GB2199290A - Auxiliary buoyancy for off-shore operations - Google Patents

Abstract

Reusable inflatable buoyancy bags having a flexible membrane are used for many offshore operations requiring a temporary application of buoyant force on offshore structures. Circumstances include launch buoyancy, uplift of floating structures for the purposes of maintenance, and the application of buoyancy during the removal of life expired bottom standing structures.

Description

IMPROVEMENTS RELATING TO OFFSHORE OPERATIONS The invention relates to auxiliary buoyancy for offshore structures.

Many offshore operations depend on provision of auxiliary buoyancy for floatation. One such typical operation is platform installation, where the platform''s inherent buoyancy is often insufficient for safely manoeuvring- and changing the platform's attitude (by upending) prior to- setting the platform down on the seabed. To increase the platform's stability during this floating phase, platforms installed to date have been fitted with buoyancy tanks and tubes, which have heretofor been fabricated in steel.

Steel buoyancy tanks have a number of drawbacks. The high density of steel makes it difficult in practise to obtain a buoyancy-to-weight ratio of the fabricated tanks of more than 4:1. and even this ratio is only obtainable for large unit tanks. Increased safety against accidental flooding of a tank can be provided by internal subdivision of the tank with bulkheads; however, this is only done at the expense of increased tank weight and fabrication cost. The problems with steel tanks can be summarised as follows: o high fabrication costs; o only large unit tanks give an economical buoyancy to weight ratio.

o large unit tanks are heavy and therefore troublesome to handle during fabrication, fitting, and removal after use; o large unit tanks are potential safety risks in the event of accidental flooding.

The invention provides auxiliary buoyancy for an offshore structure, such buoyancy comprising an inflatable float constructed of a flexible membrane with an inflation/deflation connection, and a plurality of attachment points moulded into the material of the membrane whereby the float may be attached to the structure.

The membrane may in its inflated form be shaped to conform with the exterior profile of a member of the structure.

The invention may be applied to a jacket structure requiring auxiliary buoyancy only at the time of its launch.

The invention may be a applied to pontoon members of a semi submersible structure requiring auxiliary buoyancy only during maintenance periods.

The invention may be applied to a platform sub-structure requiring auxiliary buoyancy only during removal following abandonment.

The invention of pressurised reinforced rubber buoyancy bags has evolved from an appreciation of the problems with steel tanks.

Compared to steel tanks, buoyancy bags made from rubber are expected to have the following advantages: The price per unit of net buoyancy provided is less than half of that of steel. This is based on existing rubber technology. Additional cost reductions due to increased productivity and new technology could be expected if rubber bags were fabricated in sufficient numbers.

Both cost and practical construction considerations will favour rubber buoyancy bags of moderate size. The buoyancy required for a certain operation will therefore be made up of many more units than in the case with steel tanks. As a result, the handling of the tanks will be easier, and the factors of safety can be increased.

Specific embodiments of the invention will now be described by way of example with reference to a more detailed description of the application of reinforced rubber buoyancy bags to three offshore operations: o installation of fixed steel platforms (Figures 1 to 3); o inspection of floating production facilities (Figures 4 and 5); o removal of fixed platforms.

These three operations have been identified as main application areas for reinforced rubber buoyancy bags; however, applications to other offshore operations are likely. Outside the oil and gas industry, for instance, reinforced rubber buoyancy bags might be of assistance in the installation of tunnel sections for roads or railways crossing under waterways.

A typical installation sequence for a fixed steel platform substructure (jacket) is illustrated on Figures 1(a) to 1(h). As indicated on the figure 1(c), a jacket 11 is launched off a barge 12 that has carried it to the offshore oil or gas field. After launch (Figure 1(d)), the jacket is upended (Figures 1(e) to 1(9)) and manoeuvred to the precise installation location, before being finally ballasted down to the seabed. (Figure lh) The jacket on its own is normally very close to being neutrally buoyant, i.e. the weight of the jacket nearly equals its buoyancy.

