OMAE Large Marine Lifts in Shallow Water by Alan P. Crowle CB&I

Transcription

1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE Large Marine Lifts in Shallow Water by Alan P. Crowle CB&I SUMMARY Large energy developments are taking place near shore, in locations around the world, including LNG jetties, oil production in shallow water and renewable energy projects. Crane vessels of all sizes are required to install the component parts for these projects. This paper explains current techniques and the design requirements to carry out the lifting of large units required for shallow water installations. Recent developments have seen the introduction of new vessels for offshore wind farm installation and their features are discussed. NOMENCLATURE DAF Dynamic Amplification Factor DOF Degree of Freedom DP Dynamic Positioning FPSO Floating, Production, Storage and Offloading PAU Pre-assembled Unit ROV Remotely Operated Vehicle Rpm Revolutions per Minute SEP Self Elevating Platform SSCV Semi Submersible Crane Vessel TIP Transportation and Installation Vessel 1.0 INTRODUCTION This paper discusses floating crane vessel requirements for shallow water installation of inshore pipelines, jetty construction, single point moorings, wind farms and shallow water oil and gas installations. In addition, marine heavy lifting is covered for construction yards, in particular during FPSO construction. The types of vessels considered in the paper are sheer-legs, SSCVs, self elevating platforms and cargo barges outfitted with crawler cranes Offshore wind turbines and marine current turbines are most likely to be installed from either a self-elevating platform or a sheer-leg crane vessel. The choice will depend on the water depth, the crane capability and vessel availability. The crane must be capable of lifting the structures, with hook heights greater than the level of the nacelle to enable the tower and turbine assembly to be installed. Existing crane vessels have generally not been specifically designed for installing offshore wind turbines. For large offshore wind farms, significant time (and therefore cost) savings can be realised by using an installation vessel purposely built for the task. This philosophy has been adopted elsewhere in the civil engineering industry for port and jetty construction. Significant offshore developments are taking place in the northeast Caspian Sea, where the water depths are shallow(less than four metres) but the equipment to be installed is large. This paper will identify the different methods available for the installation of equipment using crane vessels in shallow water. 2.0 SHALLOW WATER 2.1 Definition Shallow water is of such a depth that bottom topography affects surface waves. There are effects as well on current. When moving installation vessels to the site then squat is also an issue. Shallow water depth is defined as the condition in which surface waves are noticeably changed by bottom topography; typically this implies a water depth equivalent to less than half the wave length. An alternate definition would be based on developing marine terminal facilities. An LNG carrier typically has a maximum draft of 12 metres. Allowing 10% of draft for underkeel clearance this gives a water-depth at low water of 13.2 metres. There is no exact definition of shallow water but for the purposes of this paper shallow water is considered as being less than 15 metres. 2.2 Waves As waves enter shallow water, they become taller and slow down, and change shape, eventually breaking on the shore. At a depth of half their wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen. Although their period (frequency) stays the same, 1 Copyright 2011 by ASME

2 the waves slow down and their overall wave length shortens. The 'bumps' gradually steepen and finally break in the surf when depth becomes less than 1.3 times their height. Note that waves change shape in depths depending on their wave length. Surface waves can be bent (refracted) or bounced back (reflected) by solid objects. Waves do not propagate in a straight line but tend to spread outward while becoming smaller. When a wave front is large, such spreading cancels out and the parallel wave fronts are seen travelling in the same direction. When a lee shore exists, such as inside a harbour or behind an island, waves can be seen to bend towards the shore. In the lee of islands, waves can create an area where they interfere, causing steep and hazardous seas. When approaching a gently sloping shore, waves are slowed down and bent towards the shore. However, when approaching a steep rocky shore, waves are bounced back, creating a confused sea of interfering waves with twice the height and steepness. Such places may become hazardous to shipping in otherwise acceptable sea conditions. As waves move toward shore and into shallow water they begin to feel the ocean bottom. And in turn the ocean bottom causes friction; it slows the wave down from the speed the wave was moving in deep water. It turns out that the greatest wave slow down is in that part of the wave closest to the ocean bottom where bottom friction is greatest. The least slow down is at the air-water interface at the wave crest, farthest from the seafloor. The result is that the wave crest begins to move faster at its top than at the wave's base. Eventually this speed difference causes the wave to tip forward and the wave breaks. 