Swivel Stack Maintenance on Vietnam FPSO

Introduction

In May and June 2026, a major shutdown campaign was undertaken on an FPSO operating at the Chim Sao field, offshore Vietnam. The objective was to carry out maintenance and life-extension work on the vessel’s upper swivel stack, an important assembly that allows the FPSO to rotate with changing sea and weather conditions while maintaining the transfer of fluids, chemicals and utilities.

The maintenance programme involved replacing several critical components, including the utility swivel hoses and pedestals, M90 bolts, module driver arms and a damaged fire water swivel module. Some of the heaviest components weighed approximately 13.5 tonnes and required specialised lifting arrangements for removal and installation.

However, the FPSO’s existing pedestal crane could not reach the swivel stack working area from the supply vessel. This created a significant lifting challenge, particularly within the limited space available on the forward deck.

Thunder Cranes was engaged to provide a temporary offshore lifting solution using its TC20SL modular rental crane equipped with a 50 ft boom. Our scope covered the complete lifting operation, from initial site surveys and engineering preparations to crane mobilisation, installation, lifting execution and final rig-down.

The solution enabled the project team to safely transfer heavy swivel modules between the FPSO and supply vessel without requiring an additional lift vessel or extensive deck strengthening.

Challenges & Solutions

The project presented several technical and operational challenges that required careful engineering, planning and coordination between all parties.

Challenge 1: Limited Working Space on the FPSO Deck

The proposed crane installation area near module M14 was surrounded by existing structures, including a railway beam, monkey ladder, firefighting equipment cabinet, mooring bollard and large cable tray.

Solution:

Thunder Cranes selected the modular TC20SL crane because its components could be transported individually and handled using the FPSO’s existing 25-tonne pedestal crane. Its heaviest component, the lower king post, weighed only 3.3 tonnes.

This allowed the crane components to be landed from the supply vessel and skidded into the designated installation position without requiring major modifications to the surrounding deck structures.

Challenge 2: Installing the Boom Above an Existing Cable Tray

The TC20SL’s 50 ft boom consisted of five sections that needed to be assembled and connected. However, the existing cable tray obstructed the normal boom installation route.

Solution:

Thunder Cranes designed a customised 12-metre-high load-bearing scaffolding platform and gantry structure to support the boom assembly.

The five boom sections were positioned and assembled above the existing cable tray using chain blocks and designated rigging points. This allowed the crane installation to proceed without removing or relocating the cable tray.

Challenge 3: Crane Foundation and Structural Requirements

The crane required a secure foundation capable of supporting significant operating loads. Engineering calculations identified a peak uplift force of approximately 65 tonnes at a support point.

Solution:

A purpose-built foundation system was designed by the client’s structural engineers. Customised mounting stools were fabricated and welded directly onto the FPSO’s deck beam structure.

Thunder Cranes provided the required reaction-force calculations and verified the bolted connections and outrigger mounting through finite-element analysis against API Spec 2C and AISC allowable-stress limits.

The arrangement transferred the crane loads into the existing deck beams without requiring additional structural strengthening below the deck.

Challenge 4: Complex and Heavy Lifting Operations

The project involved transferring swivel modules weighing up to 13.5 tonnes between the FPSO and a moving supply vessel.

These operations were classified as complex lifts because of vessel movement, restricted visibility during certain lifting operations and the heavy load, which represented approximately 93% of the TC20SL’s rated capacity.

Solution:

Thunder Cranes’ Engineering Manager acted as the Lifting Technical Authority, reviewing the lifting plans, calculations and operating conditions.

A Hazard Identification and Risk Assessment (HIRA) and a formal Go/No-Go readiness review were completed before the lifting operations.

The dynamic lifting envelope was restricted to significant wave heights of 0.5 metres or less and wind speeds below 10 knots. Trial manoeuvres and practice positioning with an empty hook were also conducted before the actual lifts.

Challenge 5: Handshake Transfer Between Two Cranes

The old fire water swivel module had fittings underneath it, meaning it could not simply be placed on the deck during the transfer process.

Solution:

A coordinated handshake transfer was developed between the existing turret gantry crane and the TC20SL.

The gantry crane lifted the swivel module clear of the stack and transferred it directly to the TC20SL. The modular crane then handled the load and transferred it to the supply vessel.

