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Deepwater Offshore Equipment Benefit From Fatigue Load Monitoring In Challenging HPHT Conditions

In this article, we’ll share with you how we are using validated and reliable processes, procedures and software that we developed 30 years ago for application in the nuclear sector for the Load Monitoring of critical equipment used in the high pressure, high temperature (HPHT) oil and gas industry, and how we’re directly applying these methods to deep offshore well head equipment. We’ll also share some of the results and insights we’ve gained in recent HPHT applications. Those of you who have implemented Structural Integrity’s FatiguePro™ software in your nuclear plants will likely recognize some familiar benefits that can be realized in HPHT equipment.

Figure 1. Schematic of a Deep Sea HPHT Installation

Oil and gas operators view deepwater installations as the primary source of significant future discoveries of oil and gas reserves. The challenge facing the industry is that the environments for such installations present design conditions for which proven American Petroleum Institute (API) pressure rating designs are not yet available. These applications reside in ocean water that may be more than a mile deep, and the equipment is exposed to internal sour environments at pressures greater than 15,000 psig and temperatures greater than 350°F, while surrounded by near freezing ocean water (Figure 1). To further complicate design, such high pressures require thick-walled equipment that are designed to ASME Code, Section VIII and API standards; but, these emerging pressure and temperature extremes are beyond what API standards currently address. These conditions provide technical challenges to components not previously seen by in-service equipment. Inside surface initiated fatigue cracking, not previously considered a likely threat in earlier subsea applications, has a greater potential to be an influencing integrity threat in these very thick-walled components.

Figure 2. Revised API Technical Report

Using our FatiguePro™ 4 software, coupled with our in-house API and ASME Code expertise, we have developed a methodology for load monitoring and fatigue management for thick-walled, nickel-lined forgings subjected to these harsh conditions that can be applied in any HPHT application by any knowledgeable engineer. FatiguePro™ 4 uses reliable technology that has been validated over several decades of successful wide-spread use since its development in 1986 for application in the commercial nuclear power sector. In addition, we have shared our expertise with the API community by volunteering as authors of the Load Monitoring Annex A in the new API Technical Report 17TR8, Revision 2, which is currently being finalized for publication (Figure 2).

Although fatigue has traditionally not been a concern for deep sea well head equipment, small imperfections in the material and continuous exposure to the sour environment, coupled with extreme pressure and temperature fluctuations, increase the potential for fatigue damage in the form of crack initiation and environmentally-enhanced crack growth. In addition, standard equipment design practices that used stress concentrations for lower-pressure well equipment now estimate very large stresses in the high-pressure well head equipment. Subsequent growth and penetration of these small fatigue cracks through the corrosion resistant interior layer into the forging base material could then result in through-wall crack propagation, ultimately leading to leaks, which could be environmentally disastrous. Unfortunately, the location and assembly of these subsea components do not readily lend themselves to in-service nondestructive examination (NDE) after deployment. As a result, load and/or fatigue monitoring becomes a necessary engineering solution. Load monitoring with FatiguePro™ 4 provides a way to consistently and constantly monitor the condition of equipment to alert the operator before a critical condition threatens component integrity.

Figure 3. Counting and Categorizing Loads

FatiguePro™ 4 provides load monitoring of critical locations in subsea equipment – in essence, a “fatigue and load odometer” for equipment “hot spots” that serve as leading indicators of fatigue. We do this by strategically pairing, counting, categorizing, and tracking all of the actual loads to which the equipment is exposed in a more rigorous method than simply counting the extreme minimums and maximums. (Figure 3). Once the actual unique loading history is identified, it may be compared to loadings assumed in the design, or fatigue crack initiation and growth parameters may be calculated for comparison to allowable values or alarm limits. The key locations selected for sentinel monitoring are readily determined from the finite element analysis (FEA) performed as part of the ASME Code, Section VIII design analysis (Figure 4).

Figure 4. Finite Element Model and Critical Location Selection

We identify locations of highest stress, including the effects of structural or material discontinuities, through the modelling process and selected for monitoring in FatiguePro™ 4. We configure the software to utilize all of the same methods and inputs that are used to qualify the equipment to ASME standards. However, actual loading measured from installed instruments is used in place of design assumptions for loading, thus providing in-situ measurement and assessment of the actual component duty. The analysis can be performed remotely onshore at regular intervals or immediately updated after unusual operational events.

Figure 5. Use of Green’s and Transfer Functions

A key feature of our FatiguePro™ 4 software is that it uses existing instrumentation and previously developed FEA to provide remote and continuous load monitoring of critical well head equipment. This feature avoids the need for costly installation of additional instrumentation, especially in cases where routing of remote instruments and added electrical cabling may be cost-prohibitive, keeping the implementation cost of this solution low. The key to this approach is the use of Green’s Functions and transfer function logic, which provide mathematical modelling of available instrument measurements and their relationship to conditions at the monitored location of interest (Figure 5). Such modelling provides for a “virtual instrument” – that is, predicted measurements of pressure and temperature at the critical monitored location of interest as if there were instruments installed at that location (Figure 6). The technique, used also in nuclear power, can be applied as early as the design stage, or once a component has entered service.

Figure 6. Virtual Instrument Concept

FatiguePro™ 4 analyses all use measured fluctuations of pressure and temperature, or other available measured loads. Those fluctuations are identified, counted and categorized according to severity for direct comparison to the loads postulated in the equipment Design Specification. This comparison can serve as a first-level measure of the equipment’s condition, in that the loading is measured and compared against its accepted, benchmarked design standard.

Figure 7. Actual Data and Comparison to Design. (Top Plot: comparison of temperature and pressure loading; Bottom plot: comparison of allowable fatigue cycles based on stress evaluation)

A typical field observation for most equipment is that the actual field loading is much less severe than the loading postulated in the design of the equipment (Figure 7). Accounting for this difference can extend the equipment’s life, oftentimes significantly – but more importantly, provide the added level of confidence in the safety and reliability of the equipment. Using the Green’s and transfer functions, FatiguePro™ 4 may also be used to calculate both a cumulative usage factor (cuf) as a measure of fatigue crack initiation or postulated fatigue crack growth (Figue 8). Both of these parameters can be plotted real-time and trended into the future to provide insight to operational practices, equipment maintenance or for proactive planning of equipment replacement (Figure 9). Alert levels can be set to trigger other proactive measures by operators long before problems are encountered.

Figure 8. Result of Using Green’s Functions to Determine Available Margin

Our FatiguePro™ 4 software also provides evaluation of both past and future “what-if” operational practices to show the results of planned or desired operational improvements. This provides important feedback to operators ahead of time that allows for procedure adjustment or the avoidance of operating practices that can prematurely consume equipment operational life.

Figure 9. Trending and Extrapolating Component Life

Our FatiguePro™ 4 computational fatigue analysis software integrates both FEA and fracture mechanics to establish improved fatigue tolerance and fatigue life cycle management during operation. Implementing this methodology will also provide key technical data that can be used to improve future well completion designs. Properly understanding the influence and effects of HPHT environments on new-generation equipment can result in significant weight and cost savings.

To learn more about FatiguePro™ 4 and its application to your well head equipment, or to partner with us on load monitoring applications, contact us at 1-877-4SI-POWER.

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