A method for underwater transportation and installation or removal of objects at sea Field of the Invention Oil and gas field developments are experiencing a push to- wards underwater production with more of the infrastructure placed on the seabed. There is thus an increasing need for transportation of objects with subsequent installation of the objects at the field. Also, with an increasing number of oil and gas fields being decommissioned, there is a growing need for removal of objects with subsequent trans- port to shore. Some of the objects that are to be installed or removed from the offshore sites are relatively compli- cated with large dimensions and weights. One is often de- pendent on costly vessels and equipment, and the availabil- ity of such vessels may also be poor. Based on these as- pects there is a need to develop new and alternative meth- ods for transport and installation/removal of objects, as conventional methods become unfit, inadequate or very ex- pensive.

Thus, the present invention relates to a method for trans- port and installation of objects at sea, particularly re- lating to transport and installation of objects that are part of the infrastructure in oil and gas fields offshore, where the object is put in the sea at a suitable location near the shore or in sheltered waters, then towed to the installation site while being suspended in at least one buoyancy unit, and subsequently lowered to its final desti- nation.

A method of this type is known from GB 1191146. In this method two slightly different types of buoys are used for suspending a pipeline while it is being towed from the shore to its place of installation. At the installation site, the pipeline is lowered to the sea bottom by flooding

the buoys, some of them deliberately and others automati- cally through the implosion of bursting disks so as to fill their buoyancy chambers at a predetermined depth. The low- ering of the pipeline is started from one end, and if the water is deep, a substantial part of the pipeline will be suspended in the still floating part before the first end reaches the bottom, thus subjecting part of the pipeline to very substantial bending loads and possibly also causing the lowering process to become out of control. GB 1191146 is silent on the possible recovery of the buoys after the pipeline has reached the bottom. In any case, such recovery work would be quite complicated, particularly at greater depths.

The more conventional methods are normally based on trans- porting the objects to the destination on deck of the in- stallation vessel or a transportation barge, with subse- quent offshore lift from deck and lowering of object through the splashzone/sea surface using a crane. Such op- erations set high demands to crane capacity and deck space, and can be very weather sensitive operations depending on type of object to be installed and the motion characteris- tics of the installation vessel (s). They further require costly construction vessels and potentially tying them up for long periods of time depending on weather.

Thus, the purpose of the present invention is to provide a method as mentioned in the second paragraph above which is safe and simple and may be performed with the use of read- ily available equipment even when the weather conditions are not favourable.

Summary of the invention The transport and installation method may be summarised as follows: The object is transported from the fabrication yard to a suitable location inshore or a location in sheltered waters

or a location near shore with the appropriate weather to perform the transfer operations. An inshore crane vessel will then pick up the object from the transportation barge or transportation vessel and lower the object through the splashzone/sea surface with subsequent hook-up and weight transfer to the buoyancy unit and towing arrangement, ac- cording to the invention. The object will then be towed to the destination by a towing vessel while immersed and sus- pended from the buoyancy unit. On arrival at the destina- tion the towing winch wire from the towing vessel will be shortened to take the weight of the suspended object and enable release of the buoyancy unit. The buoyancy unit will be released from the object when it no longer carries any weight, and subsequently stored on deck of or alongside of the towing/installation vessel, or other vessel, with post- installation transport back to shore either on deck or towed in a horizontal position, respectively. The object will then be lowered to the seabed or to a specified target depth between the surface and the seabed for installation with the heave compensated towing winch wire from the tow- ing vessel or other vessel with heave compensated lifting arrangement.

This is obtained by a method as recited in claim 1.

The invention also relates to removal operations as defined in claim 2.

When applying the invention one achieves a number of advan- tages compared to conventional methods. In particular, one avoids the problems and weather restrictions associated with an offshore lift from deck and subsequent lowering through the splashzone/sea surface. Avoiding using crane vessels and improving the weather criteria for the instal- lation will also result in a major reduction in cost for the operation, and the invention is in principle indepen- dent of dimensions and weight of the object to be transpor- ted and installed. Further, the transport to destination is

safer and less weather sensitive since the object is sus- pended from a slender buoyancy unit and is independent of the installation vessel during transport. The buoyancy unit has a small and constant waterplane area and acts as a hea- ve compensator, thereby reducing the dynamic loads in the object and the suspension arrangement between the object and buoyancy unit. The method can be based on using the same vessel for both the tow and the installation or recov- ery, and a minimum of equipment is needed. This means that any vessel that satisfies the basic capacity requirements may be used, which increases vessel availability, opens for using low cost vessels and reduces vessel mobilisation time.

