MPU Heavy Lifter A lightweight concrete vessel for heavy offshore lifting operations

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN MPU Heavy Lifter A lightweight concrete vessel for heavy offshore lifting operations T. Landbø MPU Offshore Lift ASA E.B. Holm & H. Ludescher Olav Olsen a.s, Oslo, Norway ABSTRACT: The paper describes concept and general design of a reinforced concrete vessel which has been developed for heavy offshore lifting operations. The vessel has overall dimensions of about 110 m 87 m 55 m and is able to lift up to tonnes. It is currently under construction in Rotterdam and will be commissioned in the first quarter of Its key features are pivoting lifting frames and large flushing tanks that enable to lift entire topsides or jackets. Draft is controlled in a large scale by ballasting compartments of the concrete hull. For its propulsion, the vessel has 8 thrusters which are mostly required for positioning during lifting. For transportation the vessel will be towed by tug boats. Operations are controlled by a permanent crew that is accommodated and supplied by a living quarter with helicopter deck. 1 INTRODUCTION The conception of the MPU Heavy Lifter has its origins in the Norwegian industrial research program Lettkon, dating back to 1995/96, that aimed to promote the use of lightweight concrete. In the framework of the project, the engineering company Dr.techn.Olav Olsen a.s proposed to use concrete technology for a robust lifting vessel dedicated for offshore use. The idea was based on the market situation in offshore heavy lifting with only a few major contractors controlling the lifting marked. The trend was directed towards ever heavier lifting facilities that reduced risky offshore operations to a minimum. Consequently, the MPU Heavy Lifter was developed to install and remove entire topsides and jackets in a single lift operation. The single-lift technology facilitates recycling and reuse of existing installations, thus contributing to a more sustainable development. The intention was to render the vessel as polyvalent as possible, leading to its denomination multi-purpose unit (MPU). The considered uses included the removal of abandoned offshore structures, the installation of new or refurbished offshore structures, floating production, floating storage, Figure 1. The MPU Heavy Lifter with shocks cells for topside lifting. oil spill recovery, or as support structure for offshore oil and gas exploration facilities, floating dock. In order to enable the Heavy Lifter to fulfil these tasks, an uncommon structure was developed (see Figure 1). Basically it is a U-shaped concrete hull equipped with a set of parallel steel lifting frames located at its inside. The concrete hull is composed of two longitudinal pontoons connected by a transverse pontoon 925

2 and of four columns located in the corners. The lifting frames, weighing about tonnes each, are hinged on the longitudinal pontoons and retained on top of the columns (Figure 4). The U-shape makes it possible to enclose the payload during lifting and provides shelter during the employment for specific offshore operations. The hull is subdivided in a number of cells and compartments that may be individually ballasted with water in order to control the draft. The outline of the concrete hull is based on extensive testing on hydrodynamic models that has accompanied development of the vessel. The geometry of the brim was optimised in order to obtain good motion characteristics without exceeding the dry dock dimensions. The key feature for lifting are flushing tanks that extend from the bottom of the unit to 15 m above the top deck. They are fitted with large valves that can be opened to quickly flush the entire tank content which allows for a rapid, controlled coming up of the vessel. This is necessary to ensure quick and safe first contact and lift off which minimises the risk of repeated impacts between the floating vessel and structures fixed to the seafloor. In order to maximise the vessel s cargo capacity, a lightweight concrete with a design density of kg/m 3 was used for the hull. The slab and wall thicknesses vary between 0.3 m and 0.9 m, with typical spans of about 10 m. In order to sustain the rough conditions in the North Sea, the concrete is highly prestressed and has an average reinforcement intensity of 490 kg/m 3. For the operation of the vessel including lifting, ballasting and propulsion, it is equipped with complex mechanical and electrical systems. The main components are Hull with outfitting (including living quarter and safety installations) Movable lifting frames Sea-fastening system Shock cells and strand jacks Retractable thrusters Flushing system Power supply Control room In the following, mostly the concrete hull is described more in detail. 2 CONCRETE STRUCTURE With a volume of m 3, the concrete hull constitutes about 80% of the total lightship mass of tonnes. The base of the concrete hull measures about 110 m (width) times 87 m (length). It consists of a slab with a thickness of 0.4 m to 0.8 m. The Figure 2. Plan view of Base slab. base slab supports the main structure which is formed by two longitudinal pontoons that are connected by a transverse pontoon extending over the 48 m wide opening of the U, see Figure 2. At their outside, the longitudinal pontoons have a 4 m high brim which has mainly three functions: (1) It enhances the hydro-dynamic behaviour by damping roll, pitch and yaw motions, (2) it supports the thrusters for positioning of the vessel, and (3) although it is open to the sea its foam glass filling provides additional buoyancy to the vessel. The pontoons are about 15 m high and 21 m wide. The top slab has a thickness of 0.5 m to 0.6 m and serves as base for lifting frames and towing brackets. The lifting frames are hinged at the inner side of the U and supported by corbels. The subdivision of the pontoons in 42 cells and 10 ballast compartments results in uniform spans of about 10 m. Compartmentation is necessary for selective ballasting and to maintain hydrostatic stability. In the four corners of the hull, columns rise 25 m above the pontoons. They retain the lifting frames and accommodate the flushing tanks. One of the aft columns also supports the living quarter. At their base, the columns are nearly quadratic whereas the top deck is circular. The thickness of the column walls varies between 0.4 m and 0.8 m. At their interior, each of the columns houses 2 flushing tanks which extend to about 15 m above the top deck. Each column has watertight steel decks at elevation 25 m and on the top, adding another 8 compartments to the 10 ballast compartments in the pontoons. 926

