WO2012070937A1

WO2012070937A1 - Method of installing an offshore tower

Info

Publication number: WO2012070937A1
Application number: PCT/NL2011/050795
Authority: WO
Inventors: Mark Riemers
Original assignee: Suction Pile Technology Bv
Priority date: 2010-11-22
Filing date: 2011-11-22
Publication date: 2012-05-31
Also published as: NL2007833A; NL2007833C2; EP2643525A1

Abstract

The invention relates to a method for erecting an off-shore slender tower. Main components for tower and suction pile comprising base or foundation are assembled into one unit at a remote location other than the final location. Said tower as pre-assembled unit or as two or more, e.g. exactly two, separate parts is transported from said remote location to the final location by a vessel and placed at the final location. At least one of said parts, possibly as part of said pre-assembled unit, is largely vertically transported. E.g. said part or pre-assembled unit is suspended aside the vessel with a largely vertical orientation of the tower from a hoisting device at the vessel during shipping and possibly installation.

Description

METHOD OF INSTALLING AN OFFSHORE TOWER

The invention relates to a method of installing an upright extending shaft, mast or tower, hereafter called tower or slender tower, and similar offshore mono-pile structure, braced or not, at an offshore site, for instance a tower to support or supporting a measuring pole or the nacelle and blades of a windmill or wind turbine. The invention also relates to a method and apparatus for transporting such a slender tower and similar offshore structure to its location of use offshore. And the invention relates to a slender offshore tower. The tower is provided with delicate bulky and heavy equipment (e.g. a windmill or measuring pole) on top always projecting above the sea water at the off-shore operation site.

Particularly, the invention relates to installing an offshore structure based on a suction pile, e.g. based on one, two, three or a cluster of three or four suction piles.

Suction piles and their way of installing are a.o. known from GB-B-2300661 and EP-B-0011894, which disclosures are enclosed in here by reference. Briefly, a suction pile is a thin walled steel cylinder, closed at at least one longitudinal end, that is located on the subsea bottom with the opposite end and penetrates the subsea bottom with the aid of a suction created within the cylinder. The creation of the suction can be with the aid of a suction source, such as a pump, being on, or close to or at a distance from the suction pile. The applied level of the suction can be e.g. at least substantially constant, smoothly increase or decrease or else pulsate, for which there are convenient means. After use, the suction pile can easily be removed by creating an overpressure within the cylinder, e.g. by pumping in (sea) water.

A self installing platform applying suction piles is known from e.g. W099/51821 (SIP1) or EP-A-1 101 872 (SIP2) of the present inventor.

Instead of installing a suction pile into the under water bottom by generating a fluid pressure difference between the inside and outside of the suction pile, it is also feasible that the suction pile at least partly penetrates the under water bottom by a weight resting on it, e.g. the platform and/or a ballast body .

Therefore according to the invention it is proposed to make the marine structure self foundating or self installing by provi- ding it with one or more suction piles. So the hoisting device and the plant for installing the foundation, e.g. hammering device, can be eliminated. Thus a simpler equipment to assist during installing the marine structure is sufficient, e.g. a simpler hoisting device.

Since the structure is provided with one or more suction piles, removal after use is made easier in that by pressing out the suction pile, the anchoring of the structure to the underwater bottom can be removed.

Preferably each suction pile has one or more of: a diameter of at least 5 metres; a height of at least 5 metres; a wall thickness of at least 1 centimetre; the longitudinal axis of the suction pile and the relevant supporting leg are substantially in line. BACKGROUND

The installation of towers at offshore sites gives rise to several inherent problems.

The towers must be constructed on shore or at shore, and then moved to their intended offshore site, and there set down on the seabed. During construction, it may be convenient for a tower to be in an upright attitude. At the offshore site the upright attitude is required for operation of the tower.

