<p>This work adresses ocean current variability in relation to offshore engineering.</p><p>The offshore oil and gas activity has up until recently taken place mainly on the continental shelves around the world. During the last few years, however, the industry has moved past the continental shelf edge and down the continental slope towards increasingly deeper waters. In deep water locations, marine structures may span large spaces, marine operations may become more complicated and require longer time for completion and the effect of the surface waves is diminished. Therefore, the spatial and temporal variability of the current is expected to become more important in design and planning than before.</p><p>The flow of water in the oceans of the world takes place on a wide variety of spatial scales, from the main forms of the global ocean circulation (~km), to the microstructure (~mm) of boundary layer turbulence. Similarly, the temporal variability is also large. In one end of the scale we find variations that take place over several decades, and in the other end we find small-scale turbulence (~seconds). Different features of the flow are driven by different mechanisms. Several processes and properties (stratification¹, sloping boundary, Coriolis effect, friction, internal waves, etc.) interact on the continental slope to create a highly variable flow environment. Analysis of a set of observed data that were recorded close to the seabed on the continental slope west of Norway are presented. The data suggest that some strong and abrupt current events (changes in flow speed of ~0.4 m/s in just a couple of hours) were caused by motions of the deep pycnocline², driven by variations in the surface wind field. This conjecture is partly supported by numerical simulations of an idealised continental slope and a two-layer ocean. The data also contains an event during which the flow direction at the sea bed changed very rapidly (within a few minutes) from down-slope to up-slope flow. The change in speed during this event was as high as 0.5 m/s.</p><p>Another data set has been analyzed in order to illustrate the spatial variation in the current that can sometimes be found. It is shown that the flow in the upper layer is virtually decoupled from the flow in the lower layer at a location west of Norway. This is either caused by bottom topography, stratification or both.</p><p>High variability of the current presents new requirements to the way that the current should be modelled by the offshore engineer. For instance, it is necessary to consider which type of operation/structure that is to be carried out or installed before selecting design current conditions. Reliable methods for obtaining design current conditions for a given deep water location have yet to be developed, only a brief discussion of this topic is given herein.</p><p>It is shown, through calculations of VIV-response and simulations of typical marine operations, that the variability of the current will sometimes have a significant effect on the response/operation.</p><p>¹Vertical distribution of density. In a stratified ocean or flow, the density of the water varies in the vertical direction.</p><p>²pycnocline=density surface between water masses. The pycnocline between two water masses of different density is defined by the maximum of the density gradient.</p>

This work adresses ocean current variability in relation to offshore engineering. The offshore oil and gas activity has up until recently taken place mainly on the continental shelves around the world. During the last few years, however, the industry has moved past the continental shelf edge and down the continental slope towards increasingly deeper waters. In deep water locations, marine structures may span large spaces, marine operations may become more complicated and require longer time for completion and the effect of the surface waves is diminished. Therefore, the spatial and temporal variability of the current is expected to become more important in design and planning than before. The flow of water in the oceans of the world takes place on a wide variety of spatial scales, from the main forms of the global ocean circulation (~km), to the microstructure (~mm) of boundary layer turbulence. Similarly, the temporal variability is also large. In one end of the scale we find variations that take place over several decades, and in the other end we find small-scale turbulence (~seconds). Different features of the flow are driven by different mechanisms. Several processes and properties (stratification1, sloping boundary, Coriolis effect, friction, internal waves, etc.) interact on the continental slope to create a highly variable flow environment. Analysis of a set of observed data that were recorded close to the seabed on the continental slope west of Norway are presented. The data suggest that some strong and abrupt current events (changes in flow speed of ~0.4 m/s in just a couple of hours) were caused by motions of the deep pycnocline2, driven by variations in the surface wind field. This conjecture is partly supported by numerical simulations of an idealised continental slope and a two-layer ocean. The data also contains an event during which the flow direction at the sea bed changed very rapidly (within a few minutes) from down-slope to up-slope flow. The change in speed during this event was as high as 0.5 m/s. Another data set has been analyzed in order to illustrate the spatial variation in the current that can sometimes be found. It is shown that the flow in the upper layer is virtually decoupled from the flow in the lower layer at a location west of Norway. This is either caused by bottom topography, stratification or both. High variability of the current presents new requirements to the way that the current should be modelled by the offshore engineer. For instance, it is necessary to consider which type of operation/structure that is to be carried out or installed before selecting design current conditions. Reliable methods for obtaining design current conditions for a given deep water location have yet to be developed, only a brief discussion of this topic is given herein. It is shown, through calculations of VIV-response and simulations of typical marine operations, that the variability of the current will sometimes have a significant effect on the response/operation. 1Vertical distribution of density. In a stratified ocean or flow, the density of the water varies in the vertical direction. 2pycnocline=density surface between water masses. The pycnocline between two water masses of different density is defined by the maximum of the density gradient.

