Carl M. Larsen

A reliability-based control algorithm for dynamic positioning of floating vessels

Abstract The present paper is concerned with utilization of reliability methods in relation to on-line control of dynamic systems. The particular application is to dynamic positioning of marine vehicles in connection with reliability of mechanical subsystems. The present focus is on top and bottom angles of marine risers which are suspended between the seabed and the floating vessel. These angles are of crucial importance during, e.g. drilling and workover operations. The relationship between surface floater motion and angle responses is first considered. The possibility of reducing the maximum angular response levels by dynamic positioning of the floater is then investigated. Typically, and somewhat dependent of variation of current with depth, minimization of one of the riser top and bottom angles by adjusting the vessel position will take place at the cost of increasing the other angle. Hence, an optimum position should be defined by considering both angles but with different weight functions. An attractive approach is to determine these weights as functions of the respective reliability indices for each of the two angles. A further possibility is to apply an object function (loss function) which is purely expressed in terms of reliability indices. The viability of different schemes of this type is explored by numerical simulation for a specific riser configuration.

Comparison of models for vortex induced vibrations of slender marine structures

During recent years several different models for prediction of vortex induced vibrations of slender marine structures such as risers and cables have been proposed. The purpose of this paper is to provide a consistent discussion of some of the most commonly applied models. Results from four predefined cases were presented by seven participants in a Workshop on Vortex Induced Vibrations of Marine Risers and Cables in Trondheim, May 1994. The present paper is the final report from that Workshop.

Reynolds Number Dependence of Flexible Cylinder VIV Response Data

The response amplitude and the non-dimensional frequency of flexible cylinder vortex-induced vibrations from laboratory and field experiments show significant trends with increasing Reynolds number from 103 to 2 * 105 . The analysis uses complex data from experiments with wide variations in the physical parameters of the system, including length-to-diameter ratios from 82 to 4236, tension dominated natural frequencies and bending stiffness dominated natural frequencies, sub-critical and critical Reynolds numbers, different damping coefficients, standing wave and traveling wave vibrations, mode numbers from 1 – 25th , and different mass ratios.Copyright

Optimization of Catenary Risers

This paper describes how the design of a catenary riser can be formulated as an optimization problem by using riser costs as the criteria function, design requirements in terms of maximum allowable stress and buckling capacity as constraints, and riser dimensions as free variables. The theory has been implemented in a computer program that can generate an optimized riser design for given design parameters such as water depth, diameter, pressure, and platform excursions. The developed software consists of a conventional program for two-dimensional riser analysis and a set of standard routines to minimize a nonlinear function subjected to general constraints. A case study where design parameters and requirements have been varied is also presented. The importance of buckling versus allowable equivalent stress as the most critical constraint has been investigated for varying water depth. The Conclusion of this work is that optimization is a useful tool for riser design, and that the proposed strategy for selection of design variables and constraints will enable an engineer to identify designs with minimum costs in an efficient way.

Efficient Analysis of a Catenary Riser

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. The focus of the paper will be on understanding the riser behaviour in extreme, low-tension response and in establishing suitable analysis strategies to predict the extreme response. Results from long nonlinear stochastic simulations of many sea states with varying environmental and operating conditions may be combined to describe the long-term response of a nonlinear structure such as a catenary riser. However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. The usefulness of simpler methods such as regular wave analysis to improve understanding of the physical behaviour and to aid in concentrating the nonlinear simulations to where they are most useful, will be demonstrated.Copyright

Flexible Riser Response Induced by Combined Slug Flow and Wave Loads

Flexible risers provide optimum solutions for deep water offshore fields. Reliable dynamic analysis of this kind of slender structure is crucial to ensure safety against long time fatigue failure. Beyond the effects from wave loads, the influence from transient internal slug flow on the slender structure dynamics should also be taken into account.In this study two coupled in-house codes were used in order to identify and quantify the effects of an internal slug flow and wave loads on the flexible riser dynamics. One code carries out a global dynamic analysis of the slender structure displacements using a finite element formulation. The other program simulates the behaviour of the internal slug flow using a finite volume method. The slug flow is influenced by the dynamic shape of the riser, while the time varying forces from internal slug flow plus external waves will influence the shape. Hence, a fully coupled analysis is needed in order to solve the coupled problem. By means of the distributed simulation these two programs run synchronously and exchange information during the time integration process.A test case using hydrodynamic forces according to the linear Airy wave theory, coupled with an internal unstable slug flow was analysed and the results show: amplification of the dynamic response due to the interaction between the two load types, effects on the effective tension caused by the internal two-phase flow, and influence on the internal slug flow caused by the wave induced response.Copyright