2.3 Currents An opposing current increases wave heights and reduces wave period. Conversely, it is found that a following current decreases wave height. 2.4 Tides Tide height variation has to be taken into account in all operations at sea and in shallow water can be the dominant natural condition. In all situations shallow water tides are a convenient way of describing the shape of the tidal wave through a combination of harmonic waves. Physically, the tidal wave is still experienced with the same basic period but appears deformed, with a shorter rising tide than falling tide. When this process is driven to extremes (as a result of particular topographic circumstances), the rising tide can take on the form of a wall of water which travels up the estuary, causing a nearly instantaneous rise of water level as the water wall passes. This phenomenon is known as a bore. Tidal bores can cause great inconvenience to small vessels. When a bore occurs in an otherwise navigable estuary, ship traffic comes to a halt for significant parts of the day. Port authorities then try to eliminate the bore by changing the shape and depth of the estuary. 2.5 Wind Wind is an important agent in coastal processes. It acts in various ways: Wind causes storm surges in combination with variations of the barometric pressure. It generates waves, at sea as well as at inland waters. It generates drift currents; mainly in shallow water. It transports sand in the coastal zone. 2.6 Squat The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure under its keel that causes the vessel to squat lower in the water than would otherwise be expected. This is due to a reduction in buoyancy caused by a downward hydrodynamic force created by flow-induced pressures. Squat can lead to unexpected groundings and handling difficulties. This phenomenon is caused by hydrodynamic effects between the hull of the ship and the sea floor. Squat effect is approximately proportional to the square of the speed of the ship. Squat effect is usually felt more when the depth/draft ratio is less than four or when sailing close to a bank. When water that should normally flow under the hull encounters resistance due to the close proximity of the hull to the seabed, it causes the water to move faster, especially under the bow of the vessel, creating a low-pressure area. This counteracts the force of buoyancy, causing the vessel to dip towards the bow. The reduced pressure on the bottom of the vessel draws the vessel slightly downward until the increased displacement counteracts the force generated by the reduced pressure. Signs that a marine vessel, e.g. a tug, crane vessel or cargo barge, has entered shallow water conditions can be one or more of the following, Reference 3: Maximum vessel squat increases with speed Mean bodily sinkage increases with speed The vessel will generally develop extra trim by the bow or the stern, depending on shape Wave-making increases, especially at the forward end of the vessel The vessel becomes more sluggish in responding to the rudder For a self-propelled vessel in a confined channel, there is a decrease in rpm There will be a drop in speed. The vessel may start to vibrate suddenly. This is because of the entrained water effects causing the 2 Copyright 2011 by ASME

3 natural hull frequency to become resonant with another frequency associated with the vessel. Rolling, pitching and heaving motions will all be reduced as the vessel moves from deep water to shallow water conditions. This is because of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel. The appearance of mud could suddenly show in the water around the vessel s hull, for instance, passing over a raised shelf or a submerged wreck. For a self-propelled vessel or a towed barge, turning circle diameter increases. Stopping distances and stopping times increase. Effectiveness of the rudder helm decreases. Width of the wake increases considerably. 2.7 Ice During the exploration of the Beaufort Sea in the 1970s and 1980s, and more recently in the northeast Caspian Sea, considerable attention was paid to the large accumulations of broken ice pieces (ice rubble) that are often grounded around offshore structures. Grounding of ice rubble can lead to both an increase and a decrease of the ice load. The load increase occurs during the initial stage of the underwater pile-up formation, when its draft approaches the water depth. In the later stage, with the stationary grounded pileup against the platform, ice loads are reduced. 