Explosion Recurrence Modelling

This paper describes a recurrence law for explosions. The proposed recurrence law fits quite well to the historic explosion data in residential buildings as well as to the data on offshore installations in the North Sea. Generally quantified explosion risk assessment is performed for offshore installations, since it is believed historic data does not correspond to a specific installation and it may not be appropriate for use in performance based explosion engineering, which may in itself require realistic load description of explosion recurrence. The goodness-of-fit of the model for explosion occurrence data obtained using the quantified risk assessment method is also discussed. The paper then introduces the concept of performance-based design, which is an attempt to design structures with predictable performance under explosion loading. Performance objectives such as life safety, collapse prevention, or immediate resumption of operation are used to define the state of an installation following a design explosion. The recurrence law is then used to associate a level of explosion load to each limit state using a desirable level of probability of exceedance during the installations life time.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

Load Combination for Fatigue Analysis of Ship Structures

A methodology for calculating the correlation factors to combine the long-term dynamic stress components of ship structure from various loads in seas is presented. The methodology is based on a theory of a stationary ergodic narrow-banded Gaussian process. The total combined stress in short-tem sea states is expressed by linear summation of the component stresses with the corresponding combination factors. This expression is proven to be mathematically exact when applied to a single random sea. The long-term total stress is similarly expressed by linear summation of component stresses with appropriate combination factors. The stress components considered here are due to wave-induced vertical bending moment, wave-induced horizontal bending moment, external wave pressure and internal tank pressure. For application, the stress combination factors are calculated for longitudinal stiffeners in cargo and ballast tanks of a crude oil tanker at midship section. It is found that the combination factors strongly depend on wave heading and period in the short-term sea states. It is also found that the combination factors are not sensitive to the selected probability of exceedance level of the stress in the long-term sense.

Spatial Extreme Value Analysis of Nonlinear Simulations of Random Surface Waves

The paper first recalls the Slepian Model Representation and a theorem of V. I. Piterbarg as generic tools for analyzing the spatial characteristic of ocean waves. We then consider numerical simulations of random surface gravity waves carried out in space and time by means of the modified nonlinear Schro¨dinger equation. It is shown that the extreme waves in the simulations are steeper and more asymmetric than predicted by the Gaussian theory. Moreover, the reconstructed wave fields shows extreme crest heights well in excess of what is expected from the Gaussian theory.

A-Plasma: Use of Penetration Flux and the Plasma Process When Pipe Welding Type 316L of Thicker Dimensions

The maximum penetration depth in inert shielded gas processes as TIG (≤ 3 mm) and keyhole plasma (≤ 8 mm) limit a highly weldable corrosion resistant alloy as the standard austenitic stainless steel 316L to be welded autogenously in dimensions above these maximum penetration depths. Thus an expansion of these penetration ranges would expand the selection of autogenously welded pipes classified by ASTM A312. Also an increase of the productivity rates in pipe welding of thicker dimensions due to less extensively preparations and welding procedures would be a benefit. Using an activated flux to achieve deeper penetration in the TIG process (A-TIG) is today a well-known and established method. In keyhole A-plasma is, however, limited research performed and the use of an activated flux in connection with the partial penetration plasma process is still not evaluated. In the present paper a discussion of different proposed mechanisms affecting the penetration characteristics is made. The experimental procedures when using flux constituents of oxides as SiO2 and TiO2 in connection with the keyhole plasma process and the partial penetration plasma process are also described. A maximum depth in partial penetration A-plasma of 15 mm has been achieved. This is an improvement of 4 times compared with the ordinary partial penetration plasma process. The depth to width ratio became in this case 1.00, which increased the ratio about 7 times. The longitudinal seem welding using two-sided partial penetration A-plasma is limited by the demands of a good groove fitting but a new process for girth welding in thicker dimensions (t ≤ 25 mm) of type 316L is achieved. The keyhole A-plasma process is not recommendable due to the narrow parameter box that changes the non-penetration mode to a cutting mode too abruptly.

Qualification Approach to Unbonded Flexible Pipes With Fibre Reinforced Armour Layer

