Integrity evaluation of a 70-year-old veteran subsea pipeline
As a leading global provider of engineering and project management services and asset integrity, Penspen has implemented a series of methods to evaluate the integrity of ageing pipelines.
By Mushaid Nauman, Penspen, Richmond upon Thames, UK
In the past, pipelines were designed to operate for 25-30 years. This can now be extended to 40 or more years through a variety of methods. Pipelines are energy lifelines, transferring precious hydrocarbons which serve as a backbone for several other industries; however, they do inevitably deteriorate over time.
Considering the current downturn and rock-bottom petroleum product prices, prolonging the lifespan of an asset while maintaining a high level of quality and safety is the most sustainable and cost-effective option. Consequently, operators are looking for benchmarked asset life extension practices made by integrity management experts, such as Penspen.
The company carried out an integrity evaluation of a 65.95 km pipeline which has been in operation in the Middle East for the past seven decades after having been constructed in 1945. The pipeline is used to transfer Arab light sweet crude oil to a refinery. The majority of the pipeline is unpiggable, and it comprises onshore and offshore sections. The pipeline is coated with concrete and tape wrap and has a de-rated pressure of 700 psig.
This article highlights the methods used and outcomes presented by Penspen to extend the life of the pipeline for the offshore and onshore unpiggable pipeline sections which are in current use for crude oil transportation.
Three methods of review were initiated to evaluate the pipeline, which included HAZID (hazard identification), an ICDA (internal corrosion direct assessment), and ECDA (external corrosion direct assessment).
Both the ICDA and ECDA methods can be used as an alternative to pigging and are performed in four stages:
- Pre-assessment: collection and evaluation of historical data and pipeline characteristics
- Indirect inspection: use of over-the-line survey techniques or internal-flow and slope assessment to determine high risk areas and excavation
- Direct examination: excavation and inspection
- Assessment: assessment of defects and definition of repair and inspection plans.
The HAZID desktop exercise was carried out first, to identify major threats to the pipeline. This method is used as an initial risk assessment of the pipeline and allows risks to be measured in quantitative and qualitative data. This initial test saved time and money by justifying recommendations made and improving the safety by allowing Penspen to target the specific resources required. This assessment allows decisions to be made around clear data collated from the test.
This review was followed by an ICDA, which was implemented since there was only a small quantity of data available to be reviewed; this was then combined with expert judgment on the condition of the internal corrosion of the pipeline.
An ECDA is carried out to enhance the safety of the pipeline by managing the risk of corrosion failures while minimising the cost required for verification and repair. This assessment was carried out on both the onshore and offshore sections of the pipeline in this instance. As with any offshore pipeline system, there is a higher degree of corrosion risk associated with the external environment of the pipe within this submerged section of the network.
A central element to the integrity assessment results lies with determining a reliable value for the internal corrosion rate in the different pipeline sections to estimate the degradation process and to help foresee breakdown over time, separately from determining which recommendations should be presented to the operator.
As previously stated, this particular pipeline had limited operational and inspection data available. Penspen carried out tests to determine the internal corrosion rate and completed flow modelling for different sections of the pipeline, to help build-up more analysable data.
With each assessment, observations and outcomes shape the recommendations that are presented to the operator. Penspen concluded that the age of this pipeline would not affect its safety, based on its historical safe operation and, over time, the adoption of reliable inspection techniques and the protective internal pipeline product chemistry.
The direct inspections were carried out on sections of the pipeline that had been noted through a series of flow models in conjunction with other techniques to be at a higher risk of internal corrosion than others. A recommendation made from the outcome of this inspection was to install a specialised corrosion-monitoring sump and implement an internal corrosion-monitoring plan for several parameters on identified critical locations. A detailed inspection plan was also created and was given to the operator to be implemented by the company’s inspection department.
The ECDA uses engineering processes to proactively assess locations for corrosion and to identify and review areas where corrosion has already been located. Offshore, this method often uses remotely operated vehicles (ROVs) to carry out surveys and is coupled with previous operational data and manufacturing records to establish outcomes. The root cause of the corrosion is identified and defect mitigation is carried out, before a risk level is breached.
The data collected from the direct inspection measurements were evaluated and generated useful results for determining the external condition of the pipeline. The stability of the pipeline was also ensured through a free-pan review and an on-bottom stability analysis. The pipeline defect mechanism was also assessed. A further recommendation was made to install anode retrofitting on several locations along the pipeline to supplement the weak cathodic protection system of the pipeline at these locations and serves as a shield for the pipe, to prevent corrosion.
After conducting a full assessment of the pipeline, it was concluded that the pipeline could remain operational for the next ten years before exceeding the dimensional tolerances identified – providing that all recommendations were followed.
Transmission pipelines, which are two to three decades old, are described as ‘middle-aged’ in the industry, and even the best designed pipelines can become defective over time. This pipeline is unique in its behaviour, age, route, and operational excellence. The outcome of this integrity evaluation was positive for the operator and highlights valuable information which ensures safe operation, the effects of a dedicated corrosion-management system, the use of product-flow modelling, the statistical evaluation of inspection data, and the use of credible inspection techniques for prolonging the pipeline’s safe operation.
For more information visit the Penspen website.
This article was featured in the December edition of Pipelines International. To view the magazine on your PC, Mac, tablet, or mobile device, click here.