<p>The thesis addresses the hydrodynamic forces on cylinders where the angle between incoming flow and the cylinder axis, the angle of attack, is low. Measured results for a rigid cylinder with length to diameter ratio of 40 towed at both constant angle of attack and oscilllating in the transverse direction are used to discuss the applicability of suggested methods like the cross flow or the 2D+t principle. It is found that the longitudional flow do influence the transverse forces. The importance of the flow pattern initiated at the nose of the cylinder is clearly illustrated.</p><p>A combination of linear and quadratic dependence on the sine of the angle is used to model the response of a flexible cylinder with forced oscillation of the tow point. The result is compared to experimental result for a flexible cylinder with length to diameter ratio of 1100 and Reynolds numbers in and above the critical range. The cylinder is simulated in time domain with a Finite Element Method with second order elements. As an example of practical application of the model, the response of a part of a full scale streamer subject to irregular waves and a control device is investigated. In realistic sea states the response is found to be rather small, but not damped by the control device. </p>

The thesis addresses the hydrodynamic forces on cylinders where the angle between incoming flow and the cylinder axis, the angle of attack, is low. Measured results for a rigid cylinder with length to diameter ratio of 40 towed at both constant angle of attack and oscilllating in the transverse direction are used to discuss the applicability of suggested methods like the cross flow or the 2D+t principle. It is found that the longitudional flow do influence the transverse forces. The importance of the flow pattern initiated at the nose of the cylinder is clearly illustrated. A combination of linear and quadratic dependence on the sine of the angle is used to model the response of a flexible cylinder with forced oscillation of the tow point. The result is compared to experimental result for a flexible cylinder with length to diameter ratio of 1100 and Reynolds numbers in and above the critical range. The cylinder is simulated in time domain with a Finite Element Method with second order elements. As an example of practical application of the model, the response of a part of a full scale streamer subject to irregular waves and a control device is investigated. In realistic sea states the response is found to be rather small, but not damped by the control device.

<p>Collisions between FPSO and shuttle tanker in tandem offloading operation have caused a growing concern in the North Sea. Several recent contact incidents between FPSO/FSU and shuttle tanker have clearly demonstrated a high likelihood of contact between vessels in tandem offloading. The large masses involved, i.e. the high potential impact energy, make the collision risk large. Traditional ship/platform collision frequency modeling may not be applicable in the tandem offloading context. Moreover, offshore quantitative risk analyses generally focus more on technical aspects, little on human and organizational aspects. This leads to a hardware-dominated risk reduction approach, and it has been proved not to be effective to mitigate risks involved in complex marine operations in general.</p><p>Frequency modeling of collision between FPSO and shuttle tanker in offloading operation is carried out in this study. The collision frequency model is structured in two stages, i.e. the initiating stage and the recovery stage, where the former involves an uncontrolled forward movement of tanker, and the latter involves the recovery actions initiated from tanker and FPSO to avoid the collision.</p><p>In the <i>initiating </i>stage, this study focuses on tanker drive-off forward scenarios. Macroscopically, the frequency of tanker drive-off ahead during offshore loading and specifically during tandem offloading is portrayed by statistical data from an earlier study, recent SYNERGI incident data, and expert judgments made by tanker DP operators. Relatively high frequency values of tanker drive-off in tandem offloading are found. Microscopically, the tanker drive-off ahead scenario is investigated by examining 9 such events in tandem offloading based on investigation reports, interviews and discussions with individuals who directly or indirectly were involved. Findings reveal that in order to effectively reduce tanker drive-off in tandem offloading, efforts should be targeted on minimizing those failure prone situations, i.e. the excessive relative motions (termed as surging and yawing) between FPSO and tanker. A simulation-based study is carried out to quantitatively assess and effectively minimize the occurrence of excessive surging and yawing events. Horizontal motions of FPSO and tanker in tandem configuration are simulated via a state-of-the-art time-domain simulation code SIMO. Findings demonstrate that excessive surging and yawing events can be effectively minimized via measures such as minimizing FPSO surge and yaw motions in offloading, coordinating mean heading between FPSO and tanker, and using the dedicated DP software with the tandem loading function on tanker. Ultimately, these measures may provide a sound operational environment where the possibility of tanker drive-off can be minimized.