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.

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&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.

Non-linear analysis of wind load subjected flare tower in Abaqus

Present regulations for offshore structures on the Norwegian continental shelf havea requirement for Accidental Limit State redundancy and Fatigue Limit State redundancyin a damaged state. However, the requirement is far more defined forthe Accidental Limit State than for the Fatigue Limit State. An increased understandingof factors governing fatigue redundancy would create a basis to form acomprehensive definition.In literature, the term redundancy is defined in several ways. The different methodscan be divided into two major categories, namely deterministic and probabilisticapproaches. In general, redundancy may be defined as the absence of memberswhose failure would lead to global collapse. Within both the deterministic and theprobabilistic framework, several redundancy factors are usually defined, and thereare resemblance between some of them. In probabilistic methods, the reliabilitymethod is commonly applied through the First Order Reliability Method.Fatigue damage is a primarily concern regarding the integrity for offshore structures.A near constant subjection to cyclic loadings from wind, current and wavesinitiates a cumulative damage process which leads to a certain fatigue life for themembers in the structure. The lifetime may be calculated using either a fracturemechanics approach or a SN-curve approach. There exists several approaches tocalculate the stress levels to be used in the fatigue analyses, and the choice ofmethod is mainly based on whether or not the structure under consideration isdynamically behaving or may be regarded as quasi-static. Also, there may be nonlinearitiesthat needs to be accounted for and naturally this will affect the choiceof analysis method. However, large uncertainties are associated with fatigue calculationsregardless of analysis approach. Thus, a probabilistic framework is highlyrelevant in order to estimate the risk of failure due to fatigue.A study on how impaired integrity affects the fatigue life has been performed fortwo jackets; one highly redundant four-legged jacket and one less redundant threeleggedjacket. The main goal has been to investigate the fatigue redundancy of thestructures, in order to link up the risk of accelerated fatigue due to damage withthe risk of failure due to extreme environmental actions, which may eventually leadto structural collapse. A stochastic fatigue analysis approach was chosen, and theanalyses was in agreement with the standards governing the Norwegian continentalshelf at the time of this thesis. Pushover analyses has been performed on the jacketsto give an insight in their redundancy, and a calculation of changes in the naturalperiods under impaired integrity has also been done.The four-legged jacket was proven to be highly redundant, and had small changesin the natural period under impaired integrity. The three-legged jacket on the otherhand, had some damage cases with a rather large increase. Also, the redundancyfactor R4 was significantly lower for the three-legged jacket, thus confirming it tobe less redundant than the four-legged jacket. A large change in the natural periodwill alter the dynamic response, thus the fatigue life is vastly connected to changesin global stiffness.Trying to explain the changes in fatigue life for the two jackets under impairedintegrity without using a deterministic approach, i.e. calculate the fatigue life forthe specific damage case, was proven to be very difficult. There seems to be noeasy way to isolate the severity of the fatigue life reduction since large changesare occurring throughout almost the entire structure for several damage cases, aswell as large spread in the values them self. However, there was also found sometrends in the results. One of them, was that the closer a member is to the damagedelement, the larger is the expected reduction in fatigue life. Another trend, is thata large fatigue accelerator factor is expected in almost every damage case, thus onemay expect large changes for most of the damage scenarios.Another vastly occurring phenomenon were the location of the damaged memberswho gave the lowest fatigue life in the structures. For the four-legged jacket, thisinvolved damage in the caisson supports. The three-legged jacket, however, hadthe lowest fatigue lives occurring for damage cases in the top frame where there isa lack of deterministic redundancy.There seems to be no fatigue redundancy for the jackets, as there are large fatigueaccelerator factors occurring. There is also the very low calculated fatigue life inthe most extreme cases. However, there has been found a slight correlation betweena large reduction in fatigue life and a large initial fatigue life. What should also betaken into account though, is both the risk related to the fatigue lives found, andthe accuracy of the values due to the linearised analysis.

