The initial design of a 12-MW floating offshore wind turbine (FOWT) was made by the University of Ulsan (UOU) based on the 5-MW offshore wind turbine of the National Renewable Energy Laboratory (NREL) using the law of similarity. The tower design was checked through the eigenfrequency and fatigue strength analysis according to the GL guideline of tower design conditions. The direct expansion of the 5 MW wind turbine support structure caused a resonance problem of the tower of the 12-MW UOU FOWT and the tower length was adjusted to avoid the 3P resonance. Wind turbines are required to have a design life of more than 20 years and shall be designed to endure both ultimate and fatigue loads experienced during the design life. The platform pitch motion of FOWTs due to combined wave and wind loading may result severely in both fore-aft forces and moments at the base of the tower. In this study, we used the simplified fatigue analysis, which is generally applied when considering safety margins by stress to predict the fatigue life of tower. In order to calculate the fatigue load, the Markov matrix was constructed by using the cycle counting method to determine range, average value, and cycle number of loads from peak and valley values of actual load histories simulated by FAST v8 of the tower base. The predicted fatigue life at the tower base was follow by S/N curves for welded steel structures and it was calculated by the Palmgren-Miner’s rule.

In this research, the estimation of the fatigue life of a semi-submersible floating offshore wind platform is considered. In order to accurately estimate the fatigue life of a platform, coupled aerodynamic-hydrodynamic simulations are performed to obtain dynamic stress values. The simulations are performed at a multitude of representative environmental states, or “bins,” which can mimic the conditions the structure may endure at a given site, per ABS Floating Offshore Wind Turbine Installation guidelines. To accurately represent the variety of wind and wave conditions, the number of environmental states can be of the order of 10 3 . Unlike other offshore structures, both the wind and wave conditions must be accounted for, which are generally considered independent parameters, drastically increasing the number of states. The stress timeseries from these simulations can be used to estimate the damage at a particular location on the structure by using commonly accepted methods, such as the rainflow counting algorithm. The damage due to either the winds or the waves can be estimated by using a frequency decomposition of the stress timeseries. In this paper, a similar decoupled approach is used to attempt to recover the damages induced from these coupled simulations. Although it is well-known that a coupled, aero-hydro analysis is necessary in order to accurately simulate the nonlinear rigid-body motions of the platform, it is less clear if the same statement could be made about the fatigue properties of the platform. In one approach, the fatigue damage equivalent load is calculated independently from both scatter diagrams of the waves and a rose diagram of the wind. De-coupled simulations are performed to estimate the response at an all-encompassing range of environmental conditions. A database of responses based on these environmental conditions is constructed. The likelihood of occurrence at a case-study site is used to compare the damage equivalent from the coupled simulations. The OC5 platform in the Borssele wind farm zone is used as a case-study and the damage equivalent load from the de-coupled methods are compared to those from the coupled analysis in order to assess these methodologies.

This study evaluated, by time-domain simulations, the fatigue lives of several jacket support structures for 4 MW wind turbines distributed throughout an offshore wind farm off Taiwan’s west coast. An in-house RANS-based wind farm analysis tool, WiFa3D, has been developed to determine the effects of the wind turbine wake behaviour on the flow fields through wind farm clusters. To reduce computational cost, WiFa3D employs actuator disk models to simulate the body forces imposed on the flow field by the target wind turbines, where the actuator disk is defined by the swept region of the rotor in space, and a body force distribution representing the aerodynamic characteristics of the rotor is assigned within this virtual disk. Simulations were performed for a range of environmental conditions, which were then combined with preliminary site survey metocean data to produce a long-term statistical environment. The short-term environmental loads on the wind turbine rotors were calculated by an unsteady blade element momentum (BEM) model of the target 4 MW wind turbines. The fatigue assessment of the jacket support structure was then conducted by applying the Rainflow Counting scheme on the hot spot stresses variations, as read-out from Finite Element results, and by employing appropriate SN curves. The fatigue lives of several wind turbine support structures taken at various locations in the wind farm showed significant variations with the preliminary design condition that assumed a single wind turbine without wake disturbance from other units.