</p><p>In the <i>recovery</i> stage, this study is focused on the recovery action initiated by the tanker DP operator. Possible recovery actions are identified and evaluated. Based on calibrated tanker motion simulations, the allowable time for DP operator to initiate recovery action, so that tanker can be stopped within a separation distance, e.g. 80 m to FPSO, is found to be critically short. A 3-stage information-decision-execution model is generalized to model the DP operator’s information processing stages regarding action initiation when in a drive-off scenario. Based on this human information-processing model, expert judgment by simulator trainer and questionnaire survey among shuttle tanker captains and DP officers are conducted, reasonable estimates of the time needed for action initiation are obtained. The estimates are found to be convergent to the facts in the incidents. Findings suggest that tanker DP operators in general need more time to initiate recovery action than the allowable time window, i.e. recovery failure is likely due to lack of reaction time. Two principal recommendations are proposed to reduce the recovery failure probability, i.e. to provide a longer time window for the operator to initiate recovery action, and/or to provide various kinds of assistance to the operator to reduce the recovery action initiation time.</p><p>To increase the time window, a promising measure is to substantially increase the separation distance between FPSO and tanker, e.g. from 80 m to 150 m. The feasibility of this measure is discussed from a number of perspectives. Recovery improvement gains are assessed. The key question concerning implementation is to know how much separation distance should be configured in the operation. This has to be based on considerations of both human operators’ need for reaction time, and tanker drive-off behavior. Parametric tanker drive-off motion simulations are carried out in which human action at various times are imposed. The necessary distance values to stop the tanker are then obtained, and ideally these should correspond to the separation distance values between FPSO and tanker in tandem offloading. These findings provide decision-making support to select an optimum field configuration for FPSO-tanker tandem offloading, which may inherently minimize the collision risk.</p><p>Effective reduction of reaction time can be achieved by early detection and/or quick decision-making. This is based on the operator information-processing model generalized earlier in this study. Measures to improve early detection are identified. Discussions are guided by the human signal detection theory, and supported by the operational facts of alarm and non-alarm signals in the operation. Measures to effectively reduce the operator’s time involved in diagnosis and situation awareness are also identified. They are theoretically built on the generic human decision-making theory, and specifically designed for drive-off intervention based on the facts collected via a questionnaire survey among shuttle tanker captains and DP officers. These findings illuminate a broad area in the human factor perspective, i.e. training, procedure, crew resource management, human-machine interface, and automation support, where measures to reduce operator reaction time should be targeted. These measures may directly reduce the FPSO-tanker collision risk in tandem offloading.</p>

Collisions between FPSO and shuttle tanker in tandem offloading operation have caused a growing concern in the North Sea. Several recent contact incidents between FPSO/FSU and shuttle tanker have clearly demonstrated a high likelihood of contact between vessels in tandem offloading. The large masses involved, i.e. the high potential impact energy, make the collision risk large. Traditional ship/platform collision frequency modeling may not be applicable in the tandem offloading context. Moreover, offshore quantitative risk analyses generally focus more on technical aspects, little on human and organizational aspects. This leads to a hardware-dominated risk reduction approach, and it has been proved not to be effective to mitigate risks involved in complex marine operations in general. Frequency modeling of collision between FPSO and shuttle tanker in offloading operation is carried out in this study. The collision frequency model is structured in two stages, i.e. the initiating stage and the recovery stage, where the former involves an uncontrolled forward movement of tanker, and the latter involves the recovery actions initiated from tanker and FPSO to avoid the collision. In