Optimization of Ekofisk Crude Pumping by Offshore Cavitation Testing
Authors: Nilssen J. , Hancock W.P. 
Published: October 2008

The paper describes a unique cavitation test conducted on an Ekofisk main crude oil pump during normal producing operations.

Subsequent analysis of the better than expected results, enabled operation in a reduced suction pressure / high flow regime with significant benefits.

The authors advocate a modified design method for prediction of minimum suction conditions to crude oil pumps with the potential benefits of making future facilities simpler and cheaper.

Introduction

Ekofisk was the first Oilfield to be discovered in the North Sea by Phillips Petroleum in 1969, and had by the early 1990's already produced over 1 billion barrels of oil equivalents. Moreover enhanced oil recovery techniques would ensure commercial oil and gas production well into the 21st Century. Phillips Petroleum Norway is operator on behalf of the coventurers [Acknowledgements 1].

The field facilities for processing, treating and transportation of oil and gas were over 20 years old and becoming inefficient and maintenance intensive. The problem being compounded by the subsidence of the Ekofisk platforms. A decision was therefore taken to replace the old facilities with a new platform 2/4 J.

Advanced equipment was selected for this installation with the objectives of minimum manning, high efficiency and minimum environmental impact of reduced CO2 and NOx emissions.

The 2/4 J platform was commissioned in 1998 and the unique feature of the commissioning and early operation was that the platform was required to handle plateau production from the outset. Most field developments include a progressive build up in production rate, providing additional standby capacity, and thereby giving time for problem resolution.

Probably the most critical facility on the new platform is the crude oil pumping system which pumps over 80,000 Sm3/day (500,000 bopd) into the Norpipe Oil-pipeline 355 km to Teesside in the UK.

In order to achieve design oil export rates it was necessary from the outset to use all three 50 % booster and all three 50 % main pump units. Consequently, outage of any of the pumps meant a significant reduction in export rates. There was a considerable incentive therefore for a detailed investigation leading to a resolution of this problem. The pump testing program and especially the cavitation test (described herein) were the key elements in the resolution and would also provide invaluable information for future "de-bottlenecking" of installations.

Description of Crude Oil Stabilization and Pumping Facilities

Process Design

The relevant part of the process design (Fig. 1) shows the well head streams from Ekofisk and outlying fields being received in two separators, HP operating at 20 barg and LP operating at 10 barg. The associated flash gas is dried and it's dew point is controlled by a turboexpander process before being discharged into the Norpipe gas-pipeline to Emden (Germany), bypipeline compressors. The LNG knocked out from the dewpoint control unit is spiked back into the crude oil streamdownstream of the crude booster pumps.