Everyone agrees that the best way to deal with sand is to keep it in the formation, but when sand eludes our best efforts to contain it, it must be dealt with.

Sand and solids are often produced with the valuable hydrocarbon resources. Sand or solids, once entering the production train, will cause erosion of flow lines, manifolds, pipework, choke valves, control valves, vessel internals and will collect in areas of low flow stream velocity. The sand and solids will, over time, accumulate in vessels such as production separators, degassers, desalters, knockout drums, etc. and cause a loss of performance due to the reduced volume available for fluid separation.
This article highlights some of the process equipment options for topsides sand management including removal of sand from process streams and sand handling/cleaning. Several of these technologies are being developed for application downhole or subsea.
Sand deposition
Sand and solid deposition is most likely to occur in primary production separators due to the low fluid velocities that are required to effect phase separation compared to piping velocities. If sand and other solids accumulate in separators, the vessel capacity, and thus the separation efficiency, is reduced. Operation of instrumentation and other internal components may also be impaired.
Influencing factors. Determining the level of solids deposition in separators can prove difficult. The solids deposited in separation systems may well include sand, proppant, corrosion products, scale and production chemicals, all of which influence the settling characteristics of the solids. The type and configuration of the vessel's internal components will also have a significant effect on the location and rate at which the entrained solids settle out.
Deposition rate and profile prediction. The simplest way to approximate the rate of solids deposition is to utilize Stokes Law. As particle size and often hydraulic effects have to be estimated, inaccuracies in predictions will arise with this approach.
Increasingly, computational fluid dynamics is used to predict deposition rates. One of the advantages of computational fluid dynamics software is that the internal components of the separator can be incorporated into the simulation model enabling their effects on the fluid flow and solids deposition to be assessed.
Process streams
Produced water desanding. The use of cyclonic technologies (desanders) to remove solids such as sand from produced water has been widely used in offshore and onshore production facilities for many years. The basic principle is centrifugal separation, suspended particles are subjected to centrifugal acceleration by the carrier fluid which causes them to separate due to their different densities. Separation performance is influenced by the cyclone diameter and internal design, the fluid and solid properties, and flow rate. Therefore, correct selection of the desander size and flow range is imperative to meet the required separation performance.
Multiphase wellhead desanding. The use of a cyclone-based system for wellhead sand removal has been successfully employed upstream and downstream of the wellhead chokes. All the wellstream fluids pass through the wellhead desander prior to entering the conventional downstream separation process. Solids removed are collected in an accumulator, which is periodically flushed while the wellhead desander remains online.
Cost-effective designs can be provided for production systems, workover units and underbalanced drilling systems.
Separators and vessels
If the solids are not removed from the process streams by desanding technologies, the solids will deposit in vessels. Several options are available to operators to remove these solids:
Sand jetting. Sand jetting involves directing streams of "wash water" into the bottom section of a separator vessel, with the aim of fluidizing accumulated sand deposits and transporting them towards sand or slurry drain points in the bottom of the vessel. These systems can be custom designed to match wash water availability, and automated. Some of the disadvantages include high washwater rates, potential turbulence effects, poor recovery of solids and slurry concentration control.
Hydro transportation. Hydro transportation devices can be considered to be the "next generation" of sand removal devices. Among these is the HydroTrans device from Petreco that introduces wash water in a vortiginous manner. As well effecting initial mobilization of accumulated solid particles, an area of low pressure is produced at the center of the induced vortex, which is utilized, by means of strategically placed piping, to transport the produced slurry out of the vessel.
The device prepares a slurry of any liquid and solids to the required concentration and pressure and transports the slurry into the transportation pipeline or downstream process system as may be required. It can fluidize and transport a wide range of solids and particle sizes.
The basic unit can be fabricated to suit any nozzle type and stand off, thus allowing for new build or retro-fitting. The device's fluidizing head is normally fitted through the bottom of the vessel or hopper with the slurry delivery duct fitted immediately above. A number of units can be manifolded to minimize vessel penetrations.
Advantages
There are several advantages of using a continuous vortex sand removal device:
• Online sand removal, no disruption of fluid interfaces;
• No moving parts;
• Large zone of influence;
• No erosion of head;
• Effective slurry concentration control (max 70% by spot volume);
• No slurry pump required;
• Easy to install or retrofit (a 1-in. HydroTrans can be installed in a 2-in. nozzle);
• Reduced washwater consumption;
• No erosion of vessel surface;
• Smaller and fewer inlet and outlet nozzles are required; and
• Predictable performance.
Alternative methods
There are other ways to remove sand from topsides equipment. These include:
Manual removal. Manual removal of sand from separators is obviously the most basic method that can be employed. The main advantage is low capital and operating costs. The other benefit is that complete sand removal is assured. However, there are major drawbacks, specifically health, safety and environment issues including occupational injuries and the fact that hazardous material handling and removal has to coincide with a production shutdown.
Eduction. Eductors are established solids and liquids conveying devices. However, they have not found widespread use in the oil and gas industry for the purpose of sand removal.
Sand cleaning
In order to meet legislative requirements, removed solids should be cleaned prior to disposal. Petreco has utilized the attrition of cyclonic washing systems (desanders, HydroTrans and eductors) in numerous sand cleaning systems. This allows sand to be cleaned sufficiently to permit discharge to the sea (hydro-carbon on sand to less than 100 ppmv).
Materials
By their nature, cyclonic devices accelerate fluids and produce high velocities, which will lead to high erosive forces. Therefore materials that are resistant to sliding wear such as alumina ceramic (in various grades), silicon carbide and tungsten carbide are used for desander liners. Rubber materials and polyurethane materials can also be considered depending on the wear regime. In some applications, spray coatings and surface treatments may be considered as lower cost options, but with reduced lifetime compared to using erosion resistant materials throughout.
Petreco has developed a range of enhanced erosion resistant hydrocyclone liners using materials that have a considerably longer lifetime compared to materials typically used to manufacture liners, such as duplex stainless steels and cobalt alloy AMS 5387 (stellite 6). These liners have been installed offshore in the North Sea and have outlasted "standard" materials by a factor of three or more with very limited signs of wear.