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  1. 1. Guidelines for Offshore Structural Reliability Page No. 1-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072 LIST OF CONTENTSSection Title Page1.0 INTRODUCTION 31.1 Objective 31.2 Jack-ups in General 31.3 Modes of Operation 31.4 Important Structural Design Parameters 41.5 Arrangement of Report 62.0 RESPONSE 72.1 General 72.2 Jack-up Response in the Floating Mode 72.3 Jack-up Response in the Elevated Mode of Operation 102.3.1 Time Domain Analysis 112.3.2 Methods of Evaluating Response 122.3.3 Static Load Components 142.3.4 Sea Loadings 142.3.5 Wind Loadings 152.3.6 Foundations 163.0 UNCERTAINTY MODELLING 193.1 General 193.2 Loading Uncertainty Modelling 193.2.1 Aleatory Uncertainty 193.2.2 Epistemic Uncertainty 203.3 Response Uncertainty Modelling 213.3.1 Analysis Uncertainty 213.3.2 Damping 213.3.3 Foundation 223.4 Resistance Uncertainty Modelling 244.0 LIMIT STATES 254.1 General 254.1.1 Limit States Appropriate to Jack-up Structures 254.2 The Ultimate Limit State 274.2.1 Leg Strength 274.2.2 Foundation Bearing Failure 304.2.3 Holding System 304.2.4 Global Deflections 324.2.5 Global Leg Buckling 324.2.6 Overturning Stability 324.3 Literature Study 335.0 SUMMARY OF APPLICATION EXAMPLES 345.1 General 345.2 Overview of Analytical Procedure 345.3 Structural Reliability Example 365.4 Foundation Reliability Example 38
  2. 2. Guidelines for Offshore Structural Reliability Page No. 2-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072Section Title Page6.0 RECOMMENDATIONS FOR FURTHER WORK 416.1 General 416.2 Elevated Condition 416.3 Floating / Installation Phase Conditions 427.0 REFERENCES 44
  3. 3. Guidelines for Offshore Structural Reliability Page No. 3-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00721.0 INTRODUCTION1.1 ObjectiveThe objective of this report is to document offshore structural reliability guidelinesappropriate to self-elevating unit structures (hereafter referred to as ‘jack-ups’). With thisintention the following items are addressed ;- characteristic responses- modes of failure and related reliability analysis characteristics and parameters- typical examples of reliability analysis.The guidelines are intended for application of Level III structural reliability where the jointprobability distribution of uncertain parameters is used to compute a probability of failure.1.2 Jack-ups in GeneralThe term ‘Jack-up’ covers a large variety of offshore structures from small liftboat structures,Stewart (1991), to large deepwater designs, e.g. Bærheim (1993). The purpose of the jack-updesign is to provide a mobile, self-installing, stable working platform at an offshore (or off-land) location. The jack-up platform itself may be designed to serve any function such as, forexample ; tender assist, accommodation, drilling or production.Thus, the term jack-up may represent a structure that has a mass of a few hundred tonnes andis capable of elevating not more than a few metres above the still water surface, to a structurethat has a mass of over 20,000 tonnes and is capable of operating in water depths in excess of100 metres.· It is evident, for the above stated reasons, that statistics representing jack-up structures should be treated with a good deal of suspicion as they may not be representative for the type of structure required to be considered.· These guidelines are intended to deal primarily with conventional design, larger size jack-ups, namely those intended to operate in waterdepths in excess of, say, 50 metres. A typical arrangement of such a unit is shown in Figure 1.1 below, Bærheim (1993).1.3 Modes of OperationA jack-up generally arrives on location in the self-floating mode. The transportation of thejack-up to the site may, however, have been undertaken as a wet, or dry (piggy-back) tow, or,may have been undertaken by the use of self-propulsion. Once on location installation willtake place, which will typically involve elevating the hull structure to a predetermined heightabove the water surface, preloading, and then elevating to an operational height.Characteristically the jack-up will then remain on location for a period of 2-4 months, beforejacking down, raising the legs to the transit mode condition, and transferring to the nextlocation.· This short-term contracting of jack-up units has historically resulted in that, within its life cycle, the jack-up rarely operates to its maximum design environmental criteria.· There is a current tendency to design jack-up units for extended period operation at specific sites, Bærheim (1993), Scot Kobus (1989), e.g. as work-over or production units.
