7. CONCRETE HULL DESIGN. 7.1 Main Features of the Design

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1 7. CONCRETE HULL DESIGN 7.1 Main Features of the Design The general arrangement of the concrete hull design is shown on drg OEA8/2. The plan dimensions and elevation are as agreed with the team at progress meetings and, except for the hull width, are similar to the dimensions of the steel hull design. The hull width has had to be increased slightly with respect to the steel hull width to produce sufficient buoyancy to support the additional weight of concrete structure. The concrete hull consists of twelve modules separated by watertight bulkheads each forming one side of the dodecagon. Each module is divided into six watertight cells by a horizontal mid height floor and two intermediate bulkheads. Each module supports one flexible membrane "bag" which is connected to a 1.5m diameter common manifold duct by a single branch duct. The central upper cell in each module contains the turbine cell. The manifold duct is terminated in a flange within the turbine cell for attachment to a 1.8m diameter turbine pod. An overall diameter of 2.0m has been allowed for the clearance dimensions of the turbine generator pod. Switchgear, oil coolers and other ancillaries will be located within the turbine cell around the turbine pod. The reverse curved front of the module forms a recess to suit the wave power extraction requirements. To enable the bag to be rapidly changed the rim of the recess is shaped to accept a bag attachment frame fabricated from steel. Because of the additional hull width in the concrete option there is no need to provide additional reserve buoyancy within the bag as there is with the narrower steel hull. Y

2 The hull is reinforced to carry the transverse panel loads from hydrostatic and hydrodynamic forces as well as other local loads and is circumferentially prestressed to carry the global heave and surge bending moments and torsion. A number of different prestressing schemes were investigated, each appropriate to a specific method of construction. Two schemes are shown on drg OEA8/2 and these and other methods are discussed below. 7.2 Strength and Serviceability Transverse Design The external panels in the upper hull were designed to span both horizontally between bulkheads and vertically between decks. The lower, curved panels were designed to act as membrane shells under hydrostatic pressure with a nominal transverse bending moment and shear but large in-plane forces. The hull has been designed for a static head of water 7m above the top deck with an additional hydrodynamic head equivalent to 4m of water. A load factor of 1.5 has been used at the ultimate limit state giving a factored ultimate design pressure of SOOkN/mm^ at mid deck level. Crack widths have been limited to O.lmm under the serviceability limit state with a load factor of unity. In the upper section crack widths were limited by using small diameter bars (16mm or 20mm) at fairly close centres (typically 125mm). At the ultimate limit state transverse shear was found to govern the design and this determined the panel thickness used. Although it was found possible to meet global stress limitations using 250mm thick panels this led to excessive shear stresses requiring extensive through-thickness reinforcement. Therefore the wall thickness was generally increased to 300mm. Even with the provision of haunches as shown on drg DEA8/2 some through thickness shear reinforcement is still required in the upper hull. Y

3 The lower section has been designed to act as a membrane and the same main reinforcement has been conservatively detailed for the lower hull as for the upper hull. Through thickness reinforcement, however, is not required in the lower hull Longitudinal Design The hull is designed as a class I prestressed concrete structure in the longitudinal direction; that is no longitudinal tensile stress is permitted under serviceability loads. Because of the increase in hull width and 300mm thick walls a uniform prestress of only BN/mtn^ is required to satisfy this condition. Two basic methods of prestressing are suitable for this type of structure; external or internal stressing. Both methods are shown on Drg DEA8/2. In both methods the tendons are located within the hull and protected from the marine environment. However, whereas internally stressed tendons are subsequently grouted to form a continuous bond with the concrete, external prestressing tendons are outside the concrete structure and are physically connected to it only at the anchorages and at the saddle (or deflector) points. In this case saddles are located at the corners of the dodecagon. The difference between the systems is only apparent at the ultimate limit state when tension is allowed in the concrete. Once the concrete cracks the bonded tendons of the internal stressing system are strained locally and act as reinforcement across the crack whereas the unbonded tendons of the external system only experience the average strain between anchorages and are usually considered as applying a constant compressive force. Therefore, if the design load is exceeded and tension is induced joints could open. This problem has also been encountered when designing externally prestresses bridges. The solution is to provide unstressed reinforcement in the longitudinal direction in addition to the prestressing tendons. This unstressed reinforcement is strained locally once the concrete cracks and limits the maximum crack width. In precast segmental construction reinforcement has to be introduced across the joints between precast members once the segments have been assembled. Y

