System and method for thruster protection during transport

A system and method protect a thruster assembly disposed with a pontoon of a semi-submersible drilling vessel during dry tow transport. A cover may be positioned over the thruster assembly and secured with the pontoon. A thruster cover support structure may be disposed with the pontoon. The thruster cover support structure may be a frame positioned around the pontoon for removable attachment with the thruster cover. The thruster cover support structure may be a bracket, ring or flange fixedly attached with the pontoon. The thruster cover may be removably disposed with the bracket, ring or flange, such as by bolting or welding. A spacer barge may be positioned below the pontoon between thruster assemblies for lifting the semi-submersible drilling vessel before placement of the spacer barge and semi-submersible vessel on the dry tow transporting vessel. The thruster assemblies may be elevated and in some embodiments positioned directly over the dry tow transporting vessel.

REFERENCE TO MICROFICHE APPENDIX

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the transportation of semi-submersible drilling vessels, in particular those having fixed dynamic positioning thrusters.

2. Description of Related Art

Semi-submersible drilling vessels for conducting offshore drilling operations utilize buoyant pontoons, also known as lower hulls or floaters, which support a plurality of vertically extending columns or caissons, the upper portions of which carry a working platform. A plurality of thruster assemblies may be secured to the bottom of the pontoons for dynamically positioning the vessel at a fixed location in deep offshore waters, such as over a wellbore. Thruster assemblies are available from, for example, Wärtsilä Corporation of Helsinki, Finland and Rolls-Royce of London, England. A typical thruster assembly may cost $1.5 to $2 million, and eight (8) thruster assemblies may be attached to the pontoons of a single semi-submersible rig.

Semi-submersible rigs are typically transported in the ocean on self-propelled Heavy Transport Vessels (HTV), during what is known as a “dry tow,” since he rig is on the deck of the HTV and out of the water. While the rig is positioned on the HTV, the thrusters are typically suspended over the side of the HTV and may be exposed to the slamming and dragging forces of the waves during transport. A wave slamming force may be caused by a wave hitting the surface of the thruster and causing an impact load. A wave dragging force may be caused by dragging the thruster in the water causing a relatively steady force on the thruster.

Rig owners and/or oil and gas operators do not want to risk damage to the thrusters during transport since the damage may render the rig unusable until the thrusters are repaired. The repairs usually take a significant amount of time. Semi-submersible rigs may rent for $500,000 per day, so the loss of operational time is costly. A replacement thruster may not be available for six (6) months. Suppliers may deny warranty claims due to the excessive forces and subsequent damage experienced during transport.

One proposed solution is to remove the thrusters and reinstall them after transport. However, this solution is costly since it may take at least fourteen (14) days of critical time to remove and reinstall the thrusters, resulting in the loss of millions of dollars in wasted rig down time. In addition, the thruster supplier usually charges significant additional fees for the removal and reinstallation of the thrusters.

Another proposed solution is to tow the semi-submersible vessel in the water with one or more tug boats, known as a “wet tow.” The thrusters remain below sea level during a wet tow. However, this solution takes significantly more time than a dry tow because the semi-submersible rig has to be pulled at about one-half the speed used during a dry tow. Often the semi-submersible rig must be transported from one part of the world to another, so the loss in time and money is significant. The semi-submersible day rate is high compared to the HTV day rate. Moreover, during a wet tow, a thruster may be damaged if it impacts some obstruction below sea level.

Some semi-submersible rigs have thruster assemblies that are retractable. However, such rigs and retractable assemblies are expensive.

It would be desirable to protect the thruster assemblies during the transport of a semi-submersible vessel in a dry tow without removing the thruster assemblies.

BRIEF SUMMARY OF THE INVENTION

A system and method are provided for protecting a thruster assembly attached to a pontoon of a semi-submersible drilling vessel during dry tow transport. In one embodiment, a thruster cover support structure may be disposed with the pontoon of the semi-submersible vessel. The thruster cover support structure may be a frame positioned around the pontoon and made from any combination of steel tubulars, ropes, wires, chains, or other materials. A thruster cover may be disposed over the thruster assembly and removably secured with the frame. Alternatively, the thruster cover support structure may be a bracket, ring or flange attached with the pontoon. The thruster cover may be removably disposed with the bracket, ring or flange, such as by bolting or welding. In another embodiment, the thruster cover may be removably mounted directly to the pontoon, such as by welding.

