The need

The application area for the apparatus usually involves the lowering of heavy assemblies, such as subsea control modules, to the seabed and to locate them on existing structures. This requires considerable positioning accuracy as well as load handling.

Lowering of the component from a surface vessel is usually carried out in conjunction with a subsea Remote Operated Vehicle (ROV). The surface vessel will be subject to pitch, yaw and heave depending on the state of the sea. A previous patent by Webster and Tarr (GB2410757 – published 2005-08-10) addressed this problem with the equipment shown in Fig 1.

At this stage the component assembly (1) is ready for installation. The deployment stack includes of a hoist carriage (2) and two guide wire ropes (3) attached to an extendable parallelogram linkage (4). The whole stack is supported by a wire rope/cable (5) attached to a winch at the stern of the surface vessel.

The guide wires are attached to a fixed subsea base such as a location point for a pipe ‘tree’. The support cable (5) is tensioned so that the parallelogram linkage is at the middle of its dynamic range. Additional linkages may be required to accommodate more heave.

The inventor says that the new improvements are necessary because the above system may be unstable sometimes with low frequency oscillation, especially in the vertical. Causes include elasticity of the deployment lines and inadequate damping from fiction in the water.

The new invention is intended to provide apparatus and methods for stability in the water and to restrict the amplitude of surface vessel heave, thus eliminating the need for many parallelogram linkage assemblies.

The requirement is for a means of damping that resists movement of the carriage through the water when in use and that also has substantially neutral buoyancy. The buoyancy may (preferably) be inflatable and/or can be filled with water before or during use, and comprise of several units.

The compensation linkage can be installed between the means of damping and the base and/or between the damping unit and the hoist carriage.

How it works

Fig 2 shows the deployment system described in this invention, whilst Fig 3 shows an alternative embodiment. Fig 2 shows that a means of damping has been added between the parallelogram linkage and the carriage (2). This is a toroidal shape bag filled with water and attached to a lifting cable (8) passing through a hole in the centre of the bag. Several wires (9 and 10) provide the necessary attachment, being arranged symmetrically about the bag at its top and bottom.

In the diagram there are four wires (9) from the top of the bag and four more attached to the bottom. This arrangement provides horizontal stability of the bag to the lifting cable. Additional bags can be included in series with the first.

A typical method of use is to lower the deployment stack (not already attached to the subsea base – 6) together with the component for installation, an empty damping bag and parallelogram linkage into the water from the stern of a vessel.

The mass of the bag is relatively low at this stage. It is then filled with water using a valve (11), air being expelled through valve 12. This creates a rigid ring of substantially neutral buoyancy.

The whole assembly is then lowered further to the seabed and the guide wires attached to the subsea ‘anchor’ base using an ROV. The guide wires are then tensioned by the lifting cable, setting the parallelogram linkage in the middle of its dynamic range.

Any motion of the lifting cable caused by vessel heave is then damped, vertically and horizontally, by the friction of the water on the large surface area of the bag. The dimensions are chosen so that there is sufficient friction to provide the required damping and to eliminate system instability.

Fig 3 shows how, by fitting at least one damping bag to the cable above the linkage device, the vessel heave effect can be averaged out to reduce the amplitude of dynamic range required for the parallelogram linkage. This can also reduce the need for multiple linkages in the event of heavy seas.

It is also possible to eliminate the linkage device entirely. In the example of Fig 3 two additional bags (13 and 14) have been included on the lifting cable between the vessel and linkage at the lower end of the cable. The resistance to movement in the water of the additional bags averages the amplitude of vessel heave, substantially reducing the peak amplitude transmitted to the linkage.

The elasticity of the lifting cable may accommodate the difference between the peak amplitude of the vessel heave and the amplitude transmitted to the linkage. Factors that must be taken into consideration in determining the dynamic range of the linkage assembly (single or multiple) include the elasticity of the lifting cable material, the cable diameter, its length, the depth of sea, and the size and number of damping bags.

At substantial depths (say, 2km or more) the elasticity and length of the lifting cable can be sufficient to accommodate all of the heave so that no parallelogram linkage may be required provided the size of each bag is sufficient to provide enough damping to achieve system stability.

Alternatives also covered within the apparatus that will not affect the basis of the invention may include:

– Inflatable bags substituted by any items of neutral buoyancy including stronger, solid items to eliminate risk of rupturing;

– Alternative shapes of damping bag as long as it provides stable resistance to movement in the water;

– Increased number of damping elements deployed along the lifting cable;

– Alternative arrangements to the hoist carriage (2);

– Alternative heave compensation devices in place of the parallelogram linkage.

About the patent

This article is an edited version of GB patent 2426264, also published as a World patent publishing no. WO 2006/123086 A1. The inventor is David Webster of Monmouth, Wales. The applicant/proprietor is Vetco Gray Controls, Ltd., of Bristol, England, a subsidiary of Vetco Gray headquartered in Houston, US. Vetco Gray Controls is a leading supplier of sub-sea production control systems and Vetco Gray supplies drilling, completion and production equipment for on- and offshore oil and gas fields worldwide. It employs 5000 people in over 30 countries.

Early last month (January) a private equity syndicate of Candover, 3i and JP Morgan Partners announced the sale of Vetco Gray to GE’s Oil & Gas Division, the completion of which is expected this month.

Disclaimer

This article is an edited version of the patent and may omit legally or technically important text. To see the full patent go to www.hoistmagazine.com/patents