However, to perform the launch and upending with acceptable clearance of the seabed and sufficient stability, the jacket is required to have a reserve buoyancy (buoyancy in excess of jacket weight) of 20-30X depending on the water depth, the jacket configuration etc.. This reserve (additional) buoyancy is provided by buoyancy tanks 14 also indicated on Figure 1.

It is these auxiliary steel buoyancy tanks 14 which are to be substituted with pressurised buoyancy bags made of reinforced rubber.

Figures 2 and 3 indicate a possible scenario for fitting horse-shoe-shaped rubber buoyancy bags 15 to a flotation leg 16 of a 'self floating' tower 17. The bag sizes, shapes and attachment methods are developed to suit the tower.

Besides the direct cost savings arising from the cheaper production cost of rubber, substituting steel tanks with rubber buoyancy bags has a whole range of other advantages: In order to be economical, the steel buoyancy tanks are large units and consequently large forces must be transferred into the jacket or tower at the tank connections. This requires extra strengthening of the primary structure. The rubber buoyancy bags are much smaller units, and can be shaped such that the forces are transferred directly into the main structural members as shown on Figure 3.

The weight of steel tanks such as those shown in Figure 1 is typically 5-10% of the overall jacket launch weight. This extra weight has to be supported by the jacket with the result that jacket members have been dimensioned specifically for dynamic loads which the buoyancy tanks induce in the jacket during transportation to the offshore field. With the present invention, leaving the bags uninflated until just before launch or installation will reduce dynamic loading while the jacket 11 is on the barge 12. Substituting the steel tanks with rubber buoyancy bags reduces the auxillary buoyancy weight and associated load which the jacket will have to carry to less than one quarter of that of steel, and hence the jacket itself can be reduced in weight.

After the jacket is set on the seabed, it relies for a period of time on its temporary foundation (mudmats) until piles can be driven into the seabed and grouted connections can be made between the jacket and piles. The predominant load that determines the size of the temporary foundation mudmats is the wave loading. The wave loading on a jacket with temporary steel buoyancy tanks is often many times higher than the wave loading on the primary structure of the jacket alone.

However, the removal of the buoyancy tanks is a lengthy task, and therefore the buoyancy tanks are normally assumed to be present for the mudmat foundation design. This would no longer be the case if the steel buoyancy tanks were replaced with pressurised rubber buoyancy bags. These bags could be deflated and removed individually after jacket installation and would therefore attract no wave loading.

Significant reductions in the mudmat supports can therefore be anticipated with the use of rubber buoyancy bags.

As already mentioned, steel buoyancy tanks are large units and therefore they have to be custom-built for a specific jacket such that the attachment brackets on the tanks fit the strong points on the jacket. Because the tanks are custom-built they can very seldom be reused for installation of a second jacket, and the whole cost of buoyancy tank fabrication must be written-off against just one jacket installation. This would no longer be the case with the proposed rubber buoyancy bags where the potential for reusability is much greater.

The commonly accepted safety level for jacket installation requires that the jacket can still be installed in the case of the oss of one buoyancy compartment. Although this criterion has, in general, proved adequate, jackets have been lost when more buoyancy compartments have failed than allowed for by the designer. For instance, one of the Frigg jackets in the North Sea had to be abandoned after buoyancy tank failure during installation.

The proposed rubber buoyancy bags are much smaller units and the consequences of failure of, say, two or three bags, are less severe than the failure of just one typical steel buoyancy tank.

Introduction of rubber buoyancy bags will therefore increase the safety level during jacket installation.

Cost studies have been made which indicate that significant cost savings can be obtained by replacing the steel buoyancy tanks with the proposed rubber buoyancy bags. The initial cost estimate showed the price of the rubber buoyancy bags to be 700 per tonne of net buoyancy provided; comparatively steel buoyancy tanks cost approximately 4,000 per tonne of fabricated steel.

