All risers are subject to the same loading: internal and external pressure, current and wave actions, vessel motions, tension and drilling vibrations. Recent developments have led to the maturing of riser integrity management and monitoring. Monitoring systems are currently used on a number of drilling and completion risers, especially in the Gulf of Mexico particularly due to prevailing presence of loop currents and hurricanes.

Recent developments
The perception of integrity monitoring within the industry has changed from R&D endeavors to systems that provide critical data used for ensuring structural integrity, enhancing

Figure 1. Fatigue damage factor comparison. (All images courtesy of 2H Offshore)
maintenance and inspection programs and assisting with operations. The value of monitoring systems has resulted in their wide application. More and more operators and drilling contractors see these systems as field proven and guaranteeing return of valuable data. In the Gulf of Mexico, integrity monitoring systems by 2H Offshore alone are currently used on approximately 25% of operating drilling risers rated for 5,000 ft (1,524 m) water depth and above. These systems are particularly useful wherever operating conditions are not well defined, e.g., new, deeper reservoirs with limited data on currents and other environmental loading.

Monitoring objectives
Drilling riser monitoring is used to:
• Ensure fatigue structural integrity;
• Reduce risk to subsea equipment;
• Improve operational guidelines; and
• Ensure appropriate maintenance and inspection.

Fatigue structural integrity is ensured by measuring riser global response and calculating fatigue damage rates and accumulation over the entire length of the riser. Riser global response is typically assessed from the discrete array of transducers measuring such physical parameters as strain, acceleration and angular rates. This approach limits dependency on analysis, which inevitably inherits uncertainties in both understanding of the riser mechanical response and knowledge of environmental loading. Ensuring fatigue driven structural integrity reduces the risk of riser failure, which can have catastrophic consequences.

Access to direct measurement helps improve operations in two ways. First, the monitoring system is providing real-time information on riser response directly to rig personnel. Such information is used in several situations:
• During critical events such as retrieval of riser in high current, high seas or both;
• During normal operation of running riser while drifting, when vessel is moving during riser installation and retrieval to decrease current loading on the riser string and thus mitigate riser deflection and vortex-induced vibration (VIV). Knowing riser response enables optimization of such activities and verification of their efficiency; and
• When certain actions are required to mitigate VIV (e.g., decreasing mud weight or pulling more tension), real-time feedback of riser response enables confirmation of VIV alleviation.

Second, data collected from real-time or standalone systems during riser monitoring can be used for better understanding of relations between the riser response and environmental loading and thus improve the operating widows. Many examples of data gathered so far suggest that analytical practices used in drilling riser design can be overly conservative. Data shown in Figure 1, which summarizes a number of monitoring campaigns conducted on drilling risers, suggest that VIV predictions are an order of magnitude higher than actual response; thus operating windows are excessively tight. Integrity monitoring identifies such discrepancies and provides a basis for their improvement and extension.

Third, drilling riser monitoring is a powerful tool for defining maintenance and inspection strategies. These strategies differ among operators, contractors, equipment manufacturers
Figure 2. Motion sensor is installed on riser.
and regions of the world. Inspection schedules can vary as much as 100% for the same type of equipment. However, these plans do not take into account what type and magnitude of loading the riser has been subjected to. Integrity monitoring improves the rationale for inspection intervals by providing data on actual riser behavior during service. This may mean extending inspection intervals if the riser is not used under high loading and ensuring that inspection and maintenance is conducted before significant damage occurs.

Monitoring instrumentation
Monitoring instrumentation can be classified into standalone and real-time, depending on the program objective.

Standalone monitoring instruments consist of transducers, batteries and memory cards, which need to be periodically recovered either during riser retrieval or by remotely operated vehicles (ROV). Standalone instrumentation is particularly applicable to monitoring of drilling and completion risers for several reasons:
• No requirement for power and communication cables and simple interface with the riser, which enables fast and straightforward installation and retrieval;
• Relatively frequent access to the instrumentation during riser retrieval, which enables data download without additional time and resources; and
• Constant presence of the ROV on drilling rigs, which again enables data access between riser retrievals.

Historically, standalone monitoring was limited to motion monitoring in order to minimize power requirements. However, new developments enable standalone strain monitoring systems of equal usefulness and robustness to motion monitoring, while maintaining a low cost.

Real-time monitoring allows direct access to the riser response data from the vessel deck without the need of recovering the instruments or the limitation of data access only during riser retrieval. Data can be processed and viewed directly on the vessel deck. In terms of measured physical quantities, real-time monitoring uses same type of instrumentation as standalone. However, in addition to the transducers it requires power and communication links with the surface. This can be either through a cable or batteries and acoustic communication link. Real-time systems are significantly more complicated and expensive than standalone. Power supply and data communication are challenging problems and require a significant level of interfaces with riser installation procedures as well as additional hardware to support these functions. To the author’s knowledge, no system covering significant length of the drilling riser has been deployed to date, but there are several projects that propose such a solution. Real-time systems provide data that allow support of operational decisions, especially during riser installation and retrieval.

Data processing
Data coming from monitoring systems is in the form of motion or strain time traces at local points along the riser. This form is very far from actually meeting the objectives of ensuring structural integrity, risk reduction, operational guidelines improvement, and supplementing maintenance and inspection. Operational data has to be processed at several levels.

Usually, fatigue damage is the parameter that the monitoring programs are designed to provide. First, local measurements have to be translated to riser response and then to fatigue damage, either at discrete points or along the whole riser, depending on the program objectives and system capabilities. Detailed studies are conducted beforehand to determine the location, type and number of transducers required to capture expected riser vibrations. Such studies allow for optimization of the instrumentation arrays and thus for allocation of resources that is most favorable both from technical and commercial points of view.

Summary
Longer and deeper wells push current drilling riser technology to its limits. Development of frontier reservoirs is associated with higher risk by using incomplete data on riser response and environmental conditions. Significant effort has been undertaken in recent years to offset this risk by monitoring what remains unknown and hidden from direct observation.

Integrity monitoring technology emerging from R&D programs has been developed to become useful tool for assisting with drilling operations. Real-time monitoring, integrated with riser controls and equipped with data processing software, provides information to support decision making during operations. Standalone monitoring is used to ensure drilling riser long term integrity and to gather data for improving understanding of riser response. Integrity monitoring has become another tool in deepwater drilling operations toolbox. The value of the integrity monitoring programs is provided by asset protection and increased competitiveness following optimization of drilling activities.

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