Deepwater subsea boosting planning and application move ahead

Subsea field development example. The yellow indicates the well-to-boosting station SURF system. The green represents the boosting station-to-host SURF system.

Deepwater projects require higher boosting pressure to reach the host facility as do long tiebacks to existing facilities. Both water depth and step-out distance contribute to the pressure differential required of the boosting system. Higher intervention, replacement and repair costs as well as the need for reliability of the boosting system add to the complexity of boosting solutions.

Systems approach

DeepSea advocates a systems approach for all field development projects but it is essential for those requiring subsea boosting due to the number of variables and their influence on the operating strategy and commercial viability of the development.

Systems engineering is interdisciplinary among all the system parts. The purpose is to optimize for maximum compatibility among the parts and high performance in terms of the objectives. The holistic approach allows translation of all the technical factors into a structured field development process to provide a decision-making framework for the project.

By mapping the inter-dependence of variables, particularly those that may (or do) cross contractual boundaries, systems engineering enables evaluation of design trade-offs against their impacts on key system goals and operational targets.

Subsea boosting affects flow assurance through the entire project lifecycle – from start-up through the production plateau to the end-of-life stages. Therefore, a structured approach is necessary to understand the benefit of boosting for the overall system performance throughout the asset lifecycle.

Undertaking systems planning early at the conceptual and definition stages helps ensure that the economic and operational considerations are woven into the resulting development strategy.

LiquidBooster. Image courtesy Aker Solutions.

Another advantage of systems engineering is that it allows better interface management by providing a process road map for the development team once project execution begins. Considerations are given to the control and communication network as well as the power provision characteristics and requirements. The availability and reliability of power umbilicals for deepwater dynamic performance, subsea connectors, penetrators, and subsea transformers are directly connected to the boosting performance and critical for the system deliverability.

Similarly, this systems approach applies strongly to other subsea technologies such as subsea separation.

Operational strategy

Large greenfield development in deepwater has a well-defined and proven model consisting of three major contracts – the SPS (subsea production system), SURF (subsea umbilicals, risers, flowlines), and the host facility (hull plus topsides). While the interfaces among the three major contracts are project-dependent, there are a few parameters that require consideration. For example, umbilicals may lie in either the SPS or SURF contracts. To date this model has served well, as it is based on the general understanding that the flow characteristics do not change from exiting SPS to entering SURF.

However, introduction of subsea boosting into the process alters the product parameters on its downstream side, creating a second set of inputs to the SURF system between the boosting station and the host facility. It is management of the change in flow conditions within the conventional contractual structure that raises the question as to where, contractually, the subsea boosting system should best reside. This is a systems-level issue with the "system" defined as the entire development (SPS, SURF, and host).

To illustrate, a hypothetical field development shows two fields, North and South, separated by 40-50 km (25-31 mi) distance and each requiring a high well count. North field has close well clustering whereas South field has distributed well centers with 10+ km (6+ mi) between them. Both fields require boosted flow.

Evaluating the development of each field separately, application of the conventional packaging into EPC, SPS, and EPIC SURF would most likely place the boosting system for North field with the SPS contractor, due to near-well boosting positioning, and with the SURF contractor for South field. Both approaches require the contractor to supply the boosting system and, therefore, accept the risks associated with delivery of the overall package. Some contractors do not have much experience with the critical boosting package and are most likely to buy it from a third party. Given the importance of the boosting system to the overall field performance, the contractor's pricing to cover the risks may be substantial.

Alternatively, the operator can single the boosting system out into a separate contract and assume responsibility for interfacing it with the other three main contracts. This places a significant emphasis on interface management by the operator. Decision to follow this route requires a cost-benefit analysis early in planning to determine the most appropriate way forward in the project- and operator-specific situation.

Seamless system performance depends on operational strategy. Boosting usually is integrated into SURF infrastructure where it injects energy and changes characteristics of the flow affecting other system components.

Near-well boosting also can be a part of the SPS subsea well center where it is governed by the well pressure characteristics and could be integrated with other components such as a manifold.

Boosting methods and sequence influence the host facility and, in turn, are affected by the host vessel lift constrains and processing requirements. Boosting design could help reduce risk of unplanned intervention by covering repair and replacement scenarios, and allowing for periodic system testing.

Contract strategy

System engineering enables operators to clearly see technical and economical implications of either contracting subsea boosting to host, SURF or SPS contractor, or managing it themselves as a separate package. Managed separately, it may be better targeted at project deliverables, but could require additional resources and equipment. Placing boosting in one of the three main contract packages means the operational integration and interface risks are moved to a contractor, but generally at a price premium.

Subsea boosting configurations

No single boosting design configuration fits all project system variables; therefore, an assessment combination of boosting options is necessary applied to the operational performance targets.

Typical boosting configurations include:

• One high-capacity booster
• A series of small boosters
• Parallel small boosters
• Series wells connected to a single medium booster.

