Genebelin Valbuena
Technip USA
Oil and gas companies are extending their operational domain to deep and ultra deepwater; which challenge the performance and operational integrity of conventional subsea technology, jeopardizing the value proposition of the enterprise and increasing the downside risk to stakeholders.
The tendency to use electronic-based equipment in subsea and the high impact of unscheduled downtime caused by any failure of this electronic equipment and associated instrumentation, in terms of loss/deferred production and environmental issues, increase the necessity to consider reliability concepts not only during the operational stage of the field, but more importantly, during the conceptualization, design, and execution of the development.
Some efforts have been made to consider reliability during the first stage of a subsea development. In some cases, by defining generic reliability goals while in other cases by imposing arbitrary reliability targets. In most cases, these requirements are limited to the component level (e.g. electronic-based equipment such as trees, BOP, and HIPPS). At this level, some of the techniques and approaches used to embed reliability into the design aim to evaluate the product itself. Some other approaches look at the process by which the product is created. Probably the more integrated and holistic approach includes not only the product and the process, but also the people and the resources (e.g. tools and work processes) used to design, manufacture, install, and commission the equipment.
First stage
During the first stage of a subsea development, the emphasis should be on the system level, because the ultimate goal is to meet enterprise value proposition, which depends not only on the independent behavior and performance of individual components but on the system (integrated development) as a whole.
At the system level it is important to define how reliable the infrastructure that supports the production system should be. Not all applications require the same level of reliability and availability. Reliability requirements for the entire system, and for critical system components, should be defined progressively during development to align the design, manufacturing, and execution of these requirements.
One way to address the level of reliability/availability is the use of a risk-based approach, in which the reliability is allocated in order to maintain the level of technical risk “As Low As Reasonably Practicable” (ALARP).
This traditional risk management framework is based on the estimation of the risk associated with the subsea development of interest, the comparison of the assessed risk to the risk acceptance criteria pre-established, and the identification and implementation of prevention, control, and mitigation actions to reduce the level of risk as low as reasonably practicable. This framework has been used not only in the safety arena, but it also has been applied in decision making processes involving environmental, health, and financial risks.
The first step of this approach is the system definition. This involves developing a clear definition of the subsea development under consideration. This should include, but not be limited to, defining the system in terms of physical boundaries, objectives, regulatory requirements, environmental and operational context, and maintenance and intervention philosophy.
The second step is assessment of the subsea development technical risk. This may be addressed by systematic brainstorming sessions, formal risk identification techniques, and/or prior experience. Risks associated with the particular project should be identified and the likelihood/probability of occurrence and consequences associated with each one of the undesirable events should be assessed.
Risk can be estimated as:
RISK = Probability of Undesirable Events x Consequence
This should use a qualitative and/or quantitative approach. Initially, a qualitative approach based on a criticality matrix may be implemented. When a better understanding of the system is obtained the approach can become more quantitative. Typically, an initial screening is based on a qualitative (e.g. risk matrix) and the quantitative or semi-quantitative approach is used to analyze the events identified in the initial screening as medium to high risk.
In this context, undesirable events refer to any event or scenario associated with the design, operation, and maintenance equipment failure and/or operational context with the potential of affecting the performance and integrity of the subsea production system.
The third step must have a reference to compare against the risk. This reference or risk acceptance criteria, which should be set before risk assessment is performed, must reflect if applicable legal or statutory requirements and/or requirements of the stakeholders regarding the level of risk they will accept. The risk acceptance criteria are meaningful benchmarks that ensure consistent application of the risk management framework.
The fourth step is identification of the different options/mechanisms available to reduce the risk according to ALARP. An effective mechanism to reduce the assessed risk, and consequently reduce the gap between the assessed risk and the risk acceptance criteria, is through the improved reliability.
Reliability at the system level can be improved by increasing the inherent reliability of the system constituents; by implementing a quality assurance program during the manufacturing, installation, and commissioning stages; and by providing competent people in an organizational climate that enables their synergy.
