Jack-up Platform Research Articles

Response Spectrum Estimation for Fixed Offshore Structures With Inundation Effect Included: A Price’s Theorem Approach

Instead of a Volterra-series based method, a method based on Price’s theorem and Fourier Transforms is proposed for evaluating the nonlinear response power spectra of fixed offshore structures modeled as finite-memory systems. The numerical computation effort required for spectral estimations using the proposed method is significantly less than that of the Volterra-series method that requires multi-dimensional integrations. Nonlinear frequency-domain analyses of two jack-up platform structures are carried out to study the effects of inundation, cubic and quartic drag terms and polynomialization methods.

Residual static strength of high strength steel cracked tubular joints

Jack-up platforms are to an increasing extent being considered for use as long-term production support structures. The implications of this new application for structural integrity assessment and maintenance are significant. Traditional regular dry dock inspection and repair is no longer possible when a platform is stationed for medium to long-term production support periods. For this reason the extent to which jack-up platforms can tolerate fatigue damage must now be a design consideration. Jack-up platforms by their nature do not have the degree of structural redundancy as found in traditional fixed jacket production structures. This means that the push-over or static strength capacity of jack-ups is far more sensitive to reduction in strength due to fatigue cracking in individual members than equivalent cracking in jacket structures. In addition, high strength weldable steels are regularly used in the construction of jack-up rig legs due to their excellent strength-to-weight ratio. However, there is often little information available concerning the mechanical behaviour in fatigue and fracture particularly under protected corrosion conditions. This has, to some extent, limited the designer's ability to maximise the potential benefits offered by high strength weldable steels. This paper considers the problem of residual static strength of jack-up platform components by examining the results of two sets of static strength destructive tests performed on full-scale pre-cracked tubular welded T- and Y-joints manufactured from a commercially used high strength weldable steel. The paper also highlights the need to conduct further work so that a thorough understanding of jack-up platform defect tolerance can be attained.

Non-linear frequency-domain analysis of jack-up platforms

The autocorrelation function and higher-order cumulants of the non-Gaussian wave modal force, with a cubic approximation of the Morison drag force and a quartic approximation of the inundation force, are presented. The method utilized is based on Price's theorem for evaluating the associated joint moments of the non-linear wave forces. One- and multi-dimensional fast Fourier transform techniques are employed to obtain the power and higher-order spectra of the wave modal force and structural response. The non-linear frequency-domain analyses of two typical jack-up platforms are carried out using the correlation function based approach, which is numerically much more efficient than either the method based on Volterra-series or time-domain simulation.

Probabilistic Models Applicable to the Short-Term Extreme Response Analysis of Jack-Up Platforms

There is a steadily increasing demand for the use of jack-up units in deeper water and harsher conditions. Confidence in their use in these environments requires jack-up analysis techniques to reflect accurately the physical processes occurring. However, nearly all analyses are deterministic in nature and do not account for the inherent variability in governing parameters and models. In this paper, probabilistic models are used to develop an understanding of the response behavior of jack-ups, with particular emphasis placed on the extreme deck displacement due to a short-term event. Variables within the structural, foundation and wave loading models are assigned probability distributions and their influence on the response statistics is quantified using a response surface methodology.

Evaluation of long-term extreme response statistics of jack-up platforms

This paper is concerned with the models appropriate for the dynamic assessment of jack-ups, concentrating particularly on the long-term response due to random ocean waves and on work-hardening plasticity models used for spud-can response. A methodology for scaling of short-term statistics, calculated using a Constrained NewWave technique, is shown in a numerical experiment for an example jack-up and central North Sea location. The difference in long-term extreme response statistics due to various footing assumptions is emphasised. Results for two environmental load conditions are described (one excluding and one including wind and current effects) and the role of sea-state severity in the variation of short-term extreme response statistics is also highlighted.