3.0 CRANE REQUIREMENTS 3.1 Floating Cranes and Jack Ups Floating cranes are found in most sizeable ports and are an essential element in the port s cargo handling equipment. Floating cranes are used to assist installation work in estuaries and inshore shallow water. They have a special role in loading and unloading specially heavy or awkward loads on and off ships, which may not have their own heavy lifting equipment, since these loads are carried only occasionally. A floating crane, as opposed to one that is mounted on road wheels and a chassis, is particularly useful, as the quay might not be sufficiently strong to bear the weight of very heavy loads, and there might be obstructed access to the quayside. Invariably, it will be much more economical if the crane can be brought to the ship, rather than a ship having to be moved to a fixed heavy lifting installation. A floating crane vessel may be self-propelled, as it will have powerful diesel generators to work the crane winches which can be switched to propel the craft when moving between jobs. Otherwise, a tug will be used to tow the crane vessel. The crane itself will be mounted on a wide and very strong pontoon barge, flat bottomed but well subdivided into many tanks. A crane vessel will usually have the facility to move ballast water around in the hull to prevent it from listing when a load is lifted. Recent years have seen an increase in the size and capacity of floating cranes, which have grown to meet demand from the construction and offshore industry. The craft which were once found lifting 300 tonne loads on and off ships have grown into gigantic work vessels capable of lifting whole bridge sections and installing modules over 10,000 tonnes for offshore platforms. The enormous capacity of these cranes enable engineers to assemble very heavy sections off-site, which is much safer and infinitely faster than if the whole structure had to be carried in small pieces to the construction site. A growth in heavy engineering projects, such as major bridge building, sea defences and major port construction has seen a substantial fleet of these craft built. 3.2 Salvage Large floating harbour crane vessels have been used for salvage, jetty construction and lifting of cargo in port. Sheerleg cranes are also well suited to marine salvage work in shallow water. 3.3 Port floating cranes Floating crane solutions have been a key part of handling operations in congested ports and on major waterways for decades. Usually perceived as a supplement to landside bulk cargo operations or an aid to transshipment direct to the cargo barge, the traditional deployment of floating cranes to handle low value bulk cargoes has not been conducive to the development of new designs or technologies because stevedoring rates have not justified vast expenditure on customized solutions. Yet the rapid growth in global seaborne trade has left many terminals cramped and congested, while higher freight rates (and demurrage charges) have ramped up the pressure on loading speeds. Given that in most parts of the world it takes a lot less time and money to mount a crane on a barge or pontoon than it does to complete the permissions required for a landside development, a new market for a fast capacity fix has emerged. The development of new technology, initially used in the offshore oil and gas sector, is also now opening up new markets for floating solutions in dry cargo markets. Some equipment suppliers are now selling new generation floating transshipment options to the container sector, for example. And the challenge of exporting coal from countries with draft limitations has prompted the development of large, new floating transshipment terminals, quite often equipped with buffer storage, to cope with offshore ship-to-ship handling. In the case of container markets, the trend is to build larger and larger vessels. The floating container crane provides users with the option needed to make their operation more competitive and profitable by way of enabling them to break 3 Copyright 2011 by ASME

4 into larger shipment size markets to which they currently have no access. The barge-mounted concept could also be applied to nonbulk operations, much as a mobile harbour crane might switch between cargoes simply by changing the crane's attachment. The mobile terminal, which can be deployed on waterways and in the open sea, uses heavy-duty ship cranes mounted on barges or platforms and can be fitted with buffer storage for up to 200 tonne capacity. For container handling, boxes are stowed fore and aft to provide balance and to optimise both the crane cycle time by minimising the spreader slewing motion for container positioning and deck cargo capacity. To ensure adequate stability and longitudinal trim, large capacity ballast tanks are also fitted as are wing tanks and high power pumps to keep the pontoon and the crane within the acceptable heeling operational limit. The solution can be applied to bulk, multi-purpose and container handling in sheltered waters, including rivers and shallow drafts. 