for advanced riser analysis. Deep water oil recovery have forced a change in thesystems used to tension risers, the traditional wire-pulley systems are replacedwith direct acting hydraulic-pneumatic systems. In order to design these systemsto obtain the desired operability, analysis tools including the heave compensationsystem is necessary. As a result a pipe in pipe RIFLEX model have been developed.In this masters thesis the pipe in pipe model will be used to investigate drive-offand weak link fracture. Both subjects investigated exposes the riser to large forces,and will push the model to its limits. Another part of the thesis is focused aroundbatch execution of analyses with the use of MATLAB.The focus of the drive-off study lies mainly in investigating the dynamic behaviourof a deep water riser (3000[m] water depth) compared to a shallow water riser(300[m] water depth). Results are presented for upstream and downstream driveoff scenarios. The maximum offset is 100[m] obtained during a 50[s] period. Driveoffscenarios include cases where the vessel remains at the 100[m] offset, and caseswhere the vessel returns to its original position.Drive-off simulations revealed large differences in dynamic behaviour of the deepwater versus the shallow water system. When observing the lower riser anglefor shallow water simulations, the angle were closely related to the vessel offsetposition. Deep water simulations showed a delay of almost one minute before thelower angle responded with a rotation in the vessels movement direction. Duringthe first minute an initial effect that caused the riser to rotate away from the vesselposition was observed. Current had a limiting effect on the lower angle whendriving upstream, and increased the angle for downstream drive-off. By includinga return motion the riser angle increased more rapidly to large values. Variation inreturn motion had little effect on the maximum amplitude. The results show thatit might be difficult to take advantage of the dynamic delay in the riser response.Bear in mind that offsets of only 100[m] was investigated, the picture might changefor simulations including larger vessel offset.Weak link fracture is of concern since it will release large amounts of stored energythat is potentially harmful for personnel and equipment. Establishing analysismethods for weak link fracture can help to better understand the dynamics ofthe problem. In the present work a suitable analysis model was selected and aparametric study on the effect of drag on response was performed.A weak link fracture was simulated with 460 tonnes over pull. Weak link fracturesis a highly complex problem since the high pressure content of the riser is releasedinto the water, causing a rocket effect. The presented results are only accountingfor potential energy stored as strain in the riser and heave compensation system.A parametric study of the tangential drag versus the maximum vertical amplitudeof the riser is presented. The results are meant to be used as a starting point inan investigation of measures to limit the weak link fracture response.The weak link simulations showed that large accelerations are involved during theweak link fracture, and therefore added mass and mass can be of importance tothe response. Due to the rapid movement after fracture, it was found that specialcare needs to be taken when selecting the time incrementation for the simulations.

The importance of offshore renewable energy from wind is expected to increase in the future. Most offshore wind turbines are currently installed in shallow water up to 50 meter water depth on bottom mounted substructures. To harvest more wind energy at deeper waters, offshore floating support structures are needed. Semi-submersible floating wind turbine is one of the proposed floating concepts. Under simultaneous wind and wave loads, fatigue might be an important design consideration. Study of fatigue for such structures is thought to contribute to a better understanding of offshore wind turbines.A local part of a semi-submersible wind turbine was studied. The column-brace connection, or joint, connected a wind turbine tower to a triangular semi-submersible floater. Design, stress concentration factors and fatigue damage of the part were the main topics. To calculate stress concentration factors and fatigue damage, dynamic response analyses and finite element modelling were performed. Only the fatigue limit state was considered.Three different column-brace connection designs were analysed. For the initial design, the stress concentration factors generally were way too large - especially for out-of-plane action. For the third design a horizontal bulkhead at the brace centreline was added. This modification decreased the stress concentrations by a maximum of over 90% for out-of-plane action. The modification was only carried out for brace 1.A long-term fatigue approximation with distribution of mean wind speed in the northern North Sea was considered, while the expected significant wave height and spectral peak period for a given mean wind speed were used, to reduce the simulation effort. The critical fatigue damage was observed for brace 2, with a life time of less than a year. For brace 1 the lowest life time was several hundred years, meaning a conservative design. By reducing and optimizing the brace thickness, one could reduce such conservatism. The critical hot-spot-stresses were found at the crown toe and heel for both brace 1 and brace 2. All fatigue calculations included a design fatigue factor of 3.The modification of brace 1 with horizontal bulkheads as additional stiffening reduced the stress concentrations significantly, and increased the fatigue life considerably. Brace 2 still needs to be modified to decrease the stress concentrations, and thus increase the fatigue life.

