An extensive full-scale measurement campaign has recently been carried out in one of the two 36" gas-condensate pipelines from Troll A wellhead platform in the North Sea to Kollsnes gas process plant. The main objectives of the fullscale tests were to collect data and to study the dynamics of three-phase gas-condensate-water flow in pipelines. Seven tests covering a large span of operating conditions were run measuring pressure drop and liquid accumulation in the pipeline. In addition, data on pig dynamics were collected for five of the tests. The measurement results will be used to revise operating procedures for Statoils gas-condensate transport systems and for verification and improvements of dynamic multiphase simulation codes. This paper focuses on the design and performance of the full-scale tests.
Long distance multiphase transport has become common and proven technology in Norwegian gas-condensate field developments. Recent and future developments like Huldra, Kvitebjørn, Sigyn, Mikkel and Snøhvit all make use of gascondensate transport as an essential technology element. Indeed, this has become possible due to extensive efforts to develop multiphase flow models for thermo-hydraulic calculations, such as the OLGA2000/PeTra simulations programs. The multiphase models are particularly important in order to select the optimum pipe diameter, design temperature and defining the operational envelope with respect to minimum and maximum flow rates. The minimum flow rate before liquid starts to accumulate considerably is an important feature, which usually limits the operational envelope for a gas-condensate pipeline. Liquid accumulation in a pipeline may cause slugging problems, and the accumulated liquid may potentially overfill the slug catcher at the receiving facility when increasing the gas flow rate. Increasing pipeline pressure loss may also be a consequence of liquid accumulation. Thus, liquid accumulation is an unwanted situation that is usually solved by draining the pipeline using scraper pigs, or simply by operating the pipeline at sufficient high flow rates to avoid the problem. Figure 1 shows typical trends of liquid accumulation and pressure loss for gas-condensate pipelines. Furthermore, the operational limits of new subsea gascondensate pipelines are being stretched with respect to longer transport distances and higher liquid content. An example is the Snøhvit pipeline, which is planned to come into production in 2006. The wellstream from Snøhvit will be transported directly in a multiphase pipeline to shore over a distance of approximately 145 km [90 miles] in harsh environments in the Barents Sea. Snøhvit and similar field developments depend on reliable multiphase models for thermo-hydraulic design calculations. The uncertainties of the multiphase flow models are incorporated in the design and field development concepts. This may, however, result in narrow operational envelopes, oversized pipelines and slug catchers, or other unsuccessfull design of the pipeline and receiving facilities. It is therefore important to improve the accuracy of multiphase models to reduce uncertainty in pipeline design and to optimise the operation of pipelines and receiving facilities. Such improvements can be achieved by exploiting operational experience from existing fields combined with systematic fullscale testing of multiphase pipelines.