3.4 Wind farms Larger turbines and wind farm sizes require larger and longer sub-sea cables, and larger/more complicated foundations. Offshore construction methods are limited by water depth, cost, and installation rate and so it must be concluded that installation is the key to a successful project. The attributes of an offshore installation system can be summarised as: Sufficient crane capacity to minimise number of lifts and handle larger capacity turbines Ability to install at high rates in order to meet demand Minimal downtime due to weather and thus increase operating window Must be capable of operating in very shallow (3 4 metres) water depths Be self-supporting in the field in order to reduce costs of support vessels such as cargo barges or anchor handling tugs Capable of self loading and transporting its own cargo To work accurately and hold position in closely spaced wind turbines Onboard accommodation to allow round-the clockworking capability Easy mobilisation Typical wind farm installation work requiring lifting is as follows Installation of met-masts Installation of foundations Installation of tower, turbine and rotors Installation of substations Installation of scour protection Installation and burial of subsea cables It has been common practice to adapt vessels chartered from the offshore oil and gas sector. The vessels are rarely optimised for a wind energy role and tend to be available only at the charter rates. So far, there has been a tendency to install offshore wind farms in relatively shallow water using jack-ups. These structures are either towed flat barge-like craft or selfpropelled ship-shaped hulls which set down legs to the sea bed and jack themselves up to provide a stable working platform. These vessels must be equipped with long boom cranes and high crane capacity to erect tall turbine structures. Whether offshore wind turbines are installed from either a jack-up barge or a floating crane vessel, the choice will depend on the water depth, the crane capability and vessel availability. The crane must be capable of lifting the structures, with hook heights greater than the level of the nacelle to enable the tower and turbine assembly to be installed. Existing crane vessels have not been specifically designed for installing offshore wind turbines. For large offshore wind farms, significant time (and therefore cost) savings could be seen using an installation vessel purposely built for the task. 3.5 Marine current farms The lift vessel must have underwater hook capability and preferably be self propelled. The vessel will also need to deploy divers and remotely operated vehicles (ROVs). 3.6 Bridge constructions For bridge installation long boom cranes are required on one of the following types of crane vessel Sheer legs SSCV on pontoons 3.7 Oil and Gas The full range of installation vessels is required for shallow water installation work Self-elevating platforms Mobile cranes on cargo barges Sheer leg Monohulls SSCV on pontoons 3.8 Single point moorings Lift capacities of up to 100 tonnes are required and are usually associated with diving equipment. Self propelled crane vessels, with dynamic positioning are normally used for this purpose. 3.9 Inshore pipelines Pipeline installation vessels are usually barge-shaped and fitted with crawler cranes with crane capacities up to 50 tonnes. 4 Copyright 2011 by ASME

5 3.10 Jetty construction All types of crane vessels are used for jetty work including: (see figure 2) Spud barges (the legs of which are used to moor the vessel in one location, but are not used for jacking up) Self-elevating platforms (non self-propelled) Cargo barges (pontoons) with crawler cranes Sheer legs 3.11 Mooring in Shallow Water Generally, in case of catenary mooring in shallow water, slowly varying oscillations occur and mooring tensions become very large. Furthermore, if floating structures are set in shallow water, mooring tensions are raised by their vertical motions. Therefore, design methods for mooring systems by approximate analysis, simulation analysis and model experiment are required. From a global analysis perspective, shallow water mooring systems can be much more challenging to analyze and design than ultra-deepwater systems. This is due to the: Environmental loading on the system in shallow water "Hardening" nonlinear stiffness of the mooring system that at extreme offsets can result in large variability in loads Low level of associated damping in the system The complexity and challenges of shallow water hydrodynamic design are not always well understood, and software tools and global analysis methodologies are currently being developed for shallow water systems. The catenary system is the most common type of mooring system employed in shallow water. The catenary refers to the shape of a free-hanging mooring line assumed under the influence of gravity. The catenary system provides restoring forces through the suspended weight of the mooring lines and its change in configuration arising from vessel motion. In other words, under environmental loadings the moored vessel tries to lift the mooring lines, creating a restoring force. By catenary system the mooring line terminates at the seabed horizontally; the anchor point is only subjected to horizontal forces at the seabed Scale model testing Scale model testing is used where the structures are unique or where