Time Domain Simulations on a Single Point Moored Submerged Sphere of Variable Radius

This paper presents a different approach to the work developed by Cruz and Sarmento (2005), where the same problem was studied in the frequency domain. It concerns the same sphere, connected to the seabed by a tension line (single point moored), that oscillates with respect to the vertical direction in the plane of wave propagation. The pulsating nature of the sphere is the basic physical phenomenon that allows the use of this model as a simulation of a floating wave energy converter. The hydrodynamic coefficients and diffraction forces presented in Linton (1991) and Lopes and Sarmento (2002) for a submerged sphere are used. The equation of motion in the angular direction is solved in the time domain without any assumption about its output, allowing comparisons with the previously obtained results.

A Comparative Assessment of the Use of Spudcans and Caissons as the Foundations of Jack-Up Structures

More than ever before, operators are advancing mobile jack-up units into deeper waters and harsher environments. Constraining their use is the capacity of the shallow foundations (typically inverted conical spudcans) to withstand the larger wave, wind and current loads of these environments. The use of foundation skirts (or caissons) is often touted as an alternative, though today only a small percentage of jack-up units use caisson foundations. One restraining feature is limited understanding of their behaviour when compared to conventional spudcan. This paper addresses this by comparing the results of push-over experiments of a three-legged jack-up with similarly sized caisson and spudcan foundations. The tests were conducted on a 250:1 scale jack-up on an overconsolidated clay sample. With improved understanding of the overall response of a jack-up platform with caisson foundations, operators should have greater confidence to use these footings.

Model Tests in Brash Ice Channels

According to the present Finnish-Swedish Ice Class Rules (FSICR) the formulas for the required main engine power for tankers led to much bigger main engines than it is needed for the demanded open water speed. Therefore model tests may be performed in order to verify the vessel’s capability to sail with less required power in brash ice channels compared to the calculations. Several model test runs have been performed in order to study the performance of crude oil tankers sailing in brash ice. The tests were performed as towed propulsion tests and the brash ice channel was prepared according to the guidelines set up by the Finnish Maritime Administration (FMA). The channel width was 2 times the beam of the tanker. The model tests were carried out at a speed of 5 knots. For the tests a parental level ice sheet of adequate thickness is prepared according to HSVA’s standard model ice preparation procedure. After a predefined level ice thickness has been reached, the air temperature in the ice tank will be raised. An ice channel with straight edges will be cut into the ice sheet by means of two ice knives. The ice stripe between the two cuts will be manually broken up into relatively small ice pieces using a special ice chisel and if required the brash ice material will be compacted. Typically the brash ice thickness will be measured prior the tests at 9 positions across the channel and every two meter over the entire length of the brash ice channel with a special device, which consists of a measuring rule with a perforated plate mounted under a right angle at the lower end of the rule. As a result of the tests it could be demonstrated that tankers with a capacity of more than 50 000 tons require 50% and even less power compared to calculations using the present FSICR formulas.

Pitting Corrosion and Its Influence on Local Strength of Hull Structural Members

The objective of this study is to obtain basic data for discussing the structural integrity of aged ships, especially hold frames of aged bulk carriers. Firstly, shapes of corrosion pits observed on hold frames of bulk carriers have been investigated. It was shown that the shape of the corrosion pits is a circular cone and the ratio of the diameter to the depth is in the range between 8 to 1 and 10 to 1. Secondly, a series of tensile tests has been conducted to investigate the effect of pitting corrosion on tensile strength. It was pointed out that the tensile strength decreases gradually and the total elongation decreases drastically with the increase of thickness loss due to pitting corrosion. Thirdly, a series of 4-point bend tests with structural models which consist of shell, web and face plates simulating hold frames of bulk carriers has been carried out to investigate the effect of pitting corrosion on collapse behavior and lateral-distortional buckling behavior. Following the experiment, a series of non-linear FE-analyses has been also made. In the case where tensile load acted on the face plate, cracks were initiated at the bottom of the pits when pitting concentrated on the web near the face plate. On the other hand in the case where compression load acted on the face plate, lateral-distortional buckling has been observed and the ultimate load of the structural models where pitting developed regularly on the web was found to be almost the same as that of the structural models where the web has uniform corrosion corresponding to the average thickness loss.

Stability Enhancement of Supercavitating Projectiles by Unsteady Air Injection

In this work, numerical simulation is used to study the stability enhancement of high speed supercavitating hydrofoils. Although supercavitation is known as one of the most effective methods for drag reduction, producing the cavity, either by ventilation or by cavitator at front of the body, may cause some instabilities on cavity surface and thus on the projectile’s motion. Therefore removing these instabilities comes as an important point of discussion. First of all, we calculate the sources of instabilities and measure respective forces and then present some approaches that significantly reduce these instabilities. One of these methods that could produce more stable supercavities is injecting of the air into the cavity unsteadily which varies through the projectile’s surface. This approach is provided by arrays of slots distributed on the projectile’s surface and unsteady injection is modeled over the surface. Furthermore, the position of ventilation, dramatically affects the stability like those in aerodynamics. In all approaches it is assumed that the supercavity covers the whole of the body, however the forces caused by the wakes, formed behind the body are taken into account. The calculation is performed at three cavitation numbers with respective velocities of 40 m/s, 50 m/s, 60 m/s.