The assessment of the loading sequence on fatigue crack growth of cracked details is essential when the ship hull is subjected to random wave-induced loading during the service time. In this paper, the effect of loading sequence on the crack growth life is investigated in order to find a physical engineering method to generate fatigue loading spectrum for ship fatigue assessment. The long term distribution of fatigue loading and the unique crack growth rate curve model have been employed in the analysis. The results show that the shape parameter affects the fatigue life significantly if the long-term distribution of wave-induced stress range is modeled by two-parameter Weibull distribution. Moreover, the spectral-based method provides a reasonable fatigue loading spectrum and avoid the confusion in determining the shape parameter for different empirical formulas, which are recommended by several main ship classification societies. An example of fatigue assessment for a cracked detail in a container ship is demonstrated as a reference for fatigue assessment of a ship hull based on crack growth.

Delivering full service life performance for mooring systems of Floating Production Storage & Offloading assets (FPSO) has been a frustrating challenge to operators across the industry. Remaining strength and fatigue assessment on degraded top mooring chains of the Bonga FPSO and Single Point Mooring (SPM) loading Buoy has been investigated as part of an in-house Bonga Asset Preservation Program. Both facilities are located approximately 120 km off the coast of Nigeria in the Gulf of Guinea operating in tropical waters just North of the Equator, where top chain links have been subjected to accelerated deterioration from Sun Corals and other forms of Microbiologically Induced Corrosion (MIC). These phenomena have led to overall corrosion rates being slightly above general design requirements, but more importantly to formations of large pitting on several sections of the top chain links. Remotely Operated Vehicles (ROV) assisted inspections, chain link cleaning and underwater 3D photogrammetry have allowed capturing the surface geometry of representative degraded chain links of the mooring lines to provide detailed input data for further analyses. Reverse engineering has been performed via Finite Element Analysis and fracture mechanics methodologies using the scanned geometry of selected highly exposed critical links to estimate the residual strength and fatigue life performance of the degraded links relative to their original design criteria. To evaluate the potential impact of cracks on the capacity of degraded chains relative to a reference link, crack tip Stress Intensity Factors have been computed at worst case stress-raising pits and parametric analyses using varying initial crack sizes have been performed to calculate the number of years for the cracks to propagate to critical sizes. A baseline for benchmarking the strength, fatigue and crack growth behaviour of the degraded links investigated has been provided by analysing non-degraded and uniformly corroded links after 12 years of service with projection to end of service life capacity. The paper provides a comprehensive application of numerical methods for assessing the fitness-for-service of the chains and recommendations on in-situ performance integrity management by circumventing the need to retrieve chain samples for testing.

Time domain numerical simulations with TechnipFMC’s in-house program were performed to estimate life cycle loading on a floating platform using High Performance Computing (HPC). Direct Simulation Analysis (DSA) were conducted to estimate fatigue life of mooring lines of two Spar platforms and the results were compared with fatigue life estimated from the condensed bin approach. In the Direct Simulation Analysis, 35-year hindcast wave and wind data with 3-hour interval for Gulf of Mexico was used. The wind and wave data covers from January 1979 to December 2013 for every 3-hours. The current data was not available so associated current information was approximately estimated. The time domain direct simulation results include nonlinear loads, nonlinear responses and seasonal effects such as hurricane events. The life cycle loading analysis tool developed will reduce uncertainties and increase the accuracy and reliability of the estimation of the mooring fatigue life during the service life of platform. Fatigue life of a mooring line from condensed bin analysis is highly variable with more than 20% depending on the selected wave seed number. In addition, the fatigue life from condensed bin analysis is about 20% less than the fatigue life from the Direct Simulation Analysis. This paper presents the systematic comparisons between the conventional approach and the direct time domain simulation results and also provides some major findings. In addition to the mooring line fatigue analysis, the structural fatigue analysis is also being performed and the results will be presented in the near future.

This paper investigates the influence of system-level innovations on the fatigue life of jacket substructures for offshore wind turbines. The innovations consist of active control strategies as individual pitch control and individual flap control. Further the effect of a magnetic pseudo direct-drive generator (PDD) mounted upfront the rotor is investigated. Fully-coupled aeroelastic simulations are performed for 10 MW and 20 MW wind turbines, with the selected innovations, supported by jacket structures under specific met-ocean conditions at 50 m of water depth. Fatigue limit states at the jacket’ welded joints are evaluated based on S-N curves and Miner’s rule according to DNV-RP-C203 guideline. Results show the potential of advanced control systems to reduce stresses at jacket members. Few design challenges characterize PDD models, as increased tower base torsional loads and excitation of global bending modes may be critical for jacket’s members. Altogether, this study indicates the benefit of an integrated-iterative design approach for jacket substructures, where system-level parameters are iterated along with the jacket design parameters using fully-coupled models.