3 The minimisation of the selfweight resulted in the development of a high performance lightweight aggregate concrete with a de-forming density less than kg/m 3. The low weight was achieved by the use of expanded clay lightweight aggregates in all fractions. The mix design was based on a mixed binder consisting of blastfurnace cement blended with pulverised fly ash. The high workability and durability requirements were met by an effective water/binder ratio less than 0.38 combined with the application of silica fume, superplastiziser and air entrainment. In order to reduce the heat of hydratation implying the risk of cracking due to thermal shrinkage a separate mix dedicated hot weather concreting was developed, in which 150 kg/m 3 of the blastfurnace cement is substituted by Portland cement. The all lightweight aggregate concrete (ALWAC) has a characteristic cylinder strength of 35 MPa (grade LC 35/38). The ordinary reinforcement has steel grade B 500 C. The post-tensioning reinforcement is composed of strands with steel grade BS 5896 Super having a yield strength of f py = MPa and a tensile strength of f pk = MPa. Figure 3. Top-side removal. 3 LIFTING FOR TOPSIDE/JACKET REMOVAL The lifting of entire topsides and jackets is a complex operation that requires extensive preparations and detailed planning of every step. The marine operations are divided into 6 distinct phases: Preparation of vessel Transit to offshore location, empty Approach and docking to structure to be removed Load transfer and lift off of decommissioned installation Transit from offshore location, loaded Landing of decommissioned installation Full control of each phase, sub-phase and task is necessary to ensure that the marine operations can be stopped and returned to safe mode at any given time. The preparation includes modifications of the platform to be lifted and adjustments of the lifting gear onboard the Heavy Lifter. The lifting points are defined and the structural safety of the vessel is verified for crucial phases. For transit between field and shore, tug boats are used. They also secure the critical approach and docking phase. The thrusters are used for approach and positioning as well as for station keeping prior to and immediately after load transfer. Depending on the size and layout of the object, lifting is performed with possible combinations of flushing, jacking and deballasting. Deballasting and flushing(s) are used for top-side removal. For jacket removal jacking is used, possibly supplemented by flushing. Deballasting is used for trimming of the Figure 4. Jacket removal. Heavy Lifter to obtain even keel and to obtain optimal draught during jacking and transport. The lifting methods differ in speed and duration. Deballasting is the slowest method allowing lifting at about 4 m/h. The jacking system is faster with a jacking speed up to 12 m/h. Flushing is very rapid as tonnes can be dropped instantly enabling the MPU Heavy Lifter to gain up to 7 8 m within 30 seconds. Model tests show that the dynamic motions following rapid flushing are moderate and dampened within few oscillations. The lifting capacity depends on various factors like on the centre of gravity (position in vertical and horizontal plane), on the height of the lifting points and their layout and on the environmental conditions. The ultimate lifting capacity is governed by the lifting frames which are designed for tonnes. This limits the lifting capacity for jacket substructures. Topsides with typical air gaps of 25 m can be lifted up to a weight of tonnes. 927