  4. 4. Guidelines for Offshore Structural Reliability Page No. 4-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072 Such units may been designed to operate in extreme environmental conditions, at relatively large waterdepths for a period in excess of 20 years.Figure 1.1 : Arrangement of a Typical Harsh Environment Jack-up1.4 Important Structural Design ParametersJack-up designs varying from being monotower structures (single leg designs) to multiple legdesigns, e.g. up to six legs, although units with sixteen legs are not unknown, Boswell(1986). The supporting leg structures may be a framework design, or, may be plate profiledesign.· The conventional jack-up design has three vertical legs, each leg normally being constructed of a triangular or square framework.Jack-up basic design involves numerous choices and variables. Typically the most importantvariables may be listed as stated below.Support FootingThe legs of a jack-up are connected to structure necessary to transfer the loadings from theleg to the seafloor. This structure normally has the intended purpose to provide vertical
  5. 5. Guidelines for Offshore Structural Reliability Page No. 5-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072support and moment restraint at the base of the legs. The structural arrangement of suchfooting may take the following listed forms;-gravity based (steel or concrete),-piled-continuous foundation support, e.g. mat foundations-individual leg footings, e.g. spudcans (with or without skirts).LegsThe legs of a jack-up unit are normally vertical, however, slant leg designs also exist. Designvariables for jack-up legs may involve the following listed considerations ;-number of legs-global orientation and positioning of the legs-frame structure or plate structure-cross section shape and properties-number of chords per leg-configuration of bracings-cross-sectional shape of chords-unopposed, or opposed pinion racks-type of nodes (e.g. welded or non-welded (e.g. forged) nodes)-choice of grade of material, i.e. utilisation of extra high strength steelMethod of transferring loading from (and to) the deckbox to the legsThe method of transferring the loadings from (and to) the deckbox to the legs is critical todesign of the jack-up. Typical design are ;-utilisation and design of guides (e.g. with respect to ; number, positioning, flexibility, supporting length and plane(s), gaps, etc.)-utilisation of braking system in gearing units-support of braking units (e.g. fixed or floating systems)-utilisation of chocking systems-utilisation of holding and jacking pins and the support afforded by such.DeckboxThe deckbox is normally designed from stiffened panel elements. The shape of the deckstructure may vary considerably from being triangular in basic format to rectangular and evenoctagonal. The corners of the deckbox may be square or they may be rounded. Units intendedfor drilling are normally provided with a cantilever at the aft end of the deckbox, however,even this solution is not without exception and units with drilling derricks positioned in themiddle of the deckbox structure are not unknown.There are a large number of solutions available to the designer of a jack-up unit and, althoughseries units have been built, there exist today an extremely large number of unique jack-updesigns.
  6. 6. Guidelines for Offshore Structural Reliability Page No. 6-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00721.5 Arrangement of ReportResponse of jack-up structures is described in Section 2, together with relevant methods forcomputation of the resulting load effects. Model uncertainties associated with thecomputation of these load effects are discussed in Section 3. Important limit states togetherwith stochastic modelling of failure modes are described in Section 4. Section 5 provides asummary of two example reliability analyses undertaken for the ultimate limit state, DNV(1996b). Recommendations for further work are given in Section 6.Note :This report should be read in conjunction with the following listed documentation ;- “Guideline for Offshore Structural Reliability Analysis -General”, DNV Technical Report no.95-2018, DNV (1996a)- “Guideline for Offshore Structural Reliability Analysis- Examples for Jack-ups”, DNV Technical Report no.95-0072, DNV (1996b)Companion application guidelines are also documented covering for jacket and TLPstructures.
  7. 7. Guidelines for Offshore Structural Reliability Page No. 7-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00722.0 RESPONSE2.1 GeneralJack-up units are normally designed to function in several different operational modes. Thesemodes may be characterised as follows ;-transit-installation-retrieval-operational (including survival) condition.Response of a jack-up in the floating mode of operation is, obviously, far different from thatof the jack-up in the as-installed, elevated condition. Both of these modes are critical to thesafe operation of a jack-up unit as each mode of operation may impose its own limitingdesign criteria on certain parts of the structure.To provide relevant guidance with respect to the stochastic properties and probabilisticanalytical procedures for both of these modes of operation, is considered to be too large anundertaking to be handled by this example guidance note.· This section is therefore mainly concerned with jack-ups in the elevated mode of operation whilst it deals only in general terms with jack-ups in the floating mode. 2.2 Jack-up Response in the Floating ModeA jack-up unit may transfer from one location to another by a number of methods. For ‘field’moves a jack-up would, normally, transfer in the self-floating mode utilising either its ownpropulsion system, or, be ‘wet’ towed to the new location. For ‘ocean’ tows, on the otherhand, it is common practice to transfer by means of a dry-tow.Three major sources of accident have been identified in respect to a jack-up in the transitcondition, Standing and Rowe (1993), namely those due to;-1- Wave damage to the unit structure leading to penetration of watertight boundaries.-2- Damage to the structure as a result of shifting cargo (usually caused by direct wave impact, excessive motions and/or inadequate seafastenings).-3- Structural damage in the vicinity of the leg support structures.In the jack-up installation phase there are normally two main areas of concern, these being ;-1- Impact loadings upon contact with the seabed.-2- Foundation failure (i.e. punch-through) during preloading.Impact loadings occur when the jack-up unit is operating in the floating mode, whilstfoundation failure is a condition occurring when the jack-up is normally elevated above thestill water surface.The retrieval phase of a jack-up has not traditionally been considered as providingdimensioning load conditions. However, when a leg is held fast at the seabed, e.g. due tolarge penetrations, there may be large loadings imposed upon the jack-up structure. Suchloadings may result from the action of waves, current, wind, deballasting and jacking uploadings.Few model tests, or full-scale measurements, have been undertaken for jack-ups in thefloating mode. Indeed, recent record searches and enquiries with model basins to establish
