Designing Very High Pressure Pipelines

Every now and then your boss gives you something that becomes a definite challenge. That happened to me last year. I was asked to participate in a new project in the Sahara Desert of Algeria. Over the past 35 years, I have been there and done that a few times. I looked forward to applying previous lessons learned to this new project. The job involved a major project to gather crude oil and condensate production from various new fields to a central processing plant. At the central plant, the oil and condensate is to be processed for export, and produced water and gas is treated for distribution back to the oil fields to be re-injected for pressure maintenance. Been there and done that too.

In this project, there is a bit of a twist to what we have done in the past. Our production and export systems are ANSI 600 and 900-rated pressure design. Our water system for injection required pressures at 3,600 psig, just above ANSI 1500 ratings, so we had to go to ANSI 2500 rating design. We moved our high-pressure pumps out of the plant into the oil fields to minimize the need for very high pressure water lines. This saved us several million dollars and paid for a power distribution system out to all of the oilfields that offered us several more benefits.

What we could not do was move our compression into the field for the gas re-injection systems. For these oilfields, the reservoir engineers decided that the injection pressure would have to be on the order of 5,500 psig at the wells. For line losses to transport the gas from the central plant compression to the fields, we required a maximum allowable operating pressure of 7,325 psig. That means a system pressure rating exceeding ANSI 2500, placing it into the range of API 10000 rating. In Algeria, we must also use their gas pipeline design code which is more conservative than B31.8, which requires slightly heavier wall pipe cross country, but extremely thick pipe at facility approaches.

My experience with gas injection pipeline systems up to this project involved systems up to ANSI 2500 rating. Material and equipment is readily available for this level of gas service pressure. These systems have all been offshore or within field unit boundaries for onshore pipelines. In some previous design work, we determined that 12-in. diameter line pipe is the probable upper limit of very high pressure pipe design for economics and practical construction. As the diameter increases, the wall thickness grows such that internal diameter reduces dramatically. At 12-in. nominal diameter in X-70 line pipe, pipe wall is at 1.25 inches. If higher volumes are required, it is usually more practical to install multiple lines. Our flow assurance studies just barely qualify our gas injection line at 12-in. diameter.

For this project, we had to design cross-country pipelines up to 70 km in length across and along public roads and by adjacent facilities. In the Sahara Desert, gas pipelines must remain buried. The sand does not restrain pipelines, so there is a tendency for the buried lines to “snake” out of the soil as the pipelines flex with temperature and pressure cycling. This is referred to as “flambage” and has to be controlled by anchoring the pipe at over bends and side bends. We have developed successful anchoring designs for gas pipelines up to ANSI 2500 design. Loads for anchoring design on this project are substantially greater. Alignment design becomes much more critical to avoid buckling and upheaval.

In developing design criteria for practical design and risk considerations in the Sahara Desert, we inquired with the U.S. DOT about maximum pipeline design pressures or conditions for operating very high pressure gas pipeline systems. We determined that liquid lines up to ANSI 2500 have been permitted in the United States, but gas pipelines have only been rated up to ANSI 900 pressure levels after special testing and design studies. Studies have been conducted to increase gas pipeline design standards in the U.S. to ANSI 1500. We determined that under other international standards, most adopted codes are limited to maximum pressures equivalent to 100 barg (1,450 psig) for cross-country gas pipelines in the public sector.

Since there are few precedents for designing and operating gas pipelines of this pressure range, we have adopted a risk-based approach for developing the design criteria that we will use for the basis of our pipeline design and assessment of quantitative risks. If I survive the testing and commissioning, I will let you know how it all comes out.