In still another embodiment, a thruster cover may be disposed with the pontoon using tendons running through conduits positioned in the pontoons. A tendon attachment member may be positioned with the thruster cover if needed for attachment with one of the tendons. Alternatively, a thruster cover may be secured with the pontoon using a tendon disposed with the exterior of the thruster cover. In another alternative, a spacer barge may be positioned beneath the pontoons and between two thruster assemblies. The semi-submersible vessel may be supported and lifted with the spacer barge. The spacer barge with supported semi-submersible vessel may be positioned on the deck of the HTV. The thruster assemblies may be disposed above the elevation of the deck of the HTV. For some HTV designs, the semi-submersible vessel may be positioned so that the thruster assemblies are directly over the deck of the HTV.

The thruster cover may be a container having rigid solid or closed sides. In another embodiment, the thruster cover may have solid or closed sides that are flexible and not rigid, such as plastic. The thruster cover may also be a sheet or bag, such as made from a plastic. The thruster assembly enclosed in the thruster cover may be surrounded with a fluid for protection, such as water. In still other embodiments, the thruster cover may have partially solid or closed sides, such as a cage. The partially closed thruster cover may be rigid or flexible.

The novel system and method advantageously allow for the protection of the thruster assemblies during the transport of the semi-submersible vessel in a dry tow without removing the thruster assemblies.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1, a semi-submersible vessel2is disposed on the deck of an HTV6for dry towing. Two pontoons4used to float the semi-submersible vessel2when it is in the water extend over both sides of the HTV6. InFIG. 2, four thruster assemblies8attached with bottom surfaces10of the pontoons4extend over the side of the HTV6and may be exposed to wave forces during transport. InFIG. 3, two thruster assemblies8disposed with the bottom surface10of pontoon4are exposed to wave forces during transport. InFIG. 4, thruster assembly8is disposed with pontoon bottom surface10.

Thruster Cover

A thruster cover or container may be used to protect thruster assemblies8attached to pontoons4during transport. InFIGS. 5A-5Ea rigid solid or closed thruster cover2is shown for positioning over a thruster assembly8(not shown). Other shapes, sizes, dimensions, and designs of thruster covers are contemplated. The thruster cover12may be made of a durable material such as steel, although other materials are contemplated. InFIG. 5F, the thruster cover12is attached to the bottom surface10of pontoon4by welding. However, as will be discussed in detail below, other attachment means are contemplated. InFIGS. 5G-5H, two thruster covers12are attached to the pontoon4.

InFIG. 6, a rigid solid or closed thruster cover14is disposed over thruster assembly8. The thruster cover14may be made of a durable material such as steel, although other materials are contemplated, including plastic. The forces of the waves may be induced into the cover14and transferred to the pontoon4. InFIGS. 7-7A, a flexible solid or closed thruster cover16is disposed over thruster assembly8. The thruster cover16may be made of thin plastic, such as a bag or sheet, although other materials are contemplated. The cover16may contain a fluid such as water to surround the thruster assembly8. Standard water bags are available from, for example, Water Weights Ltd. of Aberdeen, UK. A wide range of sizes of water weights proof load bags are available. The bags are suited for inaccessible lifting points. The wave slamming forces may be absorbed by the cover16and enclosed fluid.

InFIG. 8, a rigid partially closed thruster cover18, such as a cage, is disposed over thruster assembly8. The thruster cover18may be made of a durable material such as steel, although other materials are contemplated. The forces of the waves will be dispersed, lowering the forces on the cover18and the pontoon4. InFIG. 9, a rigid partially closed thruster cover20, such as a cage, is disposed over thruster assembly8. The thruster cover20may be made of plastic, although other synthetic materials are contemplated. The forces of the waves will be dispersed, lowering the forces on the cover and the pontoon. Also, deformation of the cover20may result in lower forces transferred to the pontoon4due to elastic deformation. Other types, shapes, and sizes of thruster covers are contemplated. As can now be understood, partially closed thruster covers may also be partially open thruster covers.

