A partially coupled effective stress analysis is assessed in predicting the seismic performance of a caisson-type quay wall. Using a nonlinear finite difference program, a numerical study of shaking table tests, carried out at Tokyo University, is performed. In this paper, the formulation of an employed computational code is described, and the results of numerical simulation are compared with the measured records. The results demonstrate that the trend and magnitude of vertical and horizontal displacements of quay wall appear to be predicted reasonably well. The overall tendency is to overpredict the horizontal movement, but the vertical movement is underpredicted.

Design S-N curves in design codes are based on fatigue test data, where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further, it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. In 2006, the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.

Rational assessment of stern slamming of a large twin screw liquefied natural gas (LNG) carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behavior of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ship’s stern. Reynolds-averaged Navier–Stokes computations based on the volume of fluid method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were coupled to a nonlinear motion analysis of the ship in waves. At every time step of the transient computation, the finite volume grid was translated and rotated, simulating the actual position of the ship. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Slamming-induced pressure peaks, typically lasting for about 0.5 s, were characterized by a steep increase and decrease before and after the peak values. Shape and duration agreed favorably with full-scale measurements and model tests carried out on other ships, indicating the plausibility of our numerical predictions. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. Short-duration impact-related slamming loads excited the ship structure to vibrations in a wide range of frequencies. Excitation of the lowest fundamental eigenmode contributed most to additional stresses caused by hull girder whipping. Although, for the cases investigated, longitudinal stresses and shear stresses caused by quasisteady wave bending were uncritical, we obtained a significant amplification (up to 25%) due to the dynamic structural response.

Application of controlled weld toe profiles can be considered an option to extend the fatigue life of welded connections when ongoing tankers are converted in dry docks to serve like offshore ships (FPSOs and FSOs). Very slim chances to implement such fatigue improvement will arise when these vessels are in service, since a converted ship is designed to be inspected, maintained, and repaired in situ and not in dry dock as it is uneconomical to interrupt production. Codes recognize fatigue life extension by means of a controlled weld toe profile ( 2004, NORSOK Standard N-004 Rev. 2 October ). Application of a controlled weld toe profile during conversion in selected areas previously identified by stress analysis of the hull structure can lead to extend the converted vessel fatigue life to comply with an expected field life. The American Bureau of Shipping S-N curves allow a credit of 2.2 on fatigue life when suitable toe grinding and NDE are provided. A controlled weld toe profile can be applied during dry dock ship conversion to FSO or FPSO to welds in a noncracked condition but that were identified prone to fatigue cracking in a stress assessment analysis under new service conditions. Credit on fatigue life in various codes and results from experimental data obtained from fatigue tested specimens with a controlled weld toe profile are given. Comments on the design of a controlled weld toe profiles and recommendations based on experimental experience for the implementation of equipment to perform a controlled weld toe profile are also given. A fracture mechanics approach for the assessment of controlled weld toe profiles for fatigue life extension purposes is described. Initially, a comparison of stress concentration factors for a typical T-butt ship hull plate connection with and without weld toe profile control determined by finite element analysis (FEA) is presented. Results obtained from the FEA connection such as through plate stress distribution are used in a fracture mechanics analysis to compare the fatigue crack growth curve in as-welded condition to that with controlled weld toe profile. It is demonstrated that weld toe profile control is a feasible method to be implemented to improve fatigue life in the order of 2 of T-butt welded connections of ships, which are under conversion to serve as FPSOs or FSOs. This fatigue life extension factor should not be considered at the design stage.

The strong interest in very large floating structure (VLFS) is a result of a need to utilize effectively the ocean space for transportation, industrial use, storage, habitats, and military bases, among others. The VLFS has great width and length and relatively small flexural rigidity, therefore, investigation of its hydroelastic behavior including fluid-structure interaction is of greater importance than studies of its motion as rigid bodies. In addition to the most important wave-induced responses, the operation of the VLFS also requires determination of its dynamic responses with respect to the effect of unsteady external loading due to intense traffic, load movement, takeoffs and landings of airplanes, missile takeoffs, etc. Therefore, the transient responses of a VLFS to impulsive and moving loads must be studied by a reliable calculation method. In this study, a finite element procedure developed directly in time domain for solution of transient dynamic response of the coupled system consists of a VLFS and a fluid domain subjected to arbitrary time-dependent external loads is presented. The hydrodynamic problem is formulated based on linear, inviscid, and slightly compressible fluid theory and the structural response is analyzed under the thin plate assumption. For numerical calculations, a scaled model of the Mega-Float is exemplified. Three tests—weight pull-up test, weight drop test, and weight moving test which idealize the airplane landing and takeoff—are carried out and compared with published experimental data. The overall agreement was favorable which indicates the validation of the present method.