4 The need for this relates to the risk of the design event being exceeded and to the consequences. Unlike bridges, human life would not be at risk, and it might be shown that the likelihood of the design event being exceeded is small enough to ignore. If external prestressing is selected then this should be the subject of further study. 7.3 Construction Considerations Construction Facilities All considerations of construction methods are dominated by the method of transferring the device from the casting bed into the water. The difficulties are somewhat greater for the concrete device, with its greater draft, than for the steel device. Although some existing ship-building facilities are of a suitable size to accept single circular clam devices they are traditionally dedicated to steel construction. Existing offshore construction basins which could be used for concrete construction are generally much larger than required for small scale production. It might be physically possible to use an existing shipyard slipway for constructing concrete devices with an extension to existing ways but this has been assumed to be ruled out because of the considerable disruption which this would cause to the shipyard's normal business and the different craft which would have to be employed. Five basic methods of construction of the device have been considered:- 1. Construction in a large construction basin capable of handling complete devices (either existing or purpose made). 2. Construction in 1/4 or 1/3 segments in a small disused graving dock or slipway, and joining segments afloat. Y

5 3. Launching using a submersible pontoon. 4. Use of a purpose made factory pontoon with submersible capability. 5. Use of a temporary steel pressure dome. The four options are considered in detail below. It has not been possible to select a single preferred solution at this stage since this will depend not only on the number of devices to be constructed but also on the availability of different facilities at the time of construction. Construction Basin The use of an offshore construction basin such as Hunterston or Ardyne Point would provide the most straightforward method of construction. However, these facilities are much larger than required to construct individual devices. Hunterston for example, would contain up to six devices at a time and because of its cost it would only be economic to construct all six devices at once. Therefore to get any benefit from economies of scale it would be necessary to construct several sets of six devices. Large existing basins are generally closed with a sand bund which has to be dredged away and reinstated for each use. This operation alone will add over 100,000 to the cost of each device on top of the rental of the facility and will severely interrupt the construction programme. However, using this type of facility introduces no constraint on the device design which can therefore be optimised independently. Costs for rental and operation of suitable existing or purpose built facilities suitable for constructing approximately 20 devices range from about 300,000 to 600,000 per device. Y

6 Graving Dock or Slipway By constructing the device in segments the required width of dock is reduced. With four segments for example the width is reduced to about 14m. This is relatively modest and there are a number of suitable drydocks which could accommodate the device split into either 1/4, 1/3 or 1/2 segments. Many of these smaller graving docks are now disused, and one of the likely problems would be to ensure that the gates are still operational. However, as long as the walls are still stable it would be possible to use the docks a limited number of times with a sheet pile closure. A number of docks are long enough to contain a sufficient number of segments to form one or more complete devices when assembled. There would, of course, be additional costs incurred in the device itself since it would have to have double bulkheads at the ends of the segments and would have to be assembled afloat. It is proposed the segments would be connected by means of Macalloy prestressing bars between the two end bulkheads. The individual segments would be prestressed using internal stressing which would be anchored using conventional anchor blocks at each end. Segments would be brought together in calm water so that the external walls seal together using a rubber gasket. The water between can then be evacuated, compressing the seal and allowing man access to install the Macalloy prestressing bars. Thus, although the cost per device of a suitable graving dock has been assumed to be only 240,000, a substantial additional cost has been allowed for additional permanent works costs and other establishment costs. Y