However, during transportation across open water, a problem arises in that an upright tower could have only marginal stability. This marginal stability would be particularly critical in high wind conditions when controlling tugs might have difficulties in manoeuvring. This problem could be exacerbated in circumstances in which a very slender tower with a slender upright pile at its upper end (e.g. a measuring pole of, say, some 100 metres high) or a wind turbine, was transported to site by a small vessel. The problem could be alleviated if a larger vessel were to be used. The greater stability of the larger vessel would allow safe transportation and installation. Nevertheless, the operating costs of the larger vessel could be too expensive for a single installation, so rendering a proposed offshore development uneconomic.

A further problem arises when the tower is to be set down on the seabed. There may be a point of instability, at which the tower has its centre of buoyancy and centre of gravity coincident.

Heretofore it has been known to tow towers to their intended offshore sites in an upright attitude, with their centre of gravity below their centre of buoyancy; and then to sink them onto the seabed. The massive Condeep gravity based platforms for the Norwegian sector of the North Sea were so installed. In particular, the single column substructure for Norske Shell's Draugen platform was installed by this technique. However, this technique might be difficult to implement in shallower water or with towers of different type, e.g. much smaller dimensions and weight and/or different type of foundation.

For smaller towers to be installed in shallower waters, the tower may be carried out to its intended site in an upright attitude on the deck of a vessel. This was the case with NAM's Offshore Loading Tower for one of their developments offshore the Netherlands. An Offshore Loading Tower of this type was featured in UK patent specification 2267525. However, carrying a tower on the deck of a vessel increases the overall height of the tower above the centre of buoyancy of the vessel. This increases the stresses induced in the tower by pitch and particularly roll motions of the vessel due to wind and waves.

It has also been known to carry towers horizontally, and then to upend them at the required offshore site, refer to eg. DE 201 00 588 U. This is the simplest method of transporting and installing slender towers. Indeed this method is used for transportation and installation of conventional piles for offshore structures. However, this method would be less suitable for the transportation and installation of towers of other type for two reasons. First, additional measures are required to avoid the delicate components of the tower would be damaged in transit; and second, upending the tower requires special attention and equipment.

Mounting tall structures such as completely above the water level provided measuring poles or wind turbines in offshore locations involves a number of complexities. Because of the shape and size of the structures they are inherently difficult to transport and there are significant handling issues in achieving a correct lowering of the structure with its offshore support base, jacket or foundation to the sea bed. These difficulties are aggravated when the sea conditions are less than ideal. In relatively shallow waters it is possible to use so-called jack-up barges which, when the legs are extended to at least partially support the barge on the seabed, provide a stable platform from which the offshore structure can be mounted on its offshore support base. However, it is increasingly required to mount structures in deeper water where jack-up barges can hardly or not operate, all increasing costs. Besides, jack-up barges add costs.

It will be appreciated that the installation processes are relatively complicated and work intensive which are considerable disadvantages by off-shore works. Work conditions at sea in general are much more difficult that on land. Transport conditions and down time due to weather implies that the average cost per effective work hour is much higher than on shore. The complicated processes that have to occur in a certain succession also imply an increased risk of being surprised by bad weather or breakdown of machinery at an unpropitious time in the course of the project. Besides, it is a cost-raising factor that installation usually occurs by multiple separate process steps each giving cause to mobilisation costs. Attempts to do the installation in less steps or even a single step, targeting cost savings, is not so easy at it seems due to the complex shape and large dimensions of the structures. An often used comparison is that the costs per effective work hour is of the magnitude 10 times greater at sea than on land. Further, insurance will only be possible on the background of very comprehensive descriptions of work and methods, model experiments, safety assessments etc. which may raise the costs of the process considerably.

