Massimiliano Russo

Suppressing Full-Scale Riser VIV With the VT Suppressor

The paper reports on recent full scale experimental tests with The Ventilated Trouser (VT), a novel device for VIV suppression of cylindrical structures exposed to external fluid flow. The VT suppressor is a loose fitting sleeve in the form of a light flexible net with integral bobbins in a special arrangement (Fig 1). It is omni-directional, rugged, and made from materials compatible with the offshore environment.The tests reported here, originated in an invitation from Statoil to test the VT on a slick riser section. They were undertaken at Marintek, Norway, with a 0.53m diameter riser in current velocities up to 2.3m/s, equivalent to post-Critical Reynolds Numbers of up to 1.2 × 106.The VT suppressed the maximum VIV amplitude of the slick joint by over 90%. This was consistent with the suppression performance of the VT from previous tests with model risers varying in size from 0.1m diameter to 0.3m diameter.The test results suggest the VT is a candidate suppressor fully capable of competing with conventional suppression devices.Copyright

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

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

A Simplified Methodology for Comparing Fatigue Loading on Subsea Wellheads

This paper presents a simplified way of comparing the fatigue load on different subsea wells. The simplest comparison is done by accumulating the number of days BOP has stayed connected to the wellhead. The wellhead fatigue load is however heavily dependent on the vessels used, water depth and weather while connected to the well. An equation for deriving a benchmark load factor for each operation phase for a subsea well is proposed. This benchmark load factor takes into account the water depth, metocean season of the operation, BOP height and weight, and the stiffness of the marine riser lower flex joint. This benchmark load factor will represent a standard number of days with a BOP connected, correcting for some known effects. The goal has been to define a measure of ‘BOP days’ that accounts for the water depth, operational season, and BOP particulars. A base case (one MODU, 100 m water depth, and all year operation), equating to one standard BOP day, has been chosen as the reference for all cases discussed.The validity of the benchmark load equation will be shown through a comparison with 31 different global riser analyses intended for wellhead fatigue. For each of the 31 data sets, time domain load analysis is done for all sea states in the wave scatter diagram. The different analyses covers different rigs, water depths and two operational phases (with or without subsea XT installed). To enable a large scale comparison of the bench mark factor, an approach where the fatigue load is summarized using the bending moment standard deviation on the wellhead datum is presented. This methodology is then compared to four full fatigue calculations using a typical subsea wellhead fatigue capacity. Then the simplified fatigue calculation is performed for all 31 global riser analyses. The calculated damage is then compared with the corresponding bench mark formula in each case.Finally it is shown how this benchmark load formula has been implemented into the Statoil WellSpot database as a fatigue load criticality screening tool for the different Statoil subsea wells. It is further shown how this can be used as a tool during planning of future operations, and how to prioritize wells where a detailed fatigue analysis is recommended.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

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

Drilling Riser VIV Tests With Prototype Reynolds Numbers

For deep-water riser systems, Vortex Induced Vibrations (VIV) may cause significant fatigue damage. It appears that the knowledge gap of this phenomenon is considerable and this has caused a high level of research activity over the last decades. Small scale model tests are often used to investigate VIV behaviour. However, one substantial uncertainty in applying such results is scaling effects, i.e. differences in VIV response in full scale flow and small scale flow. To (partly) overcome this obstacle, a new innovative VIV test rig was designed and built at MARINTEK to test a rigid full scale riser model. The rigid riser model is mounted vertically and can either be elastically mounted or be given a forced motion. In the present version, the cylinder can only move in the cross-flow (CF) direction and is restricted in the in-line (IL) direction.The paper reports results from a drilling riser VIV experiment where the new rest rig has been used. The overall objective of the work is to study possible VIV suppression to improve operability of retrievable riser systems with auxiliary lines by adding riser fins. These fins are normally used as devices for protection of the auxiliary lines.The test program has recently been completed and analysis is an on-going activity. However, some results can be reported at this stage and more results are planned to be published.A bare riser model was used in a Reynolds number (Rn) scaling effect study. The riser model was elastically mounted and towed over a reduced velocity range around 4 – 10 in two different Rn ranges, 75 000 – 192 000 (subcritical regime) and 347 000 – 553 000 (critical regime). The difference in the displacement amplitude to diameter ratio, A/D, is found to be significant.The elastically mounted riser was also towed with various drilling riser configurations in order to study VIV/galloping responses. One configuration included a slick joint riser model with 6 kill & choke lines; another has added riser fins too. The riser model is based on a specific drilling riser and the kill and choke lines have various diameters and have a non-symmetrical layout.The various riser configurations have also been used in forced motion tests where the towed model has been given a sinusoidal CF motion. Forces have been measured. Determination of the force coefficients is still in progress and is planned to be reported later.Scaling effects appear to be a significant uncertainty and further research on the subject is recommended.The slick joint drilling riser configuration generally increased the displacements compared to displacements of the bare riser model. The drilling riser configuration with protection fins, kill and choke lines generally reduced the displacements compared to displacements of the bare riser model. For both riser systems, tests showed that the response is sensitive to the heading of the current.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