Second-Order Superharmonic Wave Loading on Jack-Up Platforms

Different types of second-order superharmonic wave forces acting on jack-up platforms are studied. The nonlinear effect of variable wave surface (i.e., the inundation effect) is found to be the most significant. It is shown that a concentrated force acting at the mean water level of the structure can be used to represent the effect of the variable wave surface. The nonlinear transfer function of the modal wave force, with the inundation effect included, is derived. Frequency-domain analyses of the nonlinear wave force and structural response of a jack-up platform, modeled as a linear structure, are carried out to show the effect of inundation. The response spectral density functions obtained from the frequency-domain analysis are also compared with the spectra of time-domain simulations based on wave kinematics predicted by the Wheeler stretching and vertical uniform extrapolation methods.

Full-scale measurements from a large deepwater jack-up platform

Abstract Analysis of full-scale measurements obtained from the instrumented Smedvig West Epsilon jack-up platform operating in the STATOIL Sleipner Vest field in the North Sea is described. This jack-up platform has a skirted spud-can foundation. The following topics are discussed in the paper: • Comparison between measured and calculated natural frequencies and modes of the platform based on modal analysis. • Comparison of measured foundation stiffness with the design stiffness. • Comparison between measured and simulated platform response by application of a nonlinear time domain analysis. Implications with respect to procedures and assumptions employed for design of the platform are addressed. The comparison between measured and simulated response is performed for a moderate sea state with a significant wave height of 9.3 m.

Modelling of jack-up response for fatigue under simulated service conditions

Abstract Service loading of jack-ups is mainly due to wave and wind action with variable amplitude and frequency content. As jack-ups are used in deeper waters for long-term deployment, fatigue has become an important consideration both at the design stage and during maintenance for structural integrity assessment procedures. Jack-ups are more dynamically sensitive structures when compared with conventional fixed platforms. This means that the effects of dynamics and structural response become more important and must be adequately modelled to obtain realistic and representative results for the simulated service load sequences which are required to study the fatigue performance. This paper covers the relevant analytical and experimental procedure adopted at University College London to ensure that realistic results of the kind mentioned above were obtained during a study on variable amplitude corrosion fatigue (VACF) of a typical weldable high-strength steel (SE702). It looks at the influences on fatigue resistance of welded tubular joints used in the construction of a typical jack-up platform. Great emphasis is placed on how the relevant factors which affect the fatigue performance of these welded tubular joints are modelled to produce a jack-up standard load history (JOSH). These factors especially include wave loading, structural dynamic response and the effect of corrosion under cathodic protection conditions.

Fixed and jack-up platforms: Basis for reliability assessment

Abstract The introduction of the fixed jacket Load and Resistance Factor Design code (API RP 2A-LRFD), and the Jack-up Site Assessment Recommended Practice (SNAME T&R Bulletin 5-5A), raises debates regarding differences in the reliability levels associated with the use of the two documents. In an effort to investigate whether substantial differences exist, and to understand the underlying causes of the differences, a major study has been initiated by the UK Health and Safety Executive to compare the reliabilities of these two structures when operating in similar weather conditions and in similar water depths, to the extreme weather hazard. To ensure the comparison is appropriate, both units will be assumed to be operating close to the limits of their relevant code. Both will be subject to the same analysis packages in respect to wave-current force determination, structural analysis, pushover analysis, and reliability analysis. Initially, foundation failure will be suppressed in order that structural reliabilities under extreme weather conditions can be directly compared. Foundation failure will then be introduced to quantify the effect this has on the structural safety levels. Reliabilities will be determined at both component and system level using the response surface reliability methodology. Component strength modelling will exploit the formulations currently being harmonised for the new ISO offshore code, whilst global strength will be evaluated using CAP/Seastar for both installation types, thus reducing the potential for modelling based differences occurring during the system analysis of the study.