The following two examples show a 10,000t jacket installed in 80m of water, and a 20,000t jacket installed in 170m of water, and illustrate the potential savings with rubber buoyancy bags; (both cost estimates include an allowance for valves, piping etc).

Note that the listed cost savings are for one jacket installation only, and do not take into account cost savings that could be made by reusing the buoyancy bags.

EXAMPLE 1 10,000t Jacket Installed in 85m of Water Item Savings Requirement 3,000t net buoyancy = 1,000t steel tank buoyancy weight: 4,000,000 (4,000/tonne of fabricated steel) = 3,000t net rubber buoyancy: 2,100,000 (700/tonne of net buoyancy) 1,900,000 Reduced Mudmat Foundation: 175,000 Redimensioned Jacket due to Reduced Transportation Loads 1,050,000 Total Savings 3,125,000 EXAMPLE 2 20,000t Jacket Installed in 170m of Water Item Savings Requirement 4,500t net buoyancy = 1,500t steel tank buoyancy weight: 6,000,000 (4,000/tonne of fabricated steel) = 4,500t net rubber buoyancy: 3,150,000 (700/tonne of net buoyancy) 2,850,000 Reduced Mudmat Foundation 700,000 Redimensioned Jacket due to Reduced Transportation Loads 2,450,000 Total Savings 6,000,000 The invention can also be applied to the inspection of floating production facilities.

An increasing number of offshore oil or gas field developments are developed with a floating production facility based on a converted semi-submersible drilling vessel as shown on Figures 4 and 5. When these semi-submersible vessels were originally designed, they were expected to be brought inshore regularly and inspected. They were therefore not designed with great emphasis on structural fatigue problems, as any fatigue cracks would be detected and reported at the regular inspection intervals.

In the vessel's new role as a floating production facility (Figure 4), this is no longer possible, as the vessel may be required to remain on station for the duration of the field life which is likely to be 10-20 years. As a consequence, the regulatory authorities require that, whilst in-field, the vessel must be able to be brought up to a shallow shaft where only the pontoons 21 are submerged, such that the connection between the columns 22 and the pontoons 21, and columns 22 and braces 23 can be inspected from the outside and repaired if necessary. (See Figure 5) To obtain this shallow draft, the buoyancy of the pontoons at the light draft will have to carry the whole weight of the vessel.In many cases the buoyancy of the pontoons is inadequate to equalise the platform weight, and pontoon extensions or cross pontoons must be added to the vessel before it can be used as a floating production facility. Permanent pontoon extensions increase the wave loading on the vessel, which may create structural problems and require upgrading of the mooring system, and so require quite expensive modifications to the vessel.

To avoid these modifications, either permanently fitted, or detachable rubber buoyancy bags can be attached to the pontoons and inflated whenever the shallow inspection draft is required.

The invention can also be applied to platform removal.

When an offshore oil or gas reservoir is depleted and is no longer economical to produce, a fixed platform must be removed. Removal of offshore platforms will become an increasing problem as many of the early fields are coming towards the end of their lives.

The way an obsolete platform can be dismantled can be broken down into two tasks: removal of the topsides modules, and then removal of the substructure (the jacket). Topsides modules can be lifted off by crane and will not be discussed any further here.

The substructure or jacket can be removed by one of two methods: o cut the jacket in pieces that can be removed by crane; o cut the jacket at seabed and re-float the whole jacket.

The first option involves a large amount of subsea work cutting the jacket into pieces that are light enough to be lifted. It will therefore be a quite lengthy operation that will tie up expensive offshore equipment such as an offshore crane vessel, and could also be subject to significant downtime due to adverse weather conditions. By default this option will be expensive compared to the second option of floating off the jacket in one piece. With this option the jacket could be floated to a deep inshore location where it could be cut into pieces, or it could be towed to a very deep midocean location to be dumped.

