In September 2023, Oceaneering successfully completed work for the Nexstep (National Platform for Re-use and Decommissioning in the Netherlands) Joint Campaign to decommission thirty, Dutch sector, mudline suspension (MLS) wells. The joint campaign, utilizing a vessel-based decommissioning approach, involved six major operators in the Netherlands.
As lead contractor, Oceaneering was responsible for the full range of project services, from project management and engineering through vessel charter and service provider management. The work was carried out using a phased approach to operations.
- Phase 1: Initial Survey (General Visual Inspection [GVI]) – May 2022
- Phase 2: Deep Clean and Inspection – October 2022
- Phase 3: Temporary Abandonment (TA) Cap Recovery, Circulation, and Cementing – August 2023
- Phase 4: Conductor Severance and Seabed Clearance – September 2023
- To provide an economic solution acceptable to Nexstep and their six operator partners
- To facilitate the removal of multiple TA caps without the risk that residual oil-based mud (OBM) could escape to the environment
- To facilitate the contained removal of residual OBM from well annuli before setting environmental barriers
- To deliver project documentation within the agreed schedule of six different operators and the Dutch regulatory authorities
Leveraging experience as lead contractor in previous multi-client decommissioning scopes, Oceaneering developed a comprehensive and optimized plan that addressed the operators’ requirements, inclusive of a zero-spill solution for OBM.
Multiple TA cap removal operations were required to be carried out for the Nexstep project as part of well plugging and abandonment (P&A) activities. To complete the scope using a vessel-based riserless solution, it was necessary to further develop existing technology to augment established vessel-based operations methodologies.
Consequently, and based upon Oceaneering’s field proven conductor stripper assembly (CSA), the mud containment system (MCS) was developed. The MCS facilitated both TA cap removal and annulus circulation operations by containing and diverting OBM from a well through a return line to the vessel for storage and onshore disposal.
The project plan covered all aspects of planning, preparation, vessel mobilization, offshore operations, vessel demobilization and close-out activities for the P&A activities related to the 30 MLS wells. This work was completed in four phases, with the first two phases executed in collaboration with key sub contractor Bluestream Offshore.
The project plan covered all aspects of planning, preparation, vessel mobilization, offshore operations, vessel demobilization and close-out activities for the P&A activities related to the 30 MLS wells. This work was completed in four phases, with the first two phases executed in collaboration with key sub contractor Bluestream Offshore.
Phase 1: Initial Asset Survey
The survey campaign’s purpose was to gather a general visual overview of each location with respect to the specific operator’s requirements. A right-sized, compact vessel, the DP2 ROVSV VOS Sugar, ideally suited for completing the general visual inspection (GVI) performed at each location provided a cost-efficient, lower emissions solution. The GVI gave the team an impression of the subsea structures' condition and their attachments without cleaning and enabled them to identify any major changes to the original or previously known condition.
Additionally, the information gathered de-risked the project by identifying issues up front and allowing for proper engineering analysis and solutions to be developed prior to arriving on location with a more expensive vessel.
Phase 2: Deep Clean and Inspection
Phase 2 addressed specific wells that required deep cleaning of conductors in preparation for their Phase 3 and/or Phase 4 operations. Phase 3 wells required a more intensive and detailed internal inspection down to and inclusive of the upper most temporary abandonment (TA) cap of the mudline suspension (MLS) system.
For Phase 4 conductors, deep cleaning was only performed to 8.0m below mudline (BML) to enable the conductor to be cut at 6.5m BML. The Dutch authority required wells to be severed at 6m BML to mitigate the dynamic nature of the seabed in these waters. These operations were completed using the DSV Ievoli Ivory, a vessel specifically suited for these inspections and cleaning operations, eliminating the need to use a higher day rate vessel.
Phase 3: Temporary Abandonment (TA) Cap Recovery, Circulation, and Cementing Operations
This phase encompassed the recovery of 19 TA caps and the containment and recovery of the base oil component of the oil-based mud or water-based mud. The team completed wireline runs to perforate casings prior to pumping cement to accurately place environmental barriers through a hose, enabling the cut and recovery of various casing sizes.
These operations were successfully completed and facilitated well preparation for the final phase of the project. The MSV Island Vanguard was chartered for this phase of work in line with the project’s cost optimization objectives.
Phase 4: Conductor Severance, Recovery, and Seabed Clearance
The project's final phase included, where applicable, permanent abandonment and clearance of the well location. Scopes of work were finalized by the issuance of a Completion / Seabed Clearance Certificate by the onboard operator representative. Conductors were cut at the prescribed depths using an abrasive water jet cutting method. The cut depths ranged from 5.77m BML (which was accepted under dispensation by the regulatory authority) up to 11.12m BML. Conductors and associated debris were recovered to deck, and a seabed clearance survey was carried out at each location. The MSV Island Vanguard was again used for this phase in line with the project’s cost optimization objectives.
The scope was completed safely and used a more economically and environmentally friendly approach than previous campaigns. It enabled a more efficient approval process with the Dutch regulatory authority by aligning documentation and by sharing of campaign management and offshore supervision through all phases of the project.
Use of a phased approach to complete the specified wellhead removal work scopes in a single campaign provided cost savings and when coupled with the new design developed specifically for this project. There were multiple client benefits including:
- Eliminating OBM discharges via a mud containment system.
- Reducing environmental impacts from CO2 by using vessel instead of rig and avoiding NOx deposition on Natura2000 areas by using a selective catalytic reduction (SCR) system on the vessel.
- Allowing changes in the campaign schedule to be made between phases to adjust the execution plan based on new information (where applicable).
- Reducing costs more than 30% by moving scope from rigs to vessel, sharing risk and cost between operators, and applying lessons learned across sites to optimize operations.
The project management by Oceaneering of daily operations on each phase in close collaboration with Nexstep and the six operators to best utilize available resources was essential to the project’s success. Learnings from this campaign will be used by Oceaneering in line with its continuous improvement process to best place the team to provide vessel-based P&A solutions to the global market.
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