Effects of non-Gaussian waves to the dynamic response of jack-up platforms

Abstract The paper discusses the nonlinear response effects to jack-up platforms caused by non-Gaussian waves. The basis for the paper is field and model test observations of wave characteristics and simulation of wave processes and dynamic response applying different wave and wave kinematics models. Both normal design sea states and extremely steep sea states are investigated. In the design sea states, effects to the steady state dynamic response are evaluated. For the steep wave situations transient response effects—so called ringing response— are focused. Consequences and implication for design calculations are considered in both cases.

Verification of methods for simulation of nonlinear dynamic response of jack-up platforms

The validity of mathematical and numerical models commonly applied for simulation of the nonlinear stochastic response of jack-up platforms is discussed. The basis for the discussions is the comparison of simulated response to model test data. The model test data refer to an experiment, which in full scale refers to a typical harsh environment truss leg jack-up. The test data include wave elevation, wave particle kinematics and static and dynamic response. The verification study emphasises wave kinematics, hydrodynamic loading, damping and dynamic response.

Evaluation of hydrogen assisted cracking resistance of high strength Jack-up steels

The use of high strength steels is very attractive for the fabrication of the legs of Jack-Up platforms, however, there are still some concerns on the possibility of hydrogen assisted cracking resulting from cathodic protection. An experimental programme was performed to assess the hydrogen assisted cracking sensitivity of a high strength steel in conditions of cathodic protection. Hydrogen activity resulting from cathodic protection has been evaluated from electrochemical hydrogen permeation test results and from hydrogen analysis on exposed samples. It is shown that cathodic protection in sea water results in low hydrogen activity. Comparative slow strain rate tests performed on high strength steel and on lower strength materials show that, for a given test condition, comparative behaviour is observed. Fracture mechanic tests indicate that high K ISCC values are obtained under cathodic protection. Finally, tests performed on weld structures confirm that high strength steels are suitable for use under cathodic protection. An increase of the protection potential (from −1050m V/ECS to −850m V/ECS) can still improve the behaviour.

Assessment of Analysis of an Instrumented Drilling Jackup Conductor

Summary This paper presents an assessment of some of the results of an internationally funded joint-industry project to measure the dynamic behavior of a drilling conductor run from a large jackup platform operating in deep water at the North Sea. Three different elevations spanning the conductor were instrumented with environmentally protected strain gauges while the rig was operating. The objective was to gather data for selected storms for subsequent spectral and statistical analyses and to validate finite-element analysis (FEA) of a drilling-conductor system. The response amplitude operators (RAO's) from the analysis showed good agreement with measured RAO's. This agreement increases confidence in the validity of the loading and response calculation methods currently available for the design of drilling conductors. In the frequency analysis, the computed natural periods showed good agreement with those measured under different drilling conditions. Measurements showed that the natural frequency oscillation of the conductor was mainly in the first mode of vibration. In the time-domain analysis, the computed bending loads were found in good agreement with measured values, particularly at the middle and lower gauged levels. The analytical results showed that, for small waves, the dynamic amplification was substantial when comparing static and dynamic analytical techniques. However, for large design waves, the dynamic amplification was found to be small and the static analytical technique was found to be accurate in predicting environmentally induced loads.

Nonlinear dynamic behaviour of jack-up platforms

Jack-up platforms may, under extreme wave loading conditions, exhibit significant nonlinear behaviour. This must be accounted for in the design of such platforms, in order to ensure satisfactory structural safety. In this paper an overview of the different sources of nonlinear platform behaviour is presented. Furthermore, it is outlined how those nonlinear effects may be modelled and handled in numerical simulation studies of structural response, with a special focus on the estimation of extreme response and dynamic amplification factors. Finally, it is discussed how nonlinear dynamic response may be accounted for in the design of jack-up platforms. The discussions and outlines are illustrated by examples from numerical simulation studies of jack-up behaviour.