  8. 8. Guidelines for Offshore Structural Reliability Page No. 8-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072relevant model test data, Standing and Rowe (1993), have only been able to identify sixrelevant model tests in total, with published papers on only two of these cases, Fernandes(1985, 1986). These experiments include free decay tests to provide estimates of dampingand natural periods, measurements in heave, roll and pitch motions in regular and irregularwaves at zero speed, and measurements of resistance, heave, roll and pitch in regular andirregular waves at 6 knots tow speed. A number of the tests were repeated with the legs raisedor lowered various distances. Some full scale results were also published.Comparisons with linear wave theory, based upon potential flow assumptions, predict rolland pitch responses in regular wave sea states very well at frequencies away from resonance,but may tend to overpredict the responses at the natural period (dependent upon dampingassumptions). The results from the published jack-up model test data seem to be consistentwith findings from ships and barges, i.e. that roll response at resonance is overestimatedunless due account is taken of the increased damping resulting from viscous effects.Generally, levels of measured and predicted heave motions in regular waves agreedreasonably well although there may be marked differences in the shapes of the curves.Measurements in regular waves at 6 knots showed a considerable increase in the pitchdamping, compared with similar results at zero speed, with reduced response at the naturalperiod. Heave response was similar to that at zero speed.· Conventional wave diffraction theory will, in general, predict motion responses of a jack- up unit with a reasonable degree of accuracy. If non-linear loading effects e.g. water on deck (‘green seas’), slamming, damping (especially at and around resonance periods), non-zero transit speed etc. are significant, then it is necessary to utilise time-domain simulation and/or model test data.· The use of strip theory or Morison formulation to compute the total sea loadings on a jack-up in transit will normally be inappropriate.· In connection with the prediction of motion responses, notwithstanding account taken of relevant non-linear loading effects, it seems reasonable to refer to ship or barge related reliability data (e.g. Frieze (1991), Lotsberg (1991), Wang and Moan (1993)).· When evaluating leg strength at critical connections, transfer functions for element forces and moments (or stresses) may be calculated directly from the rig’s motions analysis. A model similar to that shown in Figure 2.1 may, typically, be utilised for such purpose.· Generally, the following loads will be necessary to consider in respect to any ultimate strength analysis of a jack-up in the transit condition ; -static load components -inertia load components (as a result of motion) -wind load components.· If any significant structural non-linearities are present in the system then such non- linearities should be accounted for in the model. One such non-linearity that may be significant is the modelling of any gaps between jackhouse guides and chords.· Reliability analysis of seafastening arrangements is documented, DNV (1992). The generalities of this documented example and the procedure utilised may also be applied to seafastenings for a jack-up unit under transit. If direct wave impact on the item held by the seafastening is a possible designing load, then such loading and associated load uncertainty should additionally be included within the analysis.
  9. 9. Guidelines for Offshore Structural Reliability Page No. 9-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072Figure 2.1 : Typical Hydrodynamic/Structural Model of a Jack-up in the Transit Condition.
  10. 10. Guidelines for Offshore Structural Reliability Page No. 10-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00722.3 Jack-up Response in the Elevated Mode of OperationResponse of jack-up structures in the elevated condition has previously been extensivelystudied, Ahilan (1993), with relevant analytical methodology being described in detail in theJack-up Recommended Practice, SNAME (1993).The response of jack-up structures, when subjected to random sea excitation, is found to benon-Guassian in nature. Due to the non-linearities in the structural system the extremeresponses are generally found to be larger than the extremes of a corresponding Gaussianprocess, Karunakaran (1993).Relevant, non-linear effects that may be significant in respect to response of jack-upstructures are given as ;- non-linear loading components (e.g. drag force loadings)- bottom restraint (non-linear foundation characteristics)- damping (e.g. due to the motions of the jack-up structure, there may be significant hydrodynamic damping as a result of the relative velocity of the water particles and the leg member)- dynamics of the structure (as the natural period of the structure is typically relatively high, e.g. 5-8 seconds, there may be significant wave energy available to excite the structural system and hence relatively large inertial forces may result)- second order effects (such effects may significantly influence the response in the considered structure)- non-linearites of structural interfaces (e.g. gaps between the leg structure and guides)· For reliability analysis, in order to account for the non-linearities in jack-up loading and response, it is considered necessary that explicit time domain analysis, utilising stochastic sea simulation, is undertaken.· Foundation modelling assumptions have been shown to be an important aspect in respect to the resulting response from analytical models of jack-up units, Manuel et al. (1993). Hence, unless it can be demonstrated that the effects are not significant, non-linear characteristics in the foundation system should be explicitly modelled when undertaking analyses in connection with reliability studies.· Guidance provided in the guideline example for jacket structures, DNV (1996c), in respect to the fatigue limit state covers the state-of-the-art knowledge with respect to fatigue reliability analysis. Response in respect to the fatigue limit state is therefore not explicitly covered in this section. Due to the non-linear characteristics of jack-up loading and response, frequency domain solution techniques are however not recommended unless, either it can be demonstrated that such effects are insignificant, or, due account has been taken of such effects.