Finite element analysis is being used by designers for fatigue assessment of structures. It is therefore important that a proper link between calculated hot spot stress and fatigue capacity is established. The fatigue capacity is expressed as a hot spot stress S-N curve. This paper presents a derivation of a hot spot stress S-N curve to be used when the hot spot stress is derived from finite element analysis of plated structures. The hot spot S-N curve is linked to the methodology used for finite element analysis of plated structures that is being included in design recommendations for fatigue assessment of welded structures.

An efficient time-variant reliability formulation for the safety assessment of an aging floating production storage and offloading (FPSO) vessels with the presence of through-thickness cracks (i.e., long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. Although it has been found that stiffened panels are tolerant to fatigue cracking, the safety of such structural components with the presence of long cracks may be threatened when exposed to overload extreme conditions, i.e., brittle or ductile fracture may occur. The probability of brittle fracture of an aging hull structure, i.e., a stiffened panel at the bottom plating with the presence of long cracks is studied in this paper. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation procedure presented herein. An analytical model is established for determining the equivalent long-term stress range including the mean stress effect. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. In the present case study it is found that the time-invariant approach is a good approximation when dealing with the time-variant reliability problem. One of the main conclusions drawn from this study is that the still-water mean stress has a significant effect on the failure probabilities of stiffened panels with long cracks under brittle fracture mode.

Recent rapid advances in developing mesh-insensitive structural stress methods are summarized in this paper. The new structural stress methods have been demonstrated to be effective in reliably calculating structural stresses that can be correlated with fatigue behavior from simple weld details to complex structures. As a result, a master S–N curve approach has been developed and validated by a large amount of weld S–N data in the literature. The applications of the present structural stress methods in a number of joint types in offshore/marine structures will be illustrated in this paper. The implications on future applications in drastically simplifying fatigue design and evaluation for offshore/marine structures will also be discussed, particularly for using very coarse finite element mesh designs in ship structures.

The LargeE Admissible Perturbation (LEAP) methodology is developed further to solve static stress redesign problems. The static stress general perturbation equation, which expresses the unknown nodal stresses of the objective structure in terms of the baseline structure stresses, is derived first. This equation depends on the redesign variables for each element or group of elements; namely, the cross-sectional area and moment of inertia, and the distance between the neutral axis and the outer fiber of the cross section. This equation preserves the shape of the cross section in the redesign process. LEAP enables the designer to redesign a structure to achieve specifications on modal properties, static displacements, forced response amplitudes, and static stresses. LEAP is implemented in code RESTRUCT which post-processes the FEA results of the baseline structure. Changes on the order of 100% in the above performance particulars and in redesign variables can be achieved without repetitive finite element (FE) analyses. Several numerical applications on a simple cantilever beam and an offshore tower are used to verify the LEAP algorithm for stress redesign.

A stepwise response surface approach is proposed in this paper. The response surface is determined by modified stepwise regression, so that the square and cross terms can be absorbed into the model automatically according to their actual contribution, which is calculated by repeated variance analysis. Besides, by applying a weighting factor to the statistical value of contribution and changing the thresholds of introduction and rejection, the entry of each term can be controlled in a fairly flexible manner. None other criteria than those in the traditional statistics are needed to check the goodness of fit. Considering the relatively small sample set at the beginning, the algorithm starts with a linear response surface. As the adaptive iteration proceeds, the bar to quadratic terms is lifted gradually to allow ordered entry. Since the sampling points in one step of iteration are recycled in the succeeding ones, a simple experimental design is enough to fit a robust response surface. A double bottom hull system is analyzed with randomized Young’s modulus, load distribution, and geometric properties. The sensitivity analysis is also performed with respect to the random variables and the parameters in their distributions.

Structural reliability methods are applied to establish a measure of safety for pipelines during laying, and especially to calibrate semi-probabilistic ultimate limit state criteria based on measures of uncertainty, method of reliability, and a given target level. Ultimate collapse of thick tubes under combined external pressure, tension, and bending loads are studied applying the finite element method. Nonlinear effects of large deformations, effects of initial ovality, residual stresses, strain-hardening, yield anisotropy, and loading paths were accounted for in the analysis. A set of interaction equations is proposed. Load effects in the pipelines during installation by the S-lay method are studied. The effects of uncertainties in yield stress, mass, stiffness of the stinger, response amplitude operator and peak period for the wave spectrum were accounted for in the analysis. The major factors affecting strain concentration due to concrete coating are taken into account. A combination of design point calculation and importance sampling procedure is used to calculate the probability of failure. The study includes calibration of partial safety factors for the design format selected. The most important random variable is the model uncertainty for bending capacity, while the uncertainty of the load effect has minor importance for the probability of failure. The system effect is taken into account considering the correlation along the pipeline. The probability of failure is referred both to the total laying period as well as a 3-h period demonstrating that the target level needs to be defined in view of the reference time period. [S0892-7219(00)01501-6]