Assessing the post-hooking lifetime of hydrogen pipelines damaged by anchors

Abstract

The increasing focus on reducing greenhouse gas emissions has led to the attractiveness of offshore hydrogen pipelines in achieving sustainable energy goals. Hydrogen, as a transport medium for energy, offers a viable alternative for transmitting large amounts of energy from offshore facilities to the shore. By storing hydrogen and subsequently converting it back into electricity during periods of peak demand, this approach aligns with the goal of establishing a green, net-zero economy by 2050. However, maritime activities in the proximity of offshore pipelines introduce a serious risk of damaging pipelines by accidental or emergency anchoring scenarios. Damage from dropped or dragged anchors can displace and harm pipelines, leading to environmental risks, safety hazards, and costly repair operations. Comparing hydrogen pipelines to existing oil and gas pipelines, there are significant differences. While oil leakage from anchor hooking poses risks to the environment and marine ecosystems, hydrogen imposes new risk factors which must be taken into account. Hydrogen negatively affects the structural integrity of pipelines, and anchor hooking leads to elevated stress levels within the pipeline material, accelerating fatigue crack growth, and reducing the operational lifespan of the pipeline. This leads to the following research question: ”What is the post-hooking lifetime of a hydrogen pipeline damaged by an anchor?”

To address this research question, the methodology applied in this thesis consists of two main approaches: numerical simulations and a fatigue crack growth model. The simulations specifically consider an incident where an 8-inch pipeline was damaged by an AC-14 High Holding Power (HHP) anchor. The pipeline is internally pressurised at maximum gauge pressure, and simulations are conducted considering daily and yearly variations in loading cycles, specifically at 10% and 50% pressure reductions. Through these simulations, the stress distribution and variation within the pipeline material resulting from the anchor impact are investigated, providing insights into the behaviour of the pipeline under such conditions. The fatigue crack growth model used in this study is based on the Paris law, which describes the relationship between crack depth and the number of cycles required for crack propagation under cyclic loading conditions. The presence of hydrogen significantly accelerates the rate of fatigue crack growth. As a result, adjustments are made to the Paris law to account for this effect, particularly in determining the range in which the law remains applicable. The crack growth analysis focuses on determining a critical crack depth, which could possibly lead to pipeline failure. A Failure Assessment Diagram (FAD) is used to determine the maximum allowable crack size, ensuring the safety of the pipeline. The FAD, along with the wall thickness of the pipeline, serves as a critical criterion for assessing structural integrity. The remaining lifetime of the pipeline following an accident depends on which criterion, either the FAD or the wall thickness, indicates failure first.

The crack growth analysis conducted in this research reveals that as the crack depth progresses under the influence of hydrogen, it eventually reaches a critical depth that introduces a potential risk of pipeline failure. Specifically, when considering yearly pressure variations, the crack reaches this critical depth in slightly over 8 years. Although the attained crack depth at this point is not yet through-thickness, the crack growth rate experiences a significant increase after 8 years, ultimately resulting in a through-thickness crack 9 years after the initial impact. The findings of this study have significant implications for the future development and maintenance of offshore hydrogen pipelines. By understanding the consequences of anchor hooking incidents and their impact on the operational lifespan of hydrogen pipelines, this research contributes to the development of robust and resilient infrastructure for a sustainable energy future.