Nils Sødahl

Efficient Mooring Line Fatigue Analysis Using a Hybrid Method Time Domain Simulation Scheme

Dynamic analyses of mooring line systems are computationally expensive. Over the last decades an extensive variety of methods to reduce this computational cost have been suggested. One method that has shown promising preliminary results is a hybrid method which combines finite element analysis and artificial neural networks (ANN). The present study presents a novel strategy for selecting, arranging and normalizing training data for an ANN. With this approach one ANN can be trained to perform high speed dynamic response prediction for all fatigue relevant sea states and cover both wave frequency motion and slow drift motion. The method is tested on a mooring line system of a floating offshore platform. After training a full fatigue analysis is carried out. The results show that the ANN with high precision provides top tension force histories two orders of magnitude faster than a full dynamic analysis.Copyright

Efficient Fatigue Analysis of Helix Elements in Umbilicals and Flexible Risers: Theory and Applications

Fatigue analysis of structural components such as helix tensile armors and steel tubes is a critical design issue for dynamic umbilicals and flexible pipes. The basis for assessment of fatigue damage of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. The long-term stress cycle distribution will hence require global dynamic time domain analysis followed by a detailed cross-sectional analysis in a large number of irregular sea states. An overall computational consistent and efficient fatigue analysis scheme is outlined with due regard of the cross-sectional analysis technique required for fatigue stress calculation with particular attention to the helix elements. The global cross-section is exposed to pure bending, tensile, torsion, and pressure loading. The state of the different cross-section elements is based on the global response. Special emphasis is placed on assessment of friction stresses caused by the stick-slip behavior of helix elements in bending that are of special importance for fatigue life assessments. The described cross-sectional analysis techniques are based on an extensive literature survey and are hence considered to represent industry consensus. The performance of the described calculation scheme is illustrated by case studies.

Carcass Tear Out Load Model for Multi-Layer Pressure Sheath Risers

Recent failures of multi-layer pressure sheath risers have shown that the carcass may fail in the top termination due to excessive axial loads. This is a new failure mode for flexible risers, recently presented by the authors in more general terms. The present paper explains details of the established load model and the validation against mid-scale tests, risers failed during operation, and operating risers close to failure by this new mode. The key driver in the model is the temperature contraction of pressure sheath layers. Also influenced by changes in polymer properties over the operational history, temperature and time is explained. Other contributing factors in the load model are gravity-component and bore pressure.The prediction model for the carcass loads are developed during Statoils investigation in 2011–12. The model is regarded representative for 20% of the most exposed risers. Several of the input parameters are uncertain and a Monte Carlo simulation approach is selected to study the variability and predict the probability of failure, given that radial contact pressure is sufficiently low.The approach adopted in the model may be applicable to other risers where polymers and steel components act together, and in such circumstances act as a guide for alternative model developments.© 2014 ASME

Methodology for Disconnect Analysis of CWO Risers in Random Seas

The term ‘riser recoil’ refers to the situation when the lower end of a top tensioned riser is released, and the riser is lifted up by the riser tensioner and/or top motion compensator system on the supporting vessel. The elastic energy stored in the riser is then released, and the riser ‘recoils’. This paper focuses on the case of planned disconnect. Recoil of Marine Drilling Risers has been the subject of several research papers over the past two decades. Some examples are listed in references [2] through [7]. Completion and Work Over (CWO) risers are unique in the sense that they may be simultaneously connected to both the riser tensioner system and the top motion compensator system of a drilling vessel. A Marine Drilling riser, on the other hand, is only connected to the riser tensioner system. Typically the riser tensioner system has a stroke of ± 8–9 m, whereas the top motion compensator system has only ± 3.5–4 m. It is imperative that the connector is lifted clear of the subsea structure in order to avoid damage to the equipment after the riser has been disconnected. The operating window for planned disconnect of CWO risers is severely limited by the available stroke of the top motion compensator. One of the purposes of the disconnect analysis is to establish the maximum wave height at which there is still sufficient clearance between the connector and the subsea structure after disconnect. Previous experience has shown that this may be the governing limitation for workover operations. The current industry practice is to use a regular wave approach in the analysis. The wave frequency is varied in order to find the maximum response, and hence one is actually searching for the extreme response, without paying attention to the probability that this will occur. In this paper a new method is presented, where the analysis is based on an irregular wave approach and the Monte Carlo technique, using time-domain simulations. Acceptance criteria are established based on a stochastic analysis, and are based on target levels of probability of exceedance. The results are documented through a case study of a typical CWO riser system connected to a semi-submersible in typical North Sea environmental conditions. The semi-submersible and the CWO riser system are exposed to both regular and irregular waves. Comparison of the resulting allowable wave height indicates that using the approach presented here with irregular waves will give a considerable increase in the operating window, and the resulting operability, compared to a regular wave analysis.Copyright