  11. 11. Guidelines for Offshore Structural Reliability Page No. 11-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00722.3.1 Time Domain AnalysisTwo general methods may be utilised in time domain analysis. These two methods being ; -use of simple, single degree of freedom (SDOF) models, and, -use of multi-degree of freedom models.In both cases however the following general guidance may be given for the analysis, SNAME(1993) ;1. The generated random sea should consist of superposition of, at least, 200 regular wave components utilising divisions of equal energy of the wave spectrum.2. In order to obtain sufficiently stable response statistics, simulation time for a single simulation should generally not be less than 60 minutes.3. The integration time step should not normally be taken greater than the smaller of the following ; - one twentieth of the zero up-crossing period of the wave spectrum - one twentieth of the jack-up natural period.4. When evaluating the response of the jack-up, the transient effects at the start of the analysis should be removed. At least the smallest of 100 seconds, or 200 time steps should be removed in this connection.5. The method of evaluating the response (e.g. the Most Probable Maximum (MPM) response) should be compatible with the simulation time and sea qualification procedure adopted for the analysis. -Further guidance in connection with this item is provided in the Commentaries to the Jack-up Recommended Practice, SNAME (1993).The asymmetry of crest heights and troughs, accounted for by higher order wave theories, isnot reproduced in methods based upon random wave simulation techniques. Linear wavetheory, Sarpkaya (1981), utilised in random wave simulation, accounts for particle kinematicsupto the still water surface and ‘kinematic stretching’ is undertaken to compute thekinematics to the instantaneous free surface. It is recommended, Gudmestad and Karunakaran(1994), that Wheeler stretching, Wheeler (1969), is utilised in this connection.The extent of wave asymmetry is a function of waterdepth. For waterdepths less than 25metres, in extreme environmental conditions, irregular wave simulation is normallyconsidered to be inappropriate and regular wave analysis should be considered. Forwaterdepths greater than 25 meters wave asymmetry may be accounted for by the formulationgiven in equation 2.1 below, SNAME (1993). Hs = ( 1 + 0.5 e (-d/25) ) Hsrp (2.1)Where :Hs : adjusted significant wave height to account for wave kinematics (metres)Hsrp : significant wave height (metres)d : waterdepth (metres)
  12. 12. Guidelines for Offshore Structural Reliability Page No. 12-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072As time domain analyses are usually fairly resource demanding procedures, it is normalpractice to utilise simplified structural modelling techniques (see Figure 2.2)· A full description of the methodology and procedure utilised in creating both a simplified hydrodynamic and simplified structural model for a jack-up is included in DNV( Feb 1992) and SNAME (1993).Figure 2.2 : Typical Simplified Model of a Jack-up Structure.2.3.2 Methods of Evaluating Response· Reliability analysis of jack-up structures will generally be undertaken based upon the following considerations ; -1- Site specific environmental and foundational data should be utilised. -2- Directional and seasonal data may be utilised. In order to reduce the amount of analytical work involved, wind, wave and current load components may however normally be assumed to be coincident. -3- The selected (governing) environmental load direction may be initially identified by evaluation of relevant deterministic, ‘quasi-static’ response analyses of the jack- up structure under consideration. The standard procedure of treating wind, waves, currents and seawater level separately and combining the independent extremes as if these extremes occur simultaneously, is conservative. In most cases however, jack- up environmental loading is wave dominated and the assumption of simultaneity of the extremes of the environmental parameters is found to be satisfactory.
  13. 13. Guidelines for Offshore Structural Reliability Page No. 13-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072The probability of failure is estimated during a reference period significantly longer than theanalysed, simulated time period. An extrapolation procedure for determining the extremevalues for the reliability analysis is therefore required when several environmental variablesare to be combined.· The reference period for extreme environmental data is normally selected as being equal to the one year return period such that the results may be directly compared with annual target reliabilities.· For jack-ups, the two most appropriate procedures for estimation of extreme load events would seem to be ; -1- By use of long term statistics of independent sea states -2- By use of conditional extreme event analysis.These procedures are described in detail in Chapter 6 to the guidelines, DNV (1996a). Forconventional jack-up structures, in general, the long term response is controlled by theextreme sea states and, as such, both of these procedures are normally acceptable. Anexample of the estimation of extreme load events by use of long term statistics ofindependent sea states is provided in the jack-up examples guidelines DNV (1996b).Karunakaran (1993) documents that the short term extreme storm response is marginallyhigher than the long term response if the long term response is controlled by extreme seastates. If however the long term response is controlled by resonance sea states, the short termextreme storm response is about 10% lower than the long term response for those casestudies considered.Response from time history simulations may be characterised by the normalised statisticalmoments ; mx, sx, sx’, g3, g4, which are the mean, standard deviation, standard deviation of thetime derivative, skewness and kurtosis of the response respectively. A limit state may then bedefined from the statistical moments of the response and the estimated reliability thusobtained by the resulting response surface, DNV (1996b).· Response surface techniques are considered to provide the most appropriate methodology in the estimation of the reliability of jack-up structures for extreme load events.In order to model how the statistical moments change with realisations of the basic variables,the derivatives of these moments may be estimated by finite differences of the variables atone estimation point. As the limit state functions are highly non-linear this technique willonly give satisfactory results if a good fit is obtained around the design point.Generally, reliability analyses of jack-up structures may be undertaken by use of first andsecond order solution methods (FORM/SORM), Madsen (1986). -See also DNV (1996a),Chapters 2 and 3, for further guidance concerning utilisation of reliability methods.
