Prediction of Hydrodynamic Forces and Moments on Submarines Using Neural Networks

This paper describes a parametric identification tool for predicting the hydrodynamic forces acting on a submarine model using its motion history. The tool uses a neural network to identify the hydrodynamic forces and moments; the network was trained with data obtained from multi-degree-of-freedom captive maneuvering tests. The characteristics of the trained network are demonstrated through reconstruction of the force and moment time histories. This technique has the potential to reduce experimental time and cost by enabling a full hydrodynamic model of the vehicle to be obtained from a relatively limited number of test maneuvers.

Reflection and Transmission of Ship Waves by Floating Barriers

The reflection and transmission of ship waves by vertical floating barriers located on both sides of a fairway are investigated by the modified Dawson’s method in this paper. The free surface is specially treated to take into account the floating barriers. The wave pattern and the wave energy density between and outside the barriers are obtained. It is found that the reflection and transmission performance of a barrier is associated with its width and height. For a wider or higher barrier, more ship waves are reflected by it. A vertical floating barrier with a reasonable width and height can reduce ship waves in the outer region very efficiently.

Probabilistic Inspection Optimization of Free-Span Surveys for Subsea Gas Pipelines

Probabilistic methods have been used to develop the basis for free-span inspection of a gas pipeline system in the South China Sea. The objective of the probabilistic analysis was to study the probability of fatigue failure associated with postulated planar flaws in the HAZ of repair welds performed on some of the girth welds. The impact of flaws on the fatigue life under different free-span conditions were studied. Conventional free-span analysis involves computation of allowable free-span lengths based on onset of in-line vibrations and does not normally consider fatigue crack growth. To consider the effect of the weld flaws on the failure probability a combined probabilistic fatigue and fracture model is required. For the particular pipelines analysed automatic ultrasonic testing (AUT) was used replacing the conventional radiography of the girth welds. Conservatism in the free-span assessment can then be significantly reduced by taking into account detailed flaw sizing information from the AUT. The inspection records provide distribution of flaw height, length and position. Combined with information on current distribution, material strength and fracture toughness distribution, a detailed probabilistic fatigue crack growth and unstable fracture assessment can be conducted as per the Det Norske Veritas (DNV) 2000 Rules for Submarine Pipeline Systems [1] using the response models of the DNV Guideline 14 for free-span analyses [2]. The objective of this analysis is to estimate the critical free-span lengths and the time for fatigue cracks to penetrate the pipe wall.

Optimum Reactive Control and Control by Latching of a Wave-Absorbing Semisubmerged Heaving Sphere

The theoretical potential for maximising energy output of wave-energy converters by means of optimum control is quantified for a heaving semi-submerged sphere on deep water. The heave amplitude is constrained to not exceed 0.6 units of the radius. Sinusoidal incident waves of amplitudes up to 3 m and period in the range of 6 s to 12 s are considered, when the sphere radius is 5 m. Optimum reactive control, contrary to sub-optimal latching phase control, requires ability for reversing the energy flow through the power take-off machinery. Computed results show that, for a typical wave of 0.5 m amplitude and 9 s period, the maximum absorbed power is 24, 137, and 172 kW for the cases of no phase control (passive system), sub-optimal latching control, and ideally optimal reactive control, respectively. The ratio between the maximum/minimum instantaneous power and the average absorbed power is 2/0, 4.1/0 and 11.0/-9.0, respectively, for the three different strategies.

Flow Assurance of Wet Gas Pipelines From a Corrosion Viewpoint

In many offshore oil and gas projects under development, the pipeline costs are a considerable part of the investment and can become prohibitively high if the corrosivity of the fluid necessitates the use of corrosion resistant alloys instead of carbon steel. Development of more robust and reliable methods for internal corrosion control can increase the application range of carbon steel and therefore have a large economic impact. Corrosion control of carbon steel pipelines has traditionally often been managed by the use of corrosion inhibitors. The pH stabilization technique has been successfully used for corrosion control of several large wet gas condensate pipelines in the last few years. Precipitation of scale and salts in the pipeline and process equipment creates further challenges when formation water is produced. Different corrosion prediction models are used in the industry to assess the corrosivity of the transported fluid. An overview of the present models is given together with a link to fluid flow modeling.

