North Sea Marginal Fields: The Subsea Completions Option
Authors: Humphrey B. , Cow S. 
Published: October 2008

The paper will identify the basic elements of a sub sea completion and review the types of overall systems available for the development of a North Sea marginal field. A typical field development is reviewed in more detail, noting the importance of simple designs for installation and maintainability and also recognizing the possible need for ROV/Diver intervention. Recent developments and trends are also noted, whilst future trends and areas requiring further work are identified. The conclusions drawn from the paper are that sub sea completions systems will become even more evident in future marginal and deepwater offshore oilfield development plans especially in the North Sea.

INTRODUCTION

The last of the large North Sea fields has now been developed, and the industry is now turning its attention to the smaller fields that initially did not appear viable. To develop these fields, which can sometimes contain recoverable reserves of only 20–50 million barrels of oil, the 'new' technology of sub sea completions with a low cost topside processing facility (F.P.V., T.L.P. or tanker) will probably prove to be the only economical system. The use of sub sea completion systems to develop a marginal field is not without precedent, there is a wealth of operational experience which has been gained over the last decade in the North Sea.

A worldwide review of active sub sea completions shows that there are currently 209 active of which 52 of these completions are in the North Sea. The feasibility, reliability and maintenance costs are areas where research would show that the industry can supply a good deal of information.

Before we discuss the overall systems available, let us firstly identify the key elements of a single sub sea completion.

ELEMENTS OF SINGLE SUBSEA COMPLETION

A sub sea completion system is basically an assembly of eight individual sub-systems which can be unitized in many forms to suit the development requirements of a particular field. These sub-systems which include their own running/testing/handling tools are:-Support BaseHigh Pressure WellheadTubing HangerWellhead ConnectorValve AssemblyTree CapFlow lineControlsSupport Base (Figure 1) The support base is designed to carry the combined weight of the conductor and casing strings, the sub sea Blow Out Prevented (BOP) equipment during drilling operations and also provide guidance for the sub sea BOP stack and ultimately the production tree assembly. Guidance onto the support base is by means of 4 surface tensioned guide wires which are attached to posts on the support base itself.

It is common to use a standard exploration style drilling support base (permanent guide base) for wells which form part of a sub sea production system but which are located singly in remote areas of the field.

A template structure can also be used for field development which allows for a more rapid drilling programmed due to the virtual elimination of rig move times and the use of batch drilling techniques. There are two main types of template in use, i.e. modular and unitized. The modular template is designed to be assembled on the seabed by installing individual interlocking bases as they are required.