WO 9943956 discloses that offshore structures in one piece, with or without foundation, may be mounted on or at quay, lifted upon a transport vessel, towed to the installation locality and there, still in one piece, lifted to the final location. By the lift performed in this method, independent from the transport vessel floating cranes are used with sufficient lifting capacity, where the structure is suspended under a hook, and where the stability is secured by lifting in the approximately highest point of the structure, substantially above the centre of gravity. In a variant of this method, no transport vessel is used, and the floating crane itself carries the completely assembled structure on the way from quay to the final location. The advantage of not using any transport vessel is that, by the lift at sea, relative movements between transport vessel and floating crane caused by sea may be disregarded. Methods of this kind normally have the very significant disadvantage that they depend on use of a floating crane the lifting capacity of which corresponds to the weight of the complete structure and the lifting height of which substantially exceeds the height of the structure to be installed. If this structure is a slender tower carrying delicate, bulky and heavy equipment as with a measuring pole or windmill, a lift over the top of the tower or the windmill is required. Typically, the height is here in the magnitude 60-100 m or more, and the requirements to the floating crane therefore become great, implying the supply of suited vessels becoming rather limited. The latter may imply considerable disadvantages with respect to availability at the desired time as well as the level of cost. The problem is worsened if the floating crane itself carries the load on the way from quay to the final location. Here, the floating crane performs a function that otherwise may be performed by a much cheaper vessel which is less in demand.

WO 02.088.475 (Suction Pile Technology, SIP3) discloses a tower carrying a wind turbine at the top and suction piles as foundation, in upright, independently balanced, attitude suspending aside a vessel.

Further prior art is provided by GB 2327970, US 2009028647, EP 1101935, the disclosure of which is enclosed in here by reference .

OBJECT OF THE INVENTION

Thus there is a desire for a method of installing a slender tower with delicate bulky equipment on top always projecting above the sea water at the off-shore operation location (a windmill or measuring pole) in an upright attitude at an offshore site without incurring one or more of the above and further disadvantages known from the prior art. In the case of a MP or WT (and similarly for other tall, narrow offshore structures which typically include a shaft, mast or tower which is arranged vertically in use) , it is relatively delicate and is also stiff in the longitudinal (vertical) direction. This makes it especially important to avoid shocks when mounting it on the sea bed or on its pre-installed offshore foundation. Usually, the centre of gravity of a slender tower is at a height which is more than twice the width of the base of the structure so that in handling the structure in an upright configuration, instability of the structure is a major concern.

The present invention seeks to provide a method which is particularly suitable for the mounting of offshore structures in deeper waters and further which is especially suitable for mounting tall structures of relatively small width, such as wind turbine generators or measuring poles. The object of the invention is to provide a method for installing towers at sea, which minimises costs and risk, and which avoids the drawbacks connected with installation by the methods known hitherto.

More especially, the present invention seeks to provide a method by which a slender tower, carrying a MP (measuring pole) or wind turbine (WT) , can be fully assembled on shore, engaged by a transporting vessel and moved by the vessel to its final installation position whereat, by the vessel, the tower is mounted on the sea bed (in case the tower is provided with an integral foundation on shore) or on a pre-installed foundation or jacket. The invention transports the offshore structure in an upright configuration, suspending aside the transport vessel.

The vessel used is preferably a floating crane barge or a vessel with lift jib on deck. Because of this it is achieved that the vessel used is adapted to the structure in the way that it has a lifting height which is substantially reduced in relation to the lifting height of a normal floating crane and which may be made as a temporary installed structure on a standard vessel.

The tower, as completely assembled with foundation and measuring pole or such, is preferably at high level, preferably above its centre of gravity, provided with ballast means, preferably comprising concrete material, which ballast means are provided asymmetrically and eccentrically such that they influence the radial location of the centre of gravity of the tower, preferably cause a radial shift of the centre of gravity of at least 1 metre.

