SOLID CONTROL EQUIPMENT

SOLID CONTROL  EQUIPMENT

Recall mud is made up of fluid (water, oil or gas) and solids (bentonite, barite etc).The aim

of any efficient solids removal system is to retain the desirable components of the mud

system by separating out and discharging the unwanted drilled solids and contaminants.

Solids in drilling fluids may be classified in two separate categories based on specific gavity,

(or density) and particle size.

Solids, classified by specific gravity, may be divided into two groups:

• High Gravity Solids (H.G.S.) sg = 4.2

• Low Gravity Solids (L.G.S.) sg = 1.6 to 2.9

The solids content of a drilling fluid will be made up of a mixture of high and low gravity

solids. High gravity solids (H.G.S) are added to fluids to increase the density,e.g. barytes,

whilst low gravity solids (L.G.S) enter the mud through drilled cuttings and should be

removed by the solids control equipment.

Mud solids are also classified according to their size in units called microns (). A micron is

0.0000394 in or 0.001 mm. Particle size is important in drilling muds for the following

reasons:

• The smaller the particle size, the more pronounced the affect on fluid properties.

• The smaller the particle size, the more difficult it is to remove it or control its

effects on the fluid.

The API classification of particle sizes is:

Particle Size ()            Classification Sieve Size (mesh)

> 2000 Coarse                      10

2000 - 250 Intermediate       60

250 - 74 Medium                 200

74 – 44 Fine                        325

44-2         Ultra Fine

2-0         Colloidal

7.1 SOLIDS CONTROL EQUIPMENT

Solids contaminants and gas entrapped in mud can be removed from mud in four stages:

• Screen separation: shale shakers, scalper screens and mud cleaner screens.

• Settling separation in non-stirred compartments: sand traps and settling

pits.

• Removal of gaseous contaminants by vacuum degassers or similar equipment

• Forced settling by the action of centrifugal devices including hydrocyclones

(desanders, desilters and micro-cones) and centrifuges.

SCREEN SEPARATION DEVICES

Figure 7.6 shows a layout for solids control equipment for a weighted mud system.

Shale shakers and scalper screens (Gumbo shakers) can effectively remove up to 80% of all

solids from a drilling fluid, if the correct type of shaker is used and run in an efficient

manner. Mud laden with solids passes over the vibrating shaker (Figure 7.7) where the liquid

part of mud and small solids pass through the shaker screens and drill cuttings collect at the

bottom of the shaker to be discharged.

There are two types of shaker operation: elliptical and linear motion. Field experience

indicate that elliptical shakers work better with water based muds and linear motion shakers

are more suited to oil based muds.

An absolute minimum of three shale shakers is recommended and that these shakers are

fitted with retrofit kits to allow quick and simply replacements.

The shakers should also be in a covered, enclosed housing with a means of ventilation and

each shaker fitted with a smoke hood.

REMOVAL OF GASEOUS CONTAMINANTS 

                         

Gas entrapped in mud must be removed in order to maintain the mud weight to a level

needed to control down hole formation pressures. Gas is removed from mud using a

vacuum degasser, see Figure 7.8. The latter is a simple equipment containing a vacuum

pump and a float assembly. The vacuum pump creates a low internal pressure which

allows gas-cut mud to be drawn into the degasser vessel and it then flows in a thin

layer over an internal baffle plate. The combination of low internal pressure and thin

liquid film causes gas bubbles to expand in size, rise to the surface of the mud inside the

vessel and break from the mud. As the gas moves toward the top of the degasser it is

removed by the vacuum pump. The removed gas is routed away from the rig and is then

either vented to atmosphere or flared.

FORCED SETTLING BY CENTRIFUGAL DEVICES

Desanders and

desilters are

hydrocyclones and

work on the principle of separating solids from a liquid by reating centrifugal forces inside the hydrocyclone. Mud is injected tangentially into the hydrocyclone and the resulting centrifugal forces drive the solids to the walls of the hydrocyclone and finally discharges

them from the apex with a small volume of mud, Figure 7.9.

The fluid portion of mud leaves the top of the hydrocyclone as an overflow and is then sent to the active pit to be pumped downhole again.

(a) Desanders

Desanders are hydrocyclones with 6 in ID or larger.The primary use of desanders is in the

top hole sections when drilling with water based mud to help maintain low mud weights. Use of desanders prevents overload of the desilter cones and increases their efficiency by

reducing the mud weight and solids content of the feed inlet. Desanders should be used if the sand content of the mud rises above 0.5% to prevent abrasion of pump liners.

Desanders should never be used with oil based muds, because of its very wet solids

discharge. The desander makes a cut in the 40 to 45 micron size range. With a spray discharge, the underflow weight should be between 2.5 to 5.0 ppg heavier than the input

mud.

