Welded steel offshore jacket structures are susceptible to fatigue damage under the action of cyclic seawave loads. Fatigue damage occurs around the intersection of the tubulars, which incidentally is a region of high stress concentration, in the form of crack initiation and propagation. Application of fracture mechanics principles has been found to provide a realistic assessment of the fatigue life of welded tubular joints. The method has been employed to evaluate the fatigue life of a typical welded T-joint encountered in a jacket platform structure
Fatigue life prediction of offshore tubular joints has been traditionally performed using the well-known Stress range vs. Number of cycles
This paper introduces a unique electric heat tracing system, called "SECT (Skin Electric Current Tracing) system" for short, which was developed to ensure the operational reliability, by eliminating pumping difficulties, in transfer of waxy or heavy oils, other petroleum products or chemicals through on-shore or off-shore pipelines. It is also dealt with how the SECT system has been designed and applied to the off-shore pipelines specifically citing the 36 inch and 5.5 kilometer off-shore pipeline in the Republic of Korea, which pipeline unloads waxy crude oils from ocean tankers to on-shore storage tanks through a single point mooring buoy.
The SECT (heat tracing) system, which utilizes the skin effect of an a.c. phenomenon, was invented and developed as an original and epochal technology to provide a quite new and unique concept of heating pipelines electrically in the way completely different from the conventional types in every respect and with a variety of technical and economical features job-proven for many years as the reliable and durable system, and for those reasons has expanded and will expand increasingly its on-shore as well as off-shore applications. Up to date, the previous world-wide applications to the pipelines, from a low temperature of freeze protection to a high temperature of maintaining the liquid phase, have extended to more than 350 instances and to more than 1,000 kilometers in the total length during the last one-quarter century. The reliability and durability of its applications are proven by the fact that some of the installed systems are still in very successful routine operation over 25 years satisfactory to its original requirements. Compared to other electric heat tracing systems, the SECT system indeed provides its users with superior performances particularly in the heating of long-distance pipelines, of underground pipelines, and of high temperature pipelines.
A double-acting wave-energy pump with floating caisson was proposed by the authors in a previous paper. In order to improve the floating pump performance, the caisson was reconstructed and the pump has a submergible cylinder operating under water. In addition, the stabilizer at the lowest part of the caisson was removed to vary the natural period of the caisson. The performance of the floating pump was examined and compared with that of the fixed caisson type. The motion of the caisson itself was small in the case of no float-piston, because the natural period of the caisson was about twice larger than the incident wave period. Though, during pumping operation, the heaving amplitude of the caisson increased remarkably with the wave period. The performance of the floating device became better than that of the fixed one in almost every experimental conditions. Effective floatpiston stroke increased because the amplitude of floatpiston stroke became larger than that of caisson heaving. It was clarified that the floating pump performance can be improved by controlling the natural period of the caisson.
Utilization of renewable energy is indispensable for decrement of fossil fuel consumption with exhaust emission including CO2 and for preservation of natural environments. Various kinds of ocean wave energy converter have been investigated, for example, by Watabe and Kondo (1990), Araki et al (1991), Watanabe et al (1991), and Lockett et al (1991). Whittaker and McIlwaine (1991), Sarmento and Jacob (1991), Masuda (1990), and B¢nke and Ambli (1986) have reported about oscillating water column (OWC) power device. Furthermore, Well''s turbine mainly for OWC power generation has been studied by Gato et al (1991), and Setoguchi et al (1992). It should be emphasized that various kinds of added value are created by integrating these systems.
The mechanism of wave induced scour around large -scale offshore structures is discussed. A comparison of two types of estimation methods, the model experiment and the computation method is made. Scouring in front of the structure can be approximately estimated by both methods. The scour on the side of the structure, however, is not well simulated owing to the lack of knowledge about the Row mechanism around the structure. The two methods are complementary to each other.
Studies of scourings around offshore structures have a long history. Most of the researchers, however, have focused on such small members as jackets or pipe lines. The scour patterns around the small-scale members are related to the fonnation of two kinds of vortices: horse-shoe vortices and Karman vortices. Around a large-scale structure, where D/L>0.2 or K.C.
PREDICTION OF SCOURING EXPERIMENTAL METHOD
It is often the case that model experiments are carried out to predict morphological changes around a structure. Let us introduce the case of the experiment done by the authors. In the experiment, the prototype condition was kept in mind.