the initiating stage, this study focuses on tanker drive-off forward scenarios. Macroscopically, the frequency of tanker drive-off ahead during offshore loading and specifically during tandem offloading is portrayed by statistical data from an earlier study, recent SYNERGI incident data, and expert judgments made by tanker DP operators. Relatively high frequency values of tanker drive-off in tandem offloading are found. Microscopically, the tanker drive-off ahead scenario is investigated by examining 9 such events in tandem offloading based on investigation reports, interviews and discussions with individuals who directly or indirectly were involved. Findings reveal that in order to effectively reduce tanker drive-off in tandem offloading, efforts should be targeted on minimizing those failure prone situations, i.e. the excessive relative motions (termed as surging and yawing) between FPSO and tanker. A simulation-based study is carried out to quantitatively assess and effectively minimize the occurrence of excessive surging and yawing events. Horizontal motions of FPSO and tanker in tandem configuration are simulated via a state-of-the-art time-domain simulation code SIMO. Findings demonstrate that excessive surging and yawing events can be effectively minimized via measures such as minimizing FPSO surge and yaw motions in offloading, coordinating mean heading between FPSO and tanker, and using the dedicated DP software with the tandem loading function on tanker. Ultimately, these measures may provide a sound operational environment where the possibility of tanker drive-off can be minimized. In the recovery stage, this study is focused on the recovery action initiated by the tanker DP operator. Possible recovery actions are identified and evaluated. Based on calibrated tanker motion simulations, the allowable time for DP operator to initiate recovery action, so that tanker can be stopped within a separation distance, e.g. 80 m to FPSO, is found to be critically short. A 3-stage information-decision-execution model is generalized to model the DP operator’s information processing stages regarding action initiation when in a drive-off scenario. Based on this human information-processing model, expert judgment by simulator trainer and questionnaire survey among shuttle tanker captains and DP officers are conducted, reasonable estimates of the time needed for action initiation are obtained. The estimates are found to be convergent to the facts in the incidents. Findings suggest that tanker DP operators in general need more time to initiate recovery action than the allowable time window, i.e. recovery failure is likely due to lack of reaction time. Two principal recommendations are proposed to reduce the recovery failure probability, i.e. to provide a longer time window for the operator to initiate recovery action, and/or to provide various kinds of assistance to the operator to reduce the recovery action initiation time. To increase the time window, a promising measure is to substantially increase the separation distance between FPSO and tanker, e.g. from 80 m to 150 m. The feasibility of this measure is discussed from a number of perspectives. Recovery improvement gains are assessed. The key question concerning implementation is to know how much separation distance should be configured in the operation. This has to be based on considerations of both human operators’ need for reaction time, and tanker drive-off behavior. Parametric tanker drive-off motion simulations are carried out in which human action at various times are imposed. The necessary distance values to stop the tanker are then obtained, and ideally these should correspond to the separation distance values between FPSO and tanker in tandem offloading. These findings provide decision-making support to select an optimum field configuration for FPSO-tanker tandem offloading, which may inherently minimize the collision risk. Effective reduction of reaction time can be achieved by early detection and/or quick decision-making. This is based on the operator information-processing model generalized earlier in this study. Measures to improve early detection are identified. Discussions are guided by the human signal detection theory, and supported by the operational facts of alarm and non-alarm signals in the operation. Measures to effectively reduce the operator’s time involved in diagnosis and situation awareness are also identified. They are theoretically built on the generic human decision-making theory, and specifically designed for drive-off intervention based on the facts collected via a questionnaire survey among shuttle tanker captains and DP officers. These findings illuminate a broad area in the human factor perspective, i.e. training, procedure, crew resource management, human-machine interface, and automation support, where measures to reduce operator reaction time should be targeted. These measures may directly reduce the FPSO-tanker collision risk in tandem offloading.

