The problem of subgrid modelling, that is, of representing energy transfers from large to small eddies in terms of the large eddies only, must arise in any large eddy simulation, whether the equations of motion are open or direct (unaveraged) or closed (averaged). Models for closed calculations are derived from classical closures, and these are used to determine the effect of filter shape, grid-scale spectrum and grid-scale anisotropy on the effective eddy viscosity: the Leonard or resolvable-scale stress is calculated separately and is found to account for 14% of the total drain in a typical high Reynolds number case.The validity of using these eddy viscosities in an open calculation is considered. It is concluded that this is not unreasonable, but that the simulation would be much improved if the gross drain could be separated into net drain and backscatter.

Flow assurance challenges, mainly of hydrates and wax depositions, are amongst the key issues that must be resolved and mitigated to ensure that hydrocarbons can be efficiently and economically transported from well to processing facilities. As wells step further away from shore into deeper water, the flow assurance challenges are increasing tremendously due to prevalence of higher pressure and lower temperature conditions. Thus, the development of cutting edge technologies to cater for the ever increasing demand in exploring the hostile and technologically challenging deepwater fields is a matter of great urgency. One of the effective solutions to prevent the formation of wax or hydrates is to use active heating methods. This paper describes an overview of the available active heating methods and mechanisms which are being implemented as thermal management systems for flowline in deepwater fields. It also discusses the thermal performance calculation models available to aid the design and modelling of such systems. Some comparative studies are carried out to determine the advantages and disadvantages of each of the methods to establish a general reference source on the technology that provides the most significant economic impact without compromising the reliability and efficiency of the overall system. Active heating systems have been used in several projects in the North Sea, Gulf of Mexico and Offshore West Africa. This paper also summarizes these projects and their operating experience from open literature. In general, due to their operational flexibility and high efficiency through control of the pipeline temperature above the hydrate formation and wax deposition temperatures, active heating seems to be the most practical, economical and viable solutions in managing flow assurance issues; especially for the development of deepwater fields.

Water-supply companies monitor the state of their pipe networks and ensure that the pipes are clean and free of inert loose deposits by flushing and pigging. Flushing involves forcing high speed water through the pipes so as to carry away particulates, pigging consists of forcing an object (the pig) through the pipe so as to push or wipe away the loose material. Both systems have drawbacks; the first tends to use very large volumes of water, and it may be impossible to get the required velocities in large-diameter pipes. The second requires purpose built launch and receive stations and may run the risk of damaging the pipe walls. This paper presents an innovative alternative to water flushing or conventional pigging for the potable-water-supply industry. This alternative uses a phase change material (ice-water slurry), which can be introduced into and removed from existing pipe networks with minimal alterations. The underlying concept is that when an ice slurry is propelled through pipes at modest speeds, the wall shear is two to four orders of magnitude higher than that which would have been achieved had water (only) been travelling in the pipe at the same speed. Thus, even with relatively low speeds, the 'ice pig' is able to achieve efficient cleaning and removal of loose materials. This technology has the advantage that the ice pig changes its shape to fit the containing topology, hence it is able to navigate bends, contraction/expansions and partly open vales, while cleaning the containment walls and transporting particulates. Lastly, the ice pig is guaranteed never to get stuck, as it will simply melt away, if left for sufficient time.The paper presents laboratory experimental data, qualitatively demonstrating the capability of the technique and quantitative data enabling engineers to scale and size the ice pig for full scale trials. Finally, preliminary work from full scale trails on live water trunk mains is briefly presented and discussed.

This paper presents the results of experiments and modelling carried out on ice slurries flowing in uninsulated steel pipes with a nominal diameter of 50 mm. The slurries used were formed from 4.75% NaCl aqueous solution and had ice mass fractions in the range 18-42%, with a view to the use of thick ice slurry 'pigs' as a pipeline clearing technique. Of particular interest was the distance over which such slurries can survive as plug-like entities, before melting reduces them to ineffective thin two-phase suspensions. The experiments showed that for small volumes of slurry, survivability is directly proportional to the quantity of slurry used, but that increasing the ice fraction has a more marked effect. A simple one-dimensional numerical model that accounts for transportation, heat transfer and melting was developed that produces reasonable predictions.

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