In order to accurately estimate the fatigue life a floating structure, it is necessary to have a large set of discrete environmental conditions. If the damage to a structure largely stems from wave-induced forces, then the creation of a set of environmental conditions or ‘bins’ is trivial. However, when considering a floating platform supporting a wind turbine, it is necessary to consider not only the wave conditions, but also the wind conditions (and perhaps current, if possible). Thus, it is common to have greater than 5 dimensions in the timeseries (e.g., significant wave height, wave period, wave direction, wind speed, wind direction, etc). The creation of bins in two dimensions is quite easily solved by creating an arbitrary grid and taking the mean of all the observations which fall in a specific cell. In higher dimensions, a p-dimensional cell is not easily visualized and so the resulting set of bins cannot easily be graphically represented. In this paper, an iterative algorithm is developed to convert N observations, each with p-dimensions, into a set with M discrete bins, where M << N. The algorithm presented borrows heavily from the maximum dissimilarity algorithm used in a wide array of fields. The benefit of using this algorithm is that there is no ‘bias’ introduced by an initial grid from the user. That is, given a desired final number of clusters and a certain distance tolerance, a unique set of cluster exists for a given data set. Inherently, the algorithm selects a diverse array of observations, usually including extreme events or outliers, which may have undue impact on the fatigue life of a structure. Although the algorithm is computationally expensive O(N 2 M), reductions in computational cost are possible. Most importantly, the algorithm can be written in such a way that memory constraints are not an issue even for N = O(10 5 ). The clustering algorithm is described in both graphical and logical terms. A case study is presented, using publicly available data from the Netherlands Enterprise Agency. The data is visualized in two dimensions with the final number of bins equaling approximately 50, 100, 200, 500, 1000, and 2000 bins. These bins are compared with a previous algorithm from these authors. Various measures are presented to assess the fidelity of a set of bins with respect to the initial observations. Each set of bins are analyzed and it is clear the MDA-based algorithm outperforms the previous algorithm.

Metallic strips flexible pipe (MSFP) is widely regarded as an alternative for submarine pipelines. This paper presents a methodology for calculating the fatigue life of MSFP. Firstly, given a specific working condition of MSFP, the dynamic responses of MSFP are calculated through OrcaFlex. The obtained results from the global analysis are then implemented into a finite element model in ABAQUS to determine the stress-history curves of each steel strips layer. The estimated fatigue life is calculated by rainflow counting algorithms, S-N curve and Miner’s rule which are coded in MATLAB. Additional study about average stress correction is carried out, which might be useful for its marine engineering applications.

Irregular seabed bathymetry around subsea pipelines can lead to the formation of pipeline free spans. When exposed to on-bottom currents these free spans can be subject to Vortex-Induced Vibration (VIV), with consequential effects on the fatigue life of the pipeline. Traditional VIV suppression technologies such as strakes and fairings present installation challenges and durability concerns due to the significant increase in overall diameter associated with the geometric profiles of strakes and fairings. Longitudinally Grooved Suppression (LGS) technology was developed from a concept stage through to field deployment on active drilling risers (Johnstone et. al., OMAE 2017) [1]. The low profile and VIV suppression abilities of LGS present an opportunity for a more effective and operationally beneficial VIV suppression solution for pipeline free spans. Based on existing Class guidance for assessing pipeline free spans, a simplified framework for assessing free spans with LGS under a response based approach is presented. The simplified assessment implied a suppression efficiency (reduction in vibration amplitude) of up to 80%. An alternative comparative analysis using a force based approach was also performed in SHEAR7 of a bare pipeline and a LGS-wrapped pipeline. The requirements for qualification of new VIV mitigation technologies are also addressed and an example of an actual field installation of the device is presented, on an existing pipeline free span with low seabed clearance.