Carcass Tearing in Flexible Pipes

This paper presents a novel model of carcass tearing in flexible pipes. The model is based on a simple parameterization of the pipe design in terms of isotropic layers, together forming the composite pipe structure. The model allows evaluations of interfacial shear stresses between the inner pipe layers, as well as axial normal stress and strain levels in response to gravitational and thermal loading.Interfacial slip of a given interface, i.e. axial sliding of adjacent layers relative to each other, is accounted for by introducing a maximal possible value of the interfacial shear stress for a given interface, amounting to a static friction capacity. The model shows how a cut-off of the shear stress at the shear stress capacity implies an interfacial slip, which is followed by a significant increase in axial strain of the carcass layer.Detailed quantitative results of the model are presented for a particular 11.5 inch K-carcass riser design. In order to improve engineering practice, an analytical expression of the governing shear stress is derived in terms of the gravitational and thermal loads. This analytical expression is easily applied for particular design evaluations.The model directs attention to critical design parameters related to the carcass tearing failure mode and thereby supports continued safety in the design and operation of flexible pipes from a carcass tearing perspective.Copyright

Global Dynamic Performance of Flexible Risers Employing Coupled Analysis

Flexible risers are inevitable in the operation of floating production units in intermediate water depths in harsh environments. The operation of flexible pipes under these conditions is challenging, and reliable and accurate analysis of the global performance of risers is vital. The main purpose of this paper is to describe an analysis method for the global performance of flexible risers taking advantages from advanced floater motion modelling and comprehensive modelling of the risers. The analysis scheme has been employed on a semi-submersible platform, operated in harsh environments in the North Sea and all its flexible risers and umbilicals have been analyzed. The platform motions are generated by means of a coupled vessel/slender structure model in which the vessel force model is introduced into a finite element model of the slender structures including the mooring lines and all its flexible risers. In this way all relevant coupling effects from damping, restoring and current loading on the slender structures are consistently included in the platform motion predictions. Subsequently each riser is analyzed in separate models with high element mesh resolution. The platform motions, in terms of low frequency and wave frequency motions obtained from the coupled model, are applied consistently with wave and current loading in the detailed riser model. In the detailed riser models, bending stiffeners are modelled with non-linear material behavior. The diffracted wave field is included in the wave loading of the risers. The stick-slip bending of the flexible pipes have been modelled for a selection of risers and the effect of this on the riser hang off loads and the curvature in the bend stiffener region is discussed. The main responses looked into have been the curvature in the bend stiffener region, the hang off loads and interference between risers and platform pontoons. The riser responses obtained from the coupled analysis scheme performed in the present work is compared to more standard analysis schemes where extreme offsets and transfer functions are used to generate platform motions.Copyright

Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.Copyright

Hydrodynamic Coefficients for Straked Risers

A considerable challenge in deep water field developments is the possibility of Vortex Induced Vibrations (VIV) of flexible risers due to presence of high currents. The most common remedy is the use of VIV suppression devices such as helical strakes.The background for this study is the lack of guidelines for estimation of hydrodynamic force coefficients for straked risers. In particular there seems to be no publicly available experimental data on tangential force coefficients for straked risers. Analyses indicate that tangential drag may be important for compliant steel risers, e.g. SCRs and steel lazy-waves risers on high-motion platforms in harsh environments. For large vertical motion of the platform there will be a substantial tangential relative velocity along the riser. Adding strakes to the riser will then effectively increase the tangential drag.Simplified formulations for hydrodynamic coefficients for straked risers have been derived, including added mass and drag coefficients for both normal and tangential flow. The simplified formulations are derived from basic hydrodynamic theory without experimental calibration except for the drag coefficients for normal flow where the proposed drag coefficients are empirical based on experiments reported in the literature.An example of a global analysis of straked risers in deep water is presented to demonstrate the effect of tangential drag. Derived formulas for hydrodynamic coefficients to be used in global analysis of straked risers will be implemented in DNV GL Recommended Practices.Copyright

Carcass Failures in Multilayer PVDF Risers

Statoil have experienced failures in a number of flexible risers due to collapse, overload, tearing and unspiraling of the carcass in pipe structures with multi-layer PVDF pressure barrier. The paper will discuss the carcass failure modes that are characteristic for multilayer flexible pipes with particular focus on the failures due to carcass tearing. The nature of the carcass tearing problem is explained and suggestions for load model and operational policy for mitigation risk of new failures is presented.Copyright