When a ship-owner orders a vessel from a shipyard, a contract is written to confirm and guarantee the agreement for both parts. An important requirement of the contract is the vessel's speed at a given engine power, RPM and draught, in "ideal" conditions (i.e. calm, infinitely deep and current free water, with smooth hull and propeller surfaces at with no wind and zero drift and rudder angle). The speed capacity of the recently built ship is measured carrying out a speed trial. It is rarely possible to perform the trial under ideal, contractual conditions, and the speed will normally be reduced by environmental factors. Whenever the test is carried out in conditions deviating from those contractually specified, the speed must be corrected for, to best coincide with the contractual stipulations. These corrections can be of significant magnitude and are of great economic importance. Penalties of considerable size are given to shipyards that fail to deliver in accordance with the contract. There are various standards published providing guidelines regarding the execution of speed trials, the measurements that are to be performed during the trials and corrections for environmental factors that are to be made in retrospect. ISO (2002), Perdon (2002), Bose (2005) and B. Henk (2006) were chosen for evaluation and comparison in this thesis. The recommendations of the standards are occasionally disagreeing.The main resistance contribution is claimed to be wind and wave (Bose (2005) and B. Henk (2006)). B. Henk (2006)) states; "these corrections (small displacement deviations, shallow water, and salinity deviations) are relatively small compared to wind and wave directions". Reinertsen (2011) suspects that the added wave resistances calculated by the Hyundai shipyard are too large. This assumption is based on Haugan (2011)'s (employee of KGJS) mean wave load calculations that generally gave results 30 % lower than those found by Hyundai. An unrealistically large correction factor for wave resistance is most definitely advantageous for the shipyard. This will give a higher calculated contractual speed, and the shipyard is consequently more likely to meet the contractual requirements. The Hyundai shipyard's correction procedures were evaluated based on the relevant standards. The shipyard neglects all resistance components, but the added resistance due to wind and waves (they also correct for large discrepancies between the trim/draught obtained at speed trial and that contractually stipulated. This is however not relevant for tankers, as these generally are capable of achieving design draught at the sea trial). This is consistent with the recommendations of Perdon (2002) and B. Henk (2006). The shipyard does not have the speed trials conducted in head - or following waves, nor head - or following wind. B. Henk (2006) and Perdon (2002) underline the importance of executing the speed trials in head - or following waves. Perdon (2002) argues; "in the case when the waves do not come from the bow or the stern the correction methods are not sufficiently reliable and the effects of steering and drift on the ship's performance may be underestimated". ISO (2002) recommends performing the trials in head and following wind (note that there usually is a correlation between true wind - and wave direction).The Hyundai shipyard assumes that the wave direction with respect to the ship's centerline equals the relative wind angle. This conflicts with the recommendations of the standards. They advise to obtain the wave direction by visual observations or instruments such as buoys or sea wave analysis radars. Furthermore, Hyundai's assumption is highly illogical from a scientific standpoint. In this thesis, the added wave resistance (due to diffraction) was computed by a handful of methods proposed in the literature. The computed values obtained in this report were all substantially larger than those found by Hyundai. This denies Reinertsen (2011)'s suspicion of Hyundai's correction factors for wave resistance being unrealistically high. B. Henk (2006) emphasizes the importance of accounting for the location of the anemometer in the computations of added resistance due to wind. This is not done by the shipyard. B. Henk (2006) proposes a formula for correction of improper placements of the anemometer. In this thesis, the added wind resistance was calculated, including this correction. The added resistance found was 28 % smaller than the value obtained by Hyundai. This is relevant as the wind tends to be a key resistance contribution.Finally, the Energy Efficiency Design Index (EEDI) has been described. The EEDI estimates a ship's CO2 emission per ton-mile of goods transported; put differently, the vessel's impact on the environment in relation to its benefit for society. The EEDI is to be implemented for all new ships, 1st of January 2013. The value of this index will be determined based on results from speed trials.