One high-capacity pump is by far the most common arrangement. Compared to other configurations it is a single, relatively straightforward system with reduced interfaces and generally lower installation costs. On the flip side, it offers redundancy at a higher cost to trade off against greater downtime for repair, maintenance, and inspection.

A series of small boosters is more complex. It reduces capex by minimizing equipment expenditure, improves operational time, and allows for the use of smaller installation and workover vessels. Redundancy can be accommodated with relative ease through appropriate manifolding and adequate capacity in the individual boosters. Additionally this redundancy can be activated and controlled remotely from the host without the need to mobilize a vessel, thereby maximizing production.

Parallel small boosters can boost specific wells as required. The drawback is the limited number of possible configurations due to the lack of inter-well connectivity, and selection of individual booster capacity.

The operator (or contractor) needs to decide whether to use a dedicated booster to each wellhead or focus on low performance wellheads, boost from day one or start after the natural flow rate has dropped, mix different boosting methods and techniques, or use them in phases such as is being employed on the Chevron Jack/St. Malo development in the deepwater Gulf of Mexico. A system approach provides a clear path for this decision-making.

Deepwater boosting techniques

Boosting techniques for deepwater include:

• Gas lift systems
• Electrical submersible pumps (ESP)
• Booster pumps: Single phase, multi-phase, hybrid.

The gas lift system is characterized by high reliability and relatively low cost. It is a tried and tested valve injection technology and allows fiber-optic monitoring of performance.

However, the 2-5 ksi maximum operating pressure limits the water depth in which this technique can be applied. Gas lift does not perform if water cut is high or natural pressure is too low. The mechanism has to be periodically retrieved for valve replacement. Gas lift requires skilful installation; poor installation could lead to flow assurance problems such as hydrates.

ESPs consist of an electric motor and centrifugal pump unit. It also includes surface control system and transformer plus electrical power cables. This technique is effective in wells with low gas-to-oil ratio and high water cut. ESP units are considered to be cheap with coil-tubing installation for managing installation costs. They require replacement typically after four to six years of use. The weaker points of this technique are limited pump power, significant intervention downtime, and high costs involved. ESP efficiency drops for heavy crude.

Booster pumps include models for single- and multi-phase flow as well as hybrid pumps. They avoid the well intervention required for the previous two methods. In general, the centrifugal technology is cheaper for single-phase pumps but limited by gas content (GVF<0.1). Most single-phase models can provide typical pressure differentials of up to 4,000 – 5,000 psi and offer four to six and one-half years of continuous operation. They are optimal for smaller fields but tend to be cumbersome as capacity goes up.

Multi-phase pumps are more complex and expensive, with restricted availability. Their typical gas content limit is close to 1.0 GVF and pressure differential is in the range of 1,500 to 2,800 psi from low to high (approximately 85%) GVF, respectively. Multi-phase pumps provide four to six and one-half years of uninterrupted operation and tend to be more compact as capacity goes up. They can operate over a wide viscosity range.

The most common type of multi-phase pump uses helico-axial impellers to spin the fluid at high RPM. The high-speed fluid passes through a booster section which converts kinetic energy to pressure. The helico-axial design offers higher operation flexibility and has a better installation track record than other options.

Both single- and multi-phase pumps are available and subsea proven.

The hybrid pump can operate at a higher differential pressure than the multi-phase pumps across the range of GVF. Additionally they have higher hydraulic efficiency than helico-axial pumps.

Boosting performance

The boosting performance is influenced by variables including fluid viscosity, gas volume fraction (GVF), and required pressure differential.

Developing and delivering a boosting package

First and foremost, it is imperative to treat boosting as a system rather than as a product. Requirements are project-specific and the operational philosophy is central to the system development. For instance, boosting changes the SURF system – it is no longer a passive system.

DeepSea has been involved in subsea boosting solutions in waters up to 3,000 m (9,842 ft) for developments around the world by applying a systems approach to the development cycle and by addressing the boosting strategy within the overall project objectives.

The boosting package delivery includes the following key aspects:

• Operational philosophy definition
• Realistic flow assurance assessments
• Clear identification requirements and design variables
• Design of boosting package and accessories (these may be more challenging than the booster itself)
• Planning for qualification
• Boosting system specification documents – from pump and boosting skids through to boosting system and SURF system
• FAT & SIT set-up and supervision
• Operation procedures and manuals
• Sparing philosophy and IRM
• Asset integrity management process and data monitoring.

The key benefit is the ability to take on various interconnected elements of subsea field engineering and balance them against competing requirements to achieve operational stability.

Primary SPS companies are moving toward including boosting in their service packages. However, the qualification process dictates that this is part of a long-term strategy warranting the necessary investment.

While there are several vendors of boosting systems, some consolidation of the supplier base has occurred because of the scale of cost associated with marinization and qualification of the systems.

As the qualification for this technology becomes standardized in the coming years, supported by further project deployments, the scope for improved performance and cost of boosting systems will follow.