Under Water Abrasive Water Jet – Internal Pile Cutting Case Study

Removal of Met Mast Monopile

Sector: Civils – Marine Structures

Task: Below Sea Bed Internal Pile Cut to Remove MET Mast Monopile (1800mm) diameter, 45mm wall thickness) from Horns Rev, offshore from a Jack-Up Barge.

Location: Offshore, Denmark

Vessel: DBB WIND SERVER Jack-Up Vessel

Abrasive Water Jet Cutting sub-contractor: RGL

Precision Hydrodemolition sub-contractor: RGL

RGL plant: Ultra High Pressure Abrasive Water Jet cutting system. Bespoke Tooling for; Dredging, Proving, and Internal Pile Cutting.

 

 

Steel deck plate being removed

 

RGL were contacted by DBB Jack-Up Services A/S, who were looking for solutions to an offshore pile cutting contract they were tendering for. After

some initial discussion and meetings RGL provided a quotation for the project which involved the removal of a Monopile from Horns Rev, Offshore Denmark. DBB were subsequently successful in their tender submission for the project.

 

Preparation

RGL’s Commercial Director Jeremy Twigg and Team Leader for the project visited Denmark in advance of the works to undertake a detailed site survey of the M

Clamshell excavator at work

onopile with DBB. This enabled the relationships between DBB and RGL to strengthen and ensured both companys’ understood their individual role with the project scope for the project. For RGL this included the dredging of material from within the pile, the proving and cleaning of pile internal prior to cutting the Monopile at a depth of 24.2 metres. DBB had just taken delivery of their brand new JackUp Vessel: WIND SERVER. This is a very impressive vessel with lifting capacity of 400 tonne and cutting edge technology. It has capability to jack up in 45 metres of water. Before any works inside the pile were possible a steel deck plate needed to be removed to allow access to the Monopile

 

Dredging

The initial phase of works was to dredge material from inside the pile to approximately 1 metre below the agreed cut line. This was achieved by using a clamshell excavator bucket and dredger pump. After successfully removing all hard objects the dredger pump with UHP jet injection assistance was employed to empty sand and other fine material from within the pile.

Several cubic metres of sand were removed. This process was very effective and the pump was repositioned several times around the pile until the depth of dredge

Dredger pump in full flow

was achieved. After dipping, a strange profile was discovered with depth variations of 200-600mm. Initially it was thought the pump was digging holes in a hard sub base; however when the clam shell was used to investigte this anomaly, steel coupons were found to be causing the problem. These were then removed allowing the dredging process to be completed. 

 

 

Proving

Upon completion of the dredging, the custom made proving rig was deployed. This equipment was designed to confirm the pile had no internal obstructions. The proving rig was lowered 4 times in total, each time to a depth of 25 metres. The proving rig was fitted with a rotating High Pressure Water Jet Nozzle to enable the cleaning of the internal bore simultaneously, whilst proving the internal diameter. Once this process was complete the pile was ready for rigging with lifting equipment.

 

Rigging

The rigging of the pile was undertaken by DBB, each lifting strop was rated to 125,000kg. Total weight of the rigging was 1.5 tonnes.

Rigging being connected to the pile

Proving rig complete with HP Water Jet

Recovered steel coupons

 

 

 

 

 

 

Cutting

The RGL cutting rig was lowered into the pile and a test cut close to the top of the pile was made. After successfully completing this cut, preparations for the main cut at depth were made. As the cutting rig was lowered into the pile all service hoses and lines were clamped together. The JackUp WIND SERVER itself was raised 2.5 metres to bring it level with the pile to give safe access for the installation process. Water depth was approximately 14 metres.

 

Test cut

Cut pile lifted out of the water

Cut pile on the barge deck

 

 

 

 

 

 

 

 

 

The cut line was set to 24.2 metres from the top of the pile. It took 8 hours to complete. This involved working well into the night before the successfully cut pile was lifted clear of the water. The 44 tonnes of pile was placed onto the deck of the WIND SERVER where it was possible to see the near perfect cut achieved by RGL through the 45mm wall. The previous diamond wire sawing attempts were also visible; this method had originally been attempted, but had proven unsuccessful.

 

Completed cuts using RGL’s pile cutting rig and UHP water jetting

45mm thick wall

Previously undertaken failed diamond wire attempts visible

 

 

 

 

 

 

 

 

 

 

Jeremy Twigg Commercial Director of RGL said: “We are extremely happy with the success of this project. The tooling used was custom made and it is very pleasing when new equipment works so well”.

 

While Anders Thorvald Nielsen, Operations Manager for DBB commented: “DBB was extremely satisfied with the performance, preparation and result of the RGL operation. RGL crew was well prepared and very pleasant to work with. A job well done”.

 

 

In summary, we will do our utmost to deliver complete satisfaction to you by:

  • responding quickly to your enquiries
  • working with you to fully understand your requirements
  • providing you with the benefit of our experience
  • preparing detailed site safety, quality and environmental
    plans, including method statements and risk assessments
  • providing competitive quotations
  • delivering on what we promise for project start and
    duration
  • deploying experienced crews and proven, correctly
    maintained equipment
  • working within site safety rules and regulations
  • managing the project environment including waste water
    treatment and filtration
  • managing each project through to completion
  • COSHH Assessment and measures to avoid adverse
    environmental impact
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