Turkstra Profiles of North Sea Currents: Wider Than You’d Think

To design marine structures in deep water, currents must be modelled accurately as a function of depth. These models often take the form of T-year profiles, which assume the T-year extreme current speed occurs simultaneously at each depth. To better reflect the spatial correlation in the current speeds versus depth, we have recently introduced Turkstra current profiles. These assign the T-year speed at one depth, and “associated” speeds expected to occur simultaneously at other depths. Two essentially decoupled steps are required: (1) marginal analysis to estimate T-year extremes, and (2) some type of regression to find associated values. The result is a set of current profiles, each of which coincides with the T-year profile at a single depth and is reduced elsewhere. Our previous work with Turkstra profiles suggested that, when applied in an unbiased fashion, they could produce unconservative estimates of extreme loads. This is in direct contrast to the findings of Statoil, whose similar (“CCA”) current profiles have generally been found to yield conservative load estimates. This paper addresses this contradiction. In the process, we find considerable differences can arise in precisely how one performs steps 1 and 2 above. The net finding is to favor methods that properly emphasize the upper tails of the data—e.g., using peak-over-threshold (“POT”) data, and regression based on class means—rather than standard analyses that weigh all data equally. By applying such tail-sensitive methods to our dataset, we find the unconservative trend in Turkstra profiles to essentially vanish. For our data, these tail-fit results yield profiles with both larger marginal extremes, and broader profiles surrounding these extremes—hence the title of this paper.

Quantitative Risk Analysis and Comparison for Onshore and Offshore LNG Terminals: The Port of Suape - Brazil Case

In recent decades, natural gas has been gaining importance in world energy scene and established itself as an important source of energy. One of the biggest obstacles to increase the usage of natural gas is its transportation, mostly done in its liquid form, LNG – Liquefied Natural Gas, and storage. It involves the liquefaction of natural gas, transport by ship, its storage and subsequent regasification, in order to get natural gas in its original form and send it to the final destination through natural gas pipeline system. Nowadays, most terminals for receiving, storing and regasificating LNG, as well as sending-out natural gas are built onshore. These terminals, however, are normally built close to populated areas, where consuming centers can be found, creating safety risks to the population nearby. Apart from possible damages caused by its cryogenic temperatures, LNG spills are associated with hazards such as pool fires and ignition of drifting vapor clouds. Alternatively to onshore terminals, there are currently several offshore terminals projects in the world and some are already running. Today, Brazil owns two FSRU (Floating Storage and Regasification Unit) type offshore terminals, one in Guanabara Bay, Rio de Janeiro and the other in Pece´m, Ceara´, both contracted to PETROBRAS. The identification of the operation risks sources of LNG terminals onshore and offshore and its quantification through mathematical models can identify the most suitable terminal type for a particular location. In order to identify and compare the risks suggested by onshore and offshore LNG terminals, we have taken the example of the Suape Port and its Industrial Complex, located in Pernambuco, Brazil, which is a promising location for the installation of a LNG terminal. The present work has focused on calculating the distance to the LNG vapor cloud with the lower flammability limits (LFL), as well as thermal radiation emitted by pool fire, in case of a LNG spill from an onshore and from an offshore terminal. The calculation was made for both day and night periods, and for three types of events: operational accident, non-operational accident and worst case event, corresponding to a hole size of 0,75m, 1,5m e 5m, respectively. Even though the accidents that happen at an onshore terminal generate smaller vulnerability distances, according to the results it would not be desirable for the Suape Port, due to the location high density of industries and people working. Therefore, an offshore terminal would be more desirable, since it presents less risk to the surrounding populations, as well as for workers in this location.

Influence of Transverse Cracks on Ultimate Strength of a Steel Plate Under Compression

This study has been carried out on ultimate compressive strength of a cracked steel plate component, considering the effects of initial imperfections (transverse and longitudinal residual stresses and initial deflection, as well). The main objective of this paper is to numerically investigate the influence of crack location and crack length on ultimate strength of a steel plate under monotonic longitudinal compression. This investigation is performed through non-linear finite element (FE) analysis using ANSYS commercial finite element code in which is employed Newton-Raphson method. The FE results indicate that the length of transverse crack and especially its location can significantly affect the magnitude of ultimate strength where the steel plate is subjected to longitudinal compressive action.

An Experimental Study of Shallow Water Wave Statistics on Mild Bed Slopes

This paper describes a new series of laboratory observations, undertaken in a purpose built wave flume, in which a number of scaled simulations of realistic ocean spectra were allowed to evolve over a range of mild bed slopes. The purpose of the study was to examine the distribution of wave heights and its dependence on the local water depth, d, the local bed slope, m, and the nature of the input spectrum; the latter considering variations in the spectral peak period, Tp, the spectral bandwidth and the wave steepness. The results of the study show that for mild bed slopes the statistical distributions of wave heights are effectively independent of both the bed slope and the spectral bandwidth. However, the peak period plays a very significant role in the sense that it alters the effective water depth. Following detailed comparisons with the measured data, the statistical distributions for wave heights in relatively deep water are found to be in reasonable agreement with the Forristall [1] and Glukhovskii [2] distributions. For intermediate water depths, the Battjes & Groenendijk [3] distribution works very well. However, for the shallowest water depths none of the existing distributions provides good agreement with the measured data; all leading to an over-estimate of the largest wave heights.

