SBM CALM Changeout of CO Buoy 760 

Location: Offshore Tema, Ghana 
Duration: February 2025 
Client: RD Marine for GPMS 
Role: Diving Contractor 
Water Depth: Approximately 24 m 
Supplied: Air diving personnel and equipment 

Project Overview 

OSC Marine was appointed as the diving contractor for the SBM CALM Buoy changeout offshore Tema, Ghana. The project involved the decoupling and removal of CO Buoy 760 and the installation and reinstatement of the replacement SO Buoy 17190. 

The campaign was completed during February 2025 and required:  

  • Detailed planning 
  • Air diving support 
  • Subsea hose handling 
  • Anchor chain layback 
  • Buoy hook-up 
  • Chain tensioning 
  • Inspection 
  • Commissioning 
  • Leak testing 

The works formed part of a critical offshore marine infrastructure operation, requiring safe diver intervention and close coordination between the project, marine, and commissioning teams. 

OSC Marine successfully completed the SBM decoupling and reinstatement of the new SO Buoy for GPMS. The final “as left” dive (a final underwater inspection or measurement) was completed on 22 February 2025, following the start of the diving campaign on 8 February 2025. 

Scope of Work 

The scope covered the changeout of CO Buoy 760 and installation of SO Buoy 17190, including all associated diving support, inspection, and commissioning activities. 

The work included: 

  • As found survey of CO Buoy 760 
  • Disconnecting and laydown of subsea hoses 
  • Decoupling of CO Buoy 760 
  • Layback of anchor-leg chains 
  • Handover of the old CO Buoy 
  • Receipt and installation support for the new SO Buoy 
  • Hook-up to the anchor spread 
  • Chain tensioning 
  • Flange sealing face inspection on the floating hose and overboard elbow piping flange 
  • Subsea and floating hose installation 
  • System commissioning up to the PLEM valves 
  • Leak testing of pipework, hoses, fittings, and interfaces to the subsea PLEM (Pipeline End Manifold) 
  • As left survey of SO Buoy 17190 

The as left survey included inspection of key buoy and subsea components, including topside areas, above and below skirt areas, sealing faces, under-buoy piping flange, subsea hose string, and the PLEM. 

Project Setup and Mobilisation 

The project required engineering support, project setup, and mobilisation of assets from both regional and international locations. This ensured that all required diving equipment, personnel, and operational resources were available to support the offshore campaign. 

OSC Marine supplied air diving personnel and equipment for the works. The project team included: 

  • Offshore Project Manager 
  • Air Supervisors 
  • Dive Technicians 
  • Air Divers 
  • Lead Diver 
  • Diver Medic Technicians 

This team structure supported safe execution across inspection, disconnection, installation, survey, and commissioning stages. 

Diving Campaign 

A total of 88 dives were conducted during the campaign, with a total dive time of 5,363 minutes recorded. 

All commercial diving operations were completed in accordance with the project dive plan. No decompression diving was required, and the campaign was completed with zero diving or topside incidents. 

This safety performance reflected the importance of planning, experienced personnel, disciplined dive management, and effective offshore coordination throughout the buoy changeout operation. 

Decoupling and Buoy Changeout 

The project began with the as found survey of CO Buoy 760 to document the condition of the buoy system before removal. The subsea hoses were then disconnected and laid down in preparation for buoy decoupling. 

Once the subsea hoses were safely managed, the CO Buoy was decoupled and the anchor-leg chains were laid back. This allowed the old buoy to be handed over and the replacement SO Buoy 17190 to be received for installation. 

Following receipt of the new buoy, the system was hooked up to the anchor spread and the chains were tensioned. This was a critical stage of the operation, ensuring that the buoy was correctly positioned and secured within the CALM (Catenary Anchor Leg Mooring) buoy mooring system. 

Hose Installation and Interface Inspection 

As part of the reinstatement works, subsea and floating hoses were installed and connected to the new SBM CALM Buoy. Flange sealing faces were inspected on both the floating hose and the overboard elbow piping flange to support reliable interface integrity before commissioning and testing. 

The subsea hose connections to the new SBM were completed from the subsea PLEM, allowing the system to be prepared for pressure testing and commissioning. 

Leak Testing and Commissioning 

Following installation, OSC Marine completed and verified the pressure testing of the SBM CALM Buoy SO17190 system. The leak test covered the pipework, hoses, fittings, and interfaces through to the subsea PLEM. 

The test medium used was fresh water containing tracer dye. The test pressure was 12 bar gauge pressure, with the pressure required to be held for a minimum of 30 minutes. 

The test procedure involved pressurising the line via the floating hose with Valve 2 closed. Once Valve 2 was successfully tested, the valve was opened to confirm that Valves 3 and 4 could hold pressure. After successful testing of Valves 3 and 4, those valves were opened to confirm that Valves 5 and 6 on the PLEM were holding pressure. Once all required valves were confirmed, the 12 bar pressure hold was completed for the required duration. 

If any subsea valve, fitting, or connection had failed to hold pressure, divers would have been deployed to observe for tracer dye and identify the source of leakage. Any detected leak would then be reported, documented, and corrected where possible. 

Final Survey and Project Outcome 

OSC Marine successfully completed, verified, and reported the pressure testing, final positioning, and level of the CALM Buoy. Draft data for the CALM Buoy was submitted to GPMS following completion of the works. 

The as left survey confirmed the final condition of SO Buoy 17190 and associated system interfaces after installation and commissioning. The completed works provided GPMS with a reinstated SBM CALM Buoy system ready for continued offshore operation. 

Conclusion 

The SBM CALM Buoy Changeout offshore Tema demonstrated OSC Marine’s capability to deliver complex offshore diving support for buoy replacement, subsea hose installation, chain handling, inspection, pressure testing, and commissioning activities. 

Through experienced air diving personnel, structured project management, effective mobilisation, and disciplined offshore execution, OSC Marine successfully completed the changeout from CO Buoy 760 to SO Buoy 17190 safely and without diving or topside incidents.