The paper describes the design philosophy operating experience and debottlenecking of large water injection facilities on the Statfjord Field. Optimum system design and selection of equipment has a large cost impact since the water injection facilities represent a significant part of the total topside weight and is a large power consumer. The fundamentals for the optimum offshore installation are fully described. Optimum installed weight, reliability and inbui1t flexibility are covered in detail. Resolution of operational problems and the debottlenecking in capacity from 736 Mbwd to over 1000 Mbwd at high on-stream availability is described. The paper concludes with some of the pertinent lessons to be learnt and changes to be incorporated in future facilities in order to make them lighter more reliable and easier to operate/maintain.
The Statfjord Field is the largest oilfield discovered to date in the North Sea. It lies in 145 meters of water, about 180 kilometres Northwest of Bergen and straddles the Norwegian/UK median line. Total field reserves are approximately 470 million metric tons of oil and 70 billion cubic meters of gas of which 84% lie in the Norwegian and 16% in the U.K. sector.
Statoi1. on behalf of the Statfjord Unit. (Reference 1) is operator of the field.
The field was developed in phases from three Condeep concrete platforms (A. B and C). First production started in late 1979 followed by gas injection in 1980 and water injection in 1981. By 1986, all systems were fully operational and a plateau oil production rate of 34.35 million tons of oil/year (736 MBopd) had been achieved with a corresponding water injection plateau of 61.75 million tons of water/year (1040 Mbwd).
The purpose of the sea water injection system is to supply large quantities of high quality, high pressure sea water to the injection wells. The heart of the system is the main sea water injection pumps which boost the sea water to pressures dictated by reservoir conditions and typically in the range of 150-220 bars. Considerable total horse power is required to drive these units and typically 15-25.000 bhp./platform. This represents a significant proportion of the total installed power on an offshore platform (of the order of 25-50%).
It can be easily appreciated therefore, that the selection of the sea water injection equipment has major implications on platform topsides' weight, costs and operability. The following description addresses the major factors in the selection of the equipment. Also, the major considerations that must be made in designing a sea water injection system to provide reliability and flexibility of operation over the life of the field at optimum weight and cost.
To supply large quantities of high pressure sea water.
To provide flexibility of operation to enable considerable variation in both flow and well head pressure as dictated by reservoir considerations over the life of the field.
