A Comparison Between BS7448-CTOD and ASTM E1290-CTOD in Linepipes

ASTM E1290 previously used the plastic hinge model for the calculation of CTOD, but has changed its CTOD calculation to J-based conversion since 2002. In this study, the ratio of ASTM-CTOD to BS-CTOD was analytically predicted and experimentally evaluated in linepipes. It was demonstrated that the CTOD ratio changed according to CTOD itself, and took a minimum in a CTOD ratio curve. The minimum value of the CTOD ratio was lower than 1 for low yield-to-tensile ratios, but higher than 1 for high yield-to-tensile ratios. The CTOD ratio tends to be higher than 1 for high CTOD caused by plastic instability, but around or less than 1 for low CTOD possibly caused by brittle fracture in X65 and X80. A CTOD transformation equation, which was proposed by the authors, can transform BS-CTOD into ASTM-CTOD with reasonable accuracy.

Effect of Different Cooling Rate on HAZ Microstructure of 2205 Duplex Stainless Steels

The effects of cooling rate ω8/5 and ω12/8 on the simulated HAZ microstructure transformation in 2205 duplex stainless steel are studied in this paper. The results indicate that 1200°C ∼ 800°C is the temperature range in which the microstructure transits the most violently for 2205 steel, and is also the cooling interval, that affects the phase proportion and microstructure morphology the most distinctly. Accordingly, It is more efficient to use ω12/8 as the parameter to investigate the microstructure transformation of welding HAZ microstructure of this material. The cooling rate in this interval will affect the microstructure transformation of HAZ microstructure of 2205 steel remarkably.

Fatigue Capacity of Stiffener to Web Frame Connections

Daewoo Shipbuilding & Marine Engineering Co., Ltd. (DSME), StatoilHydro and DNV established a common project to investigate the reason for the difference between calculated fatigue lives and the in-service experience and to assess the fatigue capacity of stiffener web connections subjected mainly to web frame shear stresses. The main objective of the work was to establish fatigue test data and perform numerical analysis of collar plate connections in order to provide improved confidence in analysis methodology for fatigue life assessment. Large scale fatigue tests of different types of connections were carried out to obtain fatigue test data of collar plate connections. Finite element analyses were carried out for comparison with fatigue test data and with measured stresses on the test model. Based on this work recommendations on fatigue design analysis of connections between stiffeners and web frames have been derived. The background for this is presented in this paper.

Environmental Effect on Fatigue Life of FPSO Hull Structures

Floating production, storage and offloading systems (FPSOs) have been widely used in the development of offshore oil and gas fields because of their many attractive features. These features include a large work area and storage capacity, mobility (if desired), relatively low construction cost and good stability. They are mostly ship shaped, either converted from existing tankers or purpose built. The hull structural scantling design for tankers may be applicable to FPSOs; however, FPSOs have their own unique characteristics. FPSOs are located at specific locations with a dynamic loading that is quite different from that arising from unrestricted ocean service conditions for tankers. It is also noted that the wave conditions in recent FPSO applications may be very complicated when operating in areas such as those offshore West Africa and offshore Brazil where both seas and swells exist and propagate in different directions. In this paper, the unique FPSO operational aspects, especially the load assessment due to on-site environments will be described. The methodology of handling complicated wave conditions in fatigue assessment will be addressed. Special considerations for converted FPSOs, which need to take into account their operational history as a trading tanker and low cycle fatigue due to FPSO operations, will also be introduced. Case studies will be presented and appropriate analysis methodology will be summarized. The methodology has also been adopted by ABS Guide, see ABS [1].

Large Scale Tests of Strain Capacity of Pipe Sections With Circumferential Defects Subjected to Installation-Induced Plastic Strain History

Installation of offshore pipelines by reeling introduces plastic pre-straining. The pre-strain history is not homogenous and it will vary around the circumference of the pipe. The pre-strain history will modify the yield and flow properties. Also, the fracture toughness may be influenced by the pre-straining. The result is that the bending strain capacity of pipelines during operation will differ depending on how the bending moment coincides with pipe orientation during installation. Three full scale tests of 12″ x-60 pipes with wall thickness 19.3mm and a 3×100 mm outer surface defect were performed to investigate the effect of pre-strain history. Two pipes were pre-strained in bending to 2% strain in the outer fibre and then straightened to simulate the reeling. The final tests to establish the strain capacity during operation as a function of strain history were performed in four point bending with an internal pressure of 325 bar. The strain capacity for the side of the pipe that ends in tension and the side that ends in compression from pre-straining was 1.7% and 2.6% respectively. The strain capacity of the third test without pre-straining was 5.7%. The results show that pre-straining will modify the strain capacity and the effect must be taken into account in engineering critical assessment of pipes during operation. The effect of prestraining should be evaluated for all installation methods that involve plastic deformation during installation, and not only reeling. It is important to note that the notch size in the full scale tests was larger than what would normally be accepted for reeling. In addition the notch was positioned in base material and not in weld metal, which is a more realistic position for a notch. The welds are normally overmatched and this might reduce the effect of prestraining.

