Two new units have been designed for adapting the steel wire line method, in universal use on land, to suit subsea wellheads.
The subsea lubricator method makes use of a swell compensating unit installed on board a surface support and enables wire line operations to be performed from a standard vessel and using a standard wire line winch.
The subsea winch is a unit which is lowered into the water and connected to the wellhead. It is completely remote-controlled from the surface by means of a multi-conductor electric cable.
These two units have been successfully tested several times on both experimental and producing subsea wells. They are now operational.
There are still comparatively few oil wells in the world which are completed with subsea wellheads, no more than about a hundred in all. It is, however, very likely that this number will increase quite rapidly over the next few years with the development of subsea fields. The various systems being tested at present in several countries for the equipment and maintenance of such producing wells is evidence of this trend.
In all these systems there is generally some method of working inside the wells to carry out operations which are simple but necessary if the wells are to be operated in a rational manner, such as bottom hole pressure measurement, tubing scraping, handling bottom hole tools, etc. The methods proposed are sometimes rather cumbersome and some of them are not, in our opinion, satisfactory from the point of view of operating safety.
We have therefore studied a way of adapting for subsea operations the wire line or slick line technique, universally used on land. We would recall that this technique, which now has specialists with long experience, involves introducing equipment into the well through a lubricator and then running it to the bottom of the well at the end of a steel wire line, unwound from a winch. The seal is made around the line by means of a stuffing box.
In wells on land, as in the case of offshore wells operated from fixed platforms, the wire line technique is widely used. In wells with subsea trees, a new operating method has been developed and tested by various companies over the last few years.
This method abandons the use of slick line and makes use of a fluid to pump the tool string in the tubing. This "pump down'tools" technique probably has a great future ahead and the fact that almost all the subsea systems being tested or studied at the present time make use of this technique, demonstrates the success anticipated.