The present invention relates to a device for suspending a tubular from a floating vessel. The present invention in particular relates to a device for reducing or eliminating resonance in a tubular when suspended from a hoisting arrangement in a derrick or from a deck of a floating unit/vessel.
Drilling in deep waters involves other and more complex challenges compared to drilling in shallow waters. This is also the case for risers to be used in such deep waters, which must withstand potentially higher tension loads and pressures etc. The vessel's heave motion may also coincide, or nearly coincide, with the riser string's natural frequency, resulting in even higher tension loads. This is a challenge in situations where the riser string has a free end, i.e., when the riser string is not connected the sea bottom. Such situations may occur when using the riser for hoisting or lowering of equipment, for example, a blow out preventer (BOP) from the surface to the sea bottom, or when the riser has been disconnected from a BOP or manifold. On drilling vessels, a critical load condition is hoisting/lowering of the BOP, or during other heavy lifts, using risers. No heave compensation systems are connected in this load condition, so that the heavy load (BOP) will try to follow the vessel's heave motion. The length of the risers with BOP depends on the water depth, but could exceed 3000 meters. The riser connection to the vessel is stiff and is either connected to the DDM (Derrick Drilling Machine) or on the spider or other hang-off plate on a deck (drill floor). Due to this hard hang-off system, the vertical naturally frequency of the riser string could meet the frequency of the heave motion on the floating vessel, potentially resulting in a considerable dynamic load amplification.
GB 2294713 A describes a deep water riser string that has a central tube, peripheral lines, and a base located at the lower end of the central tube. The central tube is fitted with means for retaining the peripheral lines in a position relative to the central tube. The lower ends of the peripheral lines are linked to a device arranged on the base, permitting a certain axial movement of at least one of the lines relative to the central tube. The string is fitted with damping means.
An aspect of the present invention is to overcome the shortcomings of the prior art and to obtain further advantages. More specifically, an aspect of the present invention is to provide a solution which renders possible deep water drilling without having to replace the existing riser string by another specific riser string to be used for a hoisting/lowering operation. Another aspect of the present invention is to provide a solution which is adaptable to be used on new vessels as well as for retrofitting existing vessels.
In an embodiment, the present invention provides a device for suspending a tubular from a floating vessel. The device includes a first element configured to carry the tubular, a second element configured to be supported by the floating vessel, and at least one compression element which forms a connection between the first element and the second element. The at least one compression element is configured to be pre-tensioned in an end stroke position so that, when the at least one compression element is subjected to a force from the tubular which is below a pre-defined threshold force, the connection formed by the at least one compression element between the first element and the second element is a rigid connection, and, when the at least one compression element is subject to a force from the tubular which is higher than the pre-defined threshold force, the connection formed by the at least one compression element between the first element and the second element is a compressible connection.
The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:
The present invention relates to a device for suspending a tubular from a floating vessel, the device having at least one compression element which is pre-tensioned in an end stroke position so that when subjected to a tubular force below a pre-defined threshold force, the at least one compression element forms a rigid hang-off for the tubular, and when subject to a force higher than the threshold force, the device forms a compressible hang-off for the tubular.
A device according to embodiments of the present invention may be suitable for a free-hanging riser string suspending heavy loads, whereby it is possible to minimize the possibility that the vertical natural frequency of the riser string meets the frequency of heave motion on the floating vessel. This reduces the load in the riser string and on the supporting structure. The device according to embodiments of the present invention can, for example, be suitable for use both on new vessels or can be installed on existing vessels, i.e., any floating installation. The device can be easily installed on existing floating installations, older vessels may thus be upgraded to permit drilling in deeper waters. The hoisting weight or stress level in the risers in new projects may also be reduced.
The device according to the present invention may thus be in a “passive mode” during normal operations, i.e., remain stiff without any resonance prevention effect, but in situations of hoisting or lowering heavy loads (such as, for example, BOPs) with a riser and specific sea conditions, the device may reduce the amplitudes of the riser with load. In other words, the device according to the present invention will only influence the riser with load if excess loads are reached, and hence prevent the riser and loads from reaching larger amplitudes beyond a threshold interval by altering the natural frequency of the riser string. In other words, the device acts as a rigid support until it is made subject to a certain predefined threshold load. When the threshold load is reached, the device may act as a spring supporting the riser string, thereby altering the natural frequency of the system and preventing resonance in the riser string.
