John M. Niedzwecki

System identification of distributed-parameter marine riser models

Modeling engineering problems of interest often requires some type of discretization model of the physical system and quite naturally leads to a mathematical description involving partial differential equations whose coefficients are dependent on both time and spatial location. In this study a reverse system identification approach is presented that utilizes generalized coordinate and force functions to recover the value of the key system parameters for each mode of vibration. To illustrate the analysis procedures, a single marine riser with general damping-restoring types of non-linearities subject to random wave excitation is considered. Analytical expressions as functions of the modes for bending stiffness and tension are derived and used for comparison with the results obtained using system identification. Numerical simulations including band-limited white noise and random wave excitation are used to explore the adequacy of the methodology and the benefits of using modal analysis in the system identification procedure. Finally, the use of and comparison with experimental data is presented and the frequency variation of parameters obtained resulting from system identification procedures discussed. Collectively, the examples demonstrate that this system identification methodology accurately identifies system parameters over portions of the frequency range of interest.

Wave interaction with tension leg platforms

Abstract The design of deep water offshore platforms requires the analysis of wave-structure interaction phenomena which have not been as critical for shallower water platform designs. In the case of tension leg platforms (TLPs) interaction phenomena such as wave run-up on the vertical legs and the amplification of the waves beneath the deck are major design considerations. The research investigation reported here focuses on a series of small scale wave tank tests on four column TLP models examining these phenomena. The role of vertical leg spacing and comparative tests of the TLP models with and without pontoons was investigated. As the vertical legs were moved closer an increase in wave run-up and a shifting of the incident wave period corresponding to the maximum wave upwelling were noted. Comparisons with wave measurements for single cylinders from previous experimental studies and the TLP configurations used in this study are presented. A design formula for estimating wave run-up on TLPs is suggested based upon these experiments. The wave run-up on a leg directly in the wake of another leg is presented. A comparison of the wave upwelling measurements with previously published numerical results are discussed. A wave uplift force model which allows for the inclusion of the experimentally obtained wave upwelling measurements is presented and discussed with regard to the design specification of platform deck elevation.

Response characteristics of multi-articulated offshore towers

Abstract Compliant offshore platforms are designed to allow controlled movement of the platform as an alternative to near-rigid resistance to wind, wave and current loading by fixed platform designs. The equations of motion, valid for both single and multi-articulated towers, are derived using a Lagrange equation approach. The resulting equations are then used to study the sensitivity of a tri-articulated tower to select design parameters. A tension-restrained articulated platform (TRAP) proposed by the offshore industry for use in 914 m (3000 ft) of water is used for comparison in the sensitivity studies. Dimensionless parameters are developed and used to identify the roles of several key variables in altering the natral periods of tower vibration in order to avoid regions of significant environmental excitation.

HEAVE COMPENSATED RESPONSE OF LONG MULTI-SEGMENT DRILL STRINGS

A model for estimating the dynamic behavior of a long drill string excited by the heaving motion of the drill ship in a seaway, is presented. The drill string is modeled as an assembly of continuous rod segments of varying cross section and material properties, to which a large package can be attached. A passive heave compensation model is derived which represents the system used for deep water scientific drilling activities. The drill string model allows the specification of the seaway modeled as a single design wave or in terms of a wave spectrum. The drill string system response is characterized in terms of transfer functions representing displacement and stress at selected elevations along the drill string. The heave compensation for a variety of 20,000 ft (6,096m) drill string systems is discussed. The numerical results illustrate the sensitivity of the dynamic response estimates in near resonant conditions, and excitation of higher modes for very long drill string systems and the motion control available due to the introduction of passive heave compensation.

Design estimates of surface wave interaction with compliant deepwater platforms

The extreme behavior of surface waves as they encounter and pass compliant deepwater platforms is an important class of problems for offshore engineers attempting to specify the platform deck elevation. In this study analytical expressions for the probability density and cumulative distribution functions that utilize empirical coefficients in an attempt to accurately model surface wave runup and airgap problems are presented. The analysis focuses upon interpreting the tails of the measured data histograms using two parameter Weibull distribution models. The appropriate empirical constants, assumed to be solely dependent upon the significant wave height, were evaluated and compared for all the test data. Based upon a small select set of data, for a mini-TLP and two Spar platforms, the airgap problem was found to be adequately modeled using a Rayleigh distribution. Further, for the seven seastates analyzed, the Weibull shape parameter was nearly constant and the data confirmed that the exclusive fit of the scale parameter assuming dependence only on the significant wave height was a reasonable approach for modeling the wave runup. Finally, by combining these models with a Poisson return model for each storm the associated reliability estimates for various deck heights were estimated.