Anchor Deployment Planning Using Simulated Annealing

In this work a planning methodology for deep-water anchor deployment of anchor lines for offshore platforms and floating production systems aiming at operational resources optimization is explored, by minimizing a multi criteria objective function. A Simulated Annealing Algorithm was used to optimize the objective function. As an additional advantage, inherited from the proposed methodology, the planning automation is achieved. Planning automation overcomes the traditional way based on trial error exercise, where an engineer using an anchoring application, decides how much of work wire and anchoring line must be paid out from both the floating system and the supply boat and additionally which horizontal force must be applied to the line trying settle the anchor on a previously defined target in the ocean floor. Some cases, from anchor deployment of some MODUs operating in deep-water oil fields in Brazil, are shown demonstrating some potentialities of the proposed model.

Wave Induced Pressures on Pipelines Near a Sloping Boundary Due to Regular Waves

The hydrodynamic pressures due to regular waves around the circumference of a pipeline near a sloping rigid bed and placed parallel to the wave direction have been measured. The pressures were integrated to obtain the force time history, from which the peak horizontal and vertical forces were evaluated. The effects of relative clearance of pipe from the bed and its relative position from the toe of the sloping bed on the pressures and forces on the pipeline as a function scattering parameter and wave steepness are reported. The reflection characteristics of the sloping bed in the presence of the pipeline are reported as a function of surf similarity parameter and compared with the results from existing literature. The details of the model setup, experimental procedure, results and discussion are presented in this paper.

Development of an Oxygen “Metabolizer” to Control Oxygen Levels in the Closed Cabin of Submarine Rescue Vehicles

The U.S. Navy is committed to maintaining the capability of rescuing survivors from a disabled submarine, including situations where the disabled submarine becomes internally pressurized due to flooding, leakage of compressed gas supplies, or through use of auxiliary breathing systems. Efficient submarine rescue requires that pressurized crewmembers be decompressed more rapidly than current decompression procedures allow when using air. The Navy Experimental Diving Unit in Panama City, FL has shown that crew decompression can be accelerated significantly by pre-breathing oxygen. Unfortunately, such oxygen pre-breathing can result in oxygen buildup in the cabin atmospheres of the submarine, and/or rescue chamber through leakage around the face seal of the oxygen masks. High levels of oxygen can create hazardous conditions within the cabin atmosphere due to fire potential and/or oxygen toxicity concerns. This paper addresses the concept development of an oxygen “metabolizer” using a hydrogen catalytic reactor to consume excess oxygen within the closed cabin atmosphere of a rescue vehicle. Such a catalytic reactor has also been shown to give an effective method of reducing the level of oxygen in diver breath heating and diver whole-body heating systems.

Investigating Novel Foundations for Offshore Windpower Generation

In recent years there has been a worldwide increase in the pressure to develop sources of renewable energy. The UK government is committed to ensuring that ten percent of UK energy consumption will be supplied by renewables by the year 2010. Central to this commitment is the need to develop wind farms particularly in the offshore environment. Moving offshore will allow very large wind turbines capable of supplying 2 MW (first generation) to 5 MW (second generation) of power to be installed in large farms consisting of up to fifty or more turbines. In contrast to typical oil and gas structures the foundation may account for up to forty percent of the projected installed cost. The weight of each structure is very low, so the applied vertical load on the foundation will be small compared to the moment load derived from the wind and waves. Further, it will be necessary to have a single design that can be mass-produced over each site rather than have each foundation individually engineered. In combination these points lead to a very interesting engineering problem where the design of the foundation becomes crucial to the economics of the project. One solution is to use conventional piling. However, at some sites it may prove more economical to use shallow foundations, and, in particular suction installed skirted foundations [1]. It will be necessary to develop an adequate design framework for these no vel foundations under the relevant combinations of load so that the optimum structural configuration can be achieved. At Oxford University a program of research on skirted foundations has been underway for the last five years, and much progress has been made on the understanding of this type of foundation under combined loading. This progress has been in both experimental and theoretical areas. This paper explores various structural options that might be used for the wind turbine application. These different options lead to different loading conditions on the foundations. Experiments investigating these different loading conditions are explored. A theoretical approach that describes the experimental results in a way that can be implemented in typical structural analyses programs is outlined. Finally details of a major research program into developing the necessary design guidelines for foundations for offshore wind turbines is described.

On the Effect of Water on Deck on Ship Motion

The effect of trapped water on deck or the interior compartments of ships on ship motions is closely investigated by use of a non-linear numerical simulation method. The employed method enables the efficient simulation of the wave excited, coupled ship – trapped water motions and proves to be a very valuable tool for the assessment of the survivability of flooded ships in waves. A detailed study has been carried out to more carefully investigate the coupling effects between the ship and the floodwater mass that can be expressed through a resultant interaction force. This interaction force has been approximated both by a simplified model employed by the present simulation method and also by a more accurate CFD code and satisfactory agreement between the results of both approaches has been obtained.