As underbalanced drilling and intervention operations find offshore applications, development may be restrained by physical dimensions, operational delivery and logistics preventing the wider application of the technology and maintaining the conventional methods with its higher cost base.
The first generations of Nitrogen Membrane units were large stand-alone skids that had no built-in compression and booster capability; these were supplied in separate units. For land-based use in drilling, well intervention and pipeline applications, this presented no problems since space availability was not an issue. However for offshore use where space is at a premium, these additional components presented such a large footprint that their use was difficult in most applications and impossible for smaller jack-up rigs and production platforms, where deck space and loading are major limitations.
In order to facilitate the use of Nitrogen Membrane technology in offshore environments, it was clear that a new generation of units would be needed. Initial development centered on existing land-based units, with primary and booster compression, Membrane and cooling combined in modular two-piece units. This approach provided for a smaller footprint, but lower output per square foot of space occupied and higher individual weights. This output penalty could be made up with additional units but the space occupied was prohibitive. Subsequent development efforts were based on a three-piece unit with prime mover, primary compressor and secondary booster driven in-line through a single drive shaft. A second-generation Nitrogen Membrane unit, with increased output and efficiency, and the cooler skid, were top-mounted on the three-piece unit. A third generation unit is currently in the design and development stage, which will be fully compliant with all offshore regulations for North Sea operations.
This paper will review the existing technology of Nitrogen Membranes. It will detail the development and design of the second and third generation units on which all the major concerns such as performance per weight/area, access, zone and operator compliance requirements and safety were addressed. It will also review all the stages of the developments, the design, manufacture, testing and field problems encountered and the lessons learned from them.
Some wider application of the technology will also be demonstrated due to the improved technology and the cost savings through operational efficiency highlighted.