On the Dynamic Response of Flexible Risers Caused by Internal Slug Flow

Slug flow through flexible risers is a frequent phenomenon which occurs during production of a mixture of oil and gas. The dynamic nature of the slug pattern induces time varying forces, which leads to structural vibrations of the riser. These vibrations can produce large deflections and stresses, which can leave it to fail by fatigue, excessive bending or local buckling.In this work the influence from slug flow on the structural dynamic response of a lazy wave flexible riser is analyzed using a computational tool consisting of one program for calculation of slug flow dynamics, and another program for structural dynamic response. Both programs apply a time integration method, and since slug flow will lead to dynamic motion response of the riser, and riser motion dynamics will influence slug flow dynamics, the two codes need to exchange information during the integration process. Information exchange is established by making a federation based on High Level Architecture (HLA).The federation is composed of SLUGIT and RISANANL. SLUGGIT is a two-phase flow code written in C++ which simulates dynamic slug flow through pipes and riser using a Lagrangian tracking model. RISANANL is a FORTRAN program for static and dynamic structural analysis of slender marine structures based on a finite element formulation. Using the HLA standard these two programs can carry out synchronized time integration and exchange information for each time step.In this work the structural analysis code accomplishes the dynamic response using a linear finite element (FE) formulation. Hence, forces from centripetal acceleration of the internal flow, relative velocity between the riser and surrounding water, and varying gravity of the pipe and content will be accounted for in the dynamic analysis. Displacements, stresses, internal pressure, and outlet flow rates of liquid and gas will be accounted for. The results encourage us to carry out a fully non-linear finite element analysis, in order to have a better understanding of the dynamic behaviour of flexible risers undergoing an unsteady internal two-phase flow.Copyright

Norwegian Deepwater Program: Damping of Vortex-Induced Vibrations

Vortex-induced vibration (VIV) of a long riser in sheared current is often considered as an energy balance problem: Excitation forces in the power-in region add an equal amount of energy to the system as is dissipated by damping forces outside this region and structural damping. A riser may have different excitation and damping regions depending on the actual oscillation frequency, cross-section properties and local flow velocity. A damping model must hence be able to handle higher and lower flow velocities than the excitation velocity range. In this paper the fluid damping models proposed by Venugopal [1] are compared with the experiments conducted by Gopalkrishnan [2] and Vikestad [3]. The results show that the models are conservative at high and low reduced velocities.

VIV of Flexible Cylinder in Oscillatory Flow

VIV in oscillatory flow is experimentally investigated in the ocean basin. The flexible test cylinder was forced to harmonically oscillate in various combinations of amplitude and period. VIV responses at cross flow direction are investigated using modal decomposition and wavelet transformation. The results show that VIV in oscillatory flow is quite different from that in steady flow; novel features such as ‘intermittent VIV’, amplitude modulation, mode transition are observed. Moreover, a VIV developing process including “Building-Up”, “Lock-In” and “Dying-Out” in oscillatory flow, is further proposed and analyzed.© 2013 ASME

A New Approach for Identification of Forces on Slender Beams Subjected to Vortex Induced Vibrations

Vortex induced vibration (VIV) has been subjected to extensive research during the last 20 years. A large number of laboratory and ocean tests with long slender beams or cables have been reported. Key results from such experiments have been response frequencies and amplitudes, but also information on mode composition and traveling waves. Due to the difficulty of direct force measurement, accelerometer and bending strain measurement are used in such experiments. Formally, it should be possible to identify the forces that have created the measured response, but so far few results from such attempts have been reported. An inverse force estimation method is adopted to provide an accurate way of reconstructing the unknown hydrodynamic forces from measured dynamic response data. The method is based on state space formulation of a finite element beam model. It incorporates the Kalman filtering and recursive least squares algorithm to remove the noise from measurement and obtain force estimation in discrete time domain. The inverse force estimation method is verified with numerical simulations. The input force of a tensioned beam structure is estimated from response. The result indicates its capability to accurately estimate the input forces from stochastic response. The method is applied to the data from Rotating Rig Test to identify hydrodynamic forces along the riser. The lift force and added mass coefficients are calculated and compared with existing data.Copyright