Field of the invention

The invention relates to a fall pipe system for placing rock in selected locations on a sea bed, which may be several hundreds of meters below sea level. The invention also relates to a vessel on which such a fall pipe system is arranged, and to a method for placing rock on a sea bed using such fall pipe system or vessel. Background of the invention

Fall pipe systems that are used for placing rock on a sea bed are known. They are generally installed on a dedicated vessel used for marine rock placement purposes. Rock is, for instance, to be placed for protecting and fixing pipelines on the sea bed. Such pipelines are, for instance, used for transporting crude oil from off-shore oil production fields to the main land, for transporting natural gas over the sea bed between countries, and the like. Subsea rock placement is also carried out for sea bed leveling to allow sea bed supported placement of pipe lines, the correct placement of off-shore oil production platform and off-shore windmills, and the like.

The fall pipe systems are employed to depths of several hundreds of meters. However, such lengthy fall pipe system become quite heavy providing an extensive static weight and load on the fall pipe itself, but also on the carrying vessel. In addition to a static load the fall pipe will experience dynamic loads due to water currents. Water currents may be in different directions at various water depths, and may even be directed in opposite directions at two different depths. Such currents will impose very large dynamic loads on the fall pipe. To withstand such dynamic and static loads, the fall pipe should be made strong enough, which may further add to the weight of the fall pipe. These limitations restrict the use of known fall pipe systems to about

1,000 meters.

Marine fall pipe systems are, for instance, disclosed in EP 0 450 675 Al, WO 2009/020385 Al, WO 2010/147460 Al and EP 2 266 873 Al, which are all incorporated herein by reference.

There is a strong demand to employ fall pipe systems at larger depths over 1,000 meters at which known fall-pipe system cannot be employed. Off-shore oil productions platforms are, for instance, being planned for locations at larger water depths. At depths just below 1,000 meters the known fall pipe systems also impose limitations on their practical use due to large static loads.

US 4,829,923 discloses a steel fall pipe made up of section that are joined together. Buoyant collars around the pipe may be used to reduce the load in the pipe string and on cables supporting the pipe string.

EP 0 450 675 Al generally discloses a fall pipe system of substantially tubular pipe sections, but without any buoyancy.

GB 2 004 817 A discloses a fall pipe system that provides a telescopic connection with a filling tube end of the fall pipe system. The fall pipe may be made of a material having s specific gravity equal to or smaller than water.

Summary of the invention

It is an object of the invention to provide a marine fall pipe system and corresponding marine rock placement method, which can be employed at large depths, for instance, over 1,000 meter.

Such object is achieved by a marine fall pipe system constructed for placing blocks of stone and/or rock on a sea bed, the fall pipe system comprising a substantially tubular fall pipe, wherein the fall pipe system comprises buoyancy provided such as to, during use, be arranged along the fall pipe so as to at least partially compensate for a weight of the fall pipe system, the fall pipe system comprising substantially tubular fall pipe sections constructed and arranged for constituting the fall pipe, the buoyancy being associated with one or more fall pipe sections, wherein the buoyancy is provided by microspheres integrated in the fall pipe section(s), the microspheres having an effective specific weight (density) below the specific weight of water. Having the buoyancy being associated with one or more tubular fall pipe sections provides an efficient manner to provide buoyancy where required. The microspheres can be made very lightweight and can be effectively incorporated into the fall pipe sections to provide buoyancy.

In a preferred embodiment the buoyancy is provided by microspheres integrated in the fall pipe section(s), the microspheres comprising hollow microspheres, especially hollow glass microspheres. Such hollow spheres provide a large amount of buoyancy per occupied volume. In the event one such hollow sphere would break, the other spheres would still provide buoyancy. Spheres also provide high resistance against external forces, such as due to high pressure at large water depths. In another preferred embodiment the buoyancy is provided such as to, during use, being distributed along the fall pipe, which provides a compensation of the weight of the file pipe system where required.

In a preferred embodiment one or more fall pipe sections (110) with associated buoyancy comprise integral buoyancy, the integral buoyancy being provided integral with the respective fall pipe section(s), which is a very efficient and reliable manner of providing the buoyancy, and does not obstruct functioning of the remainder of the system.

To ensure a proper upward orientation of the fall pipe sections the buoyancy is predominantly provided in, during use, an upper half of the fall pipe section(s).

Most efficiently, the buoyancy is provided at, during use, an upper edge of the fall pipe section(s).