Challenge 6: Tight Shutdown Schedule

The shutdown was initially estimated to take 35 days, with the business plan targeting 26 days. Any delay to the lifting operations could affect the overall maintenance schedule and production restart.

Solution:

Thunder Cranes mobilised a dedicated team to provide continuous lifting support around the clock. The TC20SL was installed, load-tested and proven through trial lifting before the shutdown commenced.

With detailed preparation and coordination between the project teams, the crane operated continuously without interruption or non-productive time (NPT) throughout its lifting scope.

Project Steps

Thunder Cranes delivered the project through a series of carefully coordinated activities, ensuring that the crane installation and lifting operations were ready before the main shutdown began.

Step 1: Site Survey and Engineering Assessment

Our engineering team conducted an initial site visit in 2025 to assess the FPSO’s forward deck and identify a suitable crane installation position near module M14.

The assessment included:

• Identifying the crane installation location and available working space.
• Reviewing existing deck obstructions and access requirements.
• Developing the TC20SL rig-up concept with a 50 ft boom.
• Preparing crane reaction-force calculations and structural requirements.
• Developing lifting plans and risk assessments.
• Coordinating the foundation design and boom assembly requirements.

Step 2: Crane Mobilisation and Offshore Transportation

The TC20SL modular crane was mobilised from Songkhla, Thailand, on 28 March 2026.

The equipment arrived at PTSC’s yard in Vung Tau, Vietnam, on 15 April before being transported offshore aboard the MV Vung Tau on 18 April. The crane and its dedicated crew reached the FPSO on 20 April.

Step 3: Crane Foundation and Installation

Once offshore, the crane components were transferred from the supply vessel to the FPSO using the existing pedestal crane.

The components were then skidded into the designated installation area. The foundation stools, lower king post and outriggers were installed and secured to the prepared deck structure.

The customised scaffolding platform was also constructed to support the assembly of the 50 ft boom above the existing cable tray.

Step 4: Boom Assembly, Testing and Certification

The five boom sections were assembled using the specially designed scaffolding and gantry arrangement.

Following installation, the TC20SL underwent load testing in accordance with API RP 2D and was certified for service on 2 May 2026.

Trial vessel manoeuvres, trial lifting and the main lift were completed ahead of the shutdown to verify the lifting arrangements and operating conditions.

Step 5: Offshore Lifting Execution

The shutdown commenced on 22 May 2026, with Thunder Cranes providing dedicated lifting support around the clock.

The TC20SL supported the removal and installation of the swivel modules, including the transfer of heavy components between the FPSO and supply vessel.

One of the critical operations was the handshake transfer of the old fire water swivel module from the turret gantry crane to the TC20SL on 25 May.

Step 6: Completion and Crane Rig-Down

The TC20SL completed its lifting scope on 29 May 2026, one day ahead of plan.

Following completion of the lifting activities, the crane was prepared for removal and rig-down, which took place from 30 May to 5 June 2026.

Project Timeline

The project involved extensive preparation and coordination before the main offshore shutdown, ensuring that all lifting arrangements were ready for execution.

• 2025: Initial site survey, crane installation concept, engineering calculations and structural assessment of the FPSO deck.
• April 2025: Trial supply vessel manoeuvre alongside the FPSO bow to verify the dynamic lifting arrangement.
• 28 March 2026: TC20SL crane mobilised from Songkhla, Thailand.
• 15–20 April 2026: Crane arrived at PTSC’s Vung Tau yard, transported offshore and delivered to the FPSO.
• Late April 2026: Crane components transferred onboard, foundation installed and crane assembled.
• 2 May 2026: TC20SL successfully load-tested and certified for service.
• Early May 2026: Vessel trial, trial lift and main lift completed ahead of the shutdown.
• 22 May 2026: Main shutdown commenced, with continuous lifting support provided from 23 May.
• 25 May 2026: Old fire water swivel transferred from the turret gantry crane to the TC20SL and offloaded to the supply vessel.
• 29 May 2026: TC20SL lifting scope successfully completed, one day ahead of plan.
• 30 May – 5 June 2026: Crane rig-down activities.
• 11 June 2026: Production resumed, one day ahead of EnQuest’s stretch plan.