Lorents Reinås

Fatigue Assessment of Subsea Wells for Future and Historical Operations Based on Measured Riser Loads

Operators in the North Sea have recently strengthened their efforts in documenting the integrity of subsea wellhead systems. As a part of this effort, fatigue damage estimation of subsea wells in service has been performed. Fatigue damage estimation on subsea wells due to drilling riser dynamic loads was carried out by the use of analytical model results. The applied analytical methodology is based on a decoupled approach, where global load analyses and local stress calculations are carried out prior to a SN based fatigue accumulation. Applying such methodology on safety critical systems the analytical philosophy should ensure conservative fatigue damage. For cases where the fatigue calculations returned unfavorable estimates, one corrective action has been to measure the actual riser response and to monitor the development of fatigue damage closely. For this purpose a methodology for fatigue estimation based on measured riser response was needed. In this approach of estimating the fatigue damage, the global load analysis results are replaced by measured dynamic load time series. By combining direct riser response measurements with local stress calculations, a revised SN based fatigue accumulation can be performed. The fatigue damage derived from measured riser response is compared to the fatigue damage based only on analytical results. From this comparison the conservatism in the analysis methodology for the global riser response is shown to be significant. As this method relays on measurements, it will only yield historical fatigue damage and at best it can return updated fatigue capacity usage on the fly. Forecasting fatigue damage still have to be established based on global riser analyses results, resulting in a conservative forecast. This paper suggests an updated methodology using actual measured response to both asses fatigue damages of historical operations and forecast fatigue damages based on historic operations. By cycle counts of measured response time series (one hour response) a link between this cycle count and the coexistent significant wave height and spectral peak period can be established. This relationship between observed weather and measured response is representative for the rig and riser system on which the measurements were performed. Then forecast and measurements of the weather conditions can be used to estimate the historical damage and the future fatigue damage respectively. The paper will present results from the suggested approach by use of examples from a real North Sea well in shallow water.Copyright

Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue.The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations.By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure.This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system.The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty.It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.Copyright

The Effect of a Fatigue Failure on the Wellhead Ultimate Load Capacity

A subsea well will experience external loading during drilling operations that can lead to the development of a fatigue fracture in the primary load bearing structural members of the upper well construction. Such a fatigue fracture can occur at several fatigue hotspots which all are located in the upper part of a subsea well. There are two main load sharing structural members; the outer tubular string named the conductor (structural) casing and the next tubular string named the surface casing. Both these strings have a circumferential load bearing weld close to the top. The load sharing between these 2 tubular strings are affected by the supported weight from further tubular strings placed inside the well.This paper discusses the residual ultimate load capacity of a typical North Sea subsea well assuming that a fatigue fracture has developed. The discussion is based on FEM analysis results where a fully developed fatigue fracture has been introduced to the analytical model of a typical well either to the conductor part of the well or to the surface casing string. Then the residual ultimate load capacity is evaluated assuming a fully developed fatigue fracture. Evaluations presented herein can be important and necessary tools in considering the consequences of a possible fatigue failure of a subsea well.A reduction in ultimate load capacity due to a fatigue fracture may reduce the safety margin should an accidental or extreme loading occur. The results indicate that the location of the potential fatigue failure is important when assessing the residual ultimate load capacity. If the factored fatigue life of a subsea well is approaching its limit the presence of a fatigue fracture should be assumed. The most prudent approach would then be to perform a permanent P&A operation of the well. Planning of such operations should comprehend the possibility of reduced structural capacity of the well due to a fatigue fracture. This paper also discusses the results in an operational context. The applied methodology is outlined and illustrative results are presented from a typical North Sea well.Copyright

Wellhead Fatigue: Effect of Directional and Annual Variation in Weather for a Sequence of Drilling Operations

Subsea wellhead systems are exposed to wave induced cyclic loading when a drilling unit connects to the well with a marine riser and a BOP. When connected, access is provided to the well and reservoir, and allows for operations such as further drilling, side tracking or workover. Once the operation is completed, the BOP is disconnected from the well, and the wellhead system is not exposed to cyclic loading any longer. Over the lifetime of a well, a number of such operations take place. A wellhead system is perhaps exposed to a total duration of fatigue loading of up to a year, which comprises a sequence of operations of different durations in different seasons. Fatigue predictions for offshore structures are typically based on statistical average of environmental conditions over a large number of years. This is appropriate for permanent installations exposed to continuous wave loading over the lifetime which is often 20 years or more, since variations in the environmental conditions from one year to another is equally represented in the statistics and experienced by the structure. However, for an operation of short duration, the uncertainty in the environmental conditions for that particular period in that particular year needs to be addressed. The weather during October this year is unlikely to be the same as in October last year, and can also be significantly different from average October weather. Although there exists no standard way of doing wellhead fatigue analysis, a commonly applied approach is to do the analysis in a single plane. This is obviously conservative since the wave direction will vary over time, and the fatigue loading will be distributed more around the circumference of the pipe sections in the wellhead system. Furthermore, the environmental conditions are typically based on statistical average for the month or season when the operation is to be executed, sometimes with some conservatism of including the adjacent more severe month or using annual data. Long crested waves are often assumed. This paper address the effect of the uncertainty in the environmental conditions on the accumulated fatigue damage for single and sequences of operations of different durations at different times of the year. A drilling rig in the North Sea has been analyzed using 56 years of hind cast data of significant wave height, peak wave period and main wave direction. Statistics of the fatigue damage rates are calculated and used in a structural reliability analysis in order to estimate reasonably but not overly conservative factors that are to be multiplied with the fatigue damage estimated in a conventional design analyses. Results based on long crested and short crested sea are calculated. An annual variation factor is proposed to account for the variability from one year to another. Secondly, a directional effect factor is proposed to account for the directional variations and its uncertainty on fatigue. Both factors are first estimated considering a single operation only, where the duration is varied between 3 days and up to a year. Thereafter, a sequence of operations of different durations at different times of the year is analyzed, and it is proposed how to consider the accumulated duration of such sequences compared to a single continuous operation. The expected result is an annual variation factor which is greater or equal to unity and a directional effect factor which is less than unity, both with lower values the longer the duration. The product of the two is a quantification of the degree of conservatism associated with a deterministic design analysis using long crested head sea and statistical average omnidirectional weather for the planned drilling operations.Copyright