7 Submersible Pontoon A number of suitable submersible barges exist which could be used to transfer the device into the water. Two alternative methods are possible:- o Assemble device segmentally on beached or floating pontoons, o Slide completed device onto pontoons after construction on land. Both options require access from an adjacent quay with relatively deep water alongside. There is no barge with a large enough beam to support the full device width so if the device is to be assembled on pontoons either two pontoons will have to be beached side by side or a beam grillage will have to be used on top of the pontoon deck to support the segments before they are stressed together. Segments could be counter-cast and epoxy glued or joined with narrow in-situ concrete infill sections prior to stressing. The alternative of sliding the device onto the pontoon is the normal method of launching offshore steel jacket structures, which can be much heavier than the Clam. The self weight moments induced in the device would be greater than the heave moments experienced during service but the stresses would not be any greater because they are still less than the surge moments which govern the operational design cases. By sliding the completed device onto the pontoon the hire charge for the pontoon would be minimised at the expense of additional cost of skid beams and skidding equipment. The budget cost of a thirty day hire for a giant class submersible barge including towage from Rotterdam/Stavanger to the West Coast of Scotland and back, and submersion, is 280,000. To this must be added the costs of skidding equipment or craneage and hire of Y

8 quayside facilities. However, since more than 50% of this cost is due to towage from Europe this could prove to be an economical solution if a number of devices were constructed at once and could all be ready for launch together. Factory Pontoon A development from the previous construction method would be to use a purpose-made submersible pontoon on which the complete device could be constructed, or alternatively a floating external "shutter" in which the device can be constructed. An outline specification for the pontoon would be that it should be approximately 65m square, submersible to 8m over the main deck with access towers arranged so that the device can be floated off. The floating shutter option would require less inherent buoyancy since the device could float at its final draft when constructed. However this option would require a complex articulated external shutter and collapsible internal shutters which it has not been attempted to cost. The pontoon or shutter would be moored alongside a jetty which would provide access for men and materials. Steel forms, batching plant, craneage and other civils plant could be stored on board for the pontoon option. Based on hire rates for similar displacement submersible pontoons the annual cost of such a facility is estimated to be in the region of 1.3m. Therefore to make this method of construction competitive it would be necessary to achieve a turn around time of about 1\ months per device. This would appear to be impractical and could only be achieved with a large investment in mass production techniques appropriate to a very large production run. This option was not therefore considered further in the present context. Pressure Dome In this scheme the devices would be constructed in a bunded area approximately 4m below MHWS in an area where the tidal range exceeds 4m. When completed a steel pressure dome with a lightweight steel Y

9 frame for handling would be lifted onto the device and bolted down to form an airtight seal. Initially the dome would be vented while the bund was removed and then, when the team was ready, the vent would be closed at low tide so that the air trapped inside the dome would be compressed as the tide rose. The device would float with a draft of about 3.2m leaving sufficient time at the top of the tide for the device to be warped into the channel. When in deep water the dome vent can be opened and the dome disconnected and lifted free. Since the dome is estimated to weigh in excess of 100 tonnes this will require a pontoon mounted shear legs or similar to be available. Alternative schemes have been considered using a submersible buoyancy unit, inside the device or converting the whole device into a hovercraft. Although both alternatives, are feasible, only the scheme described above has been costed Construction Sequence Whichever construction system is adopted, other than the counter-cast and glued segments, it is proposed that the device be constructed in three lifts using purpose made travelling forms. Each side of the dodecagon would probably be cast in three bays. The corner infills and the bulkheads would be constructed as a second stage operation, thus allowing the travelling forms to be used most efficiently. The lowest lift could be economically precast and lifted into position because of the complex shape of the soffit. Alternatively, where multiple use of a single facility is envisaged, purpose made soffit formwork could be used. When the dodecagon is completed the forms will have to be dismantled and removed either through a temporary hole in the roof or through a turbine access hatch. Y