The future water depth capabilities for unbonded flexible pipes is being pushed by NKT Flexibles I/S through the development of an innovative flexible pipe structure, taking full advantage of the material characteristics of metallic, polymeric and fibre reinforced materials. The fluid tight liner and possible insulation of this pipe structure are supported by an inner armour, capable of carrying the external hydrostatic pressure, clamp and crushing loads, as well as axial compression load, and an outer armour, consisting of two cross wound layers of carbon/epoxy composites, carrying the internal pressure as well as end cap forces and applied tension. A permeable and radially flexible outer layer protects the composite armour. Combining known and well-proven flexible pipe technologies and new solutions for materials, structure and functionality of the flexible pipe, positions this future product outside the present industry standards for flexible pipes, e.g. API-17J. The analysis tools used for the conventional flexible pipes are validated by NKT according to the API-17J specification. The API-17J describes load cases and corresponding allowable utilization ratios, stated as design criteria. However, this approach is not directly applicable to the composite pipe, where the same analysis tools will be used, but the material in one of the two primary load bearing layers is made of fibre reinforced polymer, a material class not covered by the API allowable utilization factors. The DNV offshore standard DNV-OS-C501 considers any offshore structure in which the load bearing material is a composite. An accompanying Recommended Practice DNV-RP-F202 for composite risers has also been issued, but is not applicable to the composite flexible pipe. The design equations of the DNV standard are formulated in the so-called Load and Resistance Factor Design (LRFD) format, where partial safety factors are applied to the load effects and to the resistance variables that enter the design equations. The DNV standard DNV-OS-C501 covers composite materials and composite metal interfaces of a structure, metal parts should be designed according to other relevant standards. The API standard can therefore be used for the metal parts. One of the challenges in using this combined approach is the different ways loads are defined in the two standards. In short, this will result in a conventional API design check of the inner armour, the polymer layers, and the secondary layers, whereas the composite tensile armour, special intermediate layers and the interfaces will be analyzed with composite specific tools based on the criteria derived from the DNV standard. The qualification procedure is described and exemplified in the following.

Experimental Investigation and Finite Element Analysis of the Inversion of Capped End Frusta as Impact Energy Absorbers

In this paper, an innovative mode of deformation of the frusta is presented and discussed in details. A full experimental investigation for the quasi-static axial inversion of right circular frusta is given. The experimental work includes studying the effect of frusta wall thickness, angle of frusta and material type on the inversion of the frusta. The quasi-static tests were conducted on an Instron Universal testing machine and qualitative dynamic test were carried using Drop Hammer Facility. Finite element (FE) modeling of the inversion mode is carried out by using ABAQUS FE package. Analysis of the deformation modes is examined using a non-linear model of the finite element package. The FE findings are reported and modes of deformation during the inversion of aluminum frusta are described under quasi-static and dynamic cases. Furthermore, a good agreement is reported between the finite element force histories and the experimental results.

Integrity Management System for Fixed Offshore Structures Inspection Strategy

The GUPCO offshore structure management system was developed as a part of an integrated infrastructure management system. This paper presents a case study of providing an integrity management system for inspection, evaluation and repair of the fixed offshore platforms in Gulf of Suez. The management system procedure is presented focusing on the first step for defining the highly risky weight to the lower risky weight structure based on API criteria for assessment of the existing structures. The risk analysis methodology for developing design and assessment criteria for fixed offshore structure based on consequence of failure is illustrated. In our case study the assessment method is applied for a number of fixed offshore structures. The above methodology is performed after theoretical assessment and then verifying by using ROV subsea inspection for the fixed offshore structure. Comparison between the actual structure performance and the predicting risk assessment for the structure from the model will be studied. The overall management system will be illustrated in scope of predictive maintenance philosophy and reliability for all offshore structures.

Building a Global Resource for Rapid Assessment of the Wave Climate

In order to provide rapid access to reliable wave and wind climate information worldwide, a resource has been created combining: • a global offshore wind- and wave data-base, currently containing calibrated and validated spectral wave data from a wave hindcast model as well data from several satellite microwave sensors; • a simple but effective numerical model to predict nearshore wave conditions from the offshore spectra; • analysis tools to extract various climate parameters from the data such as scatter tables, extreme value analysis and persistency; • a web interface giving instantaneous access to the most commonly needed information. The resource is primarily intended for use in planning and design of operations typically requiring five years of data, but it an also be used for the design of certain structures, as there are now 16 years of significant wave height data from satellite radar altimeter available. This paper describes the components of the system and discusses their merits and limitations. We also present some results of the validation of the global satellite wave and wind data, of the global and regional wave model hindcasts, and of the nearshore wave transformation employed to obtain wave climate at sheltered or shallow-water sites.

Improved Spectral Wave Modelling of White-Capping Dissipation in Swell Sea Systems

An improved method for computing the dissipation by whitecapping in full spectral discrete wind wave models is presented. In this method the rate of dissipation of a certain spectral wave component is determined by the cumulative wave steepness of all spectral components with lower frequencies. This method is known as the cumulative wave steepness method (CSM) and was described by Van Vledder and Hurdle (2002). In this paper some improvements of the CSM are presented which are related to directional and scaling effects. The improved CSM was implemented in the latest release of the SWAN model (version 40.31 of February 2004) and a calibration against a parametric growth curve was performed. Further, the benefits of the CSM in swell-sea systems are illustrated and compared to the method of Komen et al. (1994).