An optimized flexible riser design not only requests that the stress of local cross-section shouldn’t exceed the allowable strength, but also can be compliant with the floater to improve the fatigue life. It should be particularly pointed out that the flexible riser is a typical multi-scale system, which consists of the local cross-sectional scale and the global configuration scale, which differentiates each other a lot from their geometrical scales. A bi-scale response surface model is established to perform the optimized design of flexible risers by considering the parameters of local cross-sections and global configurations simultaneously. The response surface model can be an effective surrogate model to integrate the local and global responses into one loop so that the computational efficiency can be increased significantly. In the bi-scale response model, design variables of a flexible riser are extracted and defined at both the local sectional scale and global configuration scale. Sensitivity analyses of the two objectives, ultimate tension and bending strength on the design variables are then deduced to establish the bi-scale optimization framework through the response surface methodology. Finally, the optimization framework is implemented on a flexible riser with lazy-wave configuration which is considered as a case study. The properties of the optimized flexible risers are compared with those without the optimization. It is found that the ultimate load bearing capacity and fatigue life of the optimized flexible riser are improved significantly. Moreover, the feasibility and effectiveness of the bi-scale optimization strategy are verified through numerical simulations, which indicates that the bi-scale response surface optimization methodology provides a new thought and approach to explore the design potential of flexible risers.

Accurate computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy of the results relies heavily on capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. The standard industry practice to assess the fatigue life of flexibles is to use high fidelity 3D Finite Element Models (FEMs) to capture the complex kinematics and produce accurate stresses. However, direct simulation of flexible riser detailed FEMs is limited to regular wave analyses and computation of wire stress time-histories subjected to irregular waves have been computationally infeasible. This is due to the complexity of the nonlinear FEM and the long simulation time of the irregular wave environment coupled with large number of fatigue sea states. As a result, simplified approaches which do not directly simulate the local model and instead assume that wire stresses can be interpolated based on static stress versus curvature material curves within a pre-defined tension /pressure envelope have been utilized. This paper utilizes Nonlinear Dynamic Substructuring (NDS), a simulation-based approach that that extends the framework of dynamic substructuring to nonlinear problems. NDS enables the efficient nonlinear dynamic simulation of multiple pitch lengths of detailed flexible riser FEM subjected to irregular wave inputs and the computation of wire stress time-histories at any location on the local model. In this paper, a 14-inch diameter flexible riser under consideration by ExxonMobil is subjected to vessel motion and wave load in irregular wave environments and is modeled using a detailed 3D FEM and simulated via NDS. The flexible riser design features four tensile armor layers to mitigate localized lateral buckling of the wires near the touch down point. Tension and curvature time-histories of the riser near the hang-off, calculated from a conventional beam model global analysis, is used to drive a 5.1m long local model. Irregular wave wire stress time-histories extracted at the corners of the tensile armor wires are used to compute the fatigue life of the flexible. To demonstrate the inaccuracies associated with the regular wave approach, fatigue life is computed via the regular wave approach and compared against the irregular wave approach. It is shown that the NDS capability to efficiently compute irregular waves mitigates over- and under-predictions due to environment idealizations leading to a more accurate and reliable flexible riser life prediction and structural integrity assessment.

This paper mainly focuses on a theoretical methodology to calculate the fatigue life of tensile armor of flexible pipes. This approach employs the local model of flexible pipe that converts forces and moments obtained in time-domain global analyses into stresses in the spiral tendons of tensile armor layer. The stresses are then processed by rainflow counting methods, and S-N curves are adopted to evaluate the fatigue damage of tensile armors. Finally, Miner linear cumulative damage theory is used in order to calculate the accumulated fatigue damage. A case study on the fatigue life of a flexible pipe employing this methodology is presented, and the fatigue life of flexible pipe is obtained. The main points addressed in the study are the effect of mean stress and friction coefficients. The results indicate that the inner tensile armor at suspension point is the most prone to fatigue damage, in addition, mean stress correction and friction coefficients strongly influence the fatigue life of flexible pipes.