InFIG. 9A, two thruster assemblies (8,8A) are disposed with a pontoon4, with one thruster assembly8A uncovered and one thruster assembly8covered with a closed and rigid thruster cover14. InFIG. 9B, two thruster assemblies8are disposed with a pontoon4, with one thruster assembly8covered with a closed and flexible thruster cover16and one thruster assembly8covered with a partially closed thruster cover18. It is contemplated that any thruster cover shown in any of the Figures may be used in combination with a different thruster cover on a pontoon4.

Any thruster cover shown in any of the Figures may be used with any embodiment shown in any of the Figures. Any of the thruster covers are containers for protecting or enclosing the thruster assembly. As can now be understood, the thruster cover may absorb, deflect and/or break the slamming and/or dragging wave forces or possible obstacles in the ocean. Depending on the type and size of the force as compared with the strength of the thruster8, the thruster cover may be fully closed, partially closed, stiff, or flexible. For example, for slamming forces only, the embodiment inFIG. 7may be sufficient. For dragging forces, a closed stiff solution may be desirable, like the embodiment inFIG. 6, or a partially open solution may be desirable, like the embodiments inFIGS. 8-9, to break the waves.

The thruster covers may be hoisted into place over the thruster assemblies using Strand Jacks, which are available from, for example, John Gibson Group, Ltd. of Middlesbrough, UK. It is also contemplated that the thruster covers may be floated below the thruster assemblies prior to covering the thruster assemblies. If two thruster assemblies are disposed adjacent each other, it is contemplated that the two thruster covers may be attached with each other prior to placement in the water for floating.

Securing the Thruster Cover

As shown inFIG. 5F, the thruster cover may be attached directly with the bottom surface10of the pontoon4, such as by welding. However, welding to the pontoon4may not be acceptable to the rig owner or the operator. InFIGS. 10-11, thruster cover support structure28is disposed with pontoon4to provide a structure to support thruster covers26shown disposed over thruster assemblies8. It is contemplated that support structure28may be wrapped around the pontoon4, and not fixedly attached with pontoon4. It is also alternatively contemplated that the support structure28may be fixedly attached with pontoon4. Thruster covers26may be removably attached with support structure28. Thruster cover26may be any of the thruster cover embodiments shown in any of the Figures. The support structure28shown is a frame, which may be made from steel, such as bars or tubulars. However, other materials, shapes and sizes are contemplated. It is contemplated that support structure28may be made from rope, chain, or wires, or from any combination thereof. It is contemplated that support structure28may be a net.

InFIGS. 12-13, one embodiment of attachment of the support frame28is shown using chain34tension and pad (30,32) compression. Other attachment means are contemplated. The pads (30,32) are positioned between the support structure28and the pontoon4and may be adjustable. InFIG. 14, vertical stanchions36disposed in support structure28may be adjusted, such as longitudinally. The pads (30,32) may be positioned along the length of the stanchions36. Turning toFIGS. 15-15A, thruster cover support frame28with thruster covers26and pads (30,32) attached may be floated as a single unit prior to installation. Other attachment methods are contemplated. As shown inFIGS. 15-15A, it is contemplated that support structure28may be installed without lifting.FIG. 16shows two thruster cover support structures28disposed with two pontoons4of semi-submersible vessel2during transport on a HTV6. The four thruster covers26protect the covered thruster assemblies8.

The embodiments shown inFIGS. 12-16advantageously allow for thruster cover attachment without any contact with the thruster assembly8during the installation and removal of the cover26and during the transport. Attachment to the hull or pontoon4may be made without welding. It is contemplated that dual redundant connection may be made. The wave loading may be transferred to the pontoon4at the support structure28or bulkheads. It is contemplated that installation may be achieved with tug boat assistance only. The embodiment advantageously allows for simultaneous installation of all thruster covers26with a pontoon4to shorten installation and removal time. It is contemplated that the support structure28and covers26may be self-floating. It is contemplated that no lifting for installation from the water may be required.