In an effective and efficiently manufactured embodiment the fall pipe section(s) comprise(s) a collar around the fall pipe section(s), the buoyancy being provided in the collar.

In an embodiment the fall pipe section(s) comprise a polyethylene material, which proves to be a reliable and durable material for the purpose.

In an effective embodiment that provides a fall pipe system that is very flexible, the fall pipe sections comprise bottomless buckets having a top side and a bottom side, an external perimeter of the bottom side being smaller than an internal perimeter of the top side, and the fall pipe system comprises a hoisting arrangement to which the buckets are attached such that, during use, a top side of one fall pipe section overlaps a bottom side of an adjacent fall pipe section and the bottom sides tend to face in a downward direction. Preferably the buckets have a substantially conical shape, which are easily manufactured and are easily nested to occupy less space when stored.

In a further effective embodiment the hoisting arrangement comprises at least one cable or chain to which the buckets are attached, and/or the fall pipe system comprises chains or cables interconnecting adjacent buckets. The cables or chains provide a reliable and effective means for hoisting while also providing flexibility. An embodiment of the fall pipe system comprises a driving arrangement for moving a, during use, bottom end of the fall pipe in a direction substantially along the sea bed for accurate positioning of rock being placed using the fall pipe system.

In another aspect the invention provides for a marine vessel comprising a fall pipe system as disclosed above.

In yet another aspect the invention provides for a rock placement method for placing block of stone and/or rock on a sea bed, wherein the method comprises providing a fall pipe system comprising a substantially tubular fall pipe, the fall pipe system comprising buoyancy provided such as to, during use, be arranged along the fall pipe so as to at least partially compensate for a weight of the fall pipe system, the fall pipe system being a fall pipe system as disclosed above; providing the fall pipe system on a marine vessel; travelling the marine vessel to a marine location where block of stone and/or rock are to be placed on the sea bed; and providing block of stone and/or rock (R) in a location on the sea bed using the fall pipe system.

Brief description of the drawings

The invention will further be explained with reference to the accompanying drawings in which like or same reference symbols denote like, same or corresponding parts, and in which

Figure 1 shows part of a vessel carrying a fall pipe system according to the invention;

Figure 2 shows a detail of the fall pipe system of figure 1; and

Figure 3 schematically shows a large-scale view of a marine vessel provided with a fall pipe system according to the invention.

Detailed description of embodiments

Figure 1 shows part of a marine vessel 10 on which a fall pipe system 100 is arranged. A fall pipe made up of various fall pipe sections 110 suspends down from the vessel through a shaft 11 in the vessel to a large depth just above a sea bed SB. Blocks of stone and/or rock R are provided by a conveyer 12 arranged on the vessel to a top end of the fall pipe and subsequently drop down the tubular fall pipe, as indicated by unlabelled arrows shown above the conveyer and within the fall pipe. The blocks of stone and/or rock R then leave the last fall pipe section 110L of the fall pipe to be positioned on the sea bed SB.

A driver arrangement 140 is provided at the end of the fall pipe for positioning the end of the fall pipe at the correct position of the sea bed. The driver arrangement comprises drivers 141 for positioning the driver arrangement together with the end of the fall pipe along the sea bed SB as indicated by arrows D. The driver arrangement further has a shaft 142 for accommodating the fall pipe and/or for passing blocks of stone and/or rock from the end of the fall pipe towards the sea bed. The driver arrangement is shown to be attached to the last fall pipe section 110L, but it can also be attached to a fall pipe section further up the fall pipe.

The tubular fall pipe sections 110 are connected to one another by chains or steel cables 120, which also serve to lift and lower the file pipe using hoisting

arrangement 130 on the marine vessel 10 through shaft 11. Individual fall pipe sections may be added and connected to the steel cables or chains 120 at the top end of the fall pipe, as required, when the fall pipe is lowered and disconnected and removed again when the fall pipe is being lifted. Any other suitable manner of providing or storing the fall pipe sections may be employed as well.

Figure 2 shows some fall pipe sections 110 with connecting steel cables or chains 120 in more detail. A fall pipe section 110 comprises a substantially conical shaped bottomless bucket 111. The bottomless buckets can be made of a linear medium density grade polyethylene like, for instance Rigidex HD3850UA, supplied in pellet form for use in a rotational molding process to actually manufacture the bucket. The density is about 938 kg/m ³ according to ISO 1872/1-1993. The bottomless buckets 111 are attached to the steel cables or chains 120 such that a top side or end of one bucket 111 overlaps with a bottom side or end of an adjacent bottomless bucket 111.