4 At the interface between the lifting frame and the payload, a shock damping and guiding system manages the actual load transfer. There will be an impact between the MPU Heavy Lifter and the payload at the (first) flushing which has to be absorbed both laterally and vertically in order to limit the forces transferred to the Heavy Lifter. For the loaded transit phase, the payload has to be fastened securely in order to avoid horizontal sliding or clashing against the Heavy Lifter. This is achieved with a sea fastening system that consists of screw jacks and chains in order to avoid offshore welding. After the load has been transferred, the point of no return is passed. The loaded transit phase will usually last more than 72 hours and cannot take place within a weather window, i.e. a seasonal storm has to be considered. The loaded structure including the sea fastening system is designed to resist a 100-year summer storm with a significant wave height H s = 7.5 m. The entire lifting process is controlled by an onboard crew of 30 to 40 people. They are operating the mechanical and electrical systems for ballasting, the lifting frames with shock cells or jacks, the thrusters and various other systems. They are accommodated in the living quarter which includes cabins, a control room, helicopter deck and safety equipment. 4 STRUCTURAL ANALYSIS The structural analysis is carried out in a series of global and local analyses that involve the entire vessel, possible lifting objects and the environment. For structural design, the entire vessel is subdivided into superelements like the concrete hull, the lifting frames and ballast system and mechanical outfitting represented by lumped masses. Various payloads are included in the model in order to consider their weight and inertia. In function of the actions on the structure, the structural analysis is carried out stepwise. Time-domain analysis is used to simulate the load transfer phase. In the case of a topside removal, flushing means that substantial amounts of water are released from a large floating structure. The vessel s weight is reduced and it starts to move upwards. As contact is established between the vessel and the stiff structure standing on firm ground, forces increase in the shock cells and the payload gradually starts to transfer over to the Heavy Lifter. A high number of simulations are run to combine the governing sea-state with numerous top-side configurations and various deballasting procedures. The worst scenarios are used in a separate analysis of the lifting frames to perform code-checking of the steel structure. Those scenarios causing the highest forces at their supports are used in the global analysis of the concrete hull. Figure 5. Visualisation of a design wave. Frequency domain analysis is used to simulate the effect of waves on the loaded structure in the transit phase. Waves are first classified according to their direction with respect to the Heavy Lifter. For all waves, transfer functions are established that describe their effect on the forces in selected cross sections of the concrete hull model. These transfer functions are multiplied with wave spectra for a summer storm in order to obtain response spectra. For each force component in each cross section, the significant response is determined. In detail design, the design wave method is used to reproduce all significant forces with a limited number of design waves (Figure 5). The structural analysis of the concrete hull is based on a linear elastic finite element model composed of shell elements with a total of about degrees of freedom. Based on this model, the actions are modelled with about 500 load cases. They simulate weights (self weight and live load), payload (results from time-domain analysis applied as static loads), post-tensioning, ballast (water pressure in ballast compartments) outer water pressure (quasi-static pressure of design waves) and acceleration (inertial forces). The complexity of load modelling can be illustrated with an example: in order to model the water pressure for all relevant ballasting levels in a ballast compartments up to 6 load cases are defined. Consequently, more than 100 load cases are required to reproduce the water pressure in all 18 ballast compartments. The individual load cases are assembled into design combinations that are used for code-checking of service limit state (SLS), ultimate limit state (ULS) and accidental limit state (ALS). In ULS, either the outer water pressure or the permanent load/payload are considered as dominant action (load factor 1.3). Accompanying actions are considered with a load factor 1.0. The relatively low load factors used in off-shore 928

5 Figure 6. Placement of reinforcement for Base slab. engineering are justified with intense quality control during construction (DNV-OS-C ). Altogether about design combinations are used to check about points (so called design sections). This enormous task is handled by a specially developed post-processor. More information on detail design is given in a separate paper (Ludescher et al. 2008). 5 CONSTRUCTION At the end of 2006, after extensive negotiations to secure financing, a contract over 140 million Euros was signed with the Singapore based yard company Keppel Offshore & Marine Ltd. Less than half a year later construction started in KeppelVerolme s dry dock in Rotterdam that provided appropriate dimensions (draft 11 m). The concrete hull construction worth about 60 million Euros is performed by Kombinatie Heavy Lifter, a joint venture of the Dutch contractors Van Hattum en Blankevoort BV and BAM Civiel BV, under a separate contract. Keppel Verolme BV performs the steel construction and all mechanical outfitting. The engineering is conducted by the Norwegian company Dr.techn.Olav Olsen a.s and the vessel is classed by Lloyd s Register. The first concrete pour for the Base slab took place in August The concrete is exclusively placed by crane and skip. The 14 m high walls are erected with climbing formwork (Figure 6) and the last slipforming of the columns is planned for August By that time about tonnes of prestressing steel and tonnes of conventional reinforcement will be placed. The work on site engages about 200 persons including construction workers, engineers and administrative staff. The engineering in Norway involves about 50 persons that produce the 1000 drawings required for production. Early in 2008, progress on all major items as well as procurement of long lead items etc. is on schedule and the vessel will be ready for operation in the summer of Updated information can be found under the address which also includes a simulation of the Heavy Lifter in operation. 6 CONCLUSION Innovative use of all leightweight aggregate concrete has led to the construction of a novel lifting vessel dedicated for offshore use. Using the single lift