Subsea Plug and Abandonment Operational Strategies

‍ By Grant Pierce, Intervention Performance Ltd.‍‍ ‍ ‍

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Introduction

Subsea plug and abandonment operations are becoming one of the largest work scopes facing the offshore industry. As mature subsea developments continue aging, operators are increasingly focused on reducing the cost and operational complexity associated with abandonment campaigns. 

Historically, most P&A operations were performed from drilling rigs. However, the economic sustainability of performing large-scale abandonment campaigns exclusively from MODUs has become increasingly difficult. 

As a result, the industry continues moving toward vessel-based abandonment methodologies utilizing: 

  • Light Well Intervention Vessels 

  • Well Intervention Units 

  • Purpose-built abandonment rigs 

  • Riserless intervention systems 

  • Wireline-based abandonment technologies 

The long-term objective is to perform a greater percentage of permanent abandonment operations from lighter assets while maintaining barrier integrity and regulatory compliance. 

  1. Challenges in Subsea Abandonment

No two subsea abandonment operations are identical. 

Each well presents unique technical and operational challenges, which may include: 

  • High temperatures 

  • Unconsolidated formations 

  • Reservoir depletion effects 

  • Formation strength changes 

  • Sustained casing pressure 

  • Uncertain reservoir pressure 

  • Formation permeability concerns 

  • Formation shear stress and subsidence 

  • Limited historical well data 

  • Deep section milling requirements 

  • Cement verification across multiple casing strings 

These challenges require flexible operational planning and careful selection of intervention methodology. 

2. Abandonment Categories

Subsea abandonment operations are generally divided into three categories. 

Suspended Abandonment

Suspended abandonment refers to situations where well control equipment remains intact while well operations are temporarily suspended. 

Examples include: 

  • Waiting on the weather 

  • Delayed equipment delivery 

  • Adjacent well operations 

  • Construction activities 

  • Temporary operational interruptions 

Temporary Abandonment

Temporary abandonment, sometimes referred to as long-term suspension, involves abandoning the well while retaining the possibility of future re-entry. 

  • During temporary abandonment: 

  • Well control equipment is removed 

  • Reservoir isolation barriers are installed 

  • Future access remains possible 

Temporary abandonment may remain in place from days to several years. 

Permanent Abandonment

Permanent abandonment occurs when the well is permanently plugged with no intention of future re-entry. 

Permanent abandonment requires long-term isolation of all hydrocarbon-bearing zones and elimination of future flow potential.

3. Phases of Subsea P&A Operations

Subsea abandonment campaigns are generally divided into four phases. 

Phase 0 – Pre-Planning

Pre-planning includes: 

  • Well review 

  • Barrier evaluation 

  • Equipment selection 

  • Vessel selection 

  • Risk assessment 

  • Logistics planning 

  • Regulatory alignment 

Phase 1 – Reservoir Abandonment

Reservoir abandonment generally begins with: 

  • Wellhead inspection 

  • Wireline rig-up 

  • Drift runs 

  • Caliper logging 

  • Tubing condition evaluation 

Waste-handling systems for both liquid and solid phases are also established at this stage. 

An injection test is then performed to verify well integrity. 

If integrity is confirmed, cement slurry is bullheaded into the reservoir interval to establish the required barrier. 

Once the cement plug has developed sufficient strength, pressure testing is performed to verify the barrier integrity. 

Phase 2 – Intermediate Abandonment

Intermediate abandonment may involve: 

  • Section milling 

  • Casing retrieval 

  • Installation of environmental barriers 

  • Isolation of intermediate permeable zones 

  • Partial tubing retrieval 

The objective of this phase is to secure all remaining flow potential. 

Phase 3 – Wellhead and Conductor Removal

The final abandonment phase typically involves: 

  • Cutting the conductor 

  • Cutting the wellhead below the seabed level 

  • Retrieval of subsea structures 

This is performed to eliminate hazards to future marine activities such as fishing operations. 

4. Vertical vs Horizontal Xmas Trees

One of the most important operational considerations during subsea abandonment is the type of subsea Xmas Tree installed on the well. 

Subsea XT configurations are generally divided into: 

  • Vertical Xmas Trees (VXT) 

  • Horizontal Xmas Trees (HXT) 

Vertical Xmas Trees (VXT)

In a Vertical Xmas Tree arrangement: 

  • Tubing is landed in the wellhead 

  • The VXT is landed on top of the wellhead 

  • Valves are arranged vertically 

To access the tubing during abandonment, the tree must first be removed. 

This adds: 

  • Operational complexity 

  • Additional deployment steps 

  • Increased intervention time 

Horizontal Xmas Trees (HXT)

In a Horizontal Xmas Tree arrangement: 

  • Tubing is landed within the tree itself 

  • The tree is locked directly onto the wellhead 

  • Valves are arranged horizontally 

During abandonment, the Well Control Package or BOP can be positioned above the HXT while tubing retrieval occurs without removing the tree. 

This provides: 

  • Improved operational efficiency 

  • Reduced intervention time 

  • Simplified tubing retrieval 

  • Lower operational exposure 

5. LWIV and WIU Capability in P&A Operations

Modern LWIV and WIU assets can support a significant portion of subsea abandonment activities. 

Typical onboard systems may include: 

  • Electric wireline units 

  • Slickline units 

  • Coiled tubing units 

  • High-pressure pumping systems 

  • Cementing equipment 

  • Pipe handling systems 

  • Wellhead cutting systems 

Operations that can be performed include: 

  • Logging 

  • Perforating 

  • Plug setting 

  • Fluid injection 

  • Cement placement 

  • Tubing retrieval 

  • Wellhead cutting and recovery 

Some vessels may also incorporate saturation diving spreads where required. 

6. Operational Limitations

Although vessel-based intervention capability continues expanding, limitations still exist. 

These may include: 

  • Limited pull capacity 

  • Weather restrictions 

  • Deck-space limitations 

  • Section milling requirements 

  • Heavy casing retrieval requirements 

In situations requiring more extensive heavy intervention capability, Category B units or conventional drilling rigs may still be necessary. 

7. Open-Water Tubing Retrieval and Shutoff Systems

Recent developments have introduced systems capable of retrieving tubing in open water while maintaining a dual-barrier philosophy. 

Examples include: 

  • OneSubsea/Helix ROAM 

  • TechnipFMC Subsea Shutoff Device 

  • Trendsetter Engineering Mudline Closure Device 

  • Oceaneering OSPAT concept 

These systems continue to expand the capabilities of vessel-based abandonment operations. 

8. Automation and Vessel Efficiency Improvements

Modern intervention vessels increasingly incorporate: 

  • Automated deployment systems 

  • Pipe handling systems 

  • Vessel skidding systems 

  • Automated handling equipment 

These technologies reduce: 

  • Manual lifting 

  • Personnel exposure 

  • Handling risk 

  • Operational downtime 

Integrated vessels also provide advantages, including: 

  • Reduced mobilization time 

  • Reduced demobilization time 

  • Improved crew continuity 

  • Higher operational efficiency 

  • Greater agility than drilling rigs 

An LWIV or WIU may perform all phases of reservoir abandonment depending on the scope of work involved, while Vessels of Opportunity can often support later abandonment phases at reduced cost.