocean building materials, reduces the pollution of the marine environment by the discharge of dredged sludge, turns waste into treasure and has wide applications in ocean engineering.

The assessment of pipeline free-spans may involve non-linearities, including those arising from the mechanical behaviour of the pipeline, soil properties and hydrodynamic loading; as well as the interaction between these factors. Failure to properly quantify and account for these factors may lead to inadequate design outcomes, which can lead to failure (if unconservative) or result in costly remediation when unnecessary (if conservative). This paper is part of an ongoing study into the vibration response of free spanning subsea pipelines, including the development of a numerical model that highlights the value of modal and full fatigue analyses as tools for understanding span behaviour. For quality control and enhancing the reliability of assessment methodologies, the model has been benchmarked against published data in terms of modal analysis, and against industrial fatigue assessment packages in terms of fatigue analysis - and shows excellent agreement with both. Since verification, the model was used to conduct a sensitivity study on a single free span, to explore how pipeline response and fatigue damage is affected by the value of dynamic soil stiffness and damping.