computer software is not fully developed. For shallow water tests the depth should be scaled correctly; this may impose a restriction on the maximum draft. At very small wave height to water depth ratio the roughness of the basin bottom should be less than 10% of the under keel clearance. The smoothness, stiffness and water pressure tightness of the bottom should be sufficient to not affect the results significantly. Measurement of 6-DOF motions of moored floating bodies exposed to varying wave conditions, namely: Non-contact digital video motion analysis Relative motion between two floating bodies Model system stiffness of mooring systems Measurement of loads on mooring lines, namely: Simulation of mooring line stiffness and mass per unit length Use of in-line load cells to quantify mooring line loads Accurate application of pre-tension to mooring lines Vessel Interaction in Restricted Waterways. The measurement of sinkage and trim and/or heave force and pitch moment experienced by vessels travelling in restricted water namely: Measurement of sway force and yaw moment experienced by a vessel passing alongside lateral banks to determine optimum channel configurations Measurement of resistance induced by the effects of restricted water Passing Vessel - Moored Vessel Interaction Measurement of moored vessel heave, pitch and roll Measurement of moored vessel surge force, sway force and yaw moment. The model in the foreground represents the moored vessel and the model in the background represents the passing vessel. Prediction of moored vessel surge, sway and yaw motions using a mathematical model which accounts for the dynamics of the moored vessel and mooring line and fender forces. Motions of Craft Operating in Waves in Shallow Water Environments Investigate the effect of water depth on the motions of vessels Investigate the influence of non-linearity on vessel motion 4.0 CRANE VESSELS 4.1 General Cranes are the most versatile pieces of equipment to handle heavy loads. They can be used for the load-out, load-in, assembly and final installation of Pre-assembled Units (PAU) and modules. The cautionary notes should be considered when choosing a crane vessel There are many different types of cranes and it is strongly recommended to be cautious with the figure of maximum capacity, since this equates only partially to the crane s real performance. Many cranes are often quoted in short tons whereas cargo to be lifted is in metric tonnes. Consider length of the boom as longer booms reduce lift capacity 5 Copyright 2011 by ASME

6 The term load moment is an engineering term which refers to the product of a force and its moment arm, see Figure 1. The moment arm is defined as a perpendicular distance between the force vector and a reference point. In the case of cranes, the force acts vertically through the centre of gravity of the load and the moment arm becomes the horizontal distance from this centre of gravity of the load and the centre of rotation of the crane. 4.2 Inshore Crane Barges with land based cranes Although relatively low in cost and with reasonable capacity, inshore crane barges have poor installation rate capacity with little or no cargo capability. This, along with poor bad weather operability makes such vessels only suitable for small scale, inshore working. The inshore barges are normally fitted with a crawler crane. The main points are: The crane can sometimes move around on the crane barge while fully-rigged Can add super lift to increase capacity Often tied down permanently The ground bearing pressure under the tracks can reach 100 Tonne/m2 for the largest machines. Therefore an extensive matting arrangement is usually required on the deck of the marine vessel. The one way to ensure an equal spreading of the load is a 10cm deep layer of sand and matting (single or double layer depending on the size of the load spreading mats) on top. 4.3 Sheerleg Crane Vessels See figures 3 and 4 showing typical Sheerleg crane vessels which benefit from very good lift capacity, and are very useful in installing transformer modules for wind farms. They are used for large lifts in harbours and estuaries and for jetty construction. Sheer-leg cranes are very useful in installing modules on floating, production, storage and offloading vessels (FPSOs), in sheltered water. However, their operability in open waters is often limited. 4.4 Semi Submersible Crane Vessel Semi Submersible Crane Vessels (SSCV) do have excellent capacity, however, their extremely high cost and large draft make them difficult to use in shallow water open sea conditions. However, some large lifts, e.g. bridge sections, have been carried out in sheltered harbours by SSCVs. The lift capacity is reduced by about 10% while operating on the pontoons and dynamic positioning may not be possible. Semi-submersible crane vessels represent the most stable floating platform from which to carry out offshore construction work. As wind turbines are now being installed in more than 30m of water, SSCVs are being used to install wind turbines. 