Subsea excavation provides options in deepwater

The cost of rig time, particularly in deepwater, increases the importance on operators to anticipate subsea excavation requirements and prepare adequate contingencies. Recognizing the need for cost effective subsea excavation support to the increasing number of deepwater operations in the Gulf of Mexico, Underwater Excavation Limited (PSL Group), has joined forces with Louisiana-based OSCA (Great Lakes Corporation) to form OSCA Ocean Technologies. The new joint venture company will be locate

Sep 1st, 1998

The cost of rig time, particularly in deepwater, increases the importance on operators to anticipate subsea excavation requirements and prepare adequate contingencies. Recognizing the need for cost effective subsea excavation support to the increasing number of deepwater operations in the Gulf of Mexico, Underwater Excavation Limited (PSL Group), has joined forces with Louisiana-based OSCA (Great Lakes Corporation) to form OSCA Ocean Technologies. The new joint venture company will be located in New Orleans, Lafayette, and Houston to provide subsea excavation, fluid pumping, and pipeline services to the Gulf of Mexico and deepwater operators.

After testing in the North Sea, Underwater Excavation's (UEL) subsea excavation technique was recently used on a work program for Shell Deepwater Development. The JetProp mass flow excavator uses a hydrodynamic principle to move soils in depths as great as 4,000 feet, including the following tasks:

  • Drill cutting dispersal from around wellheads, templates and platform structures
  • Pipeline construction support, including pipeline lowering, deburial and freespan correction
  • Pipeline route pre paration such as presweeping
  • Seabed preparation for templates and installation jacket footprints
  • Rock dump removal up to 14 in. depths.
The tool comes in three sizes: a 25 Series JetProp with a throughput of 25,000 cu meters of seawater; a 75 Series JetProp with a throughput of 75,000 cu meters of seawater; and a 250 Series with a throughput of 250,000 cu meters of seawater.

Seawater, at up to 3,000 psi, is pumped through drill pipe or flexible high pressure hose to the tool. The high pressure water passes through the propeller hub and through the center of the propeller blades to rearward facing jet nozzles mounted on the propeller tips. The reactive force created by the water exiting the jets cause the propeller to rotate at up to 350 rpm. This draws water from above the JetProp, through the main casing, accelerating it to create a downward moving column of water.

The principle of the design, powering the propeller from the tips, results in a tool that has virtually no rotational reaction torque on the main casing. Therefore the tool can, if required, be deployed from a crane wire and also provides a stable platform during operational conditions for mounting additional control and survey equipment.

The progression of excavation is monitored on a real-time basis with sonar equipment mounted on the side of the tool. The data obtained provides a profile of the ongoing excavation and can also be used pre- and post-excavation to produce a 3D terrain model of the site and surrounding area. Monitoring the sonar data throughout the excavation process ensures accurate excavation control.

The JetProp creates a low velocity column of water traveling at 15-20 ft/sec to excavate material below it. The water column fluidizes and carries away non-cohesive and unconsolidated soil materials such as cement, sand, mud and rock dump. The excavation technique is strictly non-contact with typical stand off distances ranging between 20-50 ft.

The JetProp can be deployed and operated at any depth depending on the drill pipe length. For shallow water applications in depths of up to 1,000 ft, the tool can be deployed from crane wire utilizing high pressure flexible hose to carry the HP water power supply. The excavation rate will depend on the consolidation of the target material, the height of the unit above the seabed, the speed of the propeller, and the motion of the support vessel. A conservative rate of material fluidisation is 3% of the water column throughput.

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