Estimating nonlinear coupled frequency-dependent parameters in offshore engineering

Abstract The design of deepwater compliant offshore structures requires engineers to address many difficult challenges including defining and modeling the local offshore environment, specifying the associated combined global loading on innovative platform designs, and numerical simulation and model test verification of the platform response characteristics. The focus of this research investigation is the recovery of key parameters from time series measured during an industry type model basin test program using the reverse multiple input/single output technique. In particular, this study confronts critical problems of practical interest and extends the methodology to address the inclusion of nonlinear coupled systems in which the parameters of interest can be frequency-dependent. The analysis is developed around the nonlinear coupled equations of motions for a deepwater mini-TLP design and includes the consideration of nonlinear stiffness, quadratic damping, surge/pitch and sway/roll coupling and the frequency dependency of both the hydrodynamic added-mass and damping coefficients. A series of complementary model test measurements for the complete compliant model and the rigidly restrained hull by itself were used as the basis for the data in the system identification procedures. In addition, behavior of the hydrodynamic added-mass and damping coefficients as a function of frequency was simulated for the mini-TLP using an industry standard radiation–diffraction software package. These results were used in evaluating the accuracy of some of the key problem parameters. The results presented demonstrate the methodology as modified in this study is quite robust and yields predications that are more accurate for the parameters associated with the largest motions of the platform. Practical issues regarding the application of this approach, utilization of both force and moment measurements, and observed strengths and weakness in dealing with data regardless of its source are discussed.

Estimating extreme tendon response using environmental contours

The environmental contour technique was used to estimate the extreme in-line responses of deep-water TLP tendons designed for the Gulf of Mexico. The simulated response estimates were then used to estimate failure probabilities and reliabilities utilizing a deterministic displacement limit state. The reliability of individual tendons and tendon systems is directly associated with their respective probabilities of failure. By designing for environmental contours identified using this technique the resulting design will be more likely to approach the intended target reliability. In this article the environmental contour theory is explained and then used to estimate the extreme tendon responses in two examples reflecting practical design uncertainties. Experimental data from large scale model tendon experiments was introduced in order to assess the numerical prediction. In the first example the problem of uncertainty associated with pretensioning of the individual tendons is investigated. Although the amount of uncertainty due to the change in tension is not known the use of contour inflation is illustrated as a means to compare the numerical prediction with the experimental data. The second example addresses the uncertainties associated with the fluid/structure interaction. The placement of tendons in close proximity results in the amplification of the tendon motions. At present, no adequate hydrodynamic model exists which can be used with confidence in design practice. Again contour inflation is explored as a means to compensate for this phenomena and to quantify in a global sense the impact of this uncertainty on design.

FILTER APPROACH TO OCEAN STRUCTURE RESPONSE PREDICTION

The design and application of shaping filters for random response analysis of offshore structures is presented. The approach is based upon Markov methods and its suitability for use in offshore engineering is demonstrated through a series of numerical examples. Shaping filters are designed for generating the wave elevation, wave kinematics and wave forces from white noise. The filter equations are combined with state-space equation models of two different types of offshore structures. Moment equations are generated using Itos rule and solved for the structural response in terms of statistical moments. This filter methodology is applied to both linear and non-linear structural models of offshore platforms. The strengths and limitations of the filter approach are examined through direct comparison with time domain simulations.

Heave response of long riserless drill strings

Abstract The sensitivity of heave response predictions for long riserless drill strings hanging from a floating vessel are examined. A general analytical procedure is presented which is suitable for a variety of deepwater pipe systems. The dynamic response behavior is characterized in terms of the dynamic magnification, phase angle and total stress. The examples presented here focus on drill string systems, which vary in length from 9000 ft (2743 m) to 27,000 ft (8230 m) and can include a large package to be lowered to the seafloor. A procedure for evaluating the undamped natural periods and the corresponding mode shapes is presented. The sensitivity of the natural period estimates to hydrodynamic added mass approximations is examined. Numerical results for both design wave and random sea simulations are used to illustrate the sensitivity of the dynamic response in near resonant conditions and the possibility of exciting higher modes for very long riserless drill string systems. Finally, the sensitivity of the displacement predictions to reasonable variations in skin friciton and viscous drag coefficients are presented and discussed.

SUPPRESSION OF FLOW-INDUCED VIBRATIONS USING RIBBON FAIRINGS

An experimental study was conducted to investigate the ability of ribbon fairings to suppress fl ow-induced vibrations on a long fl exible horizontal cylinder. The test matrix included towing the cylinder at various speeds, towing the cylinder in regular waves, and investigating the infl uence of partial coverage on the response behavior. The test cylinder was 29 m long with a length to diameter (L/D) ratio of ~760. Interior to the tensioned cylinder model were six sets of unequally spaced biaxial accelerometers in a lightly pressurized environment keeping the interior dry. A string potentiometer was externally attached at the center of the model to provide a reference for later displacement estimates based on integration of the acceleration data. The time domain decomposition method (TDD) was used to recover mode shapes, damping characteristics, and modal contribution factors. For the uniform current cases, the fi ndings illustrate that ribbon fairings are effective and provide increased damping when compared with bare cylinders. Partial coverage demonstrates that localized suppression becomes increasingly less effective as the percentage coverage is reduced. The introduction of regular waves to the towed cylinder cases illustrates the ineffectiveness of ribbon fairing to suppress the orbital motions induced by the waves, which is preferable to the amplifi cation typically observed for airfoil fairings.