The number of collisions between ships and icebergs are increasing in correlation with the increase of the number of ships sailing in Arctic areas. Due to the exploitation of gas fields in Arctic areas, the LNG shipping in these areas is expected to increase. The consequences of an iceberg impact can be critical for ship structure, the crew and the cargo, and it is important to be able to predict the result of such a scenario.Different relevant impact scenarios have been discussed, and it has been concluded that the most critical scenario for a spherical type LNG carrier is an impact at the bow shoulder where the distance between the outer hull and the cargo tanks is at its shortest. Based on this study, a bow shoulder impact has been chosen as the scenario investigated in the thesis.To investigate the influence of the surrounding water, an ALE-model representing this was therefore supposed to be modelled in LS-DYNA. After several unsuccessful tests this work was postponed to further work, but a detailed description of the ALE-modelling and the different options tested has been carried out.A detailed FE model of the bow shoulder section of an ice strengthened spherical type LNG carrier has been carried out. The mesh in this section has been modelled sufficiently fine to capture the governing deformation mechanisms and the total number of elements in the model are therefore large. This will lead to increased computation time, but this is acceptable based on the accuracy of the results obtained. In addition to the local bow shoulder model, a FE model of a global 125 000 dwt. LNG tanker has been established. The outer shell and coarse geometry has been provided by Ph.D. candidate Martin Storheim, and the detailed bow shoulder model has been included in this global model. The global model is meshed with large rectangular four-node elements, and transitions to the detailed model have been created. Properties have been added to the global model in terms of getting a model which represents the real ship to the best extent possible.The steel material model used is developed by Hagbart Alsos and includes a RTCL fracture criterion. The ice material model is developed by Zhenhui Liu and is modelled to represent a physical approach to the crushing of the ice. The aluminium material is based on studies performed by Moss Maritime and simple power law calculations.The added mass and weight of the LNG cargo has been included as increased density in the outer hull below the water line and in the tank shells respectively. The aim of the simulations has been to investigate similarities and differences in different collision approaches. Three different approaches have therefore been simulated for two different impact angles. For the local model, two simplified approaches have been simulated:•A prescribed displacement has been applied to the iceberg. This is the most common approach in collision simulations due to lower requirements to computer performance.•An initial velocity has been given to the iceberg. This approach requires more computer resources, but will probably yield less conservative results.Pinned boundary conditions have been applied to the bow shoulder model for all the local analyses. In addition to the local analyses, a full global analysis has been performed for the two different impact angles. In these simulations no boundary conditions have been applied to any of the colliding bodies, and initial velocity is given to the ship structure. The mass of the iceberg is approximately 2500 tons and for the global analyses no velocity of the iceberg has been assumed.The results obtained from these simulations have shown that both of the local simulations tends to over-estimate the indentations. The prescribed displacement analyses yields over-conservative results for long duration simulations, but at large impact angles the results obtained are reasonable for longer simulated time than for small impact angles. The initial velocity method yields more reasonable results, and this approach is therefore better to apply when the damage from a collision scenario is estimated by simplified methods.To investigate the effect of the iceberg size, two simulations including an iceberg of approximately 19 000 tons have been carried out. These simulations have shown that the damage in such collision is significantly larger than for the small iceberg. It has also been seen that there is less difference between the simplified approach and the full analysis for the large iceberg than for the small iceberg approaches. This is remarkable, since the boundary conditions have been assumed to give larger contribution to the result for the large iceberg.Unreasonable large accelerations have been found in collision simulations described in different papers. To investigate this in the simulations performed in this thesis, the accelerations and motions have been observed in different spots which is not directly impacted by the collision. No extreme values of either accelerations or motions have been found, and it is concluded that this is not a problem for the collision scenario simulated in this thesis.

