Apparatus for and method of installing subsea components

Abstract

An apparatus for installing subsea components includes the use of a motion compensator to counterbalance and offset load on the component and lowering wire caused by movement of the vessel as a result of waves and wind.

Description

BACKGROUND OF THE INVENTION[0001] To install heavy subsea components into the seafloor, a floating installation vessel is typically employed. Since the components lowered vary in weight from several tons to 200 tons or more, installation vessels such as a barge commonly utilize a crane or a derreck. Other vessels have a semi-submersible hull where the component may be lowered through an aperture in the hull ("moonpool") into the water and down to the seabed. To perform such operation with this vessel, a crane or derreck is used depending on the component. For example, a crane will lower a wet block or steel wire while the derreck lowers the drill string.[0002] More recently, Anchor Handling Tug and Supply ("AHTS") vessels are used to install heavy subsea equipment. These vessels have a relatively small ship-like hull, and often do not use a crane but rather use winches to lower components into the sea. The AHTS vessel has less capacity in load, storage and accommodation than a typic...

AI Technical Summary

Benefits of technology

[0013] The motion compensator acts to reduce the effect of heave on the component as it lowered to the seafloor and thus, reduces the potential for the component to bounce up and down on the floor. The spreader beam separates the lowering wire and assists in controlling component orientation. Use of the A-frame avoids overboarding the component as it is deployed into the water.

Problems solved by technology

While derreck and crane barges are frequently used all around the world to install heavy components, use of these vessels is typically much more expensive than the AHTS-type vessels or work boats.

Indeed, in recent years the cost of using a derreck and crane barges may be as much as 3 to 10 times more expensive per day in comparison to the relatively smaller AHTS vessel.

Hence, the major drawback to using larger vessels to install the same components is cost.

Notwithstanding, while the AHTS vessel is more cost efficient, there are problems associated with lowering subsea components from the floating platform of this vessel that crane and / or derreck barges do not suffer.

On the other hand, both derricks and AHTS vessels require relatively complicated procedures in order to ensure the component clears the vessel.

Another problem is the relative motion between the component and the sea bed prior to touch down.

Heave in particular can cause damage to the component and other structures around it, if it is moving up and down uncontrollably.

A further problem is the twisting of the component and torque on the lowering wire prior to touch down.

Excessive twisting may also cause damage to the lowering lines.

Steel wires typically have tension-induced torque characteristics, which can increase the chance of twisting.

Unfortunately, even in light of the novel and useful nature of these methods, certain problems still persist.

For example, for deep subsea installation, direction control remains a problem when installing components through the use of the method and apparatus described in U.S. Pat. No. 6,009,825.

An example of torque balanced wire is multistrand or multilayer strand, difficult to handle, fragile and expensive.

Overboarding remains a problem when only winches are used as shown in U.S. Pat. No. 6,122,847.

Moreover, control over the torque or twist of the component is missing unless a second vessel is used.

Furthermore, as taught in U.S. patent application Ser. No. 09 / 627,873 while installation of components using a single vessel is preferred, the method described lacks the ability to control the torque on the component and is not suitable for deep sea installations particularly where the environment is harsh and / or the relative motion between the component and the sea floor maybe uncontrollable.

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