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Measured Wellhead Loads During Drilling Operations: Paper 1 — Data Processing and Preliminary Results

The growing size of BOPs, longer drilling campaigns on wells, and operations in harsher environments has resulted in increased challenges in properly documenting wellhead fatigue during planned or executed drilling operations. The industry has started directing its efforts toward the calibration of analytical tools which are typically adopted for predicting wellhead fatigue. The ultimate goal for achieving this ambitious scope is to identify a benchmark set of analytical results that will predict field measurements. Early on Statoil identified a major obstacle: the absence of a good and comprehensive dataset of field measurements to serve as point of reference. Statoil and Aker Solutions cooperated on a pilot project with the intent of collecting a dataset of full scale measurements during drilling operations to be used to validate and calibrate the theoretical wellhead fatigue calculation methodologies. The main objective of the instrumentation campaign was to measure sectional forces as close as possible to typical wellhead hotspots by the use of three sets of strain gauges installed on the outside surface of the conductor and on the outside of the surface casing. With the objective of collecting an exhaustive dataset of measurements, accelerometers and inclinometers were installed on the BOP, the riser adapter, the riser below the upper flex joint and on the rig. An additional set of six strain gauges was installed on the riser to record riser tension variations. Environmental conditions were logged on board the rig and by the hindcast data provider. Operational events were carefully logged. This paper presents the following:• Data processing used for quality assurance and calibration of the measured data and the associated data challenges• Highlights of the instrumentation system capabilities to capture salient events of a typical drilling campaign and of ad-hoc performed rig operations to calibrate and validate the measured data• Effect of a controlled rig cross motion test, performed to evaluate quasi static loads on the well and calibrate strain gauge sensor orientations• A riser pull test, performed to validate strain gauge functioning• Several landing and disconnecting of the LMRP• Manipulation of the preload between the high pressure housing and the low pressure housing to investigate the effect of the preloading on the load sharing between the casingsSince King and Soloman [2], the industry is still lacking quality field data to be used in order to validate the various analytical models used in the analyses of subsea conductor and wellheads. The results will confirm the quality of the measured data and will represent a first data point of comprehensive measured field data. This data will be used for future required work in calibrating the different building blocks pertaining to the analytical tools dedicated to well head fatigue predictions [3].Copyright

Forced Oscillation Model Tests for Determination of Hydrodynamic Coefficients of Large Subsea Blowout Preventers

Large scale model tests have been conducted in a towing tank facility for the determination of the hydrodynamic coefficients of subsea blowout preventers. A subsea blowout preventer (BOP) is a large, complex device 10–15 [m] tall, weighing 200–450 [ton]. The BOP stack consists of two assemblies, the ‘lower marine riser package’ (LMRP) connected to the riser string and the BOP itself, connected to the wellhead. Together they represent a large lumped mass, which directly influences the natural frequencies and vibration modes of the riser system, particularly those of the BOP-wellhead-casing assembly.Large uncertainties in the estimates of the hydrodynamic coefficients (added mass, lift and drag or damping) result in large uncertainties in the fatigue damage predictions of the riser and wellhead system. The trend toward larger and heavier BOPs, which could place BOP-wellhead-casing oscillation frequencies in the range of wave frequencies, has motivated Statoil and BP to start a new research project on this subject. The project involves a large scale model test for experimental determination of hydrodynamic coefficients.Two different BOP designs were tested in a towing tank at model scale 1:12. The models weighed about 50 [kg] in air and were about 1.2–1.5 [m] tall. A six-degree-of-freedom oscillator was mounted under the carriage of the towing tank for oscillation of the models in different directions. Static tow tests and forced oscillation tests with and in the absence of steady current were carried out. Keulegan-Carpenter (KC) numbers ranged between 0.2 and 2.0, while the Sarpkaya frequency parameter β was in the range from 4,000 to 50,000. The Reynolds numbers of the static tow tests ranged between 50,000 and 150,000. This paper focuses particularly on tests in the surge direction with and in the absence of a steady current. Results indicate that the hydrodynamic coefficients for BOP stacks are quite different from those of simpler geometries like a circular cylinder. In addition, they provide new insight for analytical modeling of global hydrodynamic forces on BOPs in many configurations and scenarios.Copyright

Reliable, Low Footprint and Cost-Effective Monitoring of Wellhead Loads Using Autonomous IMUs for Fatigue Estimates

Wellhead systems in the North Sea are frequently subjected to large bending loads, due to shallow waters, heavy seas and semi-submersible drilling rigs. This makes fatigue of wellhead systems a challenge.The fatigue estimates of such wells are often conservative when based on global FE riser simulations. To reduce the conservatism from such simulations, measurements of the actual wellhead loads have been performed on certain occasions. This is often a costly and intricate process that usually require a subsea cable from the rig. This paper presents an alternative method for measuring WH loads by the use of autonomous motion sensors (IMUs) on the BOP and lower end of the drilling riser.Measured inclinations from the IMU on the riser are used to capture the loads acting from the riser onto the BOP. The inclinations from the sensor on the BOP are used to capture the dynamic response of the BOP and well system. The combination of the measured excitation load and the measured dynamic behavior, makes it possible to reliably estimate the WH bending moment, without the use of riser simulations.The accuracy of the proposed methodology is demonstrated in a FE riser simulation. The model acts as a controlled environment where both input parameters and WH loads are known. The sensor data is extracted from the analysis at the relevant locations, run through the proposed methodology and then compared with the WH loads in the model.As a final verification, the method is tested on data from an actual measurement campaign where both IMU and strain gauge measurements are available. The WH loads will be calculated based on the proposed method and IMU data, and the results will be compared to the data from the strain gauge based load sensors.Copyright