Mooring Analysis of a Weathervaning FPSO in Bi-Directional Sea-States

Experimental studies were carried out at the Institute for Ocean Technology, Canada, in collaboration with the University of Western Australia (UWA) to assess the response of a moored 1:60 scaled Floating Production Storage and Offloading (FPSO) model in bi-directional seastates. The seastates comprise of a regular swell approaching in the head sea condition, and a JONSWAP wind sea approaching at various angles. The FPSO was moored in position by four spring-loaded mooring lines attached to an internal turret about which the model could weathervane. Previous papers by the authors have described the unpredictable yaw instability of the FPSO driven by long period swells, which was evidenced in the experiments. This creates difficulties in comparing motions from unidirectional and bi-directional seas, because the headings alter the response. However, the mooring tensions are relatively immune to yaw instabilities and this paper discusses effects of bi-directional seas on the mooring tensions. Numerical simulations are conducted using a time domain analysis software which simulates the motions of floating and moored structures in response to irregular seas. Simulations based on the software when compared with model tests at 45, 60 and 90 deg separation between the sea and swell shows reasonable agreement in terms of mooring tensions. Simulations are then conducted for a range of separation angles, and the effects of bi-directionality are further evaluated. It is found that a linear addition of the individual seastates can produce non-conservative results, which reinforces the fact that bi-directional seastates are important considerations for offshore operations of an FPSO.

Real-Time Estimation of Directional Wave Spectra Using Non-Stationary Ship Motion Data

The Bayesian modeling procedure is modified for real-time estimation of directional wave spectra using non-stationary ship motion data. The assumption of stationary stochastic processes is applied to the seaway, but not to ship response because it also depends on ship maneuvers. Ship response is strongly affected by changes in the encounter angle and frequency of waves. Therefore, it is need to be a real-time algorithm that can deal with non-stationary stochastic processes and estimate the directional wave spectra. In the proposed algorithm, the iterative calculations of the non-linear equations were optimized and the convergence was not achieved at every time step, but was achieved gradually over several time steps. In order to examine the reliability of the proposed method, real-time estimation was conducted by using the data of onboard experiments. Comparisons between the results of the proposed algorithm and a wave monitoring radar system show good agreements.

Wave Measurements at Eastern Green Canyon During Hurricane Ike

Wave measurements were obtained at an Eastern Green Canyon location during hurricane Ike in September 2009. The eye of the hurricane passed approximately 68 nautical miles to the South West of the measurement location. The significant wave height was above 30 ft for about 20 hours and above 40 ft for about 5.5 hours. The wave time series provide an insight into the wave field as the storm approaches and leaves the location. One of the interesting features observed was that there were repetitive sequences, where each sequence consisted of a period of increased wave energy followed by periods of reduced wave energy. Each sequence lasted approximately one hour. Measured wave crest, wave trough and wave height distributions are discussed. One unique wave event was observed. This event was characterized by a predictably-sized crest followed by a very deep trough.

How Does Sedimentary Layering Affect the Generation of Tsunamis?

This article presents preliminary results on the influence of sediment layers on the process of tsunami generation. The main scope here is to demonstrate and especially quantify the effect of sedimentation on vertical displacements of the seabed due to an underwater earthquake. The effects in the far field are left for future work. The elastodynamics equations are integrated with a finite element method and the fault is modelled as a dislocation in a half-space. A comparison between two cases is performed. The first one corresponds to the classical situation of an elastic homogeneous and isotropic half-space, which is traditionally used for the generation of tsunamis. The second test case takes into account the presence of a sediment layer separating the oceanic column from the hard rock. Some important differences are revealed. We conjecture that deformations in the generation region may be amplified by sedimentary deposits, at least for some parameter values. The mechanism of amplification is studied through careful numerical simulations.

Dynamic Interactions Between the Vadose and Phreatic Zones During Breaking Solitary Wave Runup and Drawdown Over a Fine Sand Beach

The objective of this work is to investigate the dynamic interactions between the vadose and the phreatic zones during breaking solitary wave runup and drawdown over a fine sand beach. Extreme wave runup and drawdown in the nearshore region can lead to soil failure in the form of severe erosion, liquefaction, or slope instability. However, the physics of the nearshore region is difficult to simulate numerically due to the greatly varying time scales between the four governing processes: loading and unloading caused by wave runup and drawdown, propagation of the saturation front, pore pressure diffusion, and soil consolidation. Such processes are also difficult to simulate experimentally via model-scale wave tank studies due to the inability to satisfy all the similarity requirements for both the wave and the porous media in a 1g environment. Hence, the goal of this work is to perform a 1D study using a multiphase model to describe the transient responses of the species saturation, pore fluid pressure, effective stresses, and skeleton deformation. Results are shown for three simulations: (1) full-scale simulation, (2) 1:20 laboratory-scale simulation without scaling of the porous media, and (3) 1:20 laboratory-scale with consistent scaling of the soil permeability. The results suggest that the scaling of porous media between the pore fluids and soil skeleton has a significant influence on the transient response of both the vadose and the phreatic zones.