Compared with the prior art, the invention implies a number of advantages, some of which are: Tower and foundation may be mounted as a unit on land or at quay, and all mounting of cables and other accessories may be finished and test running on land may be achieved. Work as well as test running may occur under normal, favourable work and safety conditions and with much lower cost than at sea. Possible faults may also be repaired with far less costs. Land based crane assistance may be provided at normal rates if necessary. Tower and base are shipped in one piece by the same vessel performing lifting as well as transport and installation. This occurs with the tower oriented largely vertically without requirements of very high floating cranes and with avoiding the possibility of damaging the tower construction which is the risk if transported horizontally and raised at the location for installing. Standardised vessels and tools are used, and temporary buoyancy elements otherwise required are absent and thus do not have to be dismounted under difficult conditions at sea. The number of working hours at sea are reduced to the least possible. The prior art

disadvantages concerning availability at the desired time and cost level for normal floating cranes, an expensive unit in demand, are avoided. Relative movements between transport vessel and floating crane caused by sea are absent and thus may be disregarded.

As the vessel, a diving support vessel or construction support vessel (commonly called ^yDSV) can be used. Such vessel preferably has an on board crane, typically with a reach of approx. 20 metre, usually no more then 30 or 50 metre, e.g. a Liebherr BOS 7500 crane or a crane of equivalent type, having a lifting capacity of approx. 300 tons, usually no more then 500 or 1000 tons. Such crane can be used to lift the tower or part of it, such that no other crane is required at the installation site offshore.

The invention provides a method of installing a slender tower in an upright attitude at an offshore location, comprising the steps of: taking a slender tower with preferably integrated suction pile foundation positioned in an upright attitude; moving a transportation vessel, being e.g. a floating crane barge, to a position adjacent to the tower; using the crane of the barge to lift the tower, while maintaining its upright attitude so that the tower suspends from the crane aside the barge; moving the transportation vessel with the tower, still in the upright attitude and aside the vessel suspended from the crane, to the offshore location; arranging the

transportation vessel adjacent to an upper end of a preinstalled vertical pile; positioning the tower, still in the upright attitude and aside the vessel suspended from the crane, over the preinstalled pile; lowering the lower side of the tower into engagement with the upper end of the preinstalled pile; fixing the tower to the preinstalled pile, so that the tower stands upright at the offshore location; and moving the transportation vessel away from the tower.

In the alternative the slender tower is transported in two or more pieces, e.g. a top piece and a bottom piece having the suction pile foundation, which are stacked on top of each other at the operation site of the tower. At least one of the pieces is transported, e.g. loaded onto the deck of the vessel, in the upright attitude similar to the operational attitude.

A vessel of use can be a standard offshore barge, e.g. with the dimensions 90 x 30 m. The vessel preferably carries a derrick or different crane. At its upper end, the crane can carry a tackle arrangement. With suitable winches the relative orientation of the crane to vertical may be adjusted, and the suspended load may be hauled up and down. A wire runs in the tackle arrangement and has at its free end a hoist hook or different means for releasable coupling with the suspended load .

The vessel is preferably a standard vessel of which the supply on the charter market is considerable and the prices moderate. The lifting arrangement, e.g. provided by a derrick, bearings and winches, or different crane, preferably

constitutes a separate unit which may be mounted on and dismounted from the standard vessel.

The tower preferably has one or more of the following features: has a single suction pile; has a single supporting pole extending upward from the base, which base e.g. comprises a suction pile; the suction pile and the supporting pole above it have a common longitudinal axis; has a platform above the water level in operation; has an upward extending measuring pole; has a top part comprising the measuring pole and possibly the platform; the measuring pole is mounted eccentrically; the measuring pole is mounted on top of the platform; ballast weight is mounted eccentrically at the platform; the hoisting cable is coupled to the platform; the platform is provided with means for releasable coupling the hoisting cable; the coupling means at the platform are provided between the ballast means and the measuring pole; the ballast means and measuring pole keep a horizontal distance to the preferably in between provided, supporting pole or its extension; the centre of gravity is below or above the platform; has a bottom part comprising the suction pile and complete or lower part of the supporting pole; top and bottom part are separate parts; the complete tower or its top part are self balanced in the operative attitude of the tower while suspended from the hoisting cable; the measuring pole is at least 30 or 50 metre high; the ballast weight is at least 10 or 20 tons; the tower extends at least 50 metre upward from sea level; at least two of supporting pole, suction pile and hoisting cable are co axial; the supporting pole is below the mid of the platform; the maximum level of the top end of the boom of the crane is limited to a level below 30 of 50 metre above sea level or vessel deck level; the measuring pole keeps a horizontal distance to the axial line through the supporting pole of at least 1 or 2 metre; the top end of the boom of the crane is at a level above the platform level and below the measuring pole top end level; the ballast means are, preferably at least 1 metre, above or below the centre of gravity of one or more of the top part, lower part or complete tower; has three suction piles and a single hoisting cable suspending from the crane is split in three at a single point at a level above the suction piles and below the top end of the supporting pole or platform and each cable part extends to a relevant of the three suction piles.