As a result of the steadily increasing activities related to marine technology in Arctic regions, Det Norske Veritas has launched an ice load monitoring project to gather knowledge of the ice conditions and prevailing ice-induced actions in the region. The intention of the following thesis is to study different aspects related to design of ice-going vessels, in particular the design scenario where a vessel impacts an ice ridge.The introductory part of the thesis gives an overview of important aspects related to sea ice, including different types of ice features and their physical and mechanical properties. The microstructure of pure ice and formation mechanisms of sea ice are briefly described, and mechanical properties such as elasticity and compressive strength are discussed. Further, a study of existing models for estimation of ice-induced loading on ships is carried out, with focus on local hull plating pressure and global loading due to ice ridge impact.A comparative study of design rules developed by Det Norske Veritas and the International Association of Classification Societies is conducted, and important differences between the two separate rules are identified. The subdivision of class notations is described, and differences in definition of design loads and corresponding requirements are presented. A general conclusion is that the rules developed by Det Norske Veritas are more specific when it comes to governing design scenarios, while rules set forth by the International Association of Classification Societies are more universal in terms of vessel type and prevailing ice conditions.Two separate finite element models based on coastguard vessel KV Svalbard are developed, including a simplified beam element model and a detailed shell element model. Quasi-static and dynamic response analyses for ice ridge impact loading are carried out, where the duration of the load pulse is varied systematically from 0.25~s to 2.0~s. The simplified finite element model is seen to give larger overall maximum response compared with the detailed model, but the difference decreases as the pulse duration is increased.It is observed that quasi-static response is overall larger than dynamic response for both finite element models within the defined pulse duration range. However, the ratio of maximum dynamic to maximum quasi-static response is seen to be positively correlated with the load pulse duration, and a close-to-linear relationship is observed.A study of different parameter variations is performed in order to investigate the importance of various pulse shapes, mass models, damping models and solution methods. Variations are only performed using the simplified beam model. It can be concluded that the shape of the load pulse is of minor importance for dynamic response when the pulse duration is short. However, the pulse shape becomes increasingly important for longer load pulses.An opposite trend is observed when varying the mass model, where a negligible difference in dynamic response is seen for longer load pulses. The difference increases somewhat for shorter load pulses, but can be considered unimportant for dynamic response within the investigated duration interval.It is further observed that the choice of damping model is of significant importance compared with other investigated parameters, and the difference in predicted response remains constant within the investigated pulse duration interval. The choice of solution method is however unimportant for analysis using the simplified beam model.In order to verify the applicability of the finite element models, full scale sea trial measurements of global motions from KV Svalbard are analysed and compared with finite element results. Difference between measured and calculated response during ice ridge impact is seen to be significant, where the calculated maximum response is close to 4 times larger than the maximum measured response. Iterative modifications of the load pulse shape are performed in order to reproduce the measured response history following ice ridge impact, and quite strong agreement is obtained between measured and calculated response.

SummaryThis Master&#146;s Thesis goal is to present fundamental physical aspects of Vortex Induced Vibrations (VIV) of marine risers, and outline methods for suppression of VIV. Main emphasis has been given to the use of strakes, and relevant theories connected to strakes&#146; influence on excitation of riser, riser response and drag is presented. Variation of outer diameter of the riser is also studied.The theory has been put to test through case studies of two Aker Solutions in-marine workover systems of 321 and 1300 m water depth. The computer program VIVANA has been used. Several analyses have been performed for various different riser configurations. For the 321 m water system, the following configurations have been studied:-Base configuration, i.e. no use of strakes-Staggered configuration, staggered bare and buoyant joints-Bottom strakes configuration, bottom section of riser covered with strakes-Middle strakes configuration, middle section of riser covered with strakes-Top strakes configuration, top section of riser covered with strakesFor the 1300 m water depth system, the following configurations have been evaluated:-Base configuration, i.e. no use of strakes-Staggered configuration, staggered bare and buoyant joints-Middle strakes configuration, middle section of riser covered with strakes-Top 50_150 strakes configuration, top 150 m of riser bare, then coverage of strakes-Top stakes configuration, top section of riser covered with strakesFor each water depth a total of 4 different current profiles have been applied. The current profiles include both measured current profiles from the relevant oil fields, as well as several other more &#147;theoretical&#148; current models.The main findings from the 321 m analyses were:-Staggered configuration gives generally lower VIV amplitudes of the dominating frequency compared to base configuration.