In recent years, failure of subsea oil pipelines have increased considerably in Bombay Offshore. Failure investigation of some of these pipelines has shown severe internal corrosion at the "6 o''clock" position in a channel like pattern with holes. Metallographic studies, hardness testing, scanning electron microscopy, chemical analysis and microbial analysis of failed pipe samples were carried out. Chemical analysis of deposits, microbiological analysis of produced water and Scanning Electron Microscopic observation of biofilm confirmed microbial induced corrosion in Bombay Offshore subsea pipelines. A probable mechanism of microbial induced corrosion in pipelines and remedial actions were also discussed.
In Bombay Offshore, existing submarine pipeline diameters range from 4 inches (10.16 cm) to 36 inches (91.44 cm), out of which the maximum length is of 12 Inches (30.48 cm). These pipelines transport oil and gas of both sour and sweet nature. A few years after commissioning, leakages have been observed in some of these lines. In some of the lines, more than 30 leaks are reported after a short period of 3;'' years. The leak history of these lines is given in Table-1. These leaks were repaired by clamps. However, in some lines, where the number of leaks were more and the condition of pipe was not suitable for transportation of crude oil, that pipe section was replaced after cutting. In order to find out the reasons of failure of the above pipelines, two damaged pipe sections from two different lines of Bombay Offshore were selected for detailed study. One pipeline was selected from the Western region L.e. Bombay Highfield (Pipeline A) and another from the Southern region i.e. Heera field (Pipeline B) of Bombay Offshore. These lines are used for transporting crude oil from well platform to process platform.
This paper presents magnetohydrodynamics (MHD) as applied to electromagnetic marine propulsion, the history of its development, and some recent accomplishments. The focus of the paper is confined to the efforts in the US and Japan. The merit of current accomplishments with respect to the development of practical electromagnetic thrusters for surface, and underwater vehicle propulsion, and the challenges such as: management of the excessive stresses and magnetic signatures, superconducting magnet design, problems associated with low conductivity, bubble formation and transport and the associated problems in current transport, noise and signatures, and electrode erosion, are discussed. The state-of the- art and future efforts needed for this technology to reach fruition as a potential candidate for marine propulsion are described.
Magnetohydrodynamics or MHD is the science of conducting fluids interacting with electric and magnetic fields. This interdisciplinary science dates back to the early eighteen hundreds for its inception. However, it has received attention from the research community only in the past few decades. The principle of MHD (power generation) is illustrated in Fig. I. The MHD generator transforms the internal energy of a gas into electric power in much the same way as a piston engine generator or a turbogenerator does, and the basic physical phenomena is the same. In the first two devices, the energy of the gas is converted into the motion of solid conductors in a magnetic field. In the MHD generator, the gas itself is the conductor. As this conducting gas moves (by expansion through the nozzle) through a magnetic field, an electromotive force is generated and current flows according to Faraday''s laws of induction. The seawater enters a rectangular duct (could be circular or annular), with continuous electrodes on the top and bottom (could be on the sides), placed in a magnetic field.
This paper introduces the theory and computation methods for the calculation of internal forces of a large floating body with an arbitrary shape induced by dynamic wave pressure on its wetted surface and body''s motion. Based on wave pressure distribution and motion response from three-dimensional di ffraction/radiation computation, the body''s inertial force distribution is calculated according to rigid body motion theory. For each frequency, the transfer function of dynamic internal forces at each specified section of the body is calculated from the wave pressure and inertial force distributions. Finally, the total dynamic internal force prediction at the specified sections of the body is given out for the input wave spectrum by using spectrum anal ys is method. The dynamic internal forces are then added to the static internal forces for the section property check. This method was developed and used for the Preliminary Design of JZ93 Artificial Islands of Liao Dong Bay of P.R.China for their global strength analysis in towing conditions.
To check a floating body''s global strength, the usual method is to calculate the internal forces of specified sections of the body and then check the sections'' modulus of the body in accordance with relevant codes. The internal forces of the sections of a floating body can be divided into two parts: static and dynamic components. The static internal forces is that caused by static loads exerted on floating body such as weight and buoyancy. Once the weight and buoyancy distributions are decided, the static internal forces of the specified sections can be calculated easily assuming the floating body as a single beam. However, the dynamic internal forces, which is mainly induced by dynamic wave loads on the floating body, is much more complicated than static internal forces.
