Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper presents a probabilistic procedure for determining the maximum loads and pressures on a fixed offshore structure due to multi‐year ice floes during winter loading events. The approach depends on a computerized simulation procedure which is described together with mechanistic models for estimating the floe contact width, peak indentation pressure, pressure versus displacement, and interaction between the multi‐year floe and the surrounding first‐year ice. Several load scenarios involving different assumptions are investigated and the predicted loads compared on the basis of their cumulative probability distributions. The paper also presents a procedure for selecting the design ice pressure as a function of the loaded area size.

A two‐part study was conducted to help evaluate analytical methods used to assess the ductile behavior of offshore structures subjected to severe earthquake ground motion. The first part of the study was a comparison of analytical time‐history responses, predicted using the Inelastic Tower Response Analysis (INTRA) program, and experimental data gathered during a shake‐table experiment performed by the University of California at Berkeley in March 1981. (INTRA is a registered trademark of PMB Systems Engineering, Inc., and is owned by Shell Oil Co.) The experiment involved a series of tests on a scaled, two‐dimensional model of a prototype offshore platform subjected to various levels of simulated ground motion. The second part of the study consisted of a comparison of ductile behavior, predicted using simplified analysis methods (static pushover and impulse velocity), and ductile behavior demonstrated by the experimental and analytical time‐history results. First part results indicated that the analytical time‐history analyses predicted within reasonable accuracy the response of the test structure as measured during the experiment. The results of the second part demonstrated that the simplified methods produce conservative estimates of the test structure's energy absorption capacity and ductile behavior during intense ground motion.

The paper reports on a study carried out by J. P. Kenny & Partners on behalf of the U.K. Department of Energy to update the Guidance Notes on the Buckling of Offshore Steel Structures. The Guidance Notes will be incorporated in the revised edition of the Department of Energy document “Offshore Installation: Guidance on Design and Construction.” Ring‐stiffened cylinders formed one of five categories of structural elements investigated by the writers in the course of the study. The paper presents comprehensive collections of test results gathered from all available sources worldwide relevant to the buckling of unstiffened and ring‐stiffened cylinders under axial, pressure, and combined loadings. The collated results are compared with the corresponding predictions from various code recommendations. All forms of collapse, such as inter‐ring shell buckling, general instability, and ring‐stiffener failure, are considered. Conclusions are drawn as to the validity and implied safety of these design recommendations and gaps in the available experimental and design information are identified.

Modifications to vertical velocity profiles of coastal currents due to surface gravity waves were experimentally and theoretically examined. Results on these modifications were used to develop guidelines for approximating the form of current velocity profiles to be used in predictive models of wave kinematics. The validity of the guidelines was established by comparing the experimental data of wave particle velocities with the theoretical predictions of two wave‐current models which approximate the shear current by: (1) A constant vorticity current; and (2) a uniform velocity over the water depth. The results show that opposing waves decrease the current mean velocity close to the bottom and increase the mean velocity and the current shear near the free surface. Following waves increase mean velocity and velocity gradient of the current, close to the bottom, and might cause the shear at the surface to be negative. Models for constant vorticity and uniform velocity approximations are found to yield accurate predictions of horizontal particle velocities for waves on opposing and following currents, respectively.

Measured jacket brace member axial loads are compared with predictions based on conventional static structural analyses. The measured to predicted member load ratios are shown to be lognormally distributed. Slightly conservative predictions are obtained, on the average, using drag and inertia coefficients of 0.7 and 1.7, respectively, and with no consideration of current or marine growth. A study of member loads vs. total wave load (shear) demonstrated that the wave shear distributions can be directly inferred from the member load distributions. For the larger waves, a median or bias of approximately 0.93 was estimated for the wave load with a coefficient of variation of approximately 0.18. A parameter study indicates that the addition of large currents and levels of marine growth, especially if the drag coefficient is increased because of roughness, results in extremely conservative wave load estimates.

Tandem (piggyback) and parallel anchor rigging arrangements have been found that result in anchor system capacities that exceed twice the capacity of a single anchor. These tests are part of a program to determine effective methods of utilizing high efficiency anchors in multiples to satisfy expanding fleet mooring requirements. Scale models of a Navy STATO anchor were tested in multiple arrangements in a sand test bed. Anchor arrangements have been found that increase anchor system capacity by 25% over twice the single anchor capacity without decreasing anchor stability. The typical fleet mooring arrangement of side‐by‐side parallel anchors decreases the anchor system capacity to less than twice the single anchor capacity due to anchor interference. Staggering the anchors by at least an anchor length increased the anchor system capacity to more than twice the single anchor capacity. Dragging the anchors far enough for the rear anchor to penetrate the soil disturbed by the forward anchor can result in a total anchor capacity exceeding twice the single anchor capacity in sand.

This paper deals with a comparison between computed and measured motions and thether forces for a deep‐water tension leg platform. The computations are compared with model experiments which have been conducted in both regular and irregular waves in the Seakeeping Laboratory of the Netherlands Ship Model Basin. Attention is paid to prediction of second harmonic components of tether forces in regular waves and to the computation, in the time domain, of combined wave frequency and low frequency motions and tether forces in irregular seas. The time domain computations have been performed using, as input, the wave record measured in the basin. The comparison of computations and model tests results allows identification of the major cause of high frequency tether force fluctuations and demonstrates the applicability of the convolution integral method in time domain analysis of low frequency tension leg platform motions.