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Bringing Best-In-Class Fatigue Monitoring To Offshore HPHT Applications

Many engineers in the nuclear and fossil electric power industry are well aware of the viability and benefits of our SI:FatiguePro 4.0 software and methodology. For more than 25 years, Structural Integrity has been using FatiguePro to help utilities monitor and manage fatigue damage of critical, safety-related equipment. In nuclear power plants, the components inside these heavily reinforced containment structures are inaccessible for direct monitoring to directly measure the complex and multiaxial strains at the locations of interest. So, the well-known Transfer Function and Green’s Function methodologies are utilized to conservatively determine the current and projected fatigue damage, based on already existing measurements of temperature, pressure, etc.

A similar challenge exists for offshore, subsea applications, which will inevitably approach ever greater depths over the coming years. These subsea depths found in the Gulf of Mexico (up to 10,000 feet of water) expose the pressure-containing components to high temperature and high pressure (HPHT) conditions. Due to the inhospitable and remote environmental conditions, in-service NDE and direct measurements of strains are all but impossible at some critical locations. In order to perform safely under these harsh conditions, we must rely on sound engineering principles to manage the fatigue life.

Metal fatigue from cyclic stresses (e.g., pressure, thermal and live loads) can shorten the life of critical HPHT components, but SI:FatiguePro 4.0 can automatically determine environmentally-assisted fatigue usage and count events, using existing and strategically placed instrumentation. SI:FatiguePro 4.0 facilitates the periodic safety assessment related to fatigue life concerns by providing an immediate, up-to-date and continual assessment of fatigue usage (either S-N based or fracture mechanics based analyses) for any critical HPHT component.

The offshore oil & gas industry is moving into new frontiers in the Gulf of Mexico. Proposed new fields will rely on equipment that will be required to operate at increased HPHT conditions, for which the American Petroleum Institute (API) and The American Society of Mechanical Engineers (ASME) standard class are not yet adopted. The next generation of equipment designs will need to perform at temperatures up to 350°F, pressures up to 20,000 psi and in extreme sour service environments (NACE SSC Region 3). As such, this industry is breaking new ground and will benefit from transitioning the lessons and technology, proven in the electric power industry, to their emerging fatigue threats offshore.

When companies submit for a conceptual Deep Water Operations Permit (DWOP) for HPHT operations, operators will need to comply with 30CFR250.807(a). The federal regulator for such matters, the Bureau of Safety and Environmental Enforcement (BSEE), expects the operator to describe their plans to monitor all loads that affect the service life of HPHT equipment after installation. To achieve the design requirements in these harsh environments, manufacturers have considered increasing the wall thicknesses in proportion to the increased pressure. A consequence of increased thickness may be increased susceptibility to fatigue damage, primarily associated with the thermal gradients experienced during the life of the well. In addition, the increased costs and technical difficulties associated with manufacturing, inspecting and deploying components with extreme thicknesses and weight further complicate the design. Based on our leadership and experience with design and analysis of high pressure equipment, we think there is a better way.

SI has begun sharing our experience with fatigue monitoring, specifically through our participation on the API 17TR8 committee – High-Pressure High-Temperature (HPHT) Design Guidelines. We have also increased our participation in the offshore Oil & Gas sector by publishing more at industry events on such topics as Fatigue Management Alternative for HPHT Deep Water Applications and Integrating Fatigue Monitoring of HPHT Systems into a Conceptual DWOP, and serving as the principal investigators on a new research project under the offshore consortium DeepStar (www.deepstar.org). Simulated case histories are being used to understand and address specific concerns with a given subsea design, and reviewing fundamental modeling principles used to monitor the fatigue life, using existing sensors and other customarily available data in standard subsea completions. These efforts focus on demonstrating how fatigue damage of components using actual operating data can be evaluated over time, in order to satisfy BSEE’s expectation for comparing the actual load cycles experienced by the HPHT component to the load cycles used in the design verification analyses at 25 percent of the design service life and beyond. The approach leverages proven principles employed for several decades in the nuclear and fossil power industry, which have successfully met similar technical and regulatory challenges involving fatigue monitoring of remote locations using limited instrumentation.

This is an exciting time for SI, and we are looking forward to providing the offshore oil & gas industry with our technical leadership and expertise, that many of you know us for, to this new application.

For additional information on SI:FatiguePro 4.0 or Structural Integrity’s subsea capabilities see www.structint.com/subseaHPHT

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