Future Intervention Options for Remote Subsea Facilities
Authors: Reeves Harold , Siegmund Mark , Dawson Ila , Openshaw Graham 
Published: October 2008

With the increasing water depths and step-out distances from the host, subsea systems are becoming discrete and complex production facilities. This demands an innovative approach to remote intervention to ensure reliability, availability and maintainability requirements are realized. Strategies for avoiding or managing intervention are addressed. New technology solutions to improve the cost effectiveness of intervention are summarized and a holistic approach to subsea system design is recommended.

Introduction

With more complex facilities driven by the growing move towards seabed processing and the increasing number of subsea trees, the demand for cost-effective intervention techniques also increases. The evolution of intervention technology from the ROV to the AUV is presented, and a vision of future hybrid designs and other potential techniques aimed at minimizing intervention cost and risk are also explored.

Subsea facilities can be compared to "not-normally-manned" platforms where periodic maintenance and repair campaigns are necessary to ensure the health of the overall system. Design options to minimize and simplify intervention are explored, including modularity, redundancy, interfacing versus simplification, interfacing and advanced diagnostics techniques.

Subsea processing will only increase the number of interventions, as there is more on the seabed that can fail or need maintenance. In order to keep the operating costs in line, reliable and inexpensive interventions are needed. If intervention costs continue to increase, fields may be abandoned earlier than planned, as there may not be enough reserves to justify the lifting cost. This could result in a significant reduction in recovery.

This paper reviews current intervention practices, with a particular emphasis on their applicability to future subsea systems. Potential methods for both avoiding and minimizing intervention are addressed.

On the basis that some level of intervention is an inevitable requirement, new technology options for reducing the cost of those interventions are examined in more detail.

Supporting technologies such as Knowledge Management are reviewed in order to address a new holistic approach to subsea system design.

Current Intervention Practices

Remote subsea installations have always needed some level of intervention. Once beyond the range of divers, this has been accomplished with vessel-based work class ROV's. These vessel systems need to have a combination of winches, cranes, moonpools, and/or A-frames to facilitate a wide variety of deployment requirements. Accommodating this equipment as well as operator-supplied tools, requires a relatively costly vessel, while smaller vessels can be used for visual inspections and light intervention. With increasing water depths, the time taken to get the ROV and running tools to bottom have also increased, escalating the cost of interventions.

Current subsea hardware configurations have several components that are designed to be replaceable. These include the control pod, choke, sand detectors, flying leads, flow meters and assorted valves. As installed maintenance is difficult these are normally replaced after failure or, with the proper monitoring system, may be replaced prior to total failure. Preemptive interventions greatly reduce production downtime, and can be done in campaigns to reduce costs. With an unplanned intervention, it usually takes at least two weeks to evaluate the problem, gather all the equipment needed, and secure an appropriate vessel to perform the work.