10 An alternative method of construction using glued precast segments which would be counter-cast vertically and turned for assembly was also considered. This method might give a small saving in the amount of thermal cracking reinforcement but would require heavy craneage and would be more suited to the barge assembly scheme, when the cost of barge hire could be reduced to offset the additional cost, or to a very large production run. Reinforcement detailing will be affected by the method of construction. The preferred construction sequence using travelling forms does not allow starter reinforcement for the bulkheads to project through the wall form. This limitation is not critical since a full moment connection between the bulkhead and hull wall is not necessary. Starter bars can be placed against the form and bent out after stripping the form or screwed couplers could be used Prestresslng Method Both internal and external prestressing techniques are shown on drg DEA8/2. In either case the largest tendons practicable will give the most economical solution, and 18 or 19 O.Sins strand tendons have been selected. Larger tendons would require an increased bend radius which would cause problems at the corners of the device and would require larger jacks which would be more difficult to handle in the confined space within the hull. In all cases access must be provided to the anchorages for installation of tendons and jacking. The external tendons will be anchored inside the turbine cell so that the jack can be lowered through the access hatch. By using a helical arrangement of tendons all the active stressing anchors will be at mid floor level. The anchors can easily be reached by a trolley mounted jack. In the case of the internal stressing scheme, using the type of coupler anchorages used for cylindrical tanks, all the anchors will be located within recesses in the wall at four stations around the device. Each stressing station will be situated below a turbine access hatch and the mid-level floor will have to have a temporary opening to provide access to the lower anchorages. Y

11 The external tendon system has the advantage that every one of the 12 modules making up the device will have identical reinforcement, coupler anchorage layout and penetrations for the prestressing system. The internal tendon system requires four special coupler anchorage sections per device. This will require an adjustable travelling form to allow a local increase in wall thickness. A modified internal stressing system will be needed for the construction of the device in large segments joined afloat. In this case the tendons will be anchored and stressed at the exposed end bulkheads of the segments. Thus conventional anchors will be provided. A secondary system of high tensile threaded stressing bars will be used to couple the segments while afloat Prestressing Tendon Material The basic prestressing system assumed in this report uses high tensile steelwire tendons. However it must be realised that there are alternative materials for prestressing strand. There are two main alternatives: i) Glass fibre (Brand name Polystal) ii) Aramid fibre (Brand name Parafil and Arapree) Non-metallic prestressing tendons would seem to offer several significant advantages over steel tendons. The first and most significant advantage is that these materials are virtually non-reactive in aggressive environment, except for very strong alkali solutions. The second being that they are considerably lighter than steel tendons, having a specific gravity of between 1 and 2 depending on material used, compared with 7.8 for steel strand. A third advantage is that there are now several methods of monitoring the behaviour of glass fibre tendons using fibre optic techniques using its naturally good optical transmissive properties. Y

12 These materials have relatively low E-values and consequently high strains are required to develop sufficient stressing forces. This does make them suitable for post-tensioning techniques. Both aramid and glass fibres have no significant strain plateau after yield and it could be said that they are brittle materials. It is therefore recommended that they are stressed to only 40% of their ultimate load (especially in the case of glass fibre tendons which are subject to stress-rupture). However, the larger strains required for failure give rise to ductile behaviour of the composite tendon-concrete section. i) Glass fibre tendons (brand name Polystal) Polystal has a significantly higher ultimate stress than steel or aramid and even though the prestress is only 40% that of ultimate, the area of tendon required is approximately the same as that for steel. This, combined with its lower E value means that losses due to elastic shortening, creep and shrinkage of concrete are less than with steel tendons. However, the higher relaxation counterbalances these effects and give typical estimated losses 5% greater than steel tendons. ii) Aramid fibre tendon (Brand name Parafil and Arapree) Aramids have a relatively high E value being 60% that of steel. This, combined with stressing requirement of only 40% ultimate leads to a larger tendon area being used. Therefore as the tendons will have the same stiffness as steel, the creep, shrinkage and elastic shortening losses become comparable. However the relaxation losses are significantly greater for aramid tendons and will account for a difference in total losses of about 5-6%. It is also worth pointing out that as parafil does not bond with cement, it can only be used for unbonded tendons. Y

13 Recommendations The relative costs for the three types of tendon (steel, glass fibre and aramid) to carry a unit load are in the ratio 1:2:10 respectively and the non-metallic materials may therefore on the surface appear to be too expensive. But it is fair to say that, for this type of environment, further investigation when the go-ahead for detailed design has been given may be worthwhile. 7.4 Maintenance Regular structural maintenance of the concrete hull should include:- annual inspection of internal and external surfaces, stressing anchorages (where accessible) and mooring attachments. inspection of bag recesses as bag change-out allows. 3-yearly removal for inspection and replacement of a sample number of greased external tendons (not feasible for cement grouted tendons). It is generally cheaper to use cement grouted tendons, even for external prestressing. However, greased and sheathed external tendons have the advantage that they can be regularly removed and inspected or subjected to a test load. Cement grouted external tendons can be replaced, if necessary, by cutting into sections. If external tendons were to be adopted the question of whether greased or grouted tendons are to be used would be the subject of more refined study. Cement grouted internal tendons cannot be replaced but have an additional concrete protection and are therefore potentially more durable. The concrete hull requires very little ballast and although it can be trimmed to a limited degree by differentially inflating the bags it cannot be trimmed to bring one side out of the water. Since the amount of hull maintenance will be significantly lower than for the Y