In an embodiment of the present invention, the at least one compression element comprises at least one cylinder coupled to an accumulator, whereby the pressure in the accumulator can be varied to adjust the pre-determined threshold force. This advantageously permits adjusting the characteristics of the device according to the operating conditions at any time. The pre-defined threshold force may be set based on at least one of the factors: (i) a weight of the tubular, (ii) a length of the tubular, (iii) a weight of a load suspended by the tubular, and (iv) a wave period for waves acting on the floating vessel. In an embodiment of the present invention, the device is designed to be activated only when a certain load is reached, which load is dependent on the maximum static load (weight) of the riser string and BOP, the threshold force being set, for example, to be 5-30% above a maximum static load. In an embodiment, the threshold force can, for example, be set 5-10% above the maximum static load.
In an embodiment of the present invention, the device can, for example, comprise a preloaded spring which is designed to be activated when a given tension load in the riser string is reached and then limit the maximum tension load to a chosen value when handling riser and BOP.
These and other characteristics of the present invention will be clear from the following description of an embodiment, which is provided as a non-restrictive example, with reference to the attached drawings.
Throughout the description, different terms such as “riser” or “riser string” is used. The skilled person will understand that the meaning of “riser” and “riser string” is the same, namely a string of tubular steel pipe extending from a floating unit downwardly towards a sea floor, either fixed to equipment on the sea floor or free-hanging in the water. The riser may or may not support equipment, such as a blow out preventer (BOP).
The device further has a plurality of cylinders 11a and 11b connected to an accumulator 8. The cylinders 11a and 11b are arranged between the carrier 7 and the base 14, and are pre-tensioned through a fluid pressure in accumulator 8 so that in the absence of a downwards force from the riser string 13, or with a force lower than the combined fluid force acting on the cylinder pistons (the “threshold force”), the cylinders 11a and 11b will be in their mechanically limited end stroke positions (as shown in
This provides the advantage that when the riser load is below the threshold limit, the system is stiff, and will be working as a conventional hard hang-off. When the riser load is above the threshold limit, the stiffness is reduced to a level so that it will alter the natural frequency of the system by, in effect, adding a spring element to the system dynamics. Resonance can thus be reduced or prevented.
When the load from the riser string 13 on the carrier 7 and base 14 is below Fth, the device will be rigid. The carrier and base then act as a traditional hard hang off. When the load from the riser string 13 is above Fth, the air in the accumulator 8 will be compressed and allow some movement of the riser string 13 in relation to the drill floor 2.
This effectively introduces a soft spring in the system (compared to the stiffness of the riser string alone) and makes the entire dynamic system softer, increasing the natural period of the riser string from (in this case) 5.7 s to approximately 7 s. If the device is installed and activated at 7035 kN and the wave period is 5.7 seconds, the device will thus have changed the riser string natural period to 7 seconds and the system will not resonate. For other conditions, the device remains stiff, thus not affecting other operations. Advantageously, one thereby has the opportunity to change or “move” the resonance period of the riser string to a value different from the sea wave period.
In the embodiment shown in
The device could consist of a passive system with cylinders and an accumulator as described above, but is not limited thereby because other examples of passive systems may exist. Other such systems may include a pre-tensioned cellular buffer or a pre-tensioned spring, or any device with a non-linear stiffness which would produce a response similar to that shown in
The dotted line in
Locations A* in
The threshold force and the pre-tension pressure can be varied according to the system design and operating conditions. This variation can be carried out based on the static load (i.e., the riser weight), external conditions (e.g., weather conditions), the specific design and type of riser or tubular used, or according to any load (e.g., a BOP unit) carried by the riser. For example, in the design example shown in
This pressure setting can easily be regulated using known techniques. The wave period is also easy to measure using, for instance, a MRU (Motion Reference Unit). A simple dynamic simulation of the response of the riser to varying vessel heave motions or frequencies may assist in determining suitable settings for the threshold force.
The present invention has been described above in non-limiting embodiments. It is clear that the person skilled in the art may make a number of alterations and modifications to the described embodiments without diverging from the scope of the present invention as also defined in the attached claims.
Number | Date | Country | Kind |
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20150589 | May 2015 | NO | national |
This application is a U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/NO2016/050070, filed on Apr. 15, 2016 and which claims benefit to Norwegian Patent Application No. 20150589, filed on May 13, 2015. The International Application was published in English on Nov. 17, 2016 as WO 2016/182448 A1 under PCT Article 21(2).
Filing Document | Filing Date | Country | Kind |
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PCT/NO2016/050070 | 4/15/2016 | WO | 00 |