The pape r presents an experimental investigation into the effectof blade section on the performance of the Wells turbine. Theblades tested included one set of eight symmetrical constant chordNACA 0015 blades, one set of eight symmetrical optimised bladesand two rotor solidities. The aim of the experiments was to validatepredictive work which indicated that optimisation of the bladeprofiles could significantly delay the onset of separation.
The Wells turbine is an axial flow air turbine designed to extract energy in ocean wave energy devices, particularly the oscillating water column (OWC). The turbine is self-rectifying, i.e. it can produce an unidirectional time averaged torque from a reciprocating flow. Like most other turbines, the Wells turbine is sensitive to the angle of the relative airflow approaching the blades. At low flow rates (small angles of incidence), the efficiency is negative and power is absorbed by the turbine due to the dominating influence in the direction of rotation of the drag force over the lift force. For high flow rates, the operational range of all fixed pitch configurations is limited by the flow stalling around the rotor blades, culminating in a loss of torque in the turbine rotor. Therefore when designing a Wells turbine rotor for wave energy applications, one of the main points to consider is the maximisation of the range of sea conditions during which the turbine can operate efficiently (Curran et al., 1997). A numerical method for optirnising symmetrical profdes was recently proposed (Gato and Henriques, 1996) aiming at the extension of the Wells turbine operating range for large flow rates. The basic idea behind the method is to control the shape of the pressure distribution around the turbine rotor blade sections, so as to postpone blade stall.
Using a three dimensional finite element method of analysis, an analytical feasibility study on suction piles was conducted. Elastoperfectly plastic soil properties were used to evaluate the effect of various cross-sectional shapes on the overall performance. Results of soil stresses and pile displacements under vertical, horizontal, and inclined loads were evaluated and compared.
The US Navy is currently conducting a technical feasibility study pertaining to the construction of Mobile Offshore Bases (MOBs). This is expected to be a self-propelled, floating military base with a runway on top and other supporting facilities below such as living quarters, material storage areas, docking facilities for transport ships, etc. The proposed dimension of the MOB is approximately 1,500 meters by 150 meters. It is intended to be a forward-deployed, self-contained military base floating in deep waters. The South Dakota School of Mines and Technology is participating in this MOB feasibility study to provide an adequate mooring technique for this very large floating structure. The MOBs are expected to be controlled by dynamic positioning. However, during storage, repair, or lay-up periods, or for hybrid mooring, conventional mooring techniques may be needed. Suction piles are currently being investigated analytically and experimentally to provide the necessary mooring capacity. Suction piles typically have a large diameter (up to 30 meters to date) with a relatively small length-to-diameter ratio. They are installed by applying a suction pressure inside the pile, which acts as an external surcharge to push the pile into the seafloor. They may be retrieved later by applying a positive pressure inside the pile. This paper describes the results of an analytical performance study on suction piles, using a three-dimensional finite element method of analysis. Three cross-sectional shapes that were thought to be able to provide adequate bearing resistance against various external loads were selected.
A unique temporary foundation system, which combined skirted mudmats with an air lift system, was used during the installation of two fixed offshore platforms. The skirts on the mudmats allowed the use of active suction to penetrate the mats. To optimize simplicity, reliability, and cost, water removal from inside the mudmats was achieved through the careful design of an air lift system using oil well gas lift technology. This skirted mudmat system has numerous advantages: it improves on-bottom stability of the structure, allows for a controlled initial penetration of the mudmats and provides a means to quickly and accurately level jackets.
The Amoco Trinidad Mahogany "A" and "B" platforms are conventional 4 leg jacket structures with 4 skirt piles and are currently in place in the Atlantic Ocean approximately 50 miles (31 km) off the East coast of the island of Trinidad in approximately 285 ft (86 m) of water (Fig. 1). A unique temporary foundation system was utilized during the installation of these structures. This system provided a relatively high level of jacket on-bottom stability and permitted jacket leveling operations to take place at any time between jacket set down and the start of pile to sleeve grouting operations. Jacket leveling operations were accomplished with minimal installation vessel critical time and without hook assistance from the installation vessel. This system and its performance are discussed below.
Figure 2 shows two elevation views of the Mahogany "B" platform. The lift-installed jacket has mudline plan dimensions of approximately 100 x 100 ft (30 m x 30 m) while the top of the jacket is 50 x 100 ft (15 m x 30 m). The four main jacket legs are approximately 60 in. (1.52 m) in diameter. The topsides lift weight was approximately 3000 short tons (2700 tonnes).
The authors have performed many cyclic loading tests of saturated clays and investigated the relationship between the degradation in shear behavior due to cyclic loading and the clay types, based on their cyclic triaxial test data and available experimental data. As the results, the degradation in cyclic shear behavior is related to the plasticity of clays and the cyclic shear behavior dramatically changes at 25 of plasticity index. Also, with the increase in soil plasticity and activity, the degradation in cyclic stiffness decreases and the degradation in undrained strength after cyclic loading increases.