To re-float the jacket it must be provided with significant amounts of - auxiliary buoyancy, and it is for this function that inflatable rubber bags would be considered. The rubber buoyancy bags could be of the same type as those proposed for jacket installation; however, special emphasis must be paid to the way the bags could be attached to the structure and inflated with the least amount of subsea work.

Rubber has played a significant role in many products used in the offshore environment. Fenders, water-tight diaphragms, storage tanks for various liquids and grout packers are all typically made of rubber material. However rubber material is completely new in the role now proposed, where inflatable rubber bags are used to provide buoyancy for various offshore operations.

The only use of rubber in a remotely'connected type of application is the "Dracone" tanks fabricated by Dunlop for towing liquids offshore.

These Oracones have a circular cross section with a diameter of up to 3m except at the ends which are tapered off. They can be as long as 150m.

The development of the proposed inflatable rubber buoyancy bags will be based on the more than 40 years of experience gained with these rubber Dracones. However, the proposed rubber buoyancy bags for the described applications are completely different from the Dracones on a number of points: o the buoyancy bags will be inflated to a much higher pressure (possibly up to 10 atm). New materials for reinforcement of the rubber are required; O the buoyancy bags must have attachment brackets moulded into the rubber. Only one towing bracket is required for a Dracone; o the round cross sectional shape of a Dracone is not the optimum for buoyancy bags, and new shapes are needed that are feasible from both an operational and fabricational point of view.

o the buoyancy tanks are subject to a much larger variety of loads and must therefore be much more thoroughly analysed and designed.

Further specific examples of buoyancy bags were disclosed in OTC paper 5603 and are included below by way of illustration only. It is not intended that these specific examples should be within the scope of omnibus claim 6.

The rubber buoyancy bags must be inflated such that the internal pressure is always in excess of the external pressure to maintain shape and total volume.

The bags must therefore be pressurised prior to platform installation to a pressure that is at least equal to the maximum hydrostatic pressure. High internal pressures are therefore generated in the buoyancy bag structure, giving high membrane hoop stresses in the rubber skin. It is this load condition that governs the design of the buoyancy bag. Naturally, the design of the buoyancy bags will vary according to the maximum hydrostatic pressure the bag should be able to take, and three different designs have been identified: o a lw pressure buoyancy bag (- 1 bar); o a medium pressure buoyancy bag (- 7 bar); o a high pressure buoyancy bag (- 10 bar); For all three designs, safety factors on tensile strength have been set to 3.5 or more.In this context it should be noted that the maximum differential pressure, and thereby maximum stresses in the bag, occurs prior to submergence into the water. A bag rupture at this stage when the jacket is still on the barge will not be critical for the platform installation. The three conceptual designs, introduced above, are discussed individually below.

Low Pressure System Concept (Figure 6) In this context a low pressure inflatable structure is one that will withstand a pressure in free air, or a differential pressure of 1 bar.

This can be achieved using tyre cord materials with cords of either polyester or polyamid.

The most efficient shape in terms of volume/unit area and strength of material is a sphere. To transfer the buoyancy forces to the attachment point 103 requires a substructure on the rubber bag which could, for instance, be- a metal cage of webbing harness/net, the latter choice being inherently buoyant. In all cases, the optimum shape of harness was found to be conical. Combining these two factors yields a profile consisting of a hemisphere 101 combined with an inverted cone 102. The displacement volume, and hence available buoyancy can further be improved by: o constructing the conical section of similar material to the sphere; o the incorporation of a cylindrical section.

This would produce the shape shown in Figure 6. A typical structure of approximately 2.5m dia x lom long would give a buoyancy of approximately 40 tonne per unit.

The low pressure buoyancy bag concept is applicable to lift installed jackets only. The bags must be attached at or close to the top face so when the jacket is lifted in the water, the hydraulic pressure experienced by the bag is kept below 1 bar. The bags are likely to be deflated prior to upending to avoid them going through 90 degree rotation.