4.5 Inshore Jackup Vessels They are known by various names including self elevating platforms (SEP), jack -ups and lift boats. In their simplest form, spud barges -- where the spuds are used for mooring and provide some measure of stability but do not carry the weight of the vessel. Spud barges have poor bad weather capabilities. SEPs have relatively low cost but with reasonable lift capacity. They also have reasonable bad weather capability but the level of infield support required, along with little or no transportation capability and onboard accommodation, means that installation capabilities are insufficient except for small scale developments. Self elevating jack-up lift appears at first glance to be the obvious method of installing the tower, nacelle and rotor of a wind turbine. It forms a stable base from which to carry out the operation and is the preferred choice for carrying out the piling operation. However, its lack of manoeuvrability can pose problems for the installation of the wind turbine tower. Offloading tower elements from a floating barge and lifting them into place will most likely require a form of piecemeal construction with the tower, nacelle and rotor all installed as separate items. The same jack-up barge can be used for driving the monopile and for installing the turbine. Purpose-built vessels for installing offshore wind turbines, foundations and transition pieces have recently been constructed, reference 1. These new wind installation vessels offer a combination of tested technologies, including self-elevating platform, rotating crane, self propelled and in some cases dynamic positioning (DP). Sometimes referred to as Transport and Installation vessel (TIV), they also have open deck space to transport the cargo to be lifted. The newer vessels have permanent accommodation. 4.6 Heavy Lift Ships (Geared) There is a modern fleet of specialised heavy lift vessels, fitted with permanent cranes. Typical characteristics are: Harbour lifts, combined with long sea voyages, have been used for large cargoes such as reactors of 1,000 tonnes and more, e.g. fragile gas turbines for a new power plant, sailing yacht, single point mooring buoys All are equipped with their own lifting gear with capacities ranging from 500 to 1,800 tonnes Their transit speed is high and some can sail with open hatches. The vessels have shallow draft and are able to work in almost any port in the world. A unique feature of these vessels is their load moment. New ships have 2 times Copyright 2011 by ASME

7 tonne capacity per crane at a 25 metre outreach. Some new vessels are outfitted with DP2-systems and can be used for unloading cargo at sea. 4.7 Monohull with Revolving Cranes Ship shaped vessels have capacities up to 5000 tonnes and are very manoeuvrable. As a result, they are in heavy demand and attract appreciable day rates. 4.8 Harbour cranes A new generation of harbour cranes has been developed recently for loading/unloading bulk cargo. They are also used to assist in ship construction and ship repair. 5.0 DESIGN METHOD Check list for the various elements of the installation are described below 5.1 Cargo The following general data is required on the cargo: Outside dimensions Weight for load out, transport and lift Centre of gravity Radii of gyration (input for calculating motions in waves) Location of lift points Grillage and sea-fastening support locations Structural design of the cargo to include: Overall strength of the cargo Local design of padeye/padear Strength of spreader beam, if any Underwater lifting loads: Buoyancy to be calculated for different drafts Centre of buoyancy for each draft Hydrodynamic loads Wave slam 5.2 Cargo barge General data is required on the cargo barge: Length Breadth Depth Maximum allowable draft Lightship draft Ballasting arrangements Radii of gyration Transport: Does cargo fit on the barge, preferably with no overhangs Intact stability Damage stability (checking reduced freeboard/closeness to the seabed) Longitudinal strength Local strength Bollard pull Wind loads Motions in waves Mooring loads Loadout: Ballasting to keep cargo barge level with the quay Intact stability Longitudinal strength Local strength Mooring loads 5.3 Local conditions Metocean conditions are needed at the load out site, the transport route and at the installation site as follows: Water depths Tide tables for expected date of installation Wave heights, period, direction and persistence Current speed, direction and variation with depth Wind speed and direction and persistence Fog 5.4 Crane vessel The crane, crane vessel and associated equipment shall be fit to perform the planned lift operations in a safe manner. The crane shall be equipped with an accurate load monitoring device, sufficient to measure cyclic dynamic loads: General information General arrangement Lifting curves Dynamics are included in the lift curves Limits on heel and trim Intact stability limits Compartment damage Ballasting arrangements Mooring equipment Clearances need to be checked for lift and lowering: Between cargo barge and the crane vessel, which will size the fender Between the keel (or thrusters) of the crane vessel and the seabed Between the underwater hull and the underwater part of the substructure onto which the cargo is to be lifted. Between crane vessel hull and the cargo Between crane vessel boom and the cargo Between the crane vessel boom and rigging Crane vessel tugs and barges Crane vessel mooring and/or DP arrangement Minimum clearances Below module 3 m 7 Copyright 2011 by ASME