The invention is further illustrated by way of non-limiting, presently preferred embodiments providing the best way of carrying out the invention and shown in the drawings.

Fig. 1 shows a first embodiment in side view;

Fig. 2 shows a detail of fig. 1;

Fig. 3 shows a second embodiment in side view, sequential steps during transport and installation are shown;

Fig. 4 shows a third embodiment in side view, sequential steps during transport and installation are shown .

Fig. 5 and 6 show an alternative embodiment in side and top view. Fig. 1 shows a vessel 6 provided with a lift jib or derrick or rig or A-frame, the top of which is approx. 80 metres above sea level 7 (sea level 7 is also indicated for the as installed case when suction pile 1 is below and platform 3 is above sea level) . The tower suspends from the hoisting cable, the free end of which is coupled to or near the platform 3. The platform carries a measuring pole 5 of approx. 90 metre length and concrete ballast 4 of 25 tons.

Below the platform the single supporting pole 2 extends from the single suction 1 pile upward. The extension of the supporting pole crosses the mid of the platform. The measuring pole is at the one and the ballast at the opposite side of the platform, and the extension of the supporting pole extends between them. Both the measuring pole and the ballast keep a distance of at least 2 metre from the axial line through the supporting pole.

The suction pile and supporting pole and hoisting cable are co axial. In operation the platform will be sufficient above sea level to be free from sea waves. The suction pile will penetrate the sea bottom.

The measuring pole extends beyond the top of the crane. The suction pile is completely lifted above the water or partially submerged to slow down motions of the suspended structure. While suspended vertically aside the vessel, the completely assembled slender tower can be transported by the vessel to the installation site offshore, while self stabilised (independently stabilised) since the centre of gravity is at the axial line of the supporting pole .

Fig. 3 and 4 show the same tower of fig. 1, this time split into separate top (comprising ballast, measuring pole and platform) and bottom parts (comprising suction pile and supporting pole) . The top part is vertically located at the deck of the DSV. The bottom part is towed independently floating in almost horizontal attitude. At the installation site, the bottom part is first upended by the crane of the DSV and subsequently the suction pile is penetrated into the seabed. Then the crane of the DSV lifts the top part of the tower. Because of the ballast the top part of the tower has its centre of gravity at the axial line of the supporting pole, such that while suspended from the hoisting cable of the DSV crane, the top part is independently stabilised and in the same attitude as during operation (vertical ) .

With the DSV crane the top part is installed on top of the pre installed bottom part.

If convenient, the upper end of the supporting pole could be part of the top part, for ease of assembly of top and bottom part .

Although the DSV of fig. 4 has a crane of only limited reach, it can be used for installation of a slender tower extending at least 30 or 50 metre upward from sea level.

Fig. 5 and 6 show the foundation part. Now there are three suction piles 1, one of which present directly below the pole 2 supporting the platform (not shown) . A hoist cable 8 is spliced into three cables 9, each extending to and ending at a relevant suction pile 1. The suction piles 1 are mutually connected by horizontal braces 11 while inclined braces 11 extend from the two suction piles remote from the pole 2 towards said pole 2 and end at the pole 2. In top view the suction piles 1 are at the corners of a triangle. The invention is not limited to the above described and in the drawings illustrated embodiments. E.g. the marine structure can have less than four, e.g. two or three, or more than four, e.g. five or six, suction piles. The number of supporting legs is preferably equal to the number of suction piles, but this is not absolutely necessary. E.g. three suction piles are at the corners of a structure that is triangular in top view. It is not required that the suction piles and supporting legs are at the corners of the structure. The platform can be constructed and/or shaped differently.