Turning toFIGS. 17-17B, thruster cover support structure or member38is attached with the bottom surface10of pontoon4. Thruster cover40is positioned over a thruster assembly (not shown) and disposed with support structure38. Support structure38may be welded to pontoon4, although other attachment means are contemplated. Support structure38is shown as a bracket. However, other types, shapes, dimensions, and sizes of support structures are contemplated for the purpose of supporting thruster cover40. Thruster cover40may be removably attached with support structure38, such as by welding or bolting. It is also contemplated that one or more tendons (42,42A) may be used to lift cover40and/or hold cover40in position over a thruster assembly.

InFIGS. 18-18B, two thruster cover support structures or members44are attached with the bottom surface10of pontoon4. Two thruster covers46are positioned over thruster assemblies8and disposed with support structures44. Support structures44may be welded to pontoon4, although other attachment means are contemplated. Support structures44are shown as rings or flanges. However, other types, shapes, dimensions, and sizes of support structures are contemplated for the purpose of supporting thruster covers46. Thruster covers46may be removably attached with support structures44, such as by welding or bolting. It is contemplated that support structure44may be either internal or external to the hull or pontoon4. It is contemplated that any of the thruster cover support structures shown in any of the Figures, including brackets, rings, flanges, and frames, may have lifting lugs integrated with them for use in lifting the thruster covers.

Turning toFIG. 19, a thruster cover48is held with tendons52over thruster assembly8against pontoon4. Tendons52may be wires, chains, ropes, or some other device for withstanding tension forces. There are typically three openings54in the pontoon4bottom surface10spaced around the thruster assembly8. There are also typically three conduits or tubes50in the pontoon4extending from the top surface openings56of the pontoon4to the bottom surface10for installation or removal of the thruster assembly8. It is contemplated that tendons52may be run through the conduits or tubes50for attachment with the cover48. The tendons52may be tensioned by a device on or in the pontoon4or the HTV6or the semi-submersible vessel2. Advantageously, no welding to the pontoon4is required.

InFIG. 20A, a thruster assembly8is shown in the position in which it is installed with and removed from the pontoon4. Three tendons52may be attached with the thruster assembly8during such operations. As shown inFIG. 20B, during dry tow transport, the thruster assembly8may be rotated 180° about a vertical axis from the position shown inFIG. 20Ato provide for tendon52clearance for attachment with the thruster cover (not shown). InFIG. 20C, the thruster covers58are disposed against the pontoon4with tension from the tendons52pulled through tubes50. It is also contemplated that there may be no conduits or tubes50, but just openings in the top surface60and the bottom surface10of the pontoon4for the tendons52. It is also contemplated that there may be no openings in the top surface60of the pontoon4, and that the tendons52may be tensioned within the pontoon4. For installation, the thruster covers58may be floated below the thruster assemblies8, then hoisted up while being guided with the tendons52. Alternatively, it is contemplated that the thruster covers58may be pulled up with the tendons52. Other attachment methods are contemplated.

InFIG. 21A, tendon52A is disposed with a tendon attachment member64, and two tendons52are disposed with the thruster cover62. The tendon attachment member64is disposed with the thruster cover62, such as by welding. However, other attachment means are contemplated. The attachment member64may be a rod or tubular, such as made from steel. However, other types and materials are contemplated. As can now be understood, the tendon attachment member64may provide a support point for tendon52A should clearance between the thruster assembly8and the thruster cover62be limited. It is contemplated that more than one tendon attachment member64may be used. It is also contemplated that the thruster covers may be of sufficient size to insure there is no clearance issue with tendons (52,52A).