This paper investigates the effect of riser-soil interaction model selection on the assessment of steel catenary riser (SCR) fatigue life for realistic environmental conditions at a deep-water Australian North west shelf (NWS) site. Using a fatigue wave scatter diagram consisting of 100 metocean conditions at the site (combining irregular seas, swell and current), a dynamic time-domain finite element analysis is coupled with the rain-flow cycle counting algorithm in order to determine the fatigue life of SCRs due to first-order motions of the host floater. Rigid, linear elastic and nonlinear riser-soil interaction models are used in order to assess the impact of model selection on the fatigue life of example deep-water SCRs of varying diameter. It is shown that the use of a nonlinear riser-soil interaction model for a representative deep-water NWS site can give an almost two-fold increase in SCR fatigue life over a stiff linear seabed assumption, albeit at a significant computational expense. It is further shown that a recently developed method for calculating equivalent linear soil stiffness may be used in place of the computationally expensive nonlinear approach in order to estimate SCR fatigue life to a reasonable level of accuracy. A methodology for applying the equivalent linear stiffness method to irregular sea-states is proposed and general insight into the selection of the most appropriate soil-riser interaction model for the fatigue analysis of large diameter deep-water SCRs is provided.

This work presents a numerical-experimental methodology to study the fatigue behavior of dented pipes under internal pressure. A full-scale experimental program on dented pipes containing gouges were achieved. Two types of defects were studied: metal loss (plain dent) and sharp notch. Both defects acting independently reduce the fatigue life performance but their combination is highly detrimental and must be avoided . We did not find a severity threshold (e.g. dent depth or crack depth) where these defects could coexist. In addition, based on numerical analyses we proposed a new expression for stress concentration factor (SCF) in line with transversal indentation. This information was successfully integrated into a simple fatigue model where the fatigue life predictions were practically inside the window of experimental results.

Here, the performance of a steel lazy-wave riser (SLWR) was numerically investigated and compared to a conventional steel catenary riser (SCR) for an internal turret and spread-moored FPSO in extreme 100-yr return and ambient metocean conditions. As expected, the SLWR demonstrated favourable characteristics by the elimination of local dynamic buckling in the touch down zone (TDZ) via a wave configuration that decouples surface-motion, hence significantly reducing the maximum stress and fatigue damage. Non-collinear waves and current in the oblique direction relative to the vessel heading showed amplified stress concentrations in the sag and hog sections as a response to the increased compressive (heave and pitch) vessel motions. Correspondingly, in the ultimate limit state, a SLWR showed similar stresses for both turret and spread-moored FPSOs. However, the turret-mounted SLWR demonstrated a less superior fatigue life compared to its midship-mounted, spread-moored counterpart, owing to riser hangoff location differences and correspondingly varied motions experienced by the riser.

Slug flow appearance in a multiphase-carrying riser with a long tie-back distance and deeper water is inevitable, depending on the operational and environmental conditions. Several state-of-the-art technologies in mitigating the effects of internal slug flows might not be completely effective or cost-efficient. In addition to the slug excitation, the external current flows can also affect the riser structural behavior and integrity by the presence of vortex-induced vibration (VIV). This study aims to investigate and understand the behavior of slug-conveying catenary riser under uniform and random slug excitations, in combination with VIV. The steady-state slugs are considered and modelled by a series of liquid and gas phases flowing at certain rates inside the riser pipe. Each slug unit consists of a slug liquid (oil, water or their mixture) and gas pocket. In the uniform slug flow cases, all slug units have their equal slug liquid lengths. Time-domain simulations are conducted for different slug units of 20 D , 30 D , 40 D and 50 D , where D is the pipe internal diameter, and for different internal flow rates. The non-uniform slug flow case is considered by randomly generating the time-varying slug liquid and unit lengths. Multi-frequency oscillations of the catenary riser are observed, triggered by the transient slug excitations rendering the fundamental vibration mode which is sustained over the ensuing steady-state slugging period. The random slug-induced vibration (SIV) entails larger response amplitudes which are critical from the fatigue life viewpoint, especially when VIV is also accounted for. For riser SIV analysis, only in-plane response is observed; nevertheless, the interaction of riser SIV and VIV generates both in-plane and out-of-plane responses with larger 3-D dynamic responses, deformations and stresses. Such combined SIV and VIV should be specially considered during the riser analysis and design by also taking into consideration the travelling random-like or intermittent slug flows.