(b) Desilters

Desilters, in conjunction with desanders, should be used to process low mud weights used to drill top hole sections, Figure 7.10. If it is required to raise the mud weight this must be done with the additions of barytes, and not by allowing the build up of low gravity solids.

Desilters should never be used with oil based muds.

The desilter makes a cut in the 20 to 25 micron size range.

Typical throughput capacities are as follows:

Desanders 12"cone 500 gpm per cone.

6" cone 125 gpm per cone.

Desilters 4"cone 50 gpm per cone.

2" cone 15 gpm per cone.

As a visual check to see that the hydrocyclone

operations are at an optimum, the discharge should be in the form of a fine spray and a suction should be felt at the apex when covered with the hand. A rope  discharge means than the mud has lost its circular  motion and the cone is not working properly.

(c) Mud Cleaners

A mud cleaner consists of a battery of hydrocyclones placed above a high energy vibrating

screen, Figure 7.11. Mud cleaners must only be used when it becomes impossible to

maintain low mud weights by use of the shale shakers alone. It is far more efficient to use

desilters and process the underflow with a centrifuge than to use the screens of a mud

cleaner.

The use of mud cleaners with oil based muds should be minimised since experience

has shown that mud losses of 3 to 5 bbls/hr being discharged are not uncommon,

coupled with the necessity to adhere to strict environmental pollution regulations.

(d) Centrifuges Centrifuges use centrifugal forces to remove heavy solids from the liquid and

lighter components of the mud. A decanting centrifuge consists of a horizontal conical

steel bowl rotating a high speed, see Figure 7.12. The bowl contains a double-screw type conveyor which rotates in the same direction as the steel bowl, but at a slightly lower speed. When mud enters the centrifuge, the centrifugal force developed by the bowl holds the mud in a pond against the walls of the pond. In this pond the silt and sand particle settle against the walls and the conveyor blade scrapes and pushes the settled solids towards the narrow end of the bowl where they are collected as damp particles with no free liquid. The liquid and clay particles are collected as as overflow from ports at the large end of the bowl.

It is recommended to have at least one centrifuge on the rig site during all drilling

operations. For expensive muds or long term drilling operations, two centrifuges may prove

economical.

When dealing with low weight muds, the solids underflow is discarded as a means of solids

control to obtain desirable particle size distribution and reduce mud weight. Processing

capacity of the centrifuge may limit its use for this purpose to lower hole sections where the

circulation rates are low as the bowl speed must be at a maximum, so lower capacities can be

dealt with. It can also be used to process the underflow from desilters, returning an expensive

or environmentally harmful liquid phase to the active mud system, and discarding relatively

dry solid fines.

With weighted muds, the solids underflow containing barytes may be returned to the mud

system and the liquid phase containing viscosity building colloids discharged. However it is

unlikely to be used for this purpose with oil based muds for both economic and

environmental reasons.

Centrifuge efficiency is affected predominantly by the feed flow rate, but it is also affected

by the following operating parameters:

• Bowl speed (rpm).

• Bowl conveyer differential speed (rpm).

• Pool depth.

Solid Density From Retort Analysis Mud Calculation

Retort analysis is the method to determine solid and liquid components in the drilling fluid. In this article, we will adapt mass balance and retort analysis data to determine solid density in the mud.

Mass balance for mud is listed below;

We can rearrange the Euqation#1 in order to determine the solid density

In the report analysis, the volume is presented in percentage and summation of solid and liquid fraction equals to one.

The unit of each parameter is described below;

Vm = mud volume, %

ρ_m = mud density, ppg

Vw = water volume, %

ρ_w = water density, ppg

Vo = oil volume, %

ρ_o = oil density, ppg

Vs = solid volume, %

ρ_s = solid density, ppg

Note: this is not only cutting weight but it includes all weights of solid (cutting and weighting material).

Example: Mud weight used for the report is 12.0 ppg and the result from the analysis showing in the following percentage;

Base oil = 60%

Solid = 35 %

Water = 5%

Base oil weight = 7.0 ppg

Water weight = 8.6 ppg

ρ_s = 21.06 ppg

Total density of solid in the drilling mud is 21.06 ppg.

OFFSHORE PLATFORMS

Most offshore drilling vessels/platforms are designed to be moved from location to location. The majority of exploratory wells drilled are failures, so it would not make sense to build a permanent structure at the start of a drilling campaign. We use a variety of styles of temporary/mobile structures to drill exploratory wells (and often to drill the development wells, too). These are called Mobile Offshore Drilling Units (MODUs).

