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.

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.

A Benchmark Study of ISO 18072-2 on the Stiffened Panel Ultimate Strength

The paper presents a major benchmarking exercise to demonstrate the accuracy of the formulations for the ultimate strength of stiffened panels that had been proposed for inclusion in ISO TS 18072-2. The complete set of formulations addressed the ultimate strength of plates, hull girders and support members, in particular, transverse frames and webs. The stiffened panel strength formulations have been in the public domain for some time and represent the most comprehensive set yet published. Their potential application is to a wide range of structural forms, e.g. floating dock gates, caissons, bridge decks, FPSOs, etc, and as such have much to recommend for other than just ship structures. The benchmark study presents comparisons between two implementations of the formulations and the results of non-linear FEA. The structures analysed represent a range of different stiffened panels with varying overall dimensions, plate dimensions and stiffener shapes, in particular, flats, angles and Tees. Close correlation is generally realized but where differences arise, these are readily related to the approximations used in developing the strength formulations. The comparisons between the strength formulations and non-linear FEA results provides the basis for quantifying statistical uncertainties in the formulations which can be used in subsequent structural reliability analysis and partial resistance factor derivation.

Direct Calculation of Fatigue Damage of Ship Structure Details

Fatigue assessment of ships using the direct calculation approach has been investigated by numerous researchers. Normally, this approach is carried out as either a global model analysis, or as a local model structural analysis. The current investigation presents a case study of a container vessel where the global and local analyses procedures are combined. A nonlinear time-domain hydrodynamic analysis followed by a global FE analysis is employed to screen for the most severe locations of the global ship’s hull with regard to fatigue damage. Once these locations have been identified, a sub-modelling technique is employed to transfer global loads from the global FE model to local FE models that have high resolution of elements for local structure details. Results from a selection of local FE model simulations are presented. Stress concentration factors at four critical locations are calculated and compared with values recommended by classification guidelines. Results are presented from a short-term fatigue analysis which has been carried out using the rainflow counting method. Finally, a long-term fatigue analysis is performed in time-domain using a designed wave scatter diagram of representative sea states.

A Modified Structural Arrangement of Rigs for Safe Launching of Life Boats and Berthing of OSVs

It has long been experienced that Launching of lifeboat from rigs and Positioning of offshore supply vessel (OSV) near rigs are very critical and vulnerable operations as regards to the safety. In present days, sophisticated & expensive Dynamic Positioning (DP) Systems are used in order to avoid the collision of OSVs with rigs. However, it is observed that despite the provision of such sophisticated means, accidents are still occurring. Loss of control during positioning of OSVs can lead to severe accident (e.g. collision of OSV Samudra Shakti with Mumbai High North (MHN) platform in 2005). To avoid such accidents, considerable gap needs to be maintained between the OSV and the rig position. Also launching of lifeboat is known to be another safety critical operation. Particularly in rough weather, the landing of lifeboat requires to be at least 20 to 30m away from the platform. This is essential for preventing the lifeboat to drift under the platform and colliding with the structure & piping system there at. Lifeboat launching by davit lowering or freefall have got their own traditional problems as widely known in shipping industry. These are described in the paper “Safety of Lifeboat launching - Some Possible Improvements {P19J_AB27}” presented at Design for Safety Conference-2010 held in Italy. Considering both the issues as described above, it is evident that maintaining adequate distance from the rig is a crucial factor for improving safety of Life Boat launching and positioning of OSVs alongside the rigs. In order to achieve this, an additional structural part of the rig is proposed in this paper. This new structural entity is to be in the form of an inclined truss which will act as launching skid for the lifeboat. This arrangement will provide a guided slipway to enable landing of lifeboat at a considerable distance from the rig. Also this structure will be configured suitably to enable berthing & mooring of supply vessels alongside the rig. In this arrangement the DP system will be required for a short time only, at the initial stage of placing the vessel. After having the vessel positioned, she can be moored with the new proposed truss structure. Thus the dependency of DP System will reduce significantly to avoid collisions. So the authors believe that the proposed arrangement will improve the safety of offshore operation in a significant manner.

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.