Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >

Underwater Laser Cutting for Decommissioning Applications

Overview

The Energy Act 2008 requires that a large proportion of the North Sea infrastructure will need to be decommissioned in the next 30 years. Only 88 (12 percent) installations have been decommissioned in the UK continental shelf to date, reflecting the nascent nature of the decommissioning market. With more stringent environmental legislation related to global warming and the depletion of oil reserves due to rising demand, it is thought that decommissioning will become a priority for many companies. This will potentially provide substantial decommissioning opportunities for years to come.

There is currently a selection of different subsea cutting methods which could be effectively utilised for decommissioning offshore installations. Three main competing technologies for decommissioning subsea structures are abrasive water jet, diamond wire cutting and plasma arc cutting. However, to couple with varied geometries and thicknesses, a single tool with flexible functionality (ease of remote deployment, operation and maintenance), and capability to cut both from outside-in and inside-out, would be considered highly desirable.

Fibre delivered laser beam cutting has the potential to deliver these benefits by means of cutting safer, cheaper and faster. Moreover, fibre delivered, underwater laser cutting has the potential to develop a system that can cut installations at extreme water depths. As water depth increases by 10m, the hydrostatic pressure increases by approximately one bar.

Objectives

  • Develop a scientific understanding of the underwater laser cutting process and influencing parameters, up to a hydrostatic pressure of 20 bar on steel structures up to 50 mm in thickness
  • Advance an existing, state-of-the-art, underwater laser cutting technology with the capability of cutting 50 mm thickness mild steel at depths up to 200 m
  • Disseminate the project results and capabilities developed towards post project exploitation

Approach

To simulate offshore conditions, a first-of-its-kind high pressure vessel was designed and manufactured by TWI (see Figure 1). The vessel was integrated with the capacity to move the workpiece with 3-degrees of freedom, and dynamically balance the hydrostatic pressure up to 35 bar, representing a water depth of ~350 m. Moreover, the high pressure vessel provided process visual monitoring and recording capability. The underwater laser cutting system can be pressured to a desired level within minutes.

The cutting trials were carried out using a 10 kW Yb fibre laser system, with optical fibre and all associated services delivered through a prototype underwater laser cutting head, capable of withstanding the hydrostatic pressure conditions. Figure 1 shows an illustration of the experimental setup using a high-pressure vessel.

Horizontal underwater laser cutting trials were carried out in increasing hydrostatic pressure conditions of 0, 5, 10, 15 and 20 bar in the vessel. The goal was to cut at 8 bar relative compressed air pressure to that of the vessel, which varied from 13, 18, 23 and 28 bar respectively. A laser power of 10 kW was used to cut C–Mn steel workpieces from a thickness of 5-50 mm. The focus position was located 18 mm inside the material and the stand-off distance was maintained at 5 mm. All experiments were recorded using a Panasonic camera and the results were plotted.

Figure 1. Experiment setup
Figure 1. Experiment setup
Figure 2. Maximum underwater cut thicknesses as a function of set cutting speed for different laser power magnitudes at 1 atmospheric pressure conditions
Figure 2. Maximum underwater cut thicknesses as a function of set cutting speed for different laser power magnitudes at 1 atmospheric pressure conditions

Results

Figure 2 indicates the maximum underwater cut thicknesses in C-Mn steel as a function of set cutting speed for different laser power magnitudes at 1 atmospheric pressure conditions.

As expected, the maximum cut thickness is proportional to laser power and inversely proportional to cutting speed. A 50 mm thick C-Mn steel workpiece was cut using a laser power of 10 kW, a cutting gas pressure of 8 bar, a stand-off distance of 5 mm and a maximum cutting speed of 150 mm/min.

Figure 3 shows the maximum cut thickness with the corresponding cutting speed at different hydrostatic pressure conditions.

Here it can be seen that a 50 mm thickness C-Mn steel workpiece can be adequately cut with a maximum cutting speed of 200 mm/min at all hydrostatic pressures. Thereafter, the maximum thickness that could be cut decreased with the increased cutting speed for all hydrostatic pressure conditions. When using an 8 bar relative compressed air pressure to that of the vessel, the effect of the hydrostatic pressure condition of the vessel on the maximum cut thickness achieved is minimal. Overall, Figure 3 shows that underwater laser cutting is achievable, with excellent performance at a hydrostatic pressure of up to 20 bar, representing an extreme water depth of up to 200 m.

 

Conclusions

The following conclusions can be drawn from the underwater laser cutting trials in hyperbaric conditions:

  • The maximum cut thickness is proportional to laser power and inversely proportional to cutting speed
  • A 50 mm thick C-Mn steel workpiece was adequately cut with a maximum cutting speed of 200mm/min for hydrostatic pressure conditions of up to 20 bar, representing a water depth of ~200m
Figure 3. Maximum underwater cut thicknesses as a function of set cutting speed for various hydrostatic pressure conditions
Figure 3. Maximum underwater cut thicknesses as a function of set cutting speed for various hydrostatic pressure conditions
Media /

Related Insights

View all insights
}