-Staggered configuration gives generally lower maximum stress amplitudes compared to base configuration.-No clear trends when comparing fatigue life of staggered and base configuration are found, however significantly better fatigue results found for the staggered configuration in measured current.-Maximum accumulated damage is located at the WH/XMT interface.-Top and middle strakes configurations give best VIV suppression results.-Applying strakes to the top section of the riser gives significantly lower VIV amplitudes, stress amplitudes and higher fatigue lives across all current profiles.-Top strakes configuration supress VIV completely for the sheared current profile.-Very small riser deflections and corresponding low flex joint angles are found; thus no operational consequences for the 321 m water depth.&amp;#8195;The main findings from the 1300 m analyses were:-A significant increase in active response frequencies compared to the 321 m water depth, more than 30 active frequencies calculated.-No clear trends in VIV amplitudes of the dominating frequency when comparing staggered and base configuration.-Highest stress amplitudes found for the base configuration in all current profiles.-No clear trends in calculated fatigue life when comparing the staggered and base configurations. Maximum accumulated damage found in the WH/BOP interface.-Top 50_150 strakes configuration the most efficient in suppressing VIV.-Significantly lower VIV amplitudes of the dominating response frequency for the top 50_150 strakes configuration. Same result found for the maximum stress amplitudes.-Compared to the base configuration significantly better fatigue lives found for the top 50_150 strakes configuration, however for the measured current profile an increase of only 1 decade was found.-Staggered configuration gives lowest static riser deflection for all current profiles, also after drag amplification from VIV.-Percentage increase in riser deflection from VIV reduced by roughly 80% when comparing the top 50_150 base configuration to the base configuration.-LFJ angles exceeded lower limits for certain drilling and workover operations, however applying the top 50_150 strakes configuration will generally give a larger operational window compared to the base configuration.Some of the results from the 1300 m analyses revealed certain discrepancies linked to the dominating frequencies and frequencies inducing maximum stress amplitudes. These inconsistencies are probably related to the convergence limit given as input in the VIVANA module.

Deployment of a large box structure in many sea states has been investigated. Two deployment methods are compared; crane installation over the side and through moonpool installation. The structure is 12 [m] long, 6 [m] wide and 12 [m] high with a mass of 250 [t]. Normand Subsea is used as installation vessel. Both JONSWAP and Torsethaugen wave spectra are used for crane lowering while only JONSWAP is used for moonpool installation.Splash zone lowering is seen as the most critical stage of the installation because hydrodynamic forces are largest at the surface. Hydrodynamic uplift is assumed limiting for the deployment, i.e &#147;slack slings&#148;. Slings are the lower part of the lift rigging. The operation limit is that dynamic uplift should not exceed 90 % of the modules static weight. Forces in z-direction are hence most interesting. Minimum wire tension for the lowering is therefore calculated at two time instances; when the module bottom end is at mean water level and when the module is fully submerged with its top end 0.5 [m] below mean water level. These time instances are referred to as time instance 1 and 2 in the report respectively. Design significant wave heights, Hs, are taken from plots of minimum wire tension for different wave peak periods Tp and wave headings. Based on these design Hs values, which equals the operation limits, operability rosettes are plotted. It is seen that wave headings 90&amp;#8304; and 120&amp;#8304; are most critical with lowest operability for crane installation while wave heading 90&amp;#8304; is worst for moonpool deployment.The lowest design Hs for all Tp values considering wave headings 0&amp;#8304; &#177;30&amp;#8304; is used as overall operation limit for deployment when weather window statistics are computed. Time instance 1 is worst for crane deployment with resulting forecasted weather operational criterion Hs=2.8 [m]. Time instance 2 is worst for moonpool deployment with forecasted operational criterion Hs=2.5 [m] and Tp &amp;#8805; 13.0 [s]. Reference time for deployment, hence the time needed from the weather forecast is issued to the module is landed on the seabed, is 6 hours. Based on reference time and forecasted operation limits weather window statistics are estimated.Moonpool deployment annual operability is 7.24 days, hence 2.0 % of the year, and can naturally not be used. Crane deployment annual operability is 213.22 days, 58.4 % of the year. This is much better but still not very good as it is theoretically desirable to be able to install the module any day year round.