Brief description of the drawings The present invention shall be described in more detail in the following with reference to the attached schematic drawings which illustrate a preferred embodiment, wherein: Fig. 1 is a side view showing how a typical inshore crane vessel lifts the object from deck of a vessel or a trans- portation barge and lowers it into the sea for connection to the towing vessel.

Fig. 2 is a plan view showing the connection of the buoyancy unit and the towing wire to the suspension arrangement while the object is hanging below the stern of the towing vessel and the buoyancy unit lying on deck of the towing vessel.

Figs. 3A-F are side views illustrating the launch of the buoyancy unit with subsequent weight transfer from towing winch to buoyancy unit.

Fig. 4 is a side view illustrating the invention during the transport phase, with an object suspended from a slender buoyancy unit and connected via a towing arrangement to a towing vessel.

Figs. 5A-F are side views illustrating the end of tow situa- tion where the object weight is transferred from the buoy-

ancy unit to the towing winch, with subsequent disconnec- tion and recovery of the buoyancy unit.

Fig. 6 is a side view showing the lowering of the object to- wards seabed or target depth for installation.

Detailed description of the preferred embodiment Fig. 1 shows the start of the operation according to the in- vention. The object 4 is being lifted through the surface 1 into the sea in sheltered waters after having been lifted off the deck of a vessel or transportation barge by a crane vessel 12. The object will be connected to the lower part of the suspension arrangement 7 and hung off in shark jaws at the stern of the towing vessel 2. The lifting wire from the crane vessel is then disconnected from the object.

Fig. 2 shows the object 4 hanging just beneath the stern of the towing vessel 2, hung off in the shark jaws 14. The buoyancy unit 5 is lying on deck of the vessel and is con- nected to the tri-plate 8. The towing winch wire 10, run- ning from the towing winch 3 via a heave compensator 13 on deck, is also connected to the tri-plate.

Figs. 3A-F show the launch of the buoyancy unit 5 from the towing vessel 2 and the subsequent object 4 weight transfer from the towing winch 3 to the buoyancy unit. In Figs. 3A-C the towing winch wire 10 is paid out and the buoyancy unit 5 is thereby pulled off the deck by means of the object 4 weight. This launch operation is performed with the vessel 2 in forward motion. In Fig. 3D and E the towing wire con- tinues to be paid out and the buoyancy unit starts to take some of the object weight and is thereby raised towards a vertical position. In Fig. 3F the buoyancy unit 5 has taken the full weight of the object 4.

Fig. 4 shows the towing situation of the present invention wherein the towing vessel 2 is an anchor handling tug with

an onboard heave compensated winch. Further, the object 4 is shown suspended from the cylindrical buoyancy unit 5, which are connected to each other by a suspension arrange- ment 6,7, 8. The winch wire from the anchor handling tug, or other heave compensated winch, is used as towing wire 10. The towing wire is connected to a buoy 11, which re- lieves the buoyancy unit of the weight of the towing wire.

If required, the directional stability of the towed object 4 may be controlled by a guideline, denoted 9.

The buoyancy unit 5 has a long, slender, cylinder shape with a small and mainly constant waterplane area and a ta- pered lower end. The unit will preferably be a steel struc- ture that may be divided into several watertight compart- ments, a principle that will ensure continued buoyancy in case of leakage or damage to one or more of the compart- ments. In the lower tapered end of the buoyancy unit there may be a padeye arrangement for connection of the suspen- sion arrangement 6-8 between the suspended object 4 and the buoyancy unit 5. The buoyancy unit may also be fitted with a ballast system which, as required, can be utilised to ad- just the vertical position of the buoyancy unit 5 in the water, and also enable connection or disconnection of a liftline between the buoyancy unit and the object.

The main purpose of the buoyancy unit 5 is to keep the towed object 4 afloat and in the capacity of its shape act as a heave compensator and thereby minimise the dynamic loads in the towed object 4 and the suspension arrangement 6,7, 8 between the object and the buoyancy unit 5. The heave compensation achieved by this principle is not lim- ited by e. g. a defined cylinder stroke length, as large waves will wash over the buoyancy unit. Thus, the resulting changes in dynamic loads will be small in the towed object 4 and the suspension arrangement 6-8 between object and buoyancy unit.

The suspension arrangement 6,7, 8 between the slender buoyancy unit 5 and the object may consist of two parts 6,7 connected together by a link 8, preferably a tri-plate. The purpose of dividing the arrangement in such a manner is to be able to pull the tri-plate 8 onto deck for connection and disconnection of the towing wire 10 to the suspension arrangement, while the object 4 is still hanging below sur- face 1.