Medium Pressure System Concept (Figure 7) In this context a medium pressure inflatable structure is one that will withstand a pressure in free air, or differential pressure1 of approximately 7 bars. Where there are large depths of submergence, the feasibility study showed that applications for this concept are limited, as it is not suited for launch operations and it cannot be inflated during a 90 degree platform upending sequence. This conclusion was reached by considering the factors discussed below.

The precise selection of material is governed by the relationship between the diameter of the structure, the internal pressure and the acceptable hoop stress which the material can withstand. Using the polyester or polyamid materials recommended for the low pressure system would lead to very small buoyancy bags, and very stiff material would give both handling and fabrication difficulties.

Stresses in the primary bag material should therefore be relieved by enclosing the inflatable buoyancy bag 104 in a very high strength webbing harness or by a lightweight steel cage 105. This harness or cage would also be the means of attaching the buoyancy to the platform. Figure 7 shows a large diameter bag (3m diameter) based on this concept.

High Pressure System Concept (Figures 8 and 9) The high pressure inflatable structure concept is considered for differential pressures up to at least 10 bars (equivalent to 100m water depth). Figure 8 shows an example of such a high pressure system.

The system consists of two parts as indicated on Figure 9. Firstly an outer skin 106 (wrapped around the jacket leg or brace member 107) made of a heavy duty tyre cord material capable of withstanding the hoop stresses associated with a 10 bar pressure; and secondly an inflatable bag 108 in light tyre cord material filling the annulus between the jacket member and the outer skin. The system can be compared with that of a car tyre with inner tube, where the tyre carries the loads and the tube only serves as a containment for the pressurised air.

Locking the two parts of the outer skin together, after it has been wrapped around the jacket member, forms a special problem and various alternatives are currently being investigated. After fastening around the leg, the outer skin should be mechanically fastened to the jacket member at the ends to allow its position to be held during initial installation. Once inflated the internal pressure would give sufficient friction force against the jacket member to keep the structure in position.

As the hoop force in the outer skin is directly proportional to the external diameter of buoyancy leg, it is clear that the diameter is one of the limiting factors in this concept. It is therefore most efficient, in terms of buoyancy provided, if attached to a small diameter jacket member, e.g. brace-members.

With the direct load transfer into the jacket members this concept can be used for launched jackets and is independent of the jacket member orientation such that it can be kept inflated during upending.

Claims (6)

Claims:

1. Auxiliary buoyancy for an offshore structure, such buoyancy comprising an inflatable float constructed of a flexible membrane with an inflation/deflation connection, and a plurality of attachment points moulded into the material of the membrane whereby the float may be attached to the structure.

2. Auxiliary buoyancy as claimed in claim 1 i-n which the membrane is in its inflated form shaped to conform with the exterior profile of a member of'the structure.

3. Auxiliary buoyancy as claimed in claim 1 or claim 2 when applied to a jacket structure requiring auxiliary buoyancy only at the time of its launch.

4. Auxiliary buoyancy claimed in claim 1 or claim 2 when applied to pontoon members of a semi submersible structure requiring auxiliary buoyancy only during maintenance periods.

5. Auxiliary buoyancy as claimed in claim 1 or claim 2 when applied to a platform sub-structure requiring auxiliary buoyancy only during removal following abandonment.

6. Auxiliary buoyancy substantially as hereinbefore described with relevance to and as shown in Figures 1 to 5 of the accompanying drawings.

Amendments to the claims have been filed as follows Claims: 1. Auxiliary buoyancy for application to the exterior of a tubular member of an open lattice framework in an offshore structure; such buoyancy comprising an inflatable float constructed of a flexible membrane with an inflation/deflation connection (to facilitate inflation of the membrane from outwith the member), and a plurality of attachment points moulded into the material of the membrane whereby the float may be attached to the member.