Claims

1. A method for erecting an off-shore slender tower where main components for tower and suction pile comprising base or foundation are preferably assembled into one unit at a remote location other than the final location, and are transported or shipped as said possible pre-assembled unit or as two or more, e.g. exactly two, separate parts from said remote location to the final location by a vessel and placed at the final location, characterised in that at least one of said parts, possibly as part of said pre-assembled unit, is largely vertically transported, e.g. said part or pre-assembled unit is suspended aside the vessel with a largely vertical orientation of the tower from a hoisting device at the vessel during shipping and possibly installation.

2. A method according to claim 1, wherein said pre-assembled unit has a suction pile (1) and a single supporting pole (2) extending upward from the suction pile while the suction pile and the supporting pole above it have a common longitudinal axis; and has a platform above the water level in operation and an upward extending measuring pole (5) mounted

eccentrically on top of the platform (3) and ballast weight (4) is mounted eccentrically at the platform and causing a sideways shift of the centre of gravity of at least one metre and the hoisting cable (8) from the vessel crane is coupled to the platform between the ballast means and the measuring pole and at said common longitudinal axis, said ballast means and measuring pole keep a horizontal distance to the in between provided extension of said supporting pole (2) and the centre of gravity is below the platform and the pre-assembled unit is self balanced in its vertical, similar to its operative, attitude while suspended from the hoisting cable with the suction pile completely above water level and the top end, from which the cable (8) extends downwards, of the boom of the crane (6) at the vessel is at a level above the platform level and below the top end level of the measuring pole.

3. A method according to claim 2, wherein said pre-assembled unit has three suction piles and a single hoisting cable suspending from the crane is split in three at a single point at a level above the suction piles and below the top end of the supporting pole or platform and each cable part extends to a relevant of the three suction piles.

4. A method according to claim 1, wherein said tower is provided by a top and a separate bottom part, said bottom part comprising the suction pile and complete supporting pole; said top part has a platform above the water level in operation and an upward extending measuring pole (5) mounted eccentrically on top of the platform (3) and ballast weight (4) is mounted eccentrically at the platform and causing a sideways shift of the centre of gravity of at least one metre and the hoisting cable (8) from the vessel crane is coupled to the platform between the ballast means and the measuring pole and at said common longitudinal axis, said ballast means and measuring pole keep a horizontal distance to the in between provided extension of said supporting pole (2) and the centre of gravity is below the platform and the top part is self balanced in its vertical, similar to its operative, attitude while separate from said bottom part suspended while completely above water level and the top end, from which the cable (8) extends downwards, of the boom of the crane (6) at the vessel is at a level above the platform level and below the top end level of the measuring pole .

5. A method according to any of claims 1-4; the measuring pole extends at least 30 or 50 metre from the platform upward and/or the ballast weight is at least 10 or 20 tons

6. An off-shore slender tower having a foundation with oe or more suction piles (1) and a supporting pole (2) extending upward from a suction pile while the suction pile and the supporting pole above it have a common longitudinal axis; and has a platform above the water level in operation and an upward extending measuring pole (5) mounted eccentrically on top of the platform (3) and ballast weight (4) is mounted eccentrically at the platform and causing a sideways shift of the centre of gravity of at least one metre and a coupling for a hoisting cable (8) from the vessel crane is located at the platform between the ballast means and the measuring pole and at said common longitudinal axis; said ballast means and measuring pole keep a horizontal distance to the in between provided extension of said supporting pole (2) and the centre of gravity is below the platform.