InFIG. 21B, two thruster covers62are disposed with the pontoon4with the tendons (52,52A). The two tendons52A are attached with two tendon attachment members64positioned in the covers62. It is also contemplated that only two tubes50with two tendons (52,52A) may be used for each cover62, rather than three tubes. In such embodiment, it is contemplated that the two covers62may be attached together for stability, such as with the cover attachment members66. The cover attachment members66may be steel rods or tubulars, although other types and materials are contemplated. The use of only two conduits or tubes50for installation of each cover62minimizes any clearance issue with the tendons52. It is contemplated that here one tendon (52,52A) in a conduit50is shown, there may be more than one tendon in a single conduit50. InFIG. 22, the thruster cover62is shown in the position during installation when there is a tendon attachment member64.

In one embodiment of a method of loading the semi-submersible rig2on the deck of the HTV6, a cribbing may be installed on the deck and the rig2floated on it. The rig2will de-ballast and the thrusters8will surface. Each of the thrusters8may have three (3) conduits or tubes50for installation and removal purposes. Flanges or rings44for bolts may be fixedly attached with the pontoon4around the thrusters8. Protective paint may be applied. Two of the thruster covers may be attached together for ease of installation. The thruster covers may be moved into the water and floated underneath the thrusters8. The thrusters8may be rotated to create clearance between the thruster covers and the thruster assemblies8. The tendons52may be attached with the thruster covers. The covers may be hoisted into position with Stand Jacks and guided with the tendons52. Alternatively, or in addition, the covers may be pulled up with the tendons52. The tendons52may also be used to control the position of the thruster covers during installation. The covers may be bolted or otherwise removably attached with the flanges or rings44. Installation may be simultaneous on several corners with several thruster covers and with sea fastening.

Turning toFIG. 23, two thruster cover guides70are disposed with the thruster cover68. The tendons52are disposed through the pontoon4and positioned with the guides70. The guides70may be a tubular, such as made from steel. However, other types and materials are contemplated. The guides70may be fixedly attached with the cover68. It is also contemplated that the tendons52may extend through the guides70and attach with the cover68. The tendons52may be used to secure the cover68against the pontoon4around a thruster assembly (not shown) for transport. It is contemplated that there may be one or more guides70.

InFIG. 24, two thruster cover guides70A are disposed with the thruster cover68A. The tendons52B are disposed through the pontoon4and extend through the guides70A and attach with the cover68A. The guides70A may be a tubular, such as made from a durable material such as steel. However, other types and materials are contemplated. The guides70A may be fixedly attached with the cover68A. The tendons52B may be used to secure the cover68A up against the pontoon4around a thruster assembly (not shown) for transport. It is contemplated that there may be one or more guides70A.

InFIGS. 25-25A, two thruster covers68A are disposed together, such as for placement over thruster assemblies8(one shown on right inFIG. 25) positioned near each other. Other shapes, sizes, dimensions, and designs of thruster covers are contemplated. Other types of attachment means between the two thruster covers68A are contemplated. The guides70A are positioned with the covers68A. InFIG. 25B, the thruster cover guide70A is disposed with the thruster cover68A, with one end of the guide70A disposed in the pontoon4. As can now be understood, the guides70A may be used to accurately position the cover68A with the pontoon4. InFIG. 25C, a plurality of tendons52B are in thruster cover guide70A.

Turning toFIG. 26, two thruster covers68B are disposed together, such as for placement over the thruster assemblies8(not shown) positioned near each other. The thruster covers68B may be made of a durable material such as steel, although other materials are contemplated. Other shapes, sizes, dimensions, and designs of the thruster covers are contemplated. Other types of attachment means between the two thruster covers68B are contemplated. InFIGS. 26A-26D, the thruster cover guides70B are disposed with the thruster covers68B. InFIGS. 27-27Cand28, the two thruster covers68B are positioned with the pontoon4using the guides70B.

