Subsea field development

Figure 10.34 Typical Subsea Field Development Options...

Typically, a Subsea Field Development or Subsea Satellite Development would consist of a cluster of special subsea trees positioned on the seabed with produced fluids piped to the host facility. Water injection, as well as lift gas, can be provided from the host facility. Control of subsea facilities is maintained from the host facility via control umbilicals and subsea control modules. 

In 1986 when the oil price crashed to 10 a barrel, operators began to look very hard at the requirements for offshore developments and novel slimline, reduced facilities platforms began to be considered. The reduced capital outlay and early production start up capability, coupled with the added flexibility, ensured that all companies now consider subsea systems as an important field development technique. Although the interest and investment in subsea systems increased dramatically, subsea systems still had to compete with the new generation of platforms, which were becoming lighter and cheaper. 

A project division within Norskoil carries out project work. At the first decision gate (DGl), the market organisation of Norskoil, i.e. the client, commissions the project division to start up project work. Before-hand, the client has made the necessary initial evaluations of the potential resource basis of oil and gas and of the market. The scope of work of the feasibility study also has to be defined. This phase aims at demonstrating whether it is technically and economically feasible to develop the field. The project team establishes detailed reservoir models in order to assess the oil and gas reserves and to establish production rates over the field s expected lifespan (production profiles). In parallel, the project evaluates different field-development concepts such as fixed platforms, floating platforms and subsea developments for tie-in to existing platforms in the area. This evaluation is based on information about existing infrastructure, water depth, reservoir models, etc. The activities are documented in a Report of Commerciality. 

Subsea production systems are an alternative development option for an offshore field. They are often a very cost effective means of exploiting small fields which are situated close to existing infrastructure, such as production platforms and pipelines. They may also be used in combination with floating production systems. 

However, in recent years the trend has been turning towards developing much smaller fields, making use of the existing field infrastructure. This, in combination with advances in subsea completion technology and the introduction of new production equipment has further stimulated the application of subsea technology. 

Whether on land or offshore, the principle of satellite development is the same. A new field is accessed with wells, and an export link is installed to the existing (host) facility. Development is not always easier on land, as environmental restrictions mean that some onshore fields have to be developed using directional drilling techniques (originally associated with offshore developments). A vertical well can be drilled offshore away from the host facility, and the well completed using a subsea wellhead. 

In the field s appraisal that subsea wells would be required if the oil were developed. 

The 340 m water depth and 1500 m.ss. reservoir depth Imply limited reach when drilling deviated we ls from a platform. This together with the areal extent of the oil indicated at a very early stage that subsea wells would be required if the oil were developed. Therefore as a part of the appraisal of the field a number of studies were carried out to identify potential production problems associated with subsea oil wells. 

Offshore deep-water oil and gas fields are increasingly developed with subsea production systems. By subsea production systems, we understand production (X-mas) trees, templates, manifolds, pumps, separators, and associated equipment that are placed on the seabed. 

The field is developed by water injection, using an FPSO with 31 deviated or horizontal subsea injector wells and 4 injection flow lines. A single flow line generally injects in several reservoirs as well as several systems. Maximum water injection is 405 kbwpd. 

Norskoil decides to select a semi-snbmersible platform concept for further development of the Ymer field. At the second decision gate (DG2), Norskoil s board of directors makes the project go decision. To pass this gate, the investment plan has to show adeqnate profitability. This impHes that the capital and operational expenditures as well as the revenues from production of oil and gas are known with adequate certainty. It is here necessary that the project s scope has been adequately defined. The platform concept and the subsea and pipeline installations must be defined at an adequate level of detail. 

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