Marginal Subsea Development with Existing Subsea Trees
Authors: Larson Eric D. , Keosoff Kevin G. 
Published: October 2008

Abstract
Recycling of subsea trees and completion equipment is becoming a practical and efficient means of developing small subsea reservoirs that were previously considered not economical. However, there are concerns and tradeoffs that an operator may be faced with when developing a solution using existing equipment. Modular approaches to equipment design, strict management of interfaces, and the use of field proven equipment help to overcome some of these challenges. By carefully reviewing basic technical needs and selecting or reconfiguring equipment that satisfies these requirements, significant cost savings may be recognized, existing resources are utilized, and shorter times from discovery to first oil can be achieved.

Introduction
In the past, subsea production equipment was designed to meet very specific field requirements. However, the constraints placed on the design of a customized system often make it undesirable or unacceptable for use in other applications or circumstances. By employing a less constrained approach to equipment design at the component level, it is possible to develop a standard product with flexibility to meet an array of options and requirements. By standardizing on essential features and reducing the number of internal interfaces, a smaller inventory of modular products can be maintained by equipment suppliers to meet a broader range of third party interfaces and customer requirements.
Although the cost of time is often difficult to quantify, it is recognized that significant savings can be achieved with reduced equipment supply times. The use of existing equipment considerably reduces the engineering and manufacturing cycle time typically associated with subsea equipment. Also, the number of technical interfaces to be resolved is reduced by force. Features of the equipment design can meet a wide variety of applications although no single system can suit every field. Strict management and resolution of remaining interfaces is crucial to keeping a fasttrack development on schedule. Early identification of basic project needs is fundamental to ensure that the selected equipment meets the requirements.
The use of field proven equipment also has the apparent advantage of utilizing a known and tested solution. Less likely to be observed are the advantages gained with familiarity of equipment, lessons learned and operational efficiencies developed by previous experience. Additionally, many of the problems associated with first time assembly and test of equipment are eliminated and increased system reliability can be achieved.
A discussion of the application of a modular design approach, technical interface management, and the use of field proven equipment will be used to illustrate how the challenges of developing four recent marginal subsea projects were successfully overcome by equipment supplier/operator teamwork and planning.

Challenges, Concerns, and Trade-offs
Utilizing existing subsea equipment and bringing production on-line quickly has many benefits but there also challenges, concerns, and trade-offs that must be addressed. The following examples illustrate some of these and the possible outcomes:Control system pressure rating - if the control system changes, there are possible conflicts with the existing hydraulic tubing design and the valve and choke actuator pressure ratings. While the hydraulic tubing change out to match the ratings is relatively simple, changing a valve actuator can affect the delivery of the equipment.