Is your ice burning?

By Ivar Brandt
Foto: SINTEF

"Burning ice" is a member of a class of solids called "gas hydrates". Hydrates can develop into massive plugs that can block multiphase flow piplines on the seabed.

"Multiphase flow" is a general term for the interaction between flowing gases, liquids and solids. Snow in wind is familiar and the flow of skis over the snow is actually also a multiphase flow phenomenon. The skiwax is a liquid of high viscosity and the snow is either in a solid or a liquid phase in the form of tiny water droplets. The interaction between wax, solid and liquid determines the speed of the flow. Just ask cross-country skiers when they test their ski waxes every morning before competitions.

The SINTEF Multiphase Flow Laboratory is facing a different challenge - namely to make the production of oil and gas more viable from the point of view of economics, safety and the environment. In the petroleum reservoir the gases, petroleum liquids and water coexist in equilibrium. When the reservoir is penetrated by a well, the fluids start to flow through the reservoir and into the well which is typically 2000 to 3000 metres deep. Traditionally the wells have gone straight up to the deck of offshore platforms for the gas and the liquids to be separated before further transport in separate pipelines or by tankers. The multiphase flow in the reservoir and in the wells is broken into streams of single phase flow. For gigantic fields like Statfjord, Oseberg and Gullfaks this technique was more or less proven and the sensible thing to do.

However, the geologists do not expect to find more giant petroleum fields in the North Sea. On the contrary, a number of smaller fields have been found and these cannot be developed with traditional technology. So - why not take the wellstreams through pipelines from these small fields directly to the existing platforms and use their spare capacity for separation into oil and gas? This idea is one of two reasons for the formidable research efforts taking place in Norway on multiphase flow technology.

The other justification for the research effort is demonstrated by Troll, which is the name of a huge offshore gas reservoir about 67 km west of Bergen, Norway.

In 1989, the operators of Troll phase 1 decided to take the Troll gas directly to shore for processing before routing the gas to European users.

This solution offers a number of benefits:

Taken together these benefits represent substantial amounts of money saved over the lifetime of the field.

The Troll reservoir will produce small amounts of light petroleum liquids, called "condensate", in addition to the gas. Some water will also be produced. Troll Phase 1, which will be onstream in 1996, consists of a huge platform that will provide support for a minimal separation process that removes produced water. The gas and the condensate will be fed into two parallel pipelines and each will carry a two-phase mixture of gas and liquid to Kollsnes in Øygarden near Bergen. Here the gas and liquids will be separated and the gas will be conditioned before it is fed into the 11 km trunk lines that will carry it to European consumers 800 km to the south.

This operation requires very sophisticated control systems, a vital part of which is the OLGA computer model for dynamic simulation of multiphase flow in pipelines.

OLGA has been uncler development by IFE and SINTEF for the past decade. Since the model has been verified by about 8000 experiments at the large-scale SINTEF laboratory and since it has been used by a number of international and Norwegian operators since 1984, it is now regarded as an industrial standard. The operation of Troll Phase 1 will be taken over by Statoil in 1996 and the modifications of OLGA for integration in the computer control system for Troll 1 is an important aspect of the cooperation between Statoil, IFE and SINTEF.

The Troll field is expected to deliver gas for 50 years and this in itself represents a tremendous challenge. The multiphase flow challenges primarily concern the flow of gas and liquid in large diameter pipelines, particularly with rather steep inclinations when the pipelines enter the landfall zone. Furthermore, because the seawater cools the pipelines, water vapour will condense, so there is a potential corrosion problem.

A number of offshore petroleum fields operate under multiphase flow conditions. However, the full potential of the technology has not yet been harvested due to a number of unresolved problems. One of the more serious is "burning ice".

"Burning ice" is a member of a class of solids called gas hydrates. A gas hydrate looks like ice or snow and is a combination of water and a gas, but it can form under much higher temperatures than ordinary ice. As a matter of fact all gases apart from a few can form hydrates with water, including the natural gases found in petroleum reservoirs. So if you bring a lump of propane hydrate into the open the hydrate will start to melt, releasing the propane. If you put a match to the hydrate the gas will start to burn and you have burning ice. Actually, the olympic torch could have been fed by burning ice.

SINTEF´s Multiphase Flow laboratory near the city of Trondheim in mid Norway

Most multiphase flow pipelines on the seabed will sooner or later operate under conditions under which gas hydrates can form. Hydrates can develop into rather massive plugs that would block the pipeline and this has actually occurred many times all over the world. Hydrates thus represent one of the most serious problems for further multiphase flow development. Several solutions are available, including the injection of antifreezes into the pipeline, but these methods are not always viable. Scientists are looking for new chemicals which must be effective in small amounts, friendly to the environment and, of course, carrying an acceptable price tag. Suitable chemical components have actually been identified in arctic fish and plants, but there is a long way to go before these chemicals are commercially available.

Several other aspects of multiphase flow technology still need to be further developed. Accurate measurements of the flow of gas and liquid are necessary for safe operations and for the allocation of the product to various owners, when the products brought from satellites to existing platforms and products from several fields are mixed into a single separator tank.

When a reservoir has produced for a while, its pressure declines and so does production. Pressure boosting of the multiphase mixture from the wells is needed to overcome the fall in pressure. Multiphase pumps and multiphase compressors are being developed and tested by Norwegian companies and institutions, including SINTEF.

Through joint efforts with Statoil and IFE, SINTEF is a very active part in finding solutions to the problems of multiphase production. Multiphase flow development of petroleum reservoirs is of strategic importance to Norway and also to other offshore producing countries. We already play a leading part and we intend to keep it up.