V.G. Idichandy

Structural monitoring of offshore platforms using impulse and relaxation response

Monitoring offshore platforms, long span bridges, high rise buildings, TV towers and other similar structures is essential for ensuring their safety in service. Continuous monitoring assumes even greater significance in the case of offshore platforms, which are highly susceptible to damage due to the corrosive environment and the continuous action of waves. Also, since a major part of the structure is under water and covered by marine growth, even a trained diver cannot easily detect damage in the structure. In the present work, vibration criterion is adopted for structural monitoring of jacket platforms. Artificial excitation of these structures is not always practicable and ambient excitation due to wind and waves may not be sufficient for collecting the required vibration data. Alternate methods can be adopted for the same purpose, for example, the application of an impact or a sudden relaxation of an applied force for exciting the structure. For jacket platforms, impact can be applied by gently pushing the structure at the fender while relaxation can be accomplished by pulling the structure and then suddenly releasing it using a tug or a supply vessel in both cases. The present study is an experimental investigation on a laboratory model of a jacket platform, for exploring the feasibility of adapting vibration responses due to impulse and relaxation, for structural monitoring. Effects of damage in six members of the platform as well as changes in deck masses were studied. A finite element model of the structure was used to analyze all the cases for comparison of the results as well as system identification. A data acquisition and analysis procedure for obtaining the response signatures of the platform due to the impulse and relaxation procedure was also developed for possible adoption in on-line monitoring of offshore platforms. From the study, it has been concluded that both impulse and relaxation responses are useful tools for monitoring offshore jacket platforms. The present work forms the basis for the development of an automated, on-line monitoring system for offshore platforms, using neural networks.

ART-based multiple neural networks for monitoring offshore platforms

A novel scheme using artificial neural networks to automate the vibration monitoring method of detecting the occurrence and location of damage in offshore jacket platforms is presented. A multiple neural network system is adopted which enables the problem to be decomposed into smaller ones, facilitating easier solution. An adaptive resonance theory (ART) neural network is used for damage diagnosis and its advantages and limitations are investigated. A comparison between a back-propagation network and an ART network is presented. The adaptability of ART for on-line monitoring is explored for possible adaptation to monitor offshore platforms in service. The system developed is tested using data from a finite-element analysis of a scale model of a jacket platform.

Coupled dynamics of SeaStar mini tension leg platform

Abstract The role of mini tension leg platforms (TLP) in oil exploration and production in marginal deepwater fields is becoming increasingly important. SeaStar is a mini TLP that combines the simplicity of a spar and favourable response features of a TLP. In this paper, the results of a detailed numerical investigation of the coupled dynamic behaviour of SeaStar are reported with special attention to platform-tether coupling. The numerical study has been carried out using a finite element computer code developed for the nonlinear dynamic analysis of compliant offshore platforms in the time domain using Morison type wave loading. A typical SeaStar platform has been considered at two water depths, 215 m and 1000 m. Experimental investigation was conducted for a scaled model corresponding to 215 m water depth for validation of the numerical model. The model-prototype correlation studies have been conducted using the numerical model. Response amplitude operators of motion and tether tension are presented for the two typical water depths and conclusions drawn.

Modal parameters for structural integrity monitoring of fixed offshore platforms

The most commonly used offshore oil-production platforms, the jacket structures, because of their long service in a very hostile environment, require periodic inspection for structural integrity. Present-day diver inspection is highly uneconomical and has certain inherent limitations. To overcome these limitations, it has been proposed to use an instrumented monitoring system based on the dynamic response of the platform. Several investigations were carried out to remove the uncertainties in this method which arose mainly from changes in the mass on the deck and marine growth. One of the important proposals investigated was the use of modal vectors. But these investigations were based on simple analytical and physical models. In order to assess the efficiency of using modal vectors to monitor the structural integrity of the platform, a detailed study was carried out on a real platform. Finite-element and physical models of the platform were used in the investigation. This paper reports on the physical model tests. A geometric model of the platform was tested under electrodynamic and wave excitation, simulating damages in the structure and changing the mass on the deck. In all tests, modal frequencies and modal vectors in different degrees of freedom were determined by autospectral analysis of acceleration response. Baed on the results, a scheme for integrity monitoring of a fixed offshore platform is suggested which, if implemented carefully, can at least minimize, if not dispense with, the uneconomic diver inspection.