Only once an oil reservoir is 1) found, 2) appraised, and 3) assigned development funding does it make sense to build a permanent, semi-immobile structure over the field. These permanent facilities may or may not have an integral drilling rig onboard, but we don't call them drilling vessels -- we call permanent facilities "production platforms". So drilling equipment is generally mobile, while production equipment is generally stationary.

There are many different types of drilling vessel. Some are self-powered and some are towed. In increasing order of water depth: 

Swamp Barge

Barge rigs are used in extremely shallow water (5-10 ft) which usually makes them suitable for use in swamps and sheltered bays. They are moved to location, and then ballasted down to sit directly on bottom. You see these in south Louisiana a lot -- it's a pretty common sight to be stuck at a drawbridge while a drilling barge floats through.

Jack Up

Jack-up rigs are used on the continental shelf, near land. The defining feature of a jack-up rig is having 3-4 enormous legs. Once floating above the desired location, they use massive hydraulic jacks to lower the legs down to the seafloor. Then, as the jacks continue pushing the legs down, the rig gets jacked up out of the water. Once the legs settle into the seafloor mud and the rig is above the wave zone, everything is stable enough for drilling.

Semi-Submersible

This is a fully-floating rig that is usually used up to 8,000 ft water depth. Semi-subs are usually anchored in place up to about 5,000 ft water depth, but in deeper water they are more often dynamically positioned, using only an array of large directional thrusters on the lower hull section to maintain position  according to various GPS receivers and sonar beacons.
Semi-submersibles are so named because they have a large lower structure that can be filled/drained of water to ballast the rig up and down. When moving from place to place, the rig can be ballasted up out of the water to reduce hull drag. Then upon arriving at the drill site, the lower structure is filled with water to put a large amount of mass beneath the wave zone. This submerged mass stabilizes the rig so that waves have minimal ability to rock and roll the rig. This gets into some pretty complicated naval architecture, but the basic premise is that the farther the center of mass is below the center of buoyancy, the more stable the structure is. Only the legs provide buoyancy and are affected by wave action. Most of the mass is below the legs.

Semi-sub vs Drillship:

Drillship

Drillships are basically regular ship-form hulls, except they have a big hole in the middle (called the moonpool) for the drilling equipment to pass through on its way to the sea floor. They can move around exactly like a normal ship. Almost all drillships are dynamically positioned, using two sets (front and back) of 360-degree rotatable thrusters. You can anchor a drillship in shallower water, but the ability to "weathervane" the rig to keep the bow into the wind/waves/current is extremely important to the stability of the vessel. You always want the smallest cross-section pointed towards the dominant environmental load. Since anchoring usually does not permit weathervaning, it's only feasible in areas with exceedingly mild weather. 

An important consequence of dynamic positioning is that drillships are usually only good for deep water (3,000-10,000 ft). They're next to useless in shallow water, because errors in the positioning system of a few tens of feet cause large bending moments to form in the pipes going to the seafloor. It's basic trigonometry: the deeper the water, the less angular offset you get for a particular position offset. The accuracy of the GPS and sonar systems is usually within a couple feet, but like any system, occasionally there are failures. If the rig suffers a power blackout, it will start drifting off position. 

The drilling riser pipe has flex joints that are usually designed for around 6 degrees of bending at the seafloor. Your blackout recovery or emergency disconnection time needs to be faster than the time it takes to drift to a 6 degree offset. In 8,000 ft of water, that safe drifting time window is 10 times longer than in 800 ft of water. A lot of analysis goes into this sort of thing.
Aside from drilling vessels, there are also many different types of production facilities:

 Fixed Jacket Platform

This type of platform is usually built in three stages. First, massive pilings are driven into the sea floor to provide a stable base. These must secure the structure against large bending/toppling moments from wind/waves/current. Then, the pre-fabbed steel "jacket" is towed into position and set down on the seafloor, where divers secure it to the pilings. Finally, the "topsides" are lifted onto the jacket with a large crane barge and secured in place by welders.
These are built and stay in place until decommissioned many decades later. Usually the topsides are cut off and returned to land for recycling, whereas the jacket can be toppled onto its side and used as an artificial reef. Marine life loves oil rigs.  

Gravity Base Platform

Where the seafloor is extremely hard (like the North Sea), driving deep pilings is not feasible. So the force required to keep the platform from being knocked over by weather is provided by absurdly large concrete or metal weights that simply sit on the seafloor. Sometimes, these platforms are re-floated at the end of the field's productive life, but it's a difficult technical challenge to move such a massive structure after it has settled in place for a few decades.