Well intervention operations can inflict large strains on a wellhead. The seabed in the North Sea is very rigid up to the point where the sea water meets the mud or sand. This is one of the reasons why wellheads operating on the Norwegian continental shelf are more exposed to fatigue damage. In search for more oil and the wish to increase the utilization rate of existing wells the oil companies drill more and more, causing more and more fatigue life consumption. The oil companies must provide sufficient documentation that the wellhead always has enough remaining fatigue life to perform a Plug and Abandon (P&amp;A) operation.Full scale measurements has been collected for a marine drilling riser connected to a moored Aker H-3 rig operating on a field with a depth of 325 m. The angles at the bottom of the riser above the lower marine riser package are used to calculate the consumed cumulative fatigue life using rainflow cycle counting and Miner-Palmgren summation.A simulation model has been developed in the computer simulation program RIFLEX, which is a state of the art simulation program developed especially for slender structures such as a riser in a marine environment. The model was built with relatively conservative assumptions. This resulted on fatigue life assessments that gave a shorter operation life than what was found using the full scale measurements. Using such simulation is often the only tool available to document fatigue life consumption since full scale measurement tools are rarely installed and used. It is vital that the simulation yield reliable and correct results and as close to the true result as possible.A series of similar simulation models were developed where we looked at the effect of taking away some of the conservatism in the original model. First we looked at the difference between a JONSWAP wave spectrum and a Torsethaugen wave spectrum. The difference lies in the assumption of that a sea state is a superposition of wind driven waves and swell waves, where the Torsethaugen use empirical data collected from the North Sea to account for the difference. A Torsethaugen is a double-peaked spectrum while the JONSWAP spectrum is a single-peaked spectrum. The difference between the two results gave little or no effect on the motion characteristics and fatigue life.Then we introduce a directional wave spectrum, meaning that waves may be short-crested and spread around a mean wave direction. This reduced the angular motion in terms of standard deviation significantly. The reduction was between 10% - 15%. It also affected the fatigue life positively. In the next model we introduced non-linear behaviour in the lower flex joint while the waves now were unidirectional. In terms of standard deviation the reduction was the same as for the model with wave spreading.In the last comparison model we used both non-linear flex joint behaviour and wave spreading. The total reduction was again significant. For some of the simulation even up to 30% compared to the original model. All the standard deviation from the full scale data has natural variances from data set to data set and most of the computer simulation fell within this margin of error.For all simulation models the model was tested with different mean heading direction of the waves. The mean heading directions were 0 deg, 30 deg, 60 deg and 90 deg relative to the rig. While the full scale measurement had little correlation between the measured response direction and the weather direction, the simulation were very consistent on this matter.Some simulations with current and support vessel offset was performed to find the effect on the standard deviation. While the presence of current damped the angular motion, the standard deviation increased with increasing support vessel offset. A discussion around the uncertainty of the true characteristics of the non-linear model explains some of the behaviour.When comparing fatigue life the calculated fatigue life consumption became closer and closer to the measured value as we removed the conservatism. However, by a closer investigation of the angle range spectra which is used in the Miner-Palmgren summation there was found differences that need more attention. While the angle range spectrum from the full scale measurement show a close to linear relation between numbers of cycles exceeding ranges the shape for the simulated models were far from linear. In terms of the shape parameter in the Weibull distribution it was found through fitting the curve that the shape model for full scale and RIFLEX simulations were around 1.05 and 1.9, respectively.It is this difference in shape that demands a closer investigation of the simulation models. The fact that the fatigue life approached the true fatigue life so closely should so far be regarded as a coincidence and not a result of good model approximations.It was also found that the full scale motion for some of the time series are low frequency dominated, i.e. high energy in oscillating components with a frequency outside the wave spectrum. Some peak periods reach periods over a minute or even two. This is an effect that is unaccounted for in the models presented in this thesis.