# Opportunities and Challenges to Develop Digital Twins for Subsea Pipelines
## 7 Life Extension Assessments  
With the increasing maturity of the oil and gas industry, the requirement to operate a subsea pipeline beyond the design life is becoming commonplace [103]. The life extension of the subsea pipelines opens up many development opportunities. If the pipelines can be reused for future developments, significant capital expenditure reduction can be achieved [104; 105]. The process of a life extension assessment considering consolidated guidelines [106; 107; 108; 109] is summarized as follows:  
1. Definition of the operational context and premises for an extended operational period, and identification of new threats. 2. Assessment of current condition, functionality and integrity of the system (Diagnostic). 3. Reassessment of the technical lifetime (Prognostic). 4. Identification of Life Extension measures (Prescriptive). 5. Development of a Life Extension program. The basic premise of the life-extension process is similar regardless of which guideline is applied. Figure 10 displays a typical life extension process. The detailed descriptions of the key steps in the process can be found in [104]. A digital twin can be of great value to support ALCM and hence support optimal management of the asset lifetime [110]. The life extension assessment process clearly maps into the classification and implementation levels of the digital twin. A digital twin supported by the system of systems concepts was proposed in [110] to represent an aid for model-based condition assessment, and the estimation of RUL contributes with prescriptive capabilities to the identification of measures related to condition-based or predictive maintenance policies, optimal operation, and control to extend equipment lifetime. In this regard, the life cycle losses and costs can be reduced. Following the standards and guidelines for life extension evaluations, a risk picture can be automated and integrated within levels 2 and 3 of the digital twin to guarantee the visualization of current and future risks. This can then serve as inputs into level 4 to identify the risk-based extension measures and to visualize the mitigated risk picture. Another digital twin concept was proposed to provide an accurate estimation of the true fatigue life of assets to unlock potential fatigue life and ultimately extend the life of assets in the oil and gas industry [111]. The digital twin concept was divided into four tiers that allow for unlocking the RFL of the subsea asset:  
1. High-resolution modelling of the asset (RB-FEA). 2. Update the model to reflect real-world conditions (Digital Twin). 3. Fatigue calculations based on continuous monitoring. 4. Statistical correlation between sea states and fatigue damage. Retrospective fatigue calculation. The reduced basis finite element analysis (RB-FEA) technology by Akselos is faster than conventional FEA in which higher accuracy is ensured by using posterior accuracy indicators and automated model enrichment [112]. Among the existing case studies [111; 113], the Akselos digital twin is used for the life extension of subsea assets. Fluid flow assurance issues including hydrates, wax deposition, asphaltenes and naphthenates, slugging, scales, corrosion, and erosion, bring a critical operational challenge to the production and transportation of pipelines [114]. Computational fluid dynamics (CFD) can be applied, combined with different models (e.g., CSMHyK rheological model, Eulerian-Eulerian CFD-model, population balance model), to provide accurate analysis in terms of flow assurance [115]. The models can be integrated into commercial CFD packages such as FLUENT, STAR-CD, TransAT, and STAR-CCM+ for such analyses. DNV  
Figure 10: The life extension process in [107]. developed hydraulic modelling software Synerg Gas for the optimization and simulation of gas distribution and transmission networks. In the further development of digital twins in subsea pipelines, flow assurance and fluid composition tracking need to be taken into consideration.