8 Between module and crane boom 3 m Between spreader bar and the crane boom 3 m From crane vessel to platform 3 m Crane hook data: Hook underwater capability Maximum hook height Hoist speed Lowering speed Lifting: Determine lift factors for the crane vessel (single crane lift 1.03 for weight change) (dual crane lift 1.03 tilt, 1.03 weight change, 1.02 for centre of gravity shift) Are motions in waves an issue? Size of fenders between crane vessel and the cargo barge Crane radius curve Vessel handling procedures Mooring arrangement Pre-lift checklist Limiting environmental criteria Static stability If the design hook load is more than 80% of the capacity of the cranes and the crane vessel will perform the lift at its normal working draft, then a stability statement needs to be submitted for review. Motions When the limiting criteria for a lift have been derived by dynamic analysis resulting in a limiting criteria based on an allowable significant wave height, Hs, and associated wave period, it is recommended that a wave buoy or similar device be deployed at the lifting site. For lifts in air the dynamic load is normally considered to be highest at the instant when the module is being lifted off its cargo barge grillage. This load, and the appropriate dynamic amplification factor (DAF), should be substantiated by means of an analysis which considers the maximum relative motions between the hook and the cargo barge, and takes account of the elasticity of the crane falls, the slings, the crane booms and the determination of the existing sea-state. The description of such an analysis must clearly state the assumed limiting wave heights and periods such that, if the calculated value of DAF is critical to the feasibility of the operation, then those conducting the lift will be aware of the limiting sea states. For lifts with the module submerged, special investigations should be made taking account of hydrostatic and hydrodynamic effects to calculate an appropriate DAF. Typical DAF values for crane vessels Weight of Modules <100 te 100 to 1000 te Greater than 1000te Inshore SSCV Inshore Monohull Inshore Sheer Leg Rigging Rigging design is based on Guidance on the Use of Cable Laid Slings and Grommets (formerly PM20), published by the International Marine Contractors Association, Reference 2. Rigging is used many times and information on what rigging is available on the crane vessel should be obtained. 5.6 Bumpers and Guides Unless the lift is in a very sheltered harbour, consideration shall be given to the provision of bumpers and guides on the modules. The bumpers and guides: Enable the object to be positioned after the lift within the required tolerances. Protect the lifted object, the adjacent surroundings and equipment from damage during lift Particular requirements for bumpers and guides should be determined at the planning stage taking account of lifting procedures and the assessed risk of damage. Fabrication tolerances of guides need to be closely controlled. Prior to lifting an as built dimensional survey of the guide systems shall be carried out to confirm that the operation tolerances have been maintained. The bumpers and guides should be designed for any possible combination of forces, except that the total force perpendicular to the face of the bumper need not exceed 0.1 x cargo weight. 6.0 CONCLUSIONS For vessels operating in shallow water special consideration needs to be made of the effects of breaking waves, current and squat. The effects of tidal range of crane vessel operations need to be taken into account. All types of crane vessels are required for shallow water heavy lifting, which include jack-ups, sheer-leg and monohull crane vessels. New vessels have been designed in recent years for the installation of renewable energy devices. Scale model testing should be considered where the structures are unique or where computer software is not fully developed for the installation situation. 8 Copyright 2011 by ASME

9 7.0 ACKNOWLEDGEMENTS FIGURE 3 SHEERLEG CRANES LIFTING PAU OUT OF FABRICATION HALL The author thanks his colleagues in CB&I for their assistance in preparing this paper. However the paper represents the opinion and views of the author, and does not necessarily represent those of CB&I, or any of its subsidiaries. 8.0 REFERENCES 1. Offshore Deployment by P.R.Blott, Mayflower Energy Limited, Middlesbrough, UK 2. International Marine Contractors Association Guidance on the Use of Cable Laid Slings and Grommets 3. Ship Squat, various by Dr. C.B. Barras FIGURE 1 LOAD MOMENT FIGURE 4 SHEERLEG INSTALLING UNLOADING ARM FOR JETTY FIGURE 2 SELF ELEVATING PLATFORMS AND FLOATING CRANE VESSELS 9 Copyright 2011 by ASME