简介：开发海上油藏存在很多挑战和风险，在设计和作业阶段要进行风险管理。隔水管由于结构响应复杂，它也是一件非常关键的完整海上构造，尤其应当对其关注。

Developing offshore reserves presents many challenges and risks that must be managed during the design and operational phases. Risers are a particular focus due to their complex structural response and are one element of equally critical integrated subsea architecture.

Example of a deepwater riser system, grouped single line offset riser. (Images courtesy of 2H Offshore Inc.)

Riser integrity is crucial in deepwater and hostile environments where loading is high and complex and often design methods are pushed to the limit of current industry capability and experience. Catastrophic riser failure is unacceptable.

Through subsea integrity work over the last decade, 2H engineers have identified and remediated critical integrity issues including those related to fatigue loading. Analyzing riser motion and strain response has been particularly effective in identifying the rate of fatigue burn along the riser. On one occasion, a major as-built anomaly consisting of a missing centralizer within a riser system was detected by analyzing measured response data. Remediation measures were put in place to prevent a potential failure.

Multiple concerns

Experience has also shown that mudline equipment put in place during initial drilling programs is often not manufactured to the standards required for fatigue service on a long-term production development. 2H has developed monitoring programs to track fatigue damage over time to determine whether loading conditions in service would result in fatigue integrity issues over the service life.

Within the process of design, many assumptions need to be made, and often new technology or design methods need to be implemented to meet commercial or technical requirements. The manufacturing and installation phases, often set against tough schedule constraints and complex manufacturing procedures, inevitably present opportunities for deficiencies to go undetected.

Meteorologists indicate that climate cycles are moving into a period of greater severity and uncertainty; this can be seen in areas such as the Gulf of Mexico, where recent hurricanes have been severe. Hurricane Katrina resulted in the destruction of 46 platforms and damage to 20, with 100 damaged pipelines and 211 minor pollution incidents on the Outer Continental Shelf.

Uncertainties impact the known and required reliability of riser systems such that over a 20-year life, it is difficult to be confident of the risk level presented by these relatively novel and dynamic systems.

Though processes and procedures required for effective integrity management are well documented, the concept of integrity management is often not fully understood. Integrity goes beyond simple inspection or inspection management. For deepwater risers, inspection activities alone provide only a fraction of the information required to give an informed opinion regarding integrity assurance. Mitigation and monitoring requirements must be assessed when developing integrity strategies.

Integrity management process

Responsible operators have recognized the importance of implementing risk-based integrity management programs to mitigate the inherent uncertainty in the life-of-field risk profile of critical systems. Such programs are based on assessing potential failure modes and developing risk mitigation plans including specifications for inspection and monitoring as well as plans to quantify actual performance and identify anomalies.

The process begins by collating sufficient data to determine the as-built condition and the operating parameters; these can be used to determine safe operating limits, which act as trigger points in initiating intervention requirements. Legislative (depending on location of operation), operator, and partner requirements determine a level of prescriptive requirements; however, the majority of inspection, mitigation and monitoring requirements are defined based on the determined risk level to system components.

Generally, the greater the amount of data considered, the more informed the decisions, which means a more effective system is put in place to manage risk. The probability and consequence of failure for each component in turn is assessed with respect to personal safety, environment, and asset availability.

Probability and consequence are then combined to give a risk rating for the component for each failure mechanism and for each consequence in turn. Confidence is also considered when determining the inspection period and mitigation or monitoring requirements. Confidence is measured in terms of the rate of deterioration, number of previous inspections, the ability of the system to deteriorate without monitoring, quality of inspection and/or monitoring data, and knowledge of operational (process) effects.

With the confidence ranking applied to the risk ranking, the overall criticality of components is calculated. At this point, engineering competence is essential in allocating suitable inspection, mitigation, or monitoring requirements. In certain instances, design changes or mitigation measures can be required to reduce the criticality of components to acceptable levels.

During the continuous cycle of the integrity management process, data is gathered, and the assessment is updated to ensure it remains focused on managing risk effectively and assuring asset integrity. The resulting data should also be fed back to assist in optimizing integrity management programs.

Integrity management systems should indicate suitable alert levels based on the parameters affecting riser integrity. On reaching defined near-critical and critical levels, pre-determined alarms should trigger, setting off an appropriate course of action to protect personnel, the environment, and the asset (Figure 2).

Certain limits must be viewed in conjunction with other events rather than treated as isolated indicators. Operational change, such as temporary increases in operating pressures or higher than anticipated corrosion rates, can require careful assessment and reconsideration of acceptable limits. Extreme design conditions (for example, during a hurricane) could result in large bending and axial stresses.

Competence in developing systems and processes is essential because provisions must be in place for all possible failure mechanism threats. Experience has shown that competence is required at all stages of the process, including implementation. An untrained technician can pose a significant threat to asset integrity simply by being unaware of the importance of the components he is looking at or by being unaware of the significance of excessive movement and missing components.

Role of structural riser monitoring

Some consider riser monitoring to be in its infancy. Certainly, there is much to do within the industry to improve monitoring reliability, reduce cost, and gather and process data more efficiently. Despite these challenges, riser structural monitoring has the potential to offer significant benefits to risk reduction if applied through a properly structured integrity management process.

Although internal threats such as corrosion and erosion are very similar to those found in other components, there are some unique external threats to deepwater risers.

These external threats are primarily due to the dynamic nature of the riser and its components and the deepwater environment in which they work. They include structural overstress, structural fatigue, structural wear, material degradation, mechanical degradation, impact, external corrosion, and fire and/or explosion.

Since riser design is built around complex engineering analyses, the value of reliable instrumentation to measure the riser’s structural response is obvious. It provides key information on riser loads and performance on an ongoing basis, which inspection alone cannot provide. An added bonus is that monitoring technology also allows forensic engineering, which permits investigation of the actual event sequence during an incident. This capability assists in determining how and why an incident occurred to ensure there is no re-occurrence.

A successful integrity management process relies on timely and reliable data transfer and processing. For some measured data, automatic downloading can be initiated to ensure rapid data retrieval; this is particularly important during and after a significant event such as a hurricane or loop or eddy current. Critical systems should be capable of recording continuously on their own power supply without manual intervention.

As environmental conditions appear to be moving toward a more unstable period, and technology advances push for more innovative solutions, monitoring systems prove their worth in providing ongoing asset integrity assurance.