Subsea Standardisation: The Realin Of Cross Operator Sharing Of Production Riser System
Authors: Hart Brian , Cameron Cooper , Gomersall Sam 
Published: October 2008

The BP Cyrus subsea development is a typical example of a marginal field having estimated reserves of approximately 24 million bbls of oil. To enable commercially viable recovery of these reserves every effort had to be made to reduce costs. Although the purchase of new subsea production equipment was unavoidable it was recognised that significant savings could be made by sharing installation tooling from other operators. This paper describes the reality of sharing riser systems.

INTRODUCTION:

The decision was taken at the outset of the project not to purchase a production riser system as the financial outlay could not be justified on a two well development. This dictated that all the potential equipment necessary for subsea completion installation would need to be identified; rental agreements made with other operators and their programmes monitored continuously. The risk factors were identified and discussed in detail with the participating operators.

Previously installed subsea completions have typically included features and interfaces that were predominantly based on personal preference with little or no consideration given to standardisation.

Sharing riser systems on the BP Cyrus project was made possible by the subseo standardisation initiatives relating to dual bore subsea equipment agreed between BP, Shell, and the leading equipment manufacturers. Mobil also recognized the benefits of these initiatives and equipment currently being used by tliem also includes these standard interfaces.

It was acknowledged that drilling and completion programmes are very susceptible to changes or slippage so all known owners of the appropriate equipment were contacted to enable the Cyrus programme to be completed without interruption. Similarly, although every participant was willing and co-operative, reassurance and contingencies needed to be provided to safeguard against possible infringement to their programmes.

Background

The production riser system is used to install the tubing hanger and the subsea tree (Fig. A- I). For tree installation (or workover) a mandatory safety package is included and this provides the interface between the riser and the tree (Fig. A-2). The safety package features gate valves (or, less frequently, rams) with wireline cutting facility and a system for emergency disconnect of the riser.

Production riser can be adapted for use with most tubing hanger systems of different origin of manufacture, however, the safety package (LRPIEDP) needs to be fully compliant with the tree. The specific areas of interface relate to the production and annulus bore spacing; the re-entry hub profile and sealing mechanisms; and the hydraulic control couplings. Adapters could be made between the tree and "non-standard" safety packages, however, these would be costly to manufacture.

Planning

The original intention was to install both Cyrus tubing hangers and subsea trees using an existing riser system owned by Mobil. Mobil had planned to use this system for installation of new subsea equipment on their forthcoming Nevis development. To enable it to be used for Cyrus the Lower Riser package required modification. The cost of the modification was approx.fO.5 million which BP Cyrus agreed to pay in return for use of the equipment. Mobil then had the benefit of a riser system that included all the standardisation features.