The hydrodynamic-structural Jacket transportation problem has been traditionally solved as an uncoupled problem, through the introduction of important simplifications. Usually the motion analysis is performed for a rigid body jacket-barge system, determining the Response Amplitude Operators, from which maximum values for the relevant degrees of freedom are predicted. From that data, inertia forces are computed and a deterministic structural analysis, for a rigid barge, is performed. Today, with the computational formulations available, the previous methodology can be improved in several significant ways. In the present work both rigid and flexible barge conditions are taken into account. The structural analysis is performed following a stochastic approach, using spectral techniques. A code check is performed on the jacket members, for stresses computed from the spectral results, and comparisons are made for design parameters. The above methodology was applied to a deck and to four offshore structures, for water depth ranging from 56 m to 290 m, and for three different barges. The comparisons performed, which are based on the code checking, permit to formulate some interesting practical conclusions, not always coinciding with the previous understanding of the problem.
Offshore oil projects become feasible when fabrication costs are reduced. This means, oftenly, that the fabrication yard IS far away from the final platform site, requiring a long transportation process. In general steel jacket structures are transported to site on top of barges, as depicted in Figure 1 Transportation may take weeks, sometimes months. In that process the jacket-barge system is subjected to environmental actions coming from waves, winds, and currents, oscillating in different directions and giving rise to complex stress and deformation fields. In particular, the transportation process must be carefully analysed, both from the naval and structural points of view, to avoid unexpected Situations.
Unocal has been operating offshore natural gas and condensate production facilities in the Gulf of Thailand since 1981, and has employed a self-imposed inspection program since 1990 that now encompasses over 750 pressure vessels. The motivations for effecting this program are safety and loss-prevention The program is implemented through a systematic and cost-effective plan of engineering assessment, field inspection, and maintenance. The inspection scope is determined from a risk assessment that considers the generic nature of the vessels. Inspections are conducted by Unocal engineers and qualified inspectors. Maintenance and monitoring activities assist in keeping the vessels in fit-for-purpose condition.
The Unocal operation in the Gulf of Thailand has expanded considerably since start-up in 1981. Approximately 800 MMscf of natural gas and 30,000 bbl of condensate are now being produced daily on 46 wellhead platforms, transported through 300 miles of interfield pipelines, and processed through over 750 pressure vessels on 8 major processing platforms. One safeguard to protect such an extensive investment is a properly executed pressure vessel inspection and maintenance program. The basics of this plan were originally developed for offshore platforms, but much of the logic is equally applicable to pressure vessel systems (Ref. 11). The plan also builds on a program proposed by DnV Technology & Services (Ref. 9). The details of the Unocal program provide a systematic, but flexible, framework for determining vessel inspection requirements that comply with the intent of the codes and recommended practices The AIM Program is administered by the Inspection & Maintenance Group of Production Engineering, which is organized according to Fig. 2. The inspection work is carried out by Unocal engineers and a dedicated offshore inspection team that is complemented by contractor expertise when necessary. Extensive field support is also required for vessel preparation and restoration to service.
Optimal control procedures for maximum energy absorption by OWC devices require the incident wave to be known in the future, mathematically up to infinity. This is a major limitation for practical on-line control procedures, since the incident wave cannot be predicted, except for very short time intervals. The present paper presents a sub-optimal control procedure which requires only the time-history of the diffraction flow. Together with the theory, the paper presents results in the frequency domain. Linear hydrodynamic theory is assumed throughout the paper.
Activities on wave energy R,D&D have been recently reviewed in a paper by Falnes and L0vseth (1991). In January 1992 an European Community project in Wave Energy started, under JOULE programme. Among the aims of this project is the construction of an European Pilot Plant of the Oscillating Water Column (OWC) type. This kind of device for extracting energy from ocean waves is based on the exchange of energy from the water into the air that is trapped above the water free-surface by an air chamber open at the bottom. The air chamber, which we assume to be formed by a fixed, partly submerged structure, is connected with the atmosphere by a duct containing an air turbine. As a result of an incident, in general irregular wave field, the internal free surface as well as the air pressure are made to oscillate, and a flux of air is driven back and forth through the turbine. This allows to introduce a new degree of freedom in the control of the device, which may lead to a considerably increase in energy absorption. The control problem in irregular seas is to find out the instantaneous flow across the turbine and air pressure in the pneumatic chamber that maximize energy absorption.