ON THE IMPORTANCE OF CONSIDERING MULTIPLE SEASTATE REALIZATIONS WHEN ESTIMATING DESIGN LOADS IN MULTI-DIRECTIONAL SHALLOW-WATER WAVES

The objectives of this work are to close some of the knowledge gaps facing designers tasked with designing new offshore structures or upgrading older structures located in shallow waters and exposed to energetic multi-directional waves generated by passing hurricanes or cyclones. This will be accomplished by first investigating and characterizing the natural variability of the maximum wave heights and crest elevations found in multiple 2-hour long realizations of several short-crested shallow-water near-breaking seastates. Following this, the variability and repeatability of peak pressures and peak loads exerted on a 1/35 scale model of a gravity-based offshore structure are explored. The analysis focuses on establishing extreme value distributions for each realization, quantifying their variability, and exploring how the variability is diminished when results from multiple seastate realizations and repeated tests are combined. The importance of considering multiple realizations of a design wave condition when estimating peak values for use in design is investigated and highlighted.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/16bCsMd0OMc

EVOLUTION OF UNSTABLE WAVE PACKETS OVER VARIABLE BATHYMETRY

Several field observations have reported the formation of rogue waves in coastal zones, see Chien et al. (2002) for an example in Taiwanese sea. The mechanisms that lead to the occurrence of rogue waves in finite water depth to shallow water are not well understood yet under the conjecture of modulation instability. Indeed, this theory for uni-directional waves shows that when kh is lower than a threshold of 1.363 in homogeneous water depth conditions, the wave train becomes stable to side-band perturbations. Then if the wave train is stable, the appearance of rogue waves is not possible within this linear stability framework. One explanation may come from the complex wave transformation mechanisms in variable bathymetry, especially, for cases of steep slopes or near the edge between a steep slope and a gentle slope as it is the case of the continental shelf. Very few laboratory experiments have been so far addressing the influence of the bathymetry on extreme wave occurrence (Baldock and Swan (1996), Kashima et al. (2012), Ma et al. (2015)).Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/a5M4PS-Lo4Q

Improved Empirical Formulation for Seabed Trench Profile at Touchdown Zone and its Effects on Fatigue of Steel Catenary Risers

Steel catenary risers (SCRs) offer a cost-effective solution to deep water deployments. Hanging from a floater, an SCR is commonly subjected to large tension at hangoff location and large bending moments at touchdown zone (TDZ) which lead to fatigue damage. Field observations showed that the depth of a seabed trench might reach to a depth of four to five times of the diameter of a riser, however, a flat seabed was often assumed for the modeling of SCRs which surely affects simulations of fatigue behavior of the SCR at the touchdown zone. Studies on the effects of seabed trench on TDZ fatigue conducted by different researchers led to contradictory conclusions, i.e., some studies suggested that considering seabed trench reduced fatigue damage at TDZ of SCRs, while, others drew opposing conclusions. The contradiction may be explained by factors including inappropriate trench profiles and different sea states assumed in the analysis model. An iterative procedure initially developed by Wang and Low and further improved in the present work was used to estimate the position and the length of a seabed trench beneath an SCR and, then, an improved empirical formulation was generated to approximate the profile of the seabed trench. Additionally, dynamic simulations were conducted to study the effects of seabed trench on fatigue behaviors of SCRs encountering variant directional waves.

DETERMINATION OF THE ACTUAL AREA OF DRY ACOUSTIC CONTACT IN THE SYSTEM “TRANSDUCER–PRODUCT” IN LOW-FREQUENCY DEFECTOSCOPY

The conditions for the emission of acoustic energy into the pipeline environment and the reception of reflected signals from inconsistencies in dry acoustic contact cause certain dimensions of the actual contact area between the transducers and the pipe surface. The basic approaches to the determination of the actual area of ​​dry acoustic contact between the surfaces of the piezoelectric transducer and the pipe are formulated under the influence of constant static force of pressing the surfaces in low-frequency flaw detection using ultrasonic directional waves. Expressions have been proposed to determine the area of ​​actual acoustic contact for single and numerical micro projections of the pipe surface. The principle of quality control of balancing of acoustic antenna piezoelectric transducers in modern systems of low-frequency diagnostics of the technical state of longitudinal pipelines by ultrasonic directed waves is described. It is revealed that after correct balancing of all the acoustic antenna piezoelectric transducers, the column image does not appear on the display screen and the mathematical support of the system will automatically collect the technical status of the diagnosed section of the pipeline, the results of which are displayed on the display screen. It is established that the actual area of ​​dry acoustic contact in the "piezoelectric product" system in low-frequency defectoscopy depends on the magnitude of the static force of pressing the surface of the piezoelectric transducer to the surface of the product. It is revealed that the deformation of the micro protrusions of the surface of the product under the action of static clamping force is uneven, which does not allow to fully calculate the actual area of ​​dry contact by mathematical methods. It is shown that in modern systems of low-frequency ultrasonic diagnostics of extended pipelines, directional waves control the quality of dry contact of the surface of the piezoelectric transducer with the surface of the pipe by balancing acoustic antennas with the use of special test programs.