The calculation of ice-induced loads on ships is still mainly based on empirical models. In order to gain a better physical understanding of the loading on ice-going vessels, Det Norske Veritas launched an <i>ice load monitoring<i> project involving full scale trials with the coastguard vessel KV Svalbard during the winters 2006, 2007 and 2011. The results from the full scale measurements conducted with KV Svalbard has been topic of several earlier master's thesis at NTNU,The master thesis consists of four parts. The first part is a literature review of the mechanical and physical properties of sea ice.The second part is a review of the rule sets developed by DNV and the IACS regarding vessels operating in ice infested waters. Both design principles and numerical values have been evaluated. The main difference between the designs principles used, is that IACS base their rules on a plastic method of approach, while DNV uses an elastic method. Despite the difference in the design principles, when comparing their numerical values turned out to be quite similar. The DNV rules are in general most conservative for the smaller vessels and the IACS rules the most conservative for large vessels. The third part consists of a finite element study of a part of the bow on KV Svalbard. A systematic load scheme is used, consisting of 102 load cases. For each of the stress factors there where made graphs that showed the stress at the sensor location when moving the patch load. The sensor mounted on the frame were able to measure load that was within the frame loading area and sensor mounted on the stringer could measure stress for all of the load cases in the horizontal directions. One of the explanations for this is that the stringer transfers stress from the load patch area that could be measured by the sensors.The last part consists of a comparison between measurements from the full scale trials and the results from the 102 load cases. This comparison is done through a weighted summation method where 5 different load cases are combined to represent the measured result, and a load factor is calculated for each load case for its contribution of the measured results. The stress component used in this comparison is the shear stress tau xy The load cases were tested against the 11 measurements from the full scale trials. There were in total 11 load cases that gave positive factors for all of the 11 measurements at the same time. A figure was made to show which load cases were likely to contribute in the solution of the load cases. Load cases inside the frame loading area have the largest load factors for the solution of measurements.This load decision scheme is very sensitive to the selection of load cases and boundary conditions. A change of the boundary conditions for the model was tried out for 7 load cases, and with changed boundary conditions, only 5 gave positive load factors.The results of this thesis shows that is possible to find many solution to the measured result by combining many load case, but is it not possible to decide <i>the<i> solution.

The exible pipelines has been widely used by the oil and gas industry the lastdecades and this trend is expected to continue as the operating depths are increasedfurther. For deep water applications the radial and lateral buckling modes can becritical. Many studies has been carried out using nite element models to studythese failure modes.In this thesis a nite element model is created with the aim to recreate resultsobtained in a recently published article by Vaz and Rizzo. The model is built upusing pipe, beam, contact and spring elements to represent the complex behaviourof the cross section. The loading is carried out by rst applying the dry mass, thenthe external pressure and nally end compression.When comparing buckling loads generated in this thesis by the ones in thearticle by Vaz and Rizzo the observation made is that the buckling loads from thisthesis are signicantly higher. When comparing only the inclination of the endshortening versus buckling load curves it was seen that the curves from article hada only slightly larger inclination than the results from the analyses in this thesis.This indicates that there is a small dierence in the stiness used. By modifyingthe stiness it should be possible to get the same inclination of the curve.Analyses were also carried out on how the slip distance aects the bucklingloads. By increasing the slip distance by 50% and 100% it was observed thatthe buckling loads were reduced drastically. In the article by Vaz and Rizzo noinformation is given on the slip distance. By tuning the slip distance and stinessof the springs it should therefore by possible to obtain the exact same results as inthe article. This clearly illustrates the importance of stating all assumptions andinput parameters when describing models used for analysis.