A computational model for fixity of spud cans on stiff clay : H. P. Jostad, F. Nadim & K. H. Andersen, Publikasjon - Norges Geotekniske Institutt, 191, 1994, pp 151–171; reprinted from: BOSS'94, 7th international conference on the behaviour of offshore structures, Massachusetts, 1994, Vol. 1: Geotechnics, ed C. Chryssostomidis & others (Elsevier), 1994, pp 151–171

An integrated analysis of a jack-up platform and its soil foundation system may increase the overall bearing capacity and provide more favourable structural response compared to a conventional analysis based on pinned footing condition. A model for the cyclic force-displacement and moment-rotation relationships of a spud can on stiff clay is presented. The model takes into account the effects of the loading history (degradation during cyclic loading), strain rate, strength anisotropy and non-linear stress-strain relationship of the clay, and coupling between all force-displacement components. It is valid for all load levels, including the combination of the static vertical load due to the submerged weight of the platform and cyclic forces (due to waves, wind and current) that cause failure of the spud can. The model performance is illustrated by an analysis of a jack-up platform at a deep water site. The results are compared with those based on the pinned footing condition and a maximum lower bound solution of the overall bearing capacity of the platform.(

Wind environment over the helideck of an offshore platform

An investigation of the wind environment over the helideck of a model of an offshore jack-up platform was carried out in an atmospheric boundary layer wind tunnel. Hot-wire anemometry was used to measure the velocity distribution over the helideck for five different wind directions. The results showed that the mean velocities and root-mean-square values of the fluctuating velocity are quite different under various wind directions. Flow visualization by smoke-wire technique is also presented.

Site-dependent reliability of a mobile jack-up platform

Abstract The site-dependent response characteristics of a jack-up platform operating at two different locations in the North Sea are investigated. Both the water depth and wave environment differ at the two sites. The effect from uncertainty in boundary conditions at the platform base on the natural frequencies and dynamic amplification factors is highlighted. Characteristic load-effects in the jack-up legs are considered, and corresponding reliability measures are compared for the two sites. Implications for design are briefly discussed.

Dynamic Amplification of Jack-Up Platforms Subjected to Non-Gaussian Wave Loads.

Jack-up platforms are sensitive to dynamic amplification in waves because their fundamental period can be as high as 7 s-8 s. Where as the dynamic motion of the platforms is rather well described by linear theory the excitation depends nonlinearly on the wave height. The stochastic wave loading is thus far from being normally distributed. In this paper dynamic amplifications obtained by the diffusion theory extended to cover nonnormal excitations are compared with available time simulation results in irregular seaways. Generally, the time simulations seem to yield responses less nonlinear in the wave heights than the responses estimated from the diffusion theory. Explanations for these discrepancies are discussed and mainly attributed to a proper choice of total damping.

Dynamic amplification of offshore steel platform responses due to non-Gaussian wave loads

The dynamic response of offshore steel platforms subjected to random non-normal wave loads is considered. Two procedures are described. The first is a simplified quasi-static approach based on the classical one degree-of-freedom dynamic amplification factor. In the second procedure the statistical moments of the non-normal dynamic response are calculated without approximations using the theory of diffusion processes and moment closure techniques. An example jack-up platform is considered. It is found that the quasi-static approach is rather accurate for extreme value predictions, but not applicable for fatigue analysis where the second procedure should be applied. It is also observed that the dynamic amplification changes the statistical behaviour of the response significantly towards a Gaussian process.

Stochastic response of jack-up platform by the method of statistical quadratization

The use of jack-up platforms in deeper water leads to their response statistics becoming more sensitive to nonlinear drag loading from waves and current and to them deviating from Gaussian assumptions. To study the significance of these effects, the method of statistical quadratization is developed in this paper. The solution is based on the Volterra series approach and computation is done in the frequency domain. An efficient scheme was implemented for the Volterra series solution. As an extension to previous studies, the response kurtosis is also estimated using the statistical quadratization method. The results (i.e. response mean, variance, skewness and kurtosis) are compared with those obtained by both the equivalent linearization method and simulation. It is shown that the statistical quadratization method provides a notable improvement over the statistical linearization method.