Truss, Spar

Spars and truss platforms have a lower  section that is floated to place on its side and then ballasted down in what amounts to a controlled sinking process. Imagine the Titanic's end rising up into the air as sequential compartments filled with water, except you're doing it on purpose. Like semi-submersibles, the majority of the mass is far below the waterline to ensure stability in rough conditions. Anchor chains/cables are run to the seafloor to keep the structure in place. Then the topsides are lifted onto the platform and integrated via welding, pipe-fitting, etc.  

Towing a truss on its side:

Lifting a topsides module onto a spar:

The crane semi-sub used to lift this module onto Chevron's Tahiti platform, theSSCV Thialf, is the largest heavy lift vessel in the world. Absolutely huge.
Truss and spar platforms can be decommissioned and moved in the reverse process to their installation.


Tension Leg Platform

Another floating platform style is the TLP. These use high-tension vertical anchor lines attached to large weights or subsea pilings to pull the rig down somewhat below the level where it would normally float. By pulling the rig down, a large buoyant force is created that helps keep the vessel stable. These are basically immune to wave-induced heave, so fixed pipes can be run to the seafloor. (Other types of floating platform require top-tensioning motion compensators or flexible semi-buoyant risers.)
Moving a TLP would require cutting all the production riser pipes, releasing from the piles or picking up the weights, and being towed away.

Semi-Submersible
Semi-submersibles can be used as production platforms, exactly as they're used for drilling rigs. They do tend to be bigger as production  platforms, because they must support the weight of many different  production risers instead of just one drilling riser. Production semi-subs are always anchored because dynamic positioning burns a lot of fuel and is not reliable enough for decades of continuous production. The lack of integral thrusters means semi-sub platforms are either towed or carried to the field, as pictured below. 

Heavy transport ships can be used to pick up fully-built platforms and simply carry them to the proper location. This tends to be done when the platform is crossing an ocean because one large ship-form vessel is faster and safer than a bunch of tugboats towing an irregularly-shaped semi-sub hull, which isdesigned to be hard to move.

So how do they get the entire production platform onto the transport ship? Simple! Partially sink the transport ship, tow the platform over it, and then un-sink the lift ship from beneath the platform. This is an absolutely incredible process. I highly recommend you visit the Dockwise Vanguard website to watch an animation about how these ships can move mega-structures. 

Floating Production/Storage/Offloading

FPSOs are highly-specialized vessels that, unlike most production platforms, have no onboard drilling rig. They tend to be more mobile than other production facilities -- it's fairly common to disconnect an FPSO during bad weather or iceberg danger and move it off location. That can be via tugs like the round FPSO pictured, or via onboard propulsion as a ship-form FPSO pictured below. Pipeline connections generally require a permanent structure, so FPSOs offload oil via tankers.

Turreted FPSO:

FPSO "turrets"allow ship-form vessels to rotate with the weather. This is a big advantage over drillships, in that an FPSO can be anchored in position via the turret and still weathervane to minimize the impact of wind/waves/current. Turrets are highly complex structures with many rotating sealing elements to allow oil to flow up and hydraulic/electrical signals to go down. If the FPSO leaves location, the lower part of the turret will stay behind (often floating just below the surface). Then when the danger passes, the vessel comes back and reconnects to regain control of the field.

I think that's all the basics of how we move offshore oil rigs and platforms into and out of place over the reservoir. Offshore drilling is a very highly specialized pursuit with enormous equipment, so over the years we've had to push the limits of engineering and invent a lot of mind-blowing techniques.

Offshore drilling platform is depending on the water depth and remoteness of the location, these "rigs" may be jack-ups (up to 400 feet of water), or semisubmersibles, or drillships (up to 12,000 feet of water). Jack-ups are bottom-supported units; semisubmersibles and drillships are floating units 

One of the most important pieces of equipment for offshore drilling is the subsea drilling template. Essentially, this piece of equipment connects the underwater well site to the drilling platform on the surface of the water. This device, resembling a cookie cutter, consists of an open steel box with multiple holes in it, dependent on the number of wells to be drilled. This drilling template is placed over the well site, and usually lowered into the exact position required using satellite and GPS technology. A relatively shallow hole is then dug, in which the drilling template is cemented into place. The drilling template, secured to the sea floor and attached to the drilling platform above with cables, allows for accurate drilling to take place, but allows for the movement of the platform, which will inevitably be affected by shifting wind and water currents.
In addition to the drilling template, a blowout preventer is installed on the sea floor. This system, much the same as that used in onshore drilling, prevents any oil or gas from seeping out into the water. Above the blowout preventer, a specialized system known as a ‘marine riser’ extends from the sea floor to the drilling platform above. The marine riser is designed to house the drill bit and drillstring, and yet be flexible enough to deal with the movement of the drilling platform. Strategically placed slip and ball joints in the marine riser allow the subsea well to be unaffected by the pitching and rolling of the drilling platform