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Abstract

Ramp-up of large-diameter, long-distance production systems is challenging due to the large liquid holdups that can exist and the transient nature of ramp-up operations. This paper presents the results of multiphase simulations performed to optimize [TGAC1]ramp-up operations in a subsea wet gas production pipeline system. All three phases (gas, condensate, and water) of the fluid system are considered. The pipeline system consists of dual interconnected, large-diameter (20+ in.) pipelines approximately 56 miles in overall length. The optimum ramp-up scenario is reached when the gas delivery rate can be increased in minimum time while maintaining the liquid surge within the onshore slug catcher liquid handling capacity. This paper proposes a solution to this problem and illustrates it with transient multiphase simulation results. The simulation results also provide some insight into the physics of the multiphase flow phenomena in large-diameter wet gas pipelines.

Introduction

This study is part of the operability design for a new offshore gas field development, Scarab/Saffron. Scarab/Saffron is operated by the Burullus Gas Company, a Joint Venture between EGPC (Egyptian General Petroleum Company), BG-Egypt, and Edison Gas. The pipeline system comprises a long distance subsea tie-back from the offshore field, approximately 56 miles north of Alexandria, to new onshore processing facilities. Initial production will come from eight subsea wells located in a water depth ranging from 430 m to 620 m with individual 10 in. flowlines tied back to two production manifolds. Two 20 in. pipelines connect the manifolds to a Pipeline End Manifold (PLEM) located in 95 m water depth approximately 20 km nearer shore. Two larger pipelines, 24 and 36 in., connect from the PLEM to the shore (Fig. 1). The pipeline profile from the manifold to the onshore is given in Fig. 2.

Scarab/Saffron will deliver a maximum of 600 MMscf/D of gas and 3,000 B/D of condensate into this high capacity pipeline system. The export pipeline system has a total capacity of approximately 1800 MMscf/D. The excess capacity allows production of future fields to be connected into the export system at the PLEM. Liquid content and holdup in the pipelines will consist of condensate, produced water, and methyl-ethylene glycol (MEG), continuously injected for hydrate prevention.

During early life production, produced water is assumed to be condensed water; about 0.24 bbl/MMscf (barrel of water per MMscf of gas) based on saturated reservoir conditions. MEG injection rate is assumed constant at 66 B/D per well.

Problem Statement

Pipeline Operation Mode.

During early life of the field, the 36 in. export pipeline will be isolated at the PLEM and the 24 in. export pipeline alone will be used for all field production rates. After several years of production and reservoir pressure decline, the 36 in. line will be needed for higher production rates. The 24 in. line will still be needed for low production rates because liquid accumulation in the larger line would be excessive at low rates. After that point, whenever only Scarab/Saffron is producing into the pipeline system, it will be necessary to switch production from the high-rate 36 in. line to the low-rate 24 in. line (turndown) and back again (ramp-up) as gas delivery demands dictate. Line switching will be accomplished using the onshore isolation valves and not the subsea manual valves at the PLEM.

Accordingly, three pipeline operating modes have been investigated:

1. The dual 20 in. pipelines producing to the 24 in. line (designated as 2-20/24 in. lines).

2. The dual 20 in. pipelines producing to the 36 in. line (designated as 2-20/36 in. lines).

Pipeline Operation Mode.

During early life of the field, the 36 in. export pipeline will be isolated at the PLEM and the 24 in. export pipeline alone will be used for all field production rates. After several years of production and reservoir pressure decline, the 36 in. line will be needed for higher production rates. The 24 in. line will still be needed for low production rates because liquid accumulation in the larger line would be excessive at low rates. After that point, whenever only Scarab/Saffron is producing into the pipeline system, it will be necessary to switch production from the high-rate 36 in. line to the low-rate 24 in. line (turndown) and back again (ramp-up) as gas delivery demands dictate. Line switching will be accomplished using the onshore isolation valves and not the subsea manual valves at the PLEM.

Accordingly, three pipeline operating modes have been investigated:

1. The dual 20 in. pipelines producing to the 24 in. line (designated as 2-20/24 in. lines).

2. The dual 20 in. pipelines producing to the 36 in. line (designated as 2-20/36 in. lines).