Thread compounds are used for three distinct purposes:
- to prevent corrosion of the connection parts during storage;
- to facilitate the running of connections during make-up/break-out;
- to seal the helical paths in the thread profile of tapered interference fit thread seals
To serve the first purpose, storage compounds were developed. These compounds can only be used to protect against corrosion during the storage of the tubular, and should never be used as running compound.
To serve the second and third purposes, running compounds were developed. These running compounds have to fulfil the following functions:
- during make-up: prevent metal-to-metal contact and thus protect the threads and seal areas from galling and wear;
- in made-up condition: seal the helical paths between mating threads to make the non-Premium connection leak tight;
- during break-out: ensure that the connection can be broken out, after having served for a long period, without galling and at torques which do not differ excessively with the applied make-up torque.
There has always been a definite distinction between the two sorts of compounds. However, some manufacturers claim to have developed running compounds which offer corrosion inhibition, equivalent to that of a good storage compound. These compounds are called hybrid or multipurpose compounds.
In the following two paragraphs, the lubricating, sealing and environmental properties of the running compound will be discussed. At present, the use of running compounds conforming to API Bull 5A2 is generally recommended. Although the API compounds were developed for API round and buttress thread connections for which the solid constituents have a sealing function, it is also used for connections with metal-to-metal seals.
Lubricating and sealing properties
Conventional thread compounds contain relatively weak, ductile, solid particles, suspended in heavy grease. Therefore the compound can be split up in grease base and suspended solid particles. Both of these components have a specific function.
The functions of the grease are:
- to act as a carrier to hold the solid particles in a stable dispersion and permit even distribution over the surface of the connection;
- to act as an adhesive to ensure that the coating sticks to the metal surface under condition of application;
- to provide sufficient lubricity to overcome the initial friction between the connection thread and seal surfaces during make-up.
The solid particles have two functions:
- To provide reserves in lubrication in extreme operating conditions, where the boundary lubrication regime prevails and the grease component has leached out. If, after some time, the connection will have to be broken out, the solids will act as lubricants. They will then prevent excessive torque required for breaking out, by deforming when the thread surfaces move over them.
- To seal the helical leak paths of API type connections. These paths should be sealed by the compound. While being made up, the grease will tend to get forced into the leak paths of the particular thread. It has been suggested that over time the grease base will tend to disappear by leaching and evaporation of the volatile parts or fluidization due to exposure to elevated temperatures. The solids will be left as sealers. The amount of pressure which can be withstood by the solids, seems to depend on their particle size and the amount of solids suspended in the grease. Sealers used are for instance lead powder, copper flake, zinc dust or chunks of Teflon. The latter is only to be used in combination with API buttress and round thread connections, for which it is reported that the "chunky" material performs better than the "flaky" material.
Each compound will have a characteristic friction correction factor, which depends on the compound composition. Grease, oil, high pressure additives, Teflon, copper flakes, graphite and certain sulphur compounds decrease the friction coefficient. Metal oxides and silicates increase the friction coefficient. The friction correction factor also depends on the hardness, the size and shape, and the number of particles suspended in the grease base.
However it is now thought that the effect of different types of thread compound on the make-up characteristics of connections may be smaller than would be suggested by the differences in the friction correction factors. Nevertheless, in the absence of specific recommendations to the contrary from the connection manufacturers, it is recommended that a compound with a quoted friction correction factor of 1.0 be used.
The use of too much compound on a Premium connection, in order to increase the lubrication, can have adverse effects. With certain connection types it has been shown that the pressure built up by the thread compound during make-up may have an adverse effect on the sealing performance of the connection. In such instances, more care should be taken in applying the right amount of compound. Based on qualification testing, the manufacturer should provide a value for the amount of compound to be applied.
For better leak testing results, it has been suggested to use a thin oil or diluted compound for the make-up of Premium connections. The Premium connection relies on the metal-to-metal seal for sealing, and not on the compound. When testing this type of connection on its seal, the oil or diluted compound would not influence the result and hence it will be easier to assess whether the metal-to-metal seal leaks.
API Bull. 5A2 states the specifications which a thread compound should meet, e.g. a compound should perform up to 300°F (150°C). It has been noted that compounds perform even above this limit. However, one should keep in mind that most of these high temperature results were obtained under laboratory conditions, and may not reflect the actual field conditions. From field experience, a maximum working temperature of 250°F (120°C) evolved, which is the limit recommended.
Environmental aspects
Conventional running compound contains about 64% metal solids by weight. According to API Bull. 5A2, the API Modified thread compound consist of:
- grease;
- lead powder;
- zinc dust;
- copper flakes;
- powdered graphite.
However, thread compounds can contain a whole range of other chemicals, harmful to the environment, like Arsenic, Antimony and Molybdenum disulphide.
Concern over the escape of these and also of lead into the environment led to a second generation of compounds without lead. However, although several additives seal nearly as well as lead, they tend to create their own environmental problems.
The use of biodegradable grease bases has been researched as well, but the problem is that this type of grease is not stable at high temperatures, and therefore not applicable in deep wells. As a result of this study, four hybrid compound compounds are considered to be acceptable and their introduction is recommended. These are:
- Mercasol 633SR Multimake White;
- Kendex Enviro Seal;
- Bestolife copperfree PTC;
- Shell Française SF 3646.
Bestolife 2000 5% copper and Cats Paw Black 712 S are also acceptable
Recommended thread compounds
It should be noted that a storage compound is not to be used as running compound, and that a running compound is not to be used as a storage compound. Mistakes are still occurring. The amount of compound to be applied is a function of connection sealing mechanism and geometry. The actual value should be acquired from the connection manufacturer.
Storage compound
- Shell Rhodina Grease 2
- Geveko Mercasol 630
- Jet Marine Imperator 1078
- Kendex OCTG corrosion inhibitor
- TSC thread storage compound
- Cortec VC1-369
- Rust Veto heavy
- RD5
Running compound (conform to API Bull.5A2)
- Calcium hydroxy stearate grease base
- Aluminium stearate grease base
- Calcium stearate grease base
- Thread Kote no 706
- *Hybrid compound
- Kendex Enviro Seal
- Mercasol 633 SR Multimake White
- Cats Paw black 712 S
- Bestolife copperfree PTC
- Bestolife 2000 5% copper
- Shell Française SF 3646