During design, reliability can be improved by incorporating redundancy. However, redundancy does not necessarily significantly improve reliability, particularly a “common cause failure” (CCF) can jeopardize overall system reliability. One powerful aspect to consider when improving reliability is diversity. Diversity is defined as the use of different means to perform a required function.
For subsea production, application of this concept depends on the target reliability goal and the availability and readiness of existing technology. The concept of diversity can be extended to installation, commissioning, operational, and maintenance activities, where the main goal is to reduce the likelihood of CCF.
Availability.
The interrelation between risk and reliability is obvious. Undesirable events triggered by equipment or system failures can be reduced by improving component and system reliability. The key is the specific nature of risk involved in subsea developments. Risks associated with subsea equipment are varied, and in some cases such as BOP or HIPPS, the risks are safety and environmental related. In cases such as multiphase meters or wellbore intelligent instrumentation, risks are associated more with loss of revenues or costs arising from lost production. Regardless of the nature of these risks, they can be characterized as a combination of probability consequence, and as such the risk management framework can be applied.
The level of reliability required is driven by the gap between the assessed risk and the risk acceptance criteria. As a result of the comparison, system reliability goals and requirements in terms of Mean Time To Failure (MTTF), Mean Time To Repair (MTTR), production availability, reliability, and/or availability can be defined. It provides a clear understanding of the performance requirements in place. High-risk field developments will require a highly reliable subsea production system and, consequently, more resources and effort should be put in place during their development.
Effort should achieve the required level of reliability, but considering that the subsea production system may fail eventually, appropriate maintenance/ intervention strategies have to be in place to restore operations efficiently and effectively. To achieve a high level of system availability, both the reliability and maintainability should be addressed.
All aspects of maintenance resources, mobilization delays, spare part constraints, intervention philosophy, and diagnostic capacity are key to reach an appropriate level of maintainability. In subsea applications, consideration should be given to the intervention strategy and the availability of intervention vessels (e.g. intervention vessel mobilization time), which usually drive the overall availability of the field.
The effort to improve subsea system availability and to achieve the risk reduction demanded by the application will be justified by reduced risk expenditures in each of the cost elements (capex, drillex, and opex). These unexpected expenditures, referred to as riskex, represent the cost of unwanted events not commonly considered in conventional life cycle cost analysis.
Overall system reliability/availability, which represents a key driver for riskex, can significantly impact the affected party, jeopardize the value proposition of the enterprise, and increase the downside risk to stakeholders. This emphasizes the importance of improving the overall reliability/availability of the system as much as reasonably practicable.
An immediate consequence of the lifecycle cost analysis is that subsea enterprises focus on capex, drillex, and opex rather than on lifecycle cost (LCC) analysis, where the financial impact of unwanted events (riskex) are evaluated and the value of reliability/availability are considered in the economic model (reliability-value analysis).
Risk vs. lifecycle
Once the reliability/availability requirements for the subsea development are defined during the first stages of the enterprise, specific reliability and availability requirements can be allocated progressively to systems, subsystems, components, and procedures as the enterprise progresses (reliability allocation process).
It is important to mention that the implementation of a framework to assess and improve the reliability of a system, subsystem, component, or procedure, and ultimately work toward the fulfilment of a subsea development reliability goal, involves a series of interacting activities during the complete project/asset lifecycle, from the feasibility and concept design to the decommissioning phase, passing through engineering design, manufacture, and operation.
About the author
Genebelin Valbuena has over 22 years of experience in the oil and gas industry. Dr. Valbuena’s areas of expertise include process safety management, probabilistic risk analysis, asset integrity management, and decision making process. He holds a BSEE from the Army Polytechnic College Institute (IUPFAN), a MSEE from UNEXPO, a MSRE and a PhD from University of Maryland. Dr. Valbuena currently works as a principal specialist in the Risk and Integrity Management group at Technip USA ([email protected]).