The cyclic behavior of saturated clays has been investigated by many researchers. Since clayey ground is sometimes subjected to cyclic loadings through earthquakes, waves and traffic loads, the saturated clay behavior to such cyclic actions is an important problem. Many factors affecting the cyclic response of saturated clays have been investigated. The degradation in cyclic shear behavior with the decrease in clay plasticity is well known, however, its mechanism has not been elucidated yet. The relationship between degradation in cyclic shear behavior and soil properties is needed to be investigated. In this paper, the authors discuss the effect of such physical properties as plasticity index, clay fraction and activity on the degradation in cyclic shear behavior of saturated clays, based on their cyclic loading test data and available data collected from many publications in which the cyclic behavior of saturated clays is shown with different physical properties.
The cyclic stiffness degradation during cyclic loading is expressed by the degradation index 6 proposed by Idriss et al (1978). Tan and Vucetic (1989) and Bahr (1991) have observed that clays become less susceptible to the degradation in the course of cyclic load application, because of high plasticity.
The present paper present spatial-temporary distribution of ice ridging in the Okhotsk Sea in mild, moderate and severe winters.
Ice ridging is one from the important features of ice cover, which complicate the navigation, fish craft and bottom extraction of mineral resources. The full-scale Arctic observations have shown that the raise of ice ridging on one ice number increases an ice thickness on 25 %. The Okhotsk sea is not exception with largest magnitudes of tides in Pacific basin and very low air temperatures. These factors promote the ice forming, compaction and freezing. In separate water areas of the Okhotsk Sea the severe zones of ridges are formed annually with significant spatialtemporary variability.
SPATIAL DISTRIBUTION OF RIDGES
For an evaluation of this variability the regular air visual observations since autumn 1956 up to 1991 were used. At first, ice number systematization (average and extreme values) in each month of ice period was made. Then the years in a range 20 % from extreme and average values of ice number were found. Thus, the groups of years with mild, moderate and severe ice state were for each type of ice number. Then the handling ridging of ice was produced separate each square for all months of an ice phase. An outcome is annual maps of distribution of ice ridging numbers for mild, moderate and severe ice cover. Fig. 1 shows such maps for March- month with maximum ice cover distribution in the Okhotsk sea. The analysis of these maps allows to reveal sea water areas with significant and small ice ridging. The first ones are Penzhinskaya and Yamskaya Bays and also Sakhalin Gulf and region, close to south from Shantarskiye islands. Ridging here begin in 2-3 weeks after steady ice forming.
When designing the air-gap of an offshore platform, current draft ISO standards () recommend that the extreme "green" water level be set by either Reliability considerations or Experience and judgement. Account may be taken of the joint probability of tide, surge height and crest heights if the metocean database allows and a reliable model for crest statistics exists. The joint probability of these variables is however seldom studied. By analogy with the traditional method, one might thus be tempted for instance to define maximum level as the sum of MHWS (Mean High Water level Spring tide), 100-year storm surge and 100- or 10000- year crest height. As especially MHWS is not a 100- year event, the above method assumes some level of dependence between the variables. The present paper reports an investigation of green water highest levels, using the joint probability of tidal elevation and crest height, based on a case study at one of the locations studied in , where observed joint distribution of storm surge and significant wave height was provided. In that case, use of the actual joint distributions reduces the 100-year level by 0.6 m, and the 10000 year level is similar to combination of 10000-year crest, MHWS and 100-year storm surge. These differences are much smaller however than those related to the choice of the crest model, when crest measurements are not available for the location of interest. Caution and judgement should thus indeed be exercised when combining the components of the extreme "green" water level for an offshore structure , but further research is needed for the modelling and description of extreme sea states before one can reliably set water levels corresponding to the now commonly requested safety probabilities .
Gas Explosions are a major hazard to Offshore Installations, not only from the direct effects of the explosions but also from escalation due to failure of safety critical systems. It is now a design consideration requiring that the effects of hydrocarbon explosions be evaluated. This paper examines some of the uncertainties inherent in calculating the response of offshore structures to hydrocarbon explosions and thus allow better bounds on the consequences.
