Subsea system reliability and uptime are crucial for developing deepwater fields.

As subsea developments begin to make substantial bottom-line contributions, life-cycle management becomes key to success. After installation and startup, a key measure of system performance is its ability to meet forecast production targets during its producing life. The Subsea Surveillance Team, together with the asset teams within the Production and Surveillance arm of Shell Exploration and Production Co. (SEPCo), is charged with ensuring that the subsea systems' production performance is optimized - not just minimizing problems, but actively working to get the most from each asset.
Asset integrity
Asset integrity is an integral part of the corporate health, safety and environment management system. To achieve asset integrity, it is necessary that the risks of loss of hydrocarbon containment are kept as low as reasonably practical (ALARP).
The primary threats in subsea systems are internal or external corrosion or erosion caused by sand production. To demonstrate that these risks are reduced to ALARP, barriers are put in place, and the condition of the system is documented.
Barriers that can be used to reduce the risk of hydrocarbon release include:
• increases in wall thickness;
• changes to more resistant material;
• changes in design (for example, to reduce velocity); and
• an increase in system monitoring and surveillance.
To demonstrate an acceptable level of asset integrity, it is necessary for the subsea project team to deliver a system that meets specified minimum functional requirements. These requirements allow quantification of the risk and identify how they can be mitigated.
The methodology for assessing erosion and corrosion risks for subsea projects has three stages. First, determine the minimum wall thickness for pressure containment during the startup and operation of deepwater flow lines. From this, remnant excess wall thickness, or "corrosion allowance," can be determined. Next, determine the corrosion rates that may be expected during the life of the project and the operating risk regime (low, medium or high). Corrosion monitoring requirements then are defined from the risk ranking. Finally, evaluate tolerance to sand production during the life of the project and the operating risk.
Once the subsea system is producing, asset integrity needs to be demonstrated. This is achieved through adoption of an erosion and corrosion surveillance plan for each system and through regular reviews of the system performance.
The following measures are used to assess the performance of the SEPCo subsea erosion and corrosion management strategy:
• annual corrosion and erosion risk assessment results;
• annual verification of each subsea system's erosion and corrosion surveillance strategy;
• quarterly reviews of program effectiveness;
• monthly deferment of production due to corrosion or sand production; and
• weekly deviations from the corrosion monitoring strategy.
Flow assurance process
Flow assurance is an engineering analysis process that works to maximize production by ensuring the production flow path is kept unrestricted for the field life while ensuring that life-cycle costs are minimized.
Subsea systems are becoming much more complex, incorporating features such as multiple projects tied back to one host and longer offset distances between the subsea wells and the host. The ability to produce these fluids generally in multiphase flow in harsh environments of high pressure and low temperatures presents a multitude of problems. Therefore, a much greater effort must be spent developing reliable guidelines for design and operation of subsea systems to prevent these problems and keep an unrestricted flow path, an engineering analysis process referred to as flow assurance or production flow management. Flow assurance engineering must consider the following production characteristics in the design and operation for the entire life cycle of a subsea property:
• formation of solids such as hydrates, wax, asphaltene and scale;
• rheology, emulsions, slugging problems;
• erosion and the accumulation of sand; and
• corrosion.
Management of erosion and corrosion falls primarily in the area of asset integrity discussed previously but is difficult to leave out of a flow assurance discussion entirely. Erosion and corrosion are integrally linked with flowing conditions, and flow assurance is concerned with the potential buildup of corrosion or erosion products, which may affect flow. While accumulated produced sand may impede or block a flow line, products of corrosion do not usually accumulate to a sufficient magnitude to cause problems. Accumulations due to corrosion or chemical reactions are a significant issue, however, in the small-bore umbilical tubes that provide well chemicals, control hydraulics or pressure-sensing ability to the subsea system.
The following activities form the strategy for accomplishing the objectives of flow assurance:
• continuous rigorous attention and evaluation of system performance;
• understanding of capacity limits (to unlock potential);
• minimization of failures, interventions and flow obstructions;
• early detection of degradation and anomalies;
• advance planning for possible interventions and changes; and
• optimization of operating modes and parameters.