A Study on Construction of Evaluation Method for Defect Sizing Utilizing Numerical Simulation

To prevent the brittle fracture of the extremely thick plate which is used in the deck plate and the hatch side coaming of the large container ships, it is important to detect surely and repair the inner defects, which are possible to lead the brittle fracture at an early stage. Now Ultrasonic Testing is used in order to inspect the inner defect in a thick plate, and it is necessary to select adequately the probe size and oscillating frequency to evaluate the defect size accurately. The estimation of the defect size in vertical direction to plate surface is important from the viewpoint of fatigue strength. But the vertical flaw length and also the inclined flaw length are hardly to be estimated accurately. In this study, to clarify the characteristics of reflected ultrasonic wave from the defect, the wave propagation behavior is simulated with the numerical simulation program by FEM which is developed by the authors. In this program, the governing equation of elastic wave propagation is calculated in time domain with explicit method utilizing the central-difference scheme. First, the effect of the probe size and oscillating frequency on the accuracy of defect sizing are investigated utilizing the developed program. The numerical simulation is performed for imaging to examine the length of flaw which is parallel to plate surface by normal beam technique. And, the applicability of 6dB method, which is one of the methods for estimating flaw length, is examined. Moreover, a new method for estimating flaw length which cannot be estimated by 6dB method is proposed. Secondly, in order to examine the inclined flaw length, the angel beam test is performed. The accuracy of numerical simulation for angle beam technique is confirmed by comparing experimental result. And, it is examined how the inclined angle of flaw affects the echo height, and it is shown that 6dB method and L level method are useful for the defect which is perpendicular to wave beam and the tip echo method is useful for the defect which is inclined to wave beam. The actual structure usually has a paint film. Therefore, the echo height level will be affected by paint thickness, and the paint film effects on the accuracy of defect sizing. Thirdly, the effects on echo height by film thickness are clarified by experiments and numerical simulations.

Reliability Assessment for TLP Tether Overload in Nonlinear Waves

Non-linear effects caused by extreme crest loading might be governing for important structural limit states. For TLP’s, non-linear effects caused by extreme crest might lead to tether overload and tether slack. Simple scaling of ULS results is not necessarily adequate if wave in deck events (or other NL phenomena) might occur with annual probability >10−4. In this case ALS analysis should be performed, and detailed knowledge of the physics and statistics of extreme crest events, and of the loading caused by these waves, is required. Ignoring the possibility for wave in deck events and establishing the characteristic tension from data where there are no wave in deck events, might lead to unrealistic low estimates for the extreme tether tension. The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified reliability analysis of a generic TLP defined by the project. In this paper, SRA for tether overload is discussed, with emphasize on the effect of tension Twid caused by wave in deck events. Data from TLP basin test undertaken as part of the CresT JIP are applied, and parametric models for tether tension from measured crests and measured tether tension data are established. The model for Twid is used in the SRA for analysis of tether overloads for design sea states and for long term analysis for different geographic locations. Special attention is given to assessment of the effect of higher order crest non-linearities beyond 2nd order. The crest models are based on results from other work packages of the Crest JIP. Recommendations for future research needs are given.

New Insights in Extreme Crest Height Distributions: A Summary of the ‘CresT’ JIP

The objective of the CresT JIP was ‘to develop models for realistic extreme waves and a design methodology for the loading and response of floating platforms’. Within this objective the central question was: ‘What is the highest (most critical) wave crest that will be encountered by my platform in its lifetime?’ Based on the presented results for long and short-crested numerical, field and basin results in the paper, it can be concluded that the statistics of long-crested waves are different than those of short-crested waves. But also short-crested waves show a trend to reach crest heights above second order. This is in line with visual observations of the physics involved: crests are sharper than predicted by second order, waves are asymmetric (fronts are steeper) and waves are breaking. Although the development of extreme waves within short-crested sea states still needs further investigation (including the counteracting effect of breaking), at the end of the CresT project the following procedure for taking into account extreme waves in platform design is recommended: 1. For the wave height distribution, use the Forristall distribution (Forristall, 1978). 2. For the crest height distribution, use 2nd order distribution as basis. 3. Both the basin and field measurements show crest heights higher than predicted by second order theory for steeper sea states. It is therefore recommended to apply a correction to the second order distribution based on the basin results. 4. Account for the sampling variability at the tail of the distribution (and resulting remaining possibility of higher crests than given by the corrected second order distribution) in the reliability analysis. 5. Consider the fact that the maximum crest height under a complete platform deck can be considerably higher than the maximum crest at a single point.

The Effect of Sloshing in Tanks on Motions and Hull Girder Responses of Damaged Vessels

The structural reliability analysis of damaged vessels has up to now commonly been investigated by neglecting the effect of sloshing. This paper deals with the effect of sloshing in tanks on motions and hull girder responses of oil tankers in various damage conditions and represents a part of a study to assess the effect of sloshing on hull girder failure of damaged vessels, The flooded tanks are assumed to have a of rectangular shape and linear multimodal approach is adopted to deal with sloshing. It is concluded that even though the effect of sloshing in tanks on the roll motion of vessels can be neglected in certain damage conditions, the effect of sloshing on the horizontal bending moment cannot be neglected, especially when resonance motion occurs.

Reliability Assessment of TLP Air-Gap in Nonlinear Waves

The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified structure reliability analysis of a generic TLP structure defined by the project. The study demonstrates effects of wave nonlinearity beyond the second order, diffraction-radiation of incoming waves with the structure, spatial variations of crest statistics, a vertical displacement of the TLP system at the deck location as well as sea water level variations due to surge and tide. In order to account for these effects a stochastic model is suggested. Sensitivity studies are carried out to identify importance of analysed parameters and their stochastic behaviour to the failure probability. Uncertainties related to the analysis are identified and ranked given special attention to the effect of wave nonlinearities beyond the 2nd order. Recommendations for future research needs are suggested.