With the introduction of the UK goal setting regime following the loss of Piper Alpha design of topsides against gas explosions has become a major design consideration. Gas explosion was found to be the main contributor to safety risk on offshore platform (as measured by risk to individual and impairment to temporary refuge). Immediately following Piper Alpha many installations were designed on the basis of what was then considered as the ''worst case'', a module full of a stochiometric mixture of gas. Our understanding of gas explosion then was incomplete, particularly, in the area of scaling; there was only small and medium scale data. The largest scale was a 1:5 scale of a much simplified model of a gas compression module at the Christian Michelsen Research in Norway. Using codes then available, it was possible by good layout practice to control maximum over-pressures generated by congestion within a module to about 1 to 2 bar as shown by Tam and Langford (1994)., Two parallel development changed that. The first is the R&D in theoretical understanding and mathematical modelling of gas explosion. The codes have been refined, with the computing power now available much ,greater resolution of the geometric model (representing the layout of the platform) can be obtained. It is known that the cumulative effects of a large number of small objects on the over-pressure predictions can be significant.
We describe herein the basic objectives and structure of the MAST project Eurowaves. The purpose of the project is to integrate, on a European scale, high quality offshore wave statistics with appropriate coastal numerical wave models and geographical and bathymetric information, to allow a relatively inexperienced user to obtain a detailed statistical description of the ocean wave climate at any nearshore location in European waters. The Eurowaves "toolbox" will cover the entire Mediterranean Sea, the Black Sea, the Baltic and the Barents Seas. Eurowaves blends existing data of various origins and from different disciplines into an extensive background data base with user-friendly and sophisticated modelling and analysis software tools, with the aim to make high quality wave information available to a larger group of users than is the case today and at lower cost.
Many marine operations and activities are concentrated at or near the coast. One of the most frequent requirements in this coastal zone is for information on the wave conditions. Ideally, the required information would be readily available in book or electronic form. Unfortunately, the coastline''s extent and variability translate into rather large spatial variations in the wave climate both in the longshore and offshore directions. Thus, it would be an impossible task to precalculate all possible information at all locations. Eurowaves is an ongoing MAST project, which has the aim to much improve the access to good quality wave information in nearshore waters, following a new approach. The user or provider of wave data will be equipped with a PC tool, enabling the wave conditions to be evaluated at any nearshore location indicated on a map. In practice, the problem is shifted to the optimal exploitation of the available information by means of special-purpose user-friendly software.
A floating, 2-body concept consisting of a deep water mini TLP and a tender barge, is investigated experimentally. The system is designed for mild environments and with a "soft" connection between the two structures. The emphasis is put on measurements and analysis of the coupled, horizontal motion response of the system, with particular focus on statistical aspects and extreme value predictions. The paper also highlights the general performance and robustness of the coupling between the two floaters, providing strict limits for the relativ excursions.
Driven by low crude oil prices and reduced profit margins, there is currently a major drive within the oil industry towards developing robust and inexpensive concepts for offshore oil production. The mini TLP and barge in tandem system, investigated in the current paper, seems to hold some promise in this respect, being a novel design well suited for inexpensive drilling and production in benign environments. The barge is clearly a very basic design, and its lack of sophistication, in combination with the relative abundance of such vessels worldwide, makes it a rather affordable option. Similar arguments can be given in favour of the mini TLP; which is a small sized, generally unmanned structure, but still capable of supporting a considerable number of "cheap" steel risers, and offering the advantage of dry wellheads. The major engineering challenge associated with this 2-body concept is to design a system for permanent mooring of the two vessels, and to confirm that the proposed solution is practical, suitable and safe. This paper deals with the experimental verification of a given design, with particular focus on the coupled motion response of the two structures under design ULS (Ultimate Limit State) conditions.
The retardation of fatigue crack caused by crack tip local heating was studied. Steady fatigue crack growth under constant amplitude loading would be disturbed by the sudden change in plastic zone size resulting from a single overload. However, mechanical properties of metal, such as yielding stress, Young''s modulus, etc., may vary with increasing temperature. From this consideration, the fatigue crack tip plastic zone size can be altered by locally changing the mechanical properties of this area. In order to understand the thermal effect on the retardation of fatigue crack, constant amplitude fatigue propagation tests were carried out using A5083P-O aluminum alloy specimens. Tests were performed under identical applied load, but with different levels of heat input. The a-N curves were recorded to compare the degree of retardation, and the variation in crack tip opening was recorded with CCD camera. The results show that the crack growth rate was reduced, when the level of heat input to crack tip was increased. Additionally, the distribution of stress during loading and heating was calculated by FE methodology. The result shows that temperature increase at the crack tip would cause the residual compressive stress, which is believed to he responsible for the retardation of fatigue growth.
For structural components, fatigue failure is a very common problem that usually initiates at a small defect, then through a period of propagating, and finally lead to catastrophic failure. The best way to prevent the occurrence of fatigue problem is to prevent the formation of fatigue crack. Unfortunately, the existence of defect in welded structure, such as: undercut, inclusion, discontinuity, void, etc., are unavoidable. When fatigue crack is detected but can not be repaired immediately, temporary measures can reduce the propagation rate by restraining the extension damage.