14 steel option this will not affect structural maintenance costs but will make bag replacement more difficult. The consequence of this depends upon the predicted life of the membranes. Access for internal maintenance will be effected at sea through a 700m diameter manhole at one inside corner of each power compartment. Only minor internal maintenance will be possible at sea but the turbine generator power pod can be removed for maintenance ashore through the 2m x 2m hatch provided (the internal diameter of the turbine is less than 2m - only the external clearance dimension is shown on the drawings). 7.5 Costing The cost estimate for the concrete circular Clam option has been built up from quantities, unit rates and method related charges. Specific unit rates have been derived from verbal or written quotes from the following sources:- Concrete System Formwork Prestressing RMC Glasgow (delivered to Kingston Yard); Steel Moulds Ltd; Balvac Whitney Moran (UK agents for VSL); Costs for labour, reinforcement, ties, waterbar and sundry items have been derived from published rates. Budget estimates for construction facilities have been based on the use of the following sites or plant:- Offshore construction basin Large drydock Barge hire Hunterston Inchgreen Drydock (Clyde); Smit Tak BV "Giant" Class barge. Estimates for green field site facilities have been derived from designs for casting basins for the Mersey Barrage, Thames Barrier and the Rio-Antirion crossing. Y

15 Cost estimates are for civil works only, assembled floating and moored off the west coast of the UK, and are based on a production run of 12 devices. The cost base date is the second quarter of The costs include no allowance for escalation or price contingency. Plant and labour costs are based on the use of a multi-trade gang, capable of prefabrication, assembly and stressing of the units. The basic costing was carried out for the large drydock facility where the gang would have to assemble and stress the floating segments while the drydock was being dewatered for the next batch of devices. At this preliminary stage a contingency of 15% has been added for both the design and construction. Prices include supervision and main office overheads, but do not include design costs. Three prestressing options were considered:- 1. External prestressing (as shown on drg. DEA8/2) 2. Internal prestressing (as shown on drg. DEA8/2) 3. Internal prestressing in large segments Five construction facilities were considered:- A. Green field casting basin; B. Existing offshore construction facility; C. Existing drydock; D. Submersible pontoon launch. E. Pressure Dome Flotation. The matrix of estimated device costs for the various combinations of prestressing and construction facilities is given in Table 7.1. Y

16 Table 7.1 Estimated Concrete Structural Costs ( mi11ion in 2nd quarter of 1991) Facility 1 Prestressing Option 2 3 Option Cost A B C 0.21 N/A N/A 1.25 D E Costs given in Table 7.1 are based on a limited number of devices being constructed, and could be reduced somewhat for mass production. It is likely that the green field site costs, in particular, could be reduced substantially for a large production run. Other facility cost estimates are based on rental charges, and will not be significantly affected by the number of devices constructed. Y

17 A typical breakdown of the costs is given below for option B2 in Table 7.1. Item Em/device Plant Labour Concrete Formwork Reinforcement Prestressing Ancillaries Contractors Charges & Margin (33%) Contingencies (15%) Facility Shell Duct Power Tube Notes: 1. Cost of materials are for delivery to site including allowance for waste etc. 2. All costs associated with placing or erecting concrete, formwork and reinforcement are costed under the headings of plant, labour and ancillaries. 3. Ancillaries are defined as all consumable items not covered elsewhere including inserts, embedded metalwork, access covers 4. Contractors Charges and Margins includes: Supervisory Staff Offices and equipment General Plant and Labour Insurance and Bonds Head Office Costs Profit Engineers Requirements Y