The actual towing force is acting in the link 8 between the upper 6 and lower 7 part of the suspension arrangement, in such a way that neither the towed object 4 nor the buoyancy unit 5 is directly connected to the towing wire 10.

Figs. 5A-F show the end of the tow when the destination is reached, with weight transfer from buoyancy unit 5 to the towing winch 3 and subsequent recovery of the buoyancy unit. In Figs. 5A-D the towing winch wire 10 is hauled in to take the weight of the object 4. The buoyancy unit will gradually take less of the object load until it is floating horizontally on the surface without carrying any load. In Fig. 5E the buoyancy unit is disconnected from the object 4 and a recovery winch (not shown) is attached to it for re- covery of the buoyancy unit onto deck of the towing vessel 2. In Fig. 5F the buoyancy unit is lying on deck of the towing vessel while the object is hanging in the winch wire ready for deployment towards seabed.

Fig. 6 shows the object 4 being lowered towards seabed for final installation. A clump weight and orientation wires (not shown) are typically used as installation aids for po- sitioning of the object at the seabed.

The vessel 2 used for towing the object 4 will according to the invention preferably also be used for installation of the object upon arrival at the destination, as the object 4 then is lowered to the seabed using the towing winch wire 10. However, the method according to the invention is not limited to using the same vessel for transport and instal-

lation, and the method according to the invention could thus include using more than one vessel. The vessel per- forming the installation or recovery offshore may be any type of ves- sel, self-propelled or not, with a heave compensated lift- ing arrangement.

The method is intended for use with only one slender buoy- ancy unit 5 for transport and installation of relatively compact objects where the structural design is such that it can be lifted or carried in a single point using a lift bridle or similar. Moreover, the method may be used with two or more equivalent slender buoyancy units when the ob- ject has a structural design that requires two or more points for lifting/carrying to ensure the integrity of the structure during the transport and installation, according to the Invention, e. g. for transport and installation of large pipe spoolpieces.

In relation to spool piece installation, the method accord- ing to the invention is considered not only applicable for installation of single spoolpieces, but is also applicable for transport and installation of several spoolpieces si- multaneously. This is made possible by having a number of spoolpieces stacked, or in other way placed, in a basket or transportation frame that will be suspended from the buoy- ancy unit as per the method according to the invention. The transportation frame may then be lowered to the seabed and wet parked on arrival at the destination, and the spool- pieces may then be picked up from the basket one by one and installed.

In addition to transport and subsequent installation of ob- jects, the method according to the invention is also suit- able for removal of objects and transportation of these to shore. This is beneficial when a field is to be decommis- sioned or when objects are to be removed for other reasons, e. g. repair or replacement. Some objects are also unfit for

recovery to the surface and lifting onto deck of a con- struction vessel or a transportation barge due to cost or technical limitations like large object dimensions and/or weight. The step by step method for removal and subsequent transport to shore of an object will in principle be the reverse of the step by step method already outlined for in- stallation of an object. It is envisioned that removal of structures using the method also may include e. g. removal of jackets, as these may be laid down on the seabed for subsequent recovery and transport to shore using the method according to the invention. The removal method is defined in claim 2.

The method may also be used to install objects that in op- eration are to be located mid-water, i. e. in a position be- low the sea surface, between the seabed and the critical wave zone. Such an object is e. g. an Artificial Buoyant Seabed (ABS), also referred to as Atlantis. The method ac- cording to the invention is in this respect not limited to installing or recovering objects on the seabed, but also involves installing or recovering objects from mid-water positions.

Another possible utilisation of the method is to have at least one winch mounted on the buoyancy unit itself, to be used for controlled lowering of the object towards seabed or target depth between seabed and sea surface. Due to its shape, the buoyancy unit acts as a heave compensator and thereby minimises the dynamic loads in the winch wire, lift arrangement and object during the lowering and final land- ing, and makes the lowering and landing operations inde- pendent of installation vessel motions. Alternatively, the same advantages may be gained by having a sheave arrange- ment or similar mounted on the buoyancy unit itself, with at least one sheave or similar. At least one winch from at least one vessel may then be routed over the sheave ar- rangement and further connected to the object for lowering

and final landing of the object. These utilisations of the method may also be used for recovery operations.

If the tow will encounter more shallow depths underway where the submerged object 4 could possibly hit the sea bottom, it is envisioned that when passing such waters, the object 4 is raised somewhat by bringing the buoyancy unit into a horizontal position. This could be done in various ways, e. g. by connecting a wire between the triplate 8 and the top of the buoyancy unit and then shortening this wire until it has the same length as the upper suspension wire 6.

The invention is not limited to the exemplifying embodiment described herein, but may be varied and modified by the skilled person within the scope of the appended claims.