Benefit of Measurements and Structural Reliability Analysis for Wellhead Fatigue

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calculated by SRA and calibrated to a target failure probability.The current analysis methods for wellhead fatigue are associated with high sensitivity to variations in some input parameters. Some of these input parameters are difficult to assess, and sensitivity screening is often needed and the worst case is then typically used as a basis for the analysis. The degree of conservatism becomes difficult to quantify, and it is therefore equally difficult to find justification to avoid worst case assumptions.By applying SRA to the problem of wellhead fatigue, the input parameters are accounted for with their associated uncertainty given by probability distributions. In performing SRA all uncertainties are considered simultaneously, and the probability of fatigue failure is estimated and the conservatism is thereby quantified. In addition SRA also provides so-called uncertainty importance factors. These represent a relative quantification of which input parameter uncertainties contribute the most to the overall failure probability, and may serve well as guidance on where possible effort to reduce the uncertainty preferably should be made. For instance, instrumentation may be used to measure the actual structural response and thus eliminate the uncertainty that is associated with response calculations. Clearly measurements obtained from an instrumented system will have its own uncertainty. Other options could be to perform specific fatigue capacity testing or pay increased attention to logging of critical operational parameters such as the cement level in the annulus between the conductor and surface casing.This article deals with the use of measurements for fatigue life estimation. Continuous measurements of the BOP motion during the drilling operations have been obtained for a subsea well in the North Sea. These measurements are used both in conventional (deterministic) analysis and in SRA (probabilistic analysis) for fatigue in the wellhead system. From the deterministic analysis improved fatigue life results are obtained if the measured response replaces the response obtained by analysis. Furthermore, SRA is used to evaluate the appropriate magnitude of the design fatigue factor when fatigue analysis is based on measured response. It is believed that the benefit from measurements and SRA serve as an improved input to the decision making process in the event of life extension of existing subsea wells.Copyright

Wellhead Fatigue Analysis Method: A New Boundary Condition Modelling of Lateral Cement Support in Local Wellhead Models

Subsea wells are dynamically loaded during all drilling operations involving a drilling riser system which will cause fatigue loading of the subsea wellhead system and the upper structural components of a well. Estimation of loads and load resistance enables analysts to predict expected service life based on analytical models of well and riser systems. Analytical wellhead models may be a simplified representation of the reality but should still capture the essential physical behavior of the well. Ensuring sound boundary conditions for the analytical model is important and subject to debate amongst analysts. This paper addresses the importance of the well cement as a lateral supporting boundary condition of the wellhead and surface casing in local modelling of a subsea wellhead system. A modified boundary condition approach is suggested that differs from the previously published way of modelling cement in a wellhead fatigue context. This modified boundary condition is applied both to a 2D FEM wellhead model and to a more complex 3D FEM wellhead model. Analysis results with the new boundary condition and the previously suggested boundary condition are compared to understand the effect this modified cement boundary condition has on the analytical results. The effective level of cement is varied to investigate the sensitivity of cement boundary condition modelling. Results indicate less conservative analytical results for the modified boundary condition both for 2D and 3D modelling. The modified cement boundary condition suggested herein is believed to be relevant for deepwater and high latitude subsea basins as it takes into account the hindered strength development of oil well cement cured close to seabed under the influence of low seawater temperatures. We advocate that installation of BOP before cement has properly cured will improve fatigue life of surface casing weld.© 2012 ASME

Wellhead Fatigue Analysis Method: The Effect of Variation of Lower Boundary Conditions in Global Riser Load Analysis

Subsea wellhead mechanical fatigue can potentially result in a gross structural failure of barrier elements in the upper part of the well, potentially resulting in loss of well control. Several major E&P operators have acknowledged the importance of wellhead fatigue and are participating in the JIP “Structural Well Integrity”. It is within the scope of this JIP to develop a recommended practice for wellhead fatigue analysis methodology. The analysis methodology currently being investigated by the JIP is a decoupled approach, with modifications of the lower boundary to account for the stiffness of the conductor, soil and template interface. A detailed local wellhead model is used to generate the lower boundary condition for a decoupled global riser load analysis model. This lower boundary condition definition is intended to capture the overall non-linear stiffness of a site specific well in order to achieve best possible global riser loads estimate.In this article the effect of varying the lower boundary conditions on a global load estimate is studied. Global load estimates are generated from a typical North Sea case and various lower boundary conditions are introduced as the only change to the global riser model. A fixed lower boundary condition is used as a reference and load estimates generated from riser models with various lower boundary conditions are compared.The different lower boundary conditions selected for comparison in this study has been derived from the following cases:1. Fixed at WH2. As per ISO 13624-23. As per JIP “Structural Well Integrity” -Current4. As per JIP “Structural Well Integrity” -ModifiedComparing the analysis results gives indications that the lower boundary condition modelling approach affect global riser load estimate. The fixed lower end boundary conditions did not yielded the most conservative load history in a fatigue context. Modelling well specific flexibility at the riser lower end increased the total number of wellhead fatigue load cycles. This finding support the current approach suggested by the works of the JIP “Structural Well Integrity”. Ensuring that riser load results are still conservative places a higher importance on precise local modelling of the well system.Copyright