Elevating the Thruster Assembly

The higher the thruster is above the waterline, the lower the exposure to wave forces and other obstacles in the ocean. A cribbing wood (one foot or 30.5 cm in height) is typically positioned between the HTV6deck and the bottom surface10of the pontoon4with the thruster assemblies8hanging over the side of the HTV6. However, using grilling instead, the distance from the thrusters to the water surface can be lengthened. For example, a grillage of two (2) meter height instead of the cribbing of one foot height may have a significant effect on thruster exposure to wave forces. However, having the semi-submersible vessel2elevated above the HTV6deck may provide challenges to stability as well as strength. Using a spacer barge beneath the semi-submersible vessel2solves the stability and strength problems as well as decreasing the draft of the semi-submersible vessel2during loading, which may sometimes be a problem.

Turning toFIGS. 29-29B, a spacer barge74is shown that has not been modified for use in elevating a semi-submersible vessel2.FIGS. 30-30Bshow a spacer barge76that has been modified for use in elevating a semi-submersible vessel2. Modifications may include the positioning of timber or similar material on the barge deck. Casing may be added. Cylindrical marine fenders may be added. Stoppers may be added. Other modifications are contemplated.

The spacer barge76or grillage may be positioned underneath a pontoon4between thruster assemblies8. The barge76will lift the rig2to a lower draft. The barge76will position the thrusters8higher above the waves. The barge76will also allow for rotation of the rig2so as to position the thrusters8directly over the deck of the HTV6if the design of the HTV allows. It is contemplated that the barge76or grillage may be at least the height of the protrusion of the thrusters8underneath the rig2. It is contemplated that the barge may allow at least part of the thruster assembly to be at a higher elevation than the HTV deck.

FIGS. 31-31Ashow one HTV78for use with spacer barge76.FIGS. 32-37illustrate one embodiment of a method for use of the spacer barge76with HTV78. Other steps or methods are contemplated. InFIG. 32, tendons82are attached with the semi-submersible vessel or rig80and the spacer barge76. InFIG. 33, the spacer barge76is positioned with the tendons82below the rig80. InFIG. 34, the buoyancy of the spacer barge76lifts the rig80. InFIG. 35, the HTV78is positioned adjacent the spacer barge76that is supporting the drilling rig80. InFIG. 36, the spacer barge76with rig80is positioned over the deck of the HTV78. InFIG. 37, the spacer barge76with the rig80is positioned on the deck of the HTV78with the thrusters8above the surface of the water.

As can now be understood, the spacer barge76may allow for dry transport of the rig80without the thrusters8hanging over the side of the HTV. The spacer barge76also allows for the thrusters8to be elevated higher than the HTV deck.FIGS. 37A-37Cshow section views. InFIG. 37B, the thrusters8are directly above the HTV78deck. InFIG. 38, rig tie down locations are shown.FIGS. 39A-39Care detail views of the tie downs. InFIG. 39A, the thruster8is over the HTV78.

InFIG. 40, the spacer barge76with the semi-submersible rig80is positioned on the deck of a HTV84. The HTV84is a different design than the HTV78inFIGS. 31-37. Returning toFIG. 40, the thrusters8are above the surface of the HTV84deck. InFIG. 41B, the thrusters8are lifted in elevation over the HTV84deck, but the thrusters8are not directly over the HTV84deck. The width of the HTV84deck inFIG. 41Bis less than the width of HTV78deck inFIG. 37B. InFIG. 42, the rig80is disposed on the deck of HTV84.

As can now be understood, a spacer barge may be used to elevate the thrusters8on the deck of the HTV. It is contemplated that the thrusters8may be positioned at a higher elevation than the deck of the HTV. However, whether the thrusters may be positioned directly over the HTV may depend on the design of the HTV, such as the width of the HTV deck. It is contemplated for all embodiments that to save additional time the thruster covers may be installed over the thruster assemblies8while the pontoons4of the semi-submersible drilling vessel are still in the water.

The foregoing embodiments address the root causes of the problem. Some embodiments increase the distance between the thruster assemblies and the water surface, minimizing or eliminating the exposure to the damaging wave forces. Other embodiments allow for the absorption of the wave forces at least in part with a thruster cover and transfer of the forces to or through the pontoon, such as with a thruster cover support structure.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and system, and the construction and the method of operation may be made without departing from the spirit of the invention.