The present invention relates to equipment and a method for offshore overboarding of a structure for safe and efficient lifting and maneuvering of the structure over the deck of a surface vessel.
Conventional offshore installation methods are normally based on the use of tugger lines directly connecting the structure to winches or handled manually by on-deck personnel. A common challenge for such operations is to obtain adequate control of the movement of the structure and the rotation in particular when the vessel is influenced by wave loads. Especially in situations where the structure is large and there is limited deck space, the overboarding operation is critical due to the risk of clashing of structure into deck equipment. Consequently, the limiting sea state for such operations is often quite low.
A number of constant tension winches, typically five to eight winches, have previously been used to guide and overboard heavy structures. The winches have been arranged such that the structure is guided step by step by a system of winch wires connected between the structure and deck. As the winches at times will pull against each other, wires have ruptured as a result of lack of control due to the complexity of the system.
GB 2502379 A, which was not publically available at the priority date of the present application, shows overboarding a structure by means of a crane while keeping the object stationary with respect to the crane axis by means of telescopic arms extending between the structure and a rotatable ring on the pedestal of the crane. The arms are maintained in compression in order to reduce pendulum movements of the structure caused by ship movements due to the sea state. This system is complicated and cannot easily be decommissioned for use on other vessels. Besides, it cannot be used for unwieldy structures and structures stored outside the relatively short reach of the arms.
The present invention comprises a method and an apparatus as defined in the appended claims.
With the present invention, the challenges mentioned above are reduced to a large extent. The main reason for this is that the structure is connected to one or more sliders or “sleds” moving along a track or rail which is fixed to the vessel deck. The sled(s) slide along the rail. For large loads, two sleds will often be used, whereby one of the sleds will be connected to one line from each corner of the structure and will be close to the horizontal position of the structure's center of gravity. A second sled will be situated at the tail end and will guide the rotation of the structure. For smaller loads, one sled might be sufficient. The lines attached to the sleds can consist of wire and polyamide slings for shock absorption. The operation starts by lifting the structure off deck with the crane. Then, the crane can start to function and thereby pull the structure along the rail. The sled(s) function as a moving guidance system for the structure. This increases the control of the structure and thereby reduces the risk of undesired structure movements, rotations and excessive forces in any tugger lines. Consequently, the sea-state operability can be increased in some cases.
The guide rail, which is a key part of the invention, is designed in a specific manner dependent on the structure dimensions, crane properties and the deck layout. The main idea is to introduce a rail pattern which shape is fitted for the specific structure geometry. Although the rail design is not limited to a circular pattern, it may be beneficial since it eliminates the need for crane boom in/out when the crane centre corresponds with the centre of curvature of the rail. Straight rails and a combination of straight and curved rails are also included in the invention.
The present invention can be applied on a large variety of offshore construction lifts, such as, but not limited to templates, manifolds, spools, suction anchors regardless of weight and shape. Typically, large and irregularly shaped structures can be handled.
The guide rail is a more safe mechanically passive system, using a track rather than a number of winches to control horizontal movement of the lifted object during over-boarding.
For better understanding of the invention, it will be described in more detail with reference to the exemplifying embodiments shown in the appended drawings, wherein
In addition,
In
For this particular structure 1, the large size relative to available deck space creates a high demand on accuracy in the positioning of the structure during the over boarding operation. The guide rail system provides a passive positioning and rotation control during the overboard phase.
As the slewing continues, the sleds 3, 4 will eventually leave the rail 2 as the structure 1 comes clear of the deck 16. In this situation, the orientation of the structure is controlled by lines from tugger winches on deck. Subsequently, the structure is lowered into the water, with the sleds 3, 4 hanging in their lines 15. When the structure 1 has reached a suitable depth, the crane 13 is set to heave compensation, and tugger lines and sleds are disconnected from the structure 1 by an ROV and retrieved to the vessel.
In
For this particular structure 20, the large size relative to available deck space creates a high demand on accuracy in the positioning of the structure 20 during the overboarding operation. The guide rail system provides a passive positioning control during the overboard phase.
It will be understood that the invention is not limited to the exemplifying embodiment described above, but can be varied and modified by the skilled person within the scope of the following claims. For example, the track can take various forms, such as that of a railroad rail. In this case, the slider could take the form of a trolley straddling the head of the rail and having wheels or other low friction elements engaging under either side of the head. Furthermore, the track can easily be removed from the deck of the vessel after completion of the overboarding operation, e.g. for use on another or the same vessel on a later occasion. For ease of handling and storage, the track may be built in manageable sections, which are joined together in a suitable manner, e.g. with a pin and socket connection, during installation on the deck.
Number | Date | Country | Kind |
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20131373 | Oct 2013 | NO | national |
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Number | Date | Country |
---|---|---|
2502379 | Nov 2013 | GB |
WO 2010093251 | Aug 2010 | WO |
Entry |
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Office Action dated Sep. 20, 2014 in Norwegian patent application No. 20131373. |
Translation into English of Office Action dated Sep. 20, 2014 in Norwegian patent application No. 20131373. |
Norwegian search report in application No. 20131373, dated Sep. 20, 2014. |
The different phases of a subsea lift from an offshore construction vessel, presentasjon av Technip, Dec. 2, 2009, for relevant portions see Norwegian search report. |
Deck and Subsea operations, presentasjon av Subsea 7, Nov. 30, 2011, for relevant portions see Norwegian search report. |
Number | Date | Country | |
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20150104274 A1 | Apr 2015 | US |