This chapter discusses the requirements for cost-effective well service and workover associated operations on subsea completion systems.
Various well service operations are identified, and their influence on vessel design and operational requirements is discussed.
The design philosophy of the Stena Seawell class of multifunctional field support vessels is discussed with reference to these operational requirements Well service techniques are described, and the methods used to operate from Stena Seawell are explained.
A typical in-field well servicing program is used to compare operating times for a Monohull vessel of the Seawell class and a typical semisubmersible drilling ng of the type presently used for such well interventions. This comparison of operating times provides one of the major economic justifications for using a Monohull vessel to perform well service operations.
The trend towards subsea production systems for marginal field development is now well established and has led to improvements in equipment reliability, with the result that subsea development scenarios are now highly attractive.
This expansion in subsea developments has created a need to provide cost-effective methods for associated workover, or more accurately, well service operations.
Well maintenance and data gathering requirements must be satisfied if the reservoir is to be properly managed and depleted for optimum oil recovery, but to date the only vessels available for this type of work have been semisubmersible drilling rigs.
The comparatively high cost of mobilizing and operating a semisubmersible drilling rig for subsea well service interventions has created a requirement for a purpose-built well service vessel capable of being quickly and easily mobilized to the well location.
The inherent design features of a semisubmersible drilling rig do, however, accurately reflect some of the requirements for subsea well service operations:
a stable platform with a good heave and roll response
an adequate derrick, and equipment handling facilities
a Certificate of Fitness as an Offshore Installation, and compliance with Offshore Mineral Workings Act
reliability
acceptance by the industry
Despite some of the positive attributes of the semisubmersible drilling vessel for well service operations, there are significant drawbacks inherent in the design:
slow transit speed
reliance on other vessels for support, e.g. anchor handling Tugs
time-consuming anchoring operations
generally small deck area
requirement to mobilize a saturation diving spread for most well re-entry operations
Consideration of both positive and negative aspects of the semisubmersible design should form the basis for a design specification for a purpose-built well service vessel and the following sections describe in detail how these aspects influenced the design of the Stena Seawell multifunctional field support vessel and how the overall design will be utilized for well service operations.
It has been shown that a purpose-built well service vessel must embrace an exacting number of design criteria if it is to satisfy operational requirements. A large monohull based on a diver support vessel (DSV) will satisfy the necessary requirements if designed correctly.