An external perimeter and cross-section of the bottom side of the buckets 11 1 is smaller than an internal perimeter and cross-section of the top side of the buckets such that a gap G is left open between the top side of one bucket and the bottom side of an adjacent bucket. Water may flow through the gaps G as indicated by the arrows. One bucket projects into an adjacent bucket that is neighboring below. Such configuration guarantees a reliable transport of rock downwards, while providing a flexible fall pipe system made up from the bottomless buckets 111. The conical shape of the buckets also allows the buckets to be nested into one another for the reduction of storage space. At their top sides the buckets have a collar 112 from which a pin 113 projects. The pin 113 comprises some arrangement for attachment to the steel cable or chain 120, which is known as such. The attachment of pin 113 to steel cable or chain 120 is such that the bottomless bucket suspends from the cables or chains 120. A position of a fall pipe segment 110 is therefore fixed with respect the cables or chains 120 and substantially fixed with respect to adjacent fall pipe sections although it allows some movement of adjacent fall pipe sections 110 with respect to one another for providing flexibility to the fall pipe.

The collar 113 provided at the upper edge of the bucket is filled with a mixture of hollow glass microspheres 114 and resin 115 in between the microspheres. A bucket 111 can be manufactured with a hollow collar 113 by an injection molding process. Subsequently, the hollow collar can be filled with a mixture of the hollow microspheres and resin, after which the resin is allowed to solidify. The microspheres have an effective specific weight, being the weight of a microsphere divided by its volume, or density that is below the specific density of water such as to provide the buoyancy to the bucket. Especially, a hollow sphere has a very low specific weight, while still providing enough strength to withstand external forces such as compression forces at considerable water depths.

Having a collar 113 with hollow microspheres provides an internal or integral buoyancy to a fall pipe section 110, especially to a top side of the fall pipe section in the embodiment shown. The top half of the fall pipe section will therefore experience more upwards force than the bottom half when provided in water, which assists in a proper orientation of the fall pipe section when suspended in water. Further, a relative upwards force as a result of the integral buoyancy is transmitted through pins 113 to the cables or chains 120. The whole fall pipe system 110 that is suspended in water will therefore experience an upward force as a result of the integral buoyancy. The amount of buoyancy can be chosen such that it partly compensates for a downward force due to the weight of the fall pipe system as experienced when suspended in water. By having the integral buoyancy a much longer fall pipe system can be employed than would be possible without buoyancy, since the effective weight is reduced. The weight of the fall pipe system is due to the weight of the individual fall pipe section 110, of the cables or chains 120 and of the driving arrangement 140. Each fall pipe section 110 of the fall pipe may be provided with integral buoyancy. However, the fall pipe may also be made up from fall pipe sections having integral buoyancy and fall pipe sections not having integral buoyancy. There may also fall pipe sections having different amounts of integral buoyancy be provided. Such various types of fall pipe sections are to be arranged along the fall pipe such that a fall pipe is made up having a desired distributed buoyancy along the fall pipe. The desired distribution and/or amount of buoyancy may depend on the specific circumstances of the location where stone and/or rock is to be placed using the fall pipe system.

Figure 3 shows a schematic large-scale view of a marine vessel 10 provided with a fall pipe system 100 according to the invention. The vessel is travelled to a location where stone and/or rock is to be placed on the bottom of the sea. The stone and/or rock may be taken with the vessel, provided by another vessel or supplied in another suitable manner. At the desired location the stone or rock is provided to the sea bed using the fall pipe system. The blocks of stone and/or rock R are placed at a sea depth SD of about 1,500 meter in the example shown, at which depth the fall pipe system according to the invention can be employed whereas known system provide too large static loads to be used at such depths. Figure 3 also shows the influence of dynamic loads due to, for instance, water currents. The example of figure 3 shows one current closer to sea level in a direction CDl and another water current near to the sea bed SB in an opposite direction CD2. Such water currents may pose large dynamics loads on the fall pipe system, which can be handled by the fall pipe system as disclosed.

An embodiment of the invention has been disclosed with one specific type of fall pipe and fall pipe sections. The invention may be employed with other types of fall pipes and fall pipe sections as well. Various other embodiments of the invention will be apparent to the skilled person when having read the above disclosure in connection with the drawing, all of which as within the scope of the invention and accompanying claims.