Maintenance management process
To achieve maximum availability at a minimum life-cycle cost, the objectives of maintenance management are to avoid interventions and ensure readiness for them when required. The following activities form the strategy for accomplishing these objectives:
• conducting intensive surveillance;
• analyzing surveillance, subsea inspection and failure investigation data to identify opportunities for intervention and system reliability improvement;
• establishing SEPCo minimum functional requirements for projects and expediting system reliability improvements via management of change processes;
• providing knowledge transfer to technology development activities;
• identifying and implementing alternative operating modes to postpone or eliminate interventions;
• developing and maintaining statistical availability models for each subsea development to aid in forecasting and targeting of performance and optimizing operational expenditures and capital expenditures;
• managing readiness for intervention in accordance with a five-point plan; and
• applying results of after-action reviews and other knowledge.
Maintenance management has three major elements: availability management, intervention readiness and reliability improvement. The first step in SEPCo's management and improvement process is the definition of SEPCo functional requirements. The basis for a robust and reliable design with high availability is laid in these functional requirements.
The new projects are required to build a RAM (reliability, availability, maintainability) model that predicts the availability of the field over its life. The RAM simulator simulates several series of events reflecting different lifetime scenarios of the field based on mean time between failure (MTBF) and repair data.
The process contains two feedback loops, which reflect two ways to improve availability. First, how long does it take to repair a failure? The intervention readiness process reduces the downtime related to a specific failure. Second, how can failures be prevented in the first place? The reliability improvement process tries to improve MTBF of individual components.
RAM models typically are updated on an annual basis. The output from availability modeling is used to:
• show the impact of loss of redundancy on subsea systems. Because subsea systems are designed with a substantial amount of redundancy, several recent failures did not have an impact on availability. They did, however, undermine potential future availability. Availability modeling will allow users to calculate and visualize the impact of this loss in redundancy. It supports the thinking about availability as a long-term target instead of a monthly snapshot;
• visualize the differences in expected availability for different assets as they are designed in a different way. A generic target of 95% is not realistic for all assets. RAM studies will help set realistic availability targets based on the level of redundancy designed into that asset;
• perform risk-based maintenance. Combining expected availability with estimated deferment cost will reveal the areas in which SEPCo is most exposed. This allows development of a risk-based subsea maintenance and inspection strategy. For certain assets or failure modes SEPCo will have to be more prepared for interventions than for others;
• forecast intervention cost for subsea assets. The results of RAM studies for SEPCo's Gulf of Mexico portfolio will help predict the number of interventions, operations expenditures and requirements for rigs and remotely operated vessels; and
• optimize system design. New developments or modifications will be asked to guarantee a certain level of availability. RAM models help project teams evaluate the influence of design decisions on long-term field availability.
Reliability improvement
The Reliability Improvement Process is focused on maintaining availability by preventing failures and recurrences of failures.
This process begins in the operating phase in which data, from surveillance, events or failures, or from SEPCo or industry experience, is gathered and analyzed. Once anomalous system performance is identified, the path toward improvement involves three key steps: understand what's wrong, identifying solutions and making risk-based decisions.
This investigative and corrective path may be simple and straightforward or complex and time-consuming, depending on the situation. Sometimes the causes of problems and the corrective actions may be obvious from initial examination of the information. Other times, a formal root cause failure analysis involving several man-weeks and considerable calendar time may be required to ensure an adequate understanding of the problem and to identify a solution that is not likely to introduce further problems.
Any changes so identified are implemented by upgrades to SEPCo minimum functional requirements, changes via intervention to existing equipment and changes to equipment during project execution cycle.
Management of a subsea portfolio requires a focused effort and due diligence in executing a comprehensive life-cycle strategy. With a focused effort in place, the integrity and business viability of a subsea portfolio can be assured.
Editors note
This is an abridged version of OTC Paper 13120, "Life Cycle Management for Gulf of Mexico Subsea Portfolio" and has been reprinted with permission from the Society of Petroleum Engineers.

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