Wellhead Fatigue Damage Based on Indirect Measurements

Enabling safe and reliable operations of subsea wellheads has a high priority in the global oil and gas industry. The objective of the current paper is to provide a novel method for bending moment estimates at the wellhead based on indirect moment measurements; this moment, together with fatigue properties are then used for fatigue damage estimation. Indirect bending moments are based on inclinations and accelerations measured by motion reference units (MRU) attached to blowout preventer (BOP), lower marine riser package (LMRP) and lower riser joint (LRJ) immediately above the lower flexible joint (LFJ). Also, required is the tension time history in the same period at the LRJ. The proposed methodology here can be implemented and integrated into a portal for data acquisition and visualisation.In order to validate the proposed method for indirect bending moment estimation, strain gages have been attached to a BOP and marine riser during drilling operations offshore Norway. Strain gage readings are transformed to bending moment which is used as reference (the so-called direct moment). The proposed method is used to calculate the moment indirectly, the so-called indirect moment. The resulting indirect moments agree very well with the direct moments.Copyright

Comparison of Global Riser Analysis to Full Scale Measurements on the NCS

Fatigue of subsea wellhead systems due to wave-induced loads from riser and rig motions has been subjected to increased attention in recent years. It is expected that calculated riser loads are conservative, as both input parameters and methodology are associated with some uncertainty. However, it is difficult to quantify the degree of conservatism in analytical results unless reference to measurements can be made.Statoil has conducted drilling riser load instrumentation campaigns in several locations around the world over the last few years in order to gather high quality data for accurate assessment of the fatigue loads imposed on the subsea wellheads (see e.g. ref. /1/, /2/, /3/, and /17/). Four (4) of these measurements campaigns have been studied in more detail, with the intention to quantify the degree of conservatism to be expected from drilling riser analysis. Three (3) of these cases have direct bending moment measurements from strain sensors at the BOP connector elevation, giving high confidence in the results. For one (1) of the cases, the bending moments at the WH have been established by use of indirect methods (ref. /2/). The campaigns have been anonymized; They are from the Norwegian Continental Shelf (NCS), with water depths ranging from 110 to 400m. This paper presents the findings from our comparison of measurements and analytically derived drilling riser loads from these 4 campaigns. The analysis models have not been “tuned” to match the measurements. The goal is rather to get a measure of how well riser analyses are able to predict the real world.The conclusion in this paper is that the global drilling riser analyses accurately predict the cyclic loads on the subsea wellheads, provided that the input data are known with high degree of detail, including e.g. riser tension setting; drill pipe tension variation over time; and hydrodynamic loads. We found that scatter in the results is due to the uncertainty inherent to several of the input parameters.It is also shown that the accumulated fatigue damage from a full drilling campaign, can be established with sufficient degree of accuracy with somewhat lower requirement to the level of detail of the input, like e.g. using unidirectional waves instead of short crested waves. Directionality and spreading of the wave field can be handled by use of factors on the damage rate. A directionality factor is proposed, to enable comparison of directionality of the measured response to the predictions from global analysis.Copyright

Drilling Riser Model Tests for Software Verification

Marine drilling riser is subject to complicated environmental loads which include top motions due to Mobile Offshore Drilling Unit (MODU), wave loads and current loads. Cyclic dynamic loads will cause severe fatigue accumulation along the drilling riser system, especially at the subsea well head (WH).Statoil and BP have carried out a comprehensive model test program on drilling riser in MARINTEK’s Towing Tank in February 2015. The objective is to validate and verify software predictions of drilling riser behaviour under various environmental conditions by use of model test data.Six drilling riser configurations were tested, including different components such as Upper Flex Joint (UFJ), tensioner, marine riser, Lower Marine Riser Package (LMRP), Blow-Out Preventer (BOP), Lower Flex Joint (LFJ), buoyancy elements and seabed boundary model.The drilling riser models were tested in different load conditions:1. Forced top motion tests2. Regular wave test3. Combined regular wave and towing test4. Irregular wave test5. Combined irregular wave and towing test6. Towing test (VIV)Measurements were made of micro bending strains and accelerations along the riser in both In-Line (IL) and Cross-Flow (CF) directions. Video recordings were made both above and under water.In this paper, the test set-up and test program are presented. Comparisons of results between model test and RIFLEX simulation are presented on selected cases. Preliminary results show that the drilling riser model tests are able to capture the typical dynamic responses observed from field measurement, and the comparison between model test and RIFLEX simulation is promising.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

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