It is known that the fatigue strength decreases in corrosive environment and many experiments were carried out to comprehend the decrease in fatigue strength in corrosive environment. In order to comprehend the actual state, a cycle speed of fatigue test loads should correspond to a wave frequency. Therefore, an experiment in the long life region is practically difficult, then the corrosion fatigue data available for the life assessment of the structure is quite limited. In this study, the fatigue strength of the welded joints in long life service was evaluated according to the calculations of corrosion fatigue crack propagation subjected to the random loadings which followed an exponential distribution. In the crack propagation calculations, the progress of corrosion wastage from the plate surface and the resultant stress increase were considered simultaneously. In the high stress and the short life region, the decrease in fatigue strength due to the accelerated crack propagation in corrosive environment was dominant because the progress of corrosion wastage was little. On the other hand, in the low stress and the long life region, the decrease in fatigue strength became dull as longer the fatigue life because the corrosion fatigue crack propagation was suppressed by the corrosion wastage, but after that the fatigue strength showed the precipitous decrease due to the increase in stress resulted by the progress of corrosion wastage.

A key challenge in the Offshore Wind industry is assuring the life-cycle structural integrity of wind turbine foundation monopiles. This is due to harsh environmental aspects as well as the loading regime ( i.e. constant exposure to wave and wind forces introducing both fatigue and corrosion damage). Welding is a widely used joining technique for the manufacturing of offshore monopile structures. However, this is an aggressive process that introduces high levels of residual stress, which in turn may lead to reduced fatigue life, corrosion cracking resistance and accelerated degradation mechanisms. This study presents evidence that a measurement-informed strategy could be used towards developing a more reliable structural integrity assessment procedure for offshore monopile structures by taking into account the effect of residual stresses. A welded mock-up, 90 mm thick, 2600 mm wide and 800 mm long plate, was fabricated using a typical double-V welding procedure following current industrial practice. The contour method of residual stress measurement was employed to map residual stresses in the welded mock-up as well as in the CT specimens extracted from the weld region of the plate for future fatigue tests. Residual stress measurement results show that the mock-up plate contained tensile residual stresses above yield in the core of the weld, while the extracted CT specimens had lower though still significant residual stress levels. These results indicate that if the initial residual stresses are not carefully considered during fatigue or corrosion cracking tests, the results from the CT specimens alone will likely result in misleading structural life estimations.

A structured technology development process targeting to combine industry and Statoil’s experience has produced an engineering approach for wellhead fatigue analysis that is verified against measurements of load and load effects in actual subsea wells. This paper outlines Statoil’s wellhead fatigue analysis approach, which is based on the new industry standard for wellhead fatigue analyses, DNVGL-RP-E104, ref. [1]. Parts of the methodology has been presented in previous papers. The present paper provides a birds eye view, putting all the pieces together into one coherent methodology. The development and validation of an engineering approach for estimating the bending moment in the surface casing, between the wellhead housing and top of cement, will be presented in detail; this has previously been referred to as load sharing between wellhead and conductor. The wellhead fatigue analysis approach is based on a “coupled model”, which in this case means that the conductor with PY-soil springs are included in the model, compatible with industry recommendations [1], with the following main characteristics: • The lower boundary condition is modelled as a conductor in soil with a bending stiffness equivalent of the well system. • Soil and template interaction is modelled by discrete springs. • The global riser load analysis is run with long crested waves and head sea. Directionality of the waves are handled by reduction factors applied to the damage rate. Alternatively, directionality effects may be included by running multiple wave directions with short crested waves. • Fatigue capacity of the hotspots in the well system is represented by ΔM-N curves generated from detailed FE models. Typically, ΔM-N curves are established for connectors, welds between housings and casings, and for the wellhead housings. The paper includes validation against full scale measurements for a wellhead of preloaded type. In addition, it is demonstrated how the approach can be used for wellheads where the high-pressure housing may rotate inside the low-pressure housing. For this case, the validation is performed against a full 3D solid element model. The analysis approach presented is computationally effective and it will hence enable increased focus on sensitivity analyses. Analysis work is moved from time consuming local- and global analysis, to effective post-processing of data. Uncertainty in the input parameters has been found to significantly influence the fatigue estimate. Understanding these effects is considered vital for making conscious decisions on the fatigue life of a well. See e.g. [8], [10] and [20]. As pointed out already in 1985 by Valka et.al., ref. [5], and also by Milberger et. al., ref. [6], the cement level, and the relative motion of the two housings, represent large uncertainties. Macke et. al, ref. [10], showed that the additional uncertainty due to cement level and friction between housings exceeds the levels covered by the traditional fatigue safety factor of DFF = 10. A method is proposed to handle this in a consistent manner.