  14. 14. Guidelines for Offshore Structural Reliability Page No. 14-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00722.3.3 Static Loading ComponentsPrevious jack-up reliability analyses, Karunakaran (1993), Løseth et al. (1990), haveidentified that response uncertainty is not significantly affected by the choice of the staticmass model. This is further demonstrated in the example documented in DNV (1996b).· Permanent loads and variable loads are generally lumped together. For structural assessment the upper bound of this sum is normally conservatively modelled. For overturning assessment the mean variable load is combined with the permanent load.2.3.4 Sea LoadingsSea loadings on conventional jack-up structures are calculated utilising Morison’s equation,Sarpkaya (1981) ; pD 2 1 Fn ( r , t ) = r Cma n ( r , t ) + rDCd v n ( r , t ) v n ( r , t ) (2.2) 4 2Wave and current velocity components in the Morison equation are obtained by combiningthe vectorial sum of the wave particle velocity and the current velocity normal to the memberaxis. (When relative motions are involved, eqn 2.2 may be modified to reflect such motionsin the terms an(r,t) and vn(r,t)).Epistemic uncertainties related to Morison’s equation are documented in Section 3.Wave LoadingsThe basic stochastic sea description is defined by use of a wave energy spectrum. The choiceof the analytical wave spectrum and associated spectral parameters should reflect the widthand shape of the spectra and significant wave height for the site being considered. Generally,either the Pierson-Moskowitz or the Jonswap spectra will be appropriate. See DNV (1996a),Section 5.· Due to the possibility of inducing greater dynamic response at lower wave periods than that necessarily associated with storm maximum significant wave height, a range of periods and associated significant wave heights should normally be investigated.· The simulated storm length is normally to be taken as 3 hours, SNAME (1993) or 6 hours, NPD (1992).
  15. 15. Guidelines for Offshore Structural Reliability Page No. 15-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072For the extreme load event it is normally, conservatively assumed that a long crested seasimulation is undertaken, NPD (1992), however, in accordance with SNAME (1993) thefollowing directionality function F(a) may be utilised ; F(a) = C. cos2na for -p/2 £ a £ p/2 (2.3)where ;n : 2.0 for fatigue analysis 4.0 for extreme analysis p /2C : constant chosen such that : å -p / 2 F (a )da = 10 .Current Loadings· Current velocity should include all relevant components, DNV (1996). Normally, however, it is acceptable to divide the total current into two components, namely, that of wind and wave generated current, V(w,w) and that of residual (e.g. tidal) current, Vr. The first of these two current components may be assumed to be fully correlated with the significant wave height, whilst the latter current component, Vr, is assumed to be completely independent of the other environmental characteristics. See DNV (1996a), Section 5.1.3.2, for a full description of this procedure.Unless site specific data indicate otherwise the current profile should be described accordingto the procedure documented in SNAME (1993).2.3.5 Wind LoadingsSingh (1989) has found a number of inconsistencies in existing wind loading calculationprocedures. Based upon this finding it has been concluded that wind tunnel measurementsappear to provide the only viable method for accurately estimating loads on complex offshorestructures.· For jack-up structures, if it is not possible to utilise model test data, either by direct testing, or from scaling of geosim models, then, assuming that wave loading is the dominating load effect, it is normally acceptable to base such loading on simplified, direct calculation methods.SNAME (1993) documents an acceptable procedure for the calculation of wind loadings,where the wind loading, Fwi , is calculated as a static load contribution by use of the equation; Fwi = ½ r Vref² Ch Cs Aw (2.4)wherer : density of airVref : the 1 minute sustained wind velocity at 10 meters above sea levelCh : height coefficientCs : shape coefficientAw : projected area of the block consideredIn locations where wind loading may be the dominating load effect (e.g. due to cyclones etc.)this load effect should be specially considered.