Numerical Simulation on Mooring Performance of LNG-FPSO System in Realistic Seas

The LNG-FPSO concept is receiving much attention in recent years, due to its active usage to exploit oil and gas resources. The FPSO offloads LNG to an LNG carrier that is located close to the FPSO, and during this transfer process two large vessels are in close proximity to each other for daylong periods of time. Due to the presence of neighboring vessel, the motion response of both the vessels will be affected significantly. Hydrodynamic interactions related to wave effects may result in unfavorable responses or the risk of collisions in a multi-body floating system. Not only the motion behavior but also the second order drift forces are influenced by the neighboring structures due to interactions of the waves among the structures. A study is made on the time domain analysis to assess the behavior and the operational capability of the FPSO system moored in the sea having an LNG carrier alongside under environmental conditions such as waves, wind and currents. This paper presents an analysis tool to predict the dynamic motion response and non-linear connecting and mooring forces on a parallel-connected LNG-FPSO system due to non-linear exciting forces of wave, wind and current. Simulation for the mooring performance is also investigated. The three-dimensional source-sink technique has been applied to obtain the radiation forces and the transfer function of wave exciting forces on floating multi-bodies. The hydrodynamic interaction effect between the FPSO and the LNG carrier is included to calculate the hydrodynamic forces. For the simulation of a random sea and also for the generation of time depended wind velocity, a fully probabilistic simulation technique has been applied. Wind and current loads are estimated according to OCIMF. The effects of variations in wave, wind and current loads and direction on the slowly varying oscillations of the LNG and FPSO are also investigated in this paper. Finally, some conclusions are drawn based on the numerical results obtained from the present time domain simulations.

On the Effects of Aero Boundary Layer Control on Pressure Drag Reduction in Supercavitating Bodies

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we’ll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present some approaches by which this probable separation can be controlled. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to delay probable separation and so significant pressure drag reduction is achieved. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered. Moreover, electromagnetic forces cause to delay or even prevent separation in high pressure gradient flows and interesting results obtained in this regard shows significant drag reduction in supercavitating vehicles.

Accurate Numerical Methods for Calculating the Response Statistics of Compliant Offshore Structures

The state-of-the-art representation of the horizontal motions of e.g. a TLP in random seas is in terms of a second order stochastic Volterra series. Until recently, there has been no method available for accurately calculating the mean level upcrossing rate of such response processes. Since the mean upcrossing rate is a key parameter for estimating the large and extreme responses it is clearly of importance to develop methods for its calculation. The paper describes numerical methods for calculating the mean level upcrossing rate of a stochastic response process represented as a second order stochastic Volterra series. Since no approximations are made, the only source of inaccuracy is in the numerical calculation, which can be controlled.

The Behaviour of Tugs in Waves Assisting LNG Carriers During Berthing Along Offshore LNG Terminals

For future offshore LNG terminals tugs are planned to assist LNG carriers during berthing and offloading operations. A model test study was carried out to better understand the tug behaviour in waves and to make a first step in the quantification of the related weather limits. Scale 1:35 model tests were performed in the two important ‘modes’ of a tug during this type of operation: the ‘push’ mode and the ‘pull’ mode. Realistic weather conditions were used and the tugs were working at the unshielded and shielded sides of the LNG carrier. Based on the results presented in this paper, it can be concluded that the motions of tugs in waves are significant, even in wave conditions that are considered to be mild for the berthing and offloading LNG carriers. The resulting push or pull loads may hamper these tug operations significantly. Special measures are necessary to take this behaviour into account in tug design, LNG carrier design and development of operational procedures and equipment. The paper gives insight in the typical tug behaviour in different weather conditions. One should be careful, however, to generalize the present results: with an optimised tug design and operation the tugs can be used in more severe conditions.

ATLAS AUVs for Offshore Applications: Status and Perspectives

The scientific and economic significance of underwater technology is considerable, particularly as the oceans are becoming more and more important not only as a source of energy, raw materials and food but also due to their influence on climatic changes. Underwater technology, especially at great depths, represents a very particular challenge which is being tackled by ATLAS ELEKTRONIK Autonomous Underwater Vehicles (AUVs) DeepC©, SeaOtter Mk1 and SeaOtter Mk2. Many actions that would normally require high cost ROV systems or which are not possible (e.g. under ice investigations) can now accomplished using AUVs, at less cost and manpower. DeepC© is a high efficient AUV system for ultra deep missions (4000m), with long endurance (40–60h) and for a lot of applications. The development objectives are: Low vehicle weight, modular design, high efficient energy generation, autonomous mission execution, obstacle avoidance, in mission re-planning capability, precise navigation, advanced diagnosis and fault recovery system, high manoeuvrability, hover capability and containerized payload. The system is final integrated and ready for water tests and demonstrations till end of 2004. The paper deals with the system performance and shows the development status. The AUV SeaOtter Mk1, based on the ATLAS Maridan M600 vehicle and adapted to modern requirements in navigation software and mission management, is the ideal payload sensor carrier for military and commercial purposes. It ensures precise positioning, stability and manoeuvrability for accurate data collection. The on-board system provides all necessary position, attitude, status and time information to ensure effective and efficient performance throughout the mission. High positional accuracy and accurate stabilization of the vehicle’s attitude throughout the collection of data allows the mapping of raw data to be implemented directly, without post-processing, to obtain the necessary correlation of the survey mosaic. The advanced version of the SeaOtter AUV, the Mk2 is designed for various purposes, strictly adhering to a modular approach with regard to superstructure, propulsion, energy, communication, navigation and payload. The basic vehicle can easily be extended by the means of one or two extension modules to carry heavier payloads. The paper shows the SeaOtter Mk1 performance and gives an overview over the Mk2 Philosophy and the development status.