Strain-gauge-based force transducers in hydrodynamic research

This paper presents three typical resistance strain gauge based force transducers developed with specific requirements of hydrodynamic model testing and have been very useful in test which otherwise would have been impossible or uneconomical. The transducers presented include those for measuring single and multi-component forces and moments.

On experimental investigation of load-out, launching and upending of offshore steel jackets

Abstract Load-out, launching and upending are major sequential installation operations of the offshore steel jackets. Due to vital engineering importance of these high risk operations, a thorough scientific understanding of their mechanics is desirable and a physical simulation approach using scaled models is a powerful method to achieve this end. This paper presents such an approach in the context of a case study which is oriented towards gaining adequate understanding of the modelling principles, model design and simulation of operations. It also highlights the role of decisive parameters as they affect the operational performance. Some of the experimental techniques of interest are also presented.

Perforated-ball velocity meter measurements of the water wave kinematics in nonlinear, long and short crested waves

Direct measurement of the wave kinematics in laboratory and field has always occupied a central place in experimental research in water waves. In this paper, a laboratory scale, multi-stage three component Perforated-ball Velocity Meter (PVM) has been used to measure the wave kinematics in linear as well as nonlinear waves, in long crested as well as short crested random waves. Special attention is devoted to measurement of the near surface kinematics, especially in regular nonlinear waves. The laboratory measurements in deep and shallow water wave flumes and wave basin have been conducted using the PVM and the measured kinematics have been compared with the calculated kinematics based on several semi-empirical methods as well as ultrasonic current meter measurements. The comparisons have been found to be excellent at all water depths.

A perforated-ball velocity meter for underwater kinematics measurement in waves and current

A perforated-ball velocity meter (PVM) has been developed for measuring all three components of water particle velocities at five different vertical locations simultaneously in regular, long crested as well as short crested free surface wave field in laboratory scale. The device measures the wave induced forces on a cantilevered ball-tube assembly. The wave kinematics is computed from these forces by solution of an inverse problem given by the Morison equation. The drag and inertia coefficients of both the ball and the tube are experimentally determined and the forces on both the ball and the tube are used in the inverse Morison problem. Measurements in wave flume and wave basin have been conducted using the multistage PVM and the measured kinematics has been successfully compared with theory using wave elevation measurements. The device can be used to measure current speed as well as wave kinematics in a wave-cum-current field.

Studies on structural monitoring of offshore jacket platforms

The paper describes a series of analytical and physical model studies conducted to develop an online monitoring system for offshore platforms. An actual offshore jacket platform situated in a water depth of 88m was selected for the study. A detailed 3D finite element analysis of the platform involving free and forced vibration revealed that there were dynamic characteristics of the platform which could be used to identify the structural damages in the structure. Having established the feasibility of the method, further work was carried out on a physical model of the platform. Dynamic characteristics of the model were determined by spectral analysis of the response data, simulating various changes including compete and partial damages on individuals elements of the model. Simultaneously work was also initiated to apply the result of the physical model tests for interpreting the causes of the changes in the dynamic characteristics using ANN trained with the database created through experiments and analysis. The final outcome of these comprehensive studies was a scheme for integrity monitoring of jacket type of fixed offshore platforms using vibration characteristics of the structure.

Inverse problem of Morison equation for a perforated-ball velocity meter : an improved method

A Perforated-ball Velocity Meter (PVM) device measures the wave induced forces on a cantilevered ball-tube assembly and the wave kinematics is computed from these forces by solution of an inverse problem given by the Morisons equation. The drag and inertia coefficients of both the ball and the tube are experimentally determined and the forces on both the ball and the tube are used in the inverse Morison problem. The behaviour and convergence of the numerical scheme for this inverse problem has been studied in detail. The laboratory experiments conducted using the PVM and the measured kinematics have been compared with calculated kinematics obtained from the free surface elevation measurements using the linear wave theory and the Wheeler stretching theory. The comparisons have been satisfactory. It is also shown that the kinematics measured by the PVM can yield the free surface elevation with good accuracy.