  16. 16. Guidelines for Offshore Structural Reliability Page No. 16-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00722.3.6 FoundationsThe uncertainty in jack-up response is greatly influenced by the uncertainties in the soilcharacteristics that determine the resistance of the foundation to the forces imposed by thejack-up structure. Ronold (1990) showed that, for a jack-up, the total uncertainty governingthe safety against foundation failure is dominated by the uncertainty in the loading. Nadim etal. (1994), on the other hand, showed that the response of a jack-up structure subjected to acombination of static and cyclic loads is just as much influenced by the uncertainties in theloads as by the uncertainties in the soil resistance. The significant discrepancy between theseresults is due to the different assumptions made with respect to the uncertainties in thevariables. One should therefore be careful in generalising the results obtained for a specificsite to other environmental and soil conditions.For traditional jack-up foundation solutions, the stability and performance of a jack-upfoundation is primarily determined by the installation procedure for the unit. This operationinvolves elevating the hull and pumping water ballast into the preload tanks, causing thespudcans to penetrate into soil and thereby increasing their bearing capacity.· The geotechnical areas of concern for jack-up foundations are: -Prediction of footing penetration during preloading. -Jack-up foundation capacity under various load combinations after preloading. -Foundation stiffness characteristics under the design storm.The recent trend in using jack-up structures in deeper waters and on a more permanent basishas resulted in another type of foundation solution, namely spud-cans equipped with skirts.The installation of skirted footings is normally achieved by suction, not preloading. Theskirted footings not only provide more predictable capacity, they also increase the footingfixity significantly. The procedure for estimating the capacity of the individual footings isbased upon analytical procedures similar to that undertaken for foundation of gravity basedstructures. For jack-up foundation systems, however, it is important to look at the completefoundation ‘system’ because at loads close to failure, significant re-distribution of reactionsamong the footings may take place. (Refer to the foundation example in DNV (1996c) formore information in respect to this item.)It is evident from statistics, Sharples et al. (1989), Arnesen et al. (1988), that punch-throughduring preloading is the most frequently encountered foundation problem for jack-ups.Punch-through occurs when a weak soil layer is encountered beneath a strong surficial soillayer.· The only way to avoid punch-through is to undertake a thorough site investigation at the jack-up location prior to installation in order to identify the potentially problematic weak soil layers.The total amount of preload used in the installation is often used as a checking parameter forthe spudcan capacity to withstand extreme loads. The so-called “100% preload check”requires that the foundation reaction during preloading on any leg should be equal to, orgreater than, the maximum vertical reaction arising from gravity loads and 100% ofenvironmental loads. The preload defines the static foundation capacity under pure verticalloading immediately after installation. Under the design storm the footing is subjected tosimultaneous action of vertical and horizontal loads, and overturning moment. The storminduced loads are cyclic with a short duration and the supporting soil may have a higherreference static shear strength than right after installation due to consolidation under the jack-
  17. 17. Guidelines for Offshore Structural Reliability Page No. 17-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072up weight. On the other hand, for equal degrees of consolidation, the vertical capacity of afooting will be greater during pure vertical loading than during a combination of vertical,horizontal and moment loadings.Having regard to the oversimplification of the l00% preload check, SNAME (1993) suggestsa phased method with three steps, increasing in the order of complexity, for the evaluation offoundation capacity, as follows :Step 1. Preload CheckThe foundation capacity check is based on the preloading capability - assuming pinnedfootings.Step 2. Bearing Capacity CheckBearing capacity check based on resultant loading on the footing under the design storm.Step 3. Displacement CheckThe displacement check requires the calculation of displacements associated with anoverload situation arising from Step 2.Any higher level check need only be performed if the lower level checks fail to meet thefoundation acceptance criteria.It is difficult to quantify the uncertainties associated with the “preload check” approach.Nadim and Lacasse (1992) developed a procedure for reliability analysis of the foundationbearing capacity of jack-ups. The procedure, which may be categorised as a Step 2 approach,is based on a prior calculation of the bearing capacity under different load combinations(interaction diagram) and updating the interaction diagram from the measured verticalpreload. The bearing capacity calculations are performed probabilistically using the FORMapproximation. The procedure developed by Nadim and Lacasse (1992) was used by Nadimet al. (1994) to study the reliability of a jack-up at a dense sand site in the North Sea.An important result of the FORM analyses is the correlation between the foundation capacityunder a given combination of horizontal and vertical loads (and overturning moment ifspudcan fixity is significant) and the foundation capacity under pure vertical loading. Thedegree of correlation determines the significance of the measured preload on reducing theuncertainty associated with foundation capacity for a given load combination.· For a given loading combination (vertical, horizontal and moment), the lognormal distribution function appears to provide a good fit to the foundation capacity, Nadim and Lacasse (1992).· The properties of the volume of soil under the footing fluctuate spatially and can be represented by a random field. The effects of this are accounted for by spatial averaging, Vanmarcke (1977, 1984), and by using stochastic interpolation techniques, Matheron (1963), if enough data exist.· Otherwise, the uncertainties in the soil parameters are based on the statistics of the available data. Mean and standard deviation are calculated by ordinary statistical methods, e.g. Ang and Tang (1975). Usually the probability distribution function used to represent geological processes follows a normal or lognormal law. More often than not however, and especially in the case of jack-up structures, there are not enough data
  18. 18. Guidelines for Offshore Structural Reliability Page No. 18-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072 available, and the designer needs to use correlations or normalised properties as a function of the type of soil to establish consistent soil profiles.See also DNV (1996a), Section 7.3.As an example the undrained shear strength of soft sedimentary clay normalised to the in-situoverburden stress is about 0.23 ± 0.03 for a horizontal failure mode; the friction angle of sandcan be selected on the basis of its relative density and an in-situ penetration test.
  19. 19. Guidelines for Offshore Structural Reliability Page No. 19-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00723.0 UNCERTAINTY MODELLING3.1 GeneralThis section provides general guidance in respect to uncertainty modelling as appropriate tothe extreme load event for a jack-up structure.3.2 LoadingUncertaintyModellingUncertainty in the load process may be attributed to either aleatory uncertainty (inherentvariability and natural randomness of a quantity) or epistemic uncertainty (uncertainty owingto limited knowledge). In respect to jack-up reliability analysis, guidance appropriate to themost significant of the uncertain variables associated with the load process is given below.3.2.1 Aleatory UncertaintyTables 3.1 to 3.3 below document a summary of recommended distributions for selectedstochastic variables. It should be noted however that site specific evaluation of environmentalvariables may dictate use of variable distributions other than those recommended in the tablesbelow. For further guidance see also DNV (1996a), Chapter 5.Description DistributionRandomness of storm extremes PoissonWaterdepth (D) Uniform (tidal effects), or, Normal (storm surge effects - conditional on Hs)Marine Growth LognormalTable 3.1 : General Environmental Variable DistributionsDescription DistributionSignificant wave height (Hs) 3-parameter Weibull/LognormalZero up-crossing period (Tz) Lognormal (conditional on Hs)Spectral peak period (Tp) Lognormal (conditional on Hs)Joint distribution (Hs,Tz) or (Hs,Tp) 3-parameter Weibull for Hs and Lognormal for Tz or Tp (conditional on Hs)Tidal current speed (Vt) UniformWind generated current speed (Vw) Normal (conditional on U10m)Average wind speed (U10m) Weibull (conditional on Hs)Table 3.2 : Long Term Analysis Variable Distributions
  20. 20. Guidelines for Offshore Structural Reliability Page No. 20-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072Description DistributionSignificant wave height (Hs) Gumbel *1, 2Total current speed (Vc) Gumbel *1, 2Average wind speed (U10m) Gumbel *1, 2Table 3.3 : Extreme Analysis Variable DistributionsKEY :*1 : Normally it is sufficient to consider the extreme dominating variable being either ; -the significant wave height, -the current, or, -the wind speed, in combination with this extreme distribution the remaining two variables are assigned the distribution according to Table 3.2.*2 : Instead of a Gumbel distribution, a Weibull distribution (see the long term analysis variables in table 3.2), raised to the power of the number of considered seastates in one year, NSea, may be utilised in practice. (See DNV (1996a), Section 6.7.)3.2.2 Epistemic Uncertainty· The following listed time independent, basic load variables have been identified as being possible significant contributors to the overall reliability of a jack-up structures, Løseth (1990), Karunakaran (1993), Dalane (1993) ; -Drag coefficient -Inertia coefficient -Marine growth -Mass of structure.Guidance to selection of distribution type and distribution parameters for random modeluncertainty factors associated with these basic load variables is given in Table 3.4 below.Basic Variable Name Distribution m1 C.o.V.Drag coefficient 2 (CD) Lognormal 1.0 0.2 3Inertia coefficient (CI) Lognormal 1.0 0.1Marine growth 4 Lognormal 1.0 0.2Mass of structure 5 Lognormal 1.0 0.14Table 3.4 : Load Model Uncertainty VariablesKEY :1: The absolute value of the distribution variables are given relative to the value applied in the structural analysis.2: The selection of appropriate drag coefficients for the structural analysis are stated in SNAME (1993).3: For extreme value jack-up analysis, without loss of any generality, it is normally considered acceptable to select the inertia coefficient as a fixed quantitiy. An inertia coefficient of 1.8 may be utilised.4: The selection of the appropriate value for the marine growth should be evaluated based upon a site specific evaluation, e.g. NPD (1992).5: See also section 2.3.3
  21. 21. Guidelines for Offshore Structural Reliability Page No. 21-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-00723.3 Response Uncertainty Modelling· Significant contributions to response model uncertainty may be attributed to the following causes, Nadim (1994), Løseth (1990), Karunakaran (1993); -Analytical uncertainty -Damping ratio -Foundation stiffness3.3.1 Analysis UncertaintyAnalytical uncertainty accounts for the model uncertainty resulting from the statisticalaccuracy of a single analytical simulation (i.e. the variability resulting from differentengineers, utilising different software, undertaking exactly the same analysis). With respectto jack-up response analysis this uncertainty is documented in DNV (1996a), Chapter 6.Guidance to selection of distribution type and distribution parameters for random analyticaluncertainty factors is given in Table 3.5 below.Basic Variable Name Distribution m C.o.V.Analytical uncertainty Lognormal 1.0 0.18Table 3.5 : Analytical Model Uncertainty Variables3.3.2 DampingDamping model uncertainty may vary depending upon the procedure adopted for includingdamping within the response analysis, Langen (1979). Relative velocity, hydrodynamicdamping should generally not be used if Eqn. 3.1 below is not satisfied, SNAME (1993). uTn/Di ³ 20 (3.1)whereu : water particle velocityTn : first natural period in surge/swayDi : diameter of leg chord· For extreme response analysis, in general, hydrodynamic damping may normally be explicitly accounted for by use of the relative velocity formulation in Morison’s equation.· A value for total global damping may be obtained by summation of those appropriate damping component percentages stated in Table 3.6, SNAME (1993).
  22. 22. Guidelines for Offshore Structural Reliability Page No. 22-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072Damping Source Global Damping (% of critical damping)Structure, holding system etc. 2%Foundation 2% or 0% 1Hydrodynamic 3% or 0% 2Table 3.6 : Table of Recommend Critical DampingKEY :1: Where a non-linear foundation model is adopted the hysteresis foundation damping will be accounted for directly and should not be included in the global damping.2: In cases where the Morison, relative velocity formulation is utilised the hydrodynamic damping will be accounted for directly and should not be included in the global damping.Guidance to selection of distribution type and distribution parameters for random dampinguncertainty factor associated with the response basic variables is given in Table 3.7 below.Basic Variable Name Distribution m1 C.o.V.Damping ratio Lognormal 1.0 0.25Table 3.7 : Damping Model Uncertainty VariablesKEY :1: The absolute value of the distribution variables are given relative to the value applied in the structural analysis.3.3.3 FoundationFor geotechnical analysis, model uncertainty is difficult to assess as there are few comparablefull scale prototypes that have actually gone to failure and where there was enoughknowledge about the site conditions and the load characteristics to enable calculation of theuncertainty.· Therefore to evaluate model uncertainty, comparisons of relevant scaled model tests with deterministic calculations, expert opinions and information from literature, in addition to any field observations that are available for similar structures on comparable soil conditions, are normally utilised.Using "traditional" analysis methods to undertake the bearing capacity analysis of thespudcan of a jack-up foundation results in large model uncertainties, as was documented byEndley et al. (1981). They compared, for 70 case studies on soft clays and 15 case studies onlayered profiles consisting of soft clay over stiff clay, predicted rig footing penetration withobserved penetrations. The comparisons suggest a model uncertainty with mean value 1.0and standard deviation 0.33, as based on the 70 cases studied. The observed data rangedbetween 0.4 and 1.55 times the predicted values.McClelland et al. (1982) undertook similar comparisons for jack-ups on uniform clay profilesand for jack-ups on layered profiles. In this study the standard deviation was about 0.20 to0.25 about a mean of 1.0.
  23. 23. Guidelines for Offshore Structural Reliability Page No. 23-DNV Application to Jackup Structures------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Report No. 95-0072The “traditional" methods of analysis are the so-called "bearing capacity formulas” which donot account for strength anisotropy, cyclic loading, soil layering, nor variation of soilproperties with depth or laterally. The model uncertainty values quoted above are valid for afailure mode under vertical loading only.In the method proposed by Nadim and Lacasse (1992), a more rigorous bearing capacityapproach than the "traditional" approach is used. The analysis uses a limiting equilibriummethod of slices. Effects of anisotropy and cycling loading, the uncertainty in the calculationmodel for both vertical and horizontal (moment) loading and combined static and cyclicloading are included. The uncertainty in this calculation model was studied in detail withseries of model tests at different scales.On the basis of the work carried-out by Andersen and his co-workers, Andersen et al. (1988),(l989), (1992), (1993), Dyvik et al. (1989), (1993), model uncertainty for bearing capacity ofa footing in clay may be mean 1.00, standard deviation 0.05 for failure under static loadingonly, and mean 1.05, standard deviation 0.15 for failure under combined static and cyclicloading. For footings installed in sand, much less information exists, and tentative values maybe mean 1.00, standard deviation 0.20 to 0.25, based on engineering judgement and theresults of recent centrifuge model tests, Andersen et al. (1994). The model uncertainty mayvary according to the failure surface. It should be noted that the mean of model uncertaintyfactor for most offshore foundations (e.g. piles in sand and clay, shallow foundations onsand) is greater than 1.0, i.e. the analytical models tend to be conservative. The methodsdeveloped for shallow foundations on clay, however, have been fine-tuned and calibratedagainst large-scale tests in the past 20 years, and much of the inherent conservatism in themethods has been removed.Little information exists on the model uncertainty associated with the foundationdisplacement of a jack-up structure (see step 3 in section 2.3.6) and the model uncertainty canonly be guessed for those cases. A model uncertainty with a coefficient of variation of at least50 % is expected.Guidance to selection of distributions associated with the foundation parameters is given inTable 3.8 below. Reference should also be made to DNV (1996a), Section 7.3.Description Distribution*1Rotational stiffness LognormalHorizontal stiffness LognormalVertical stiffness LognormalTable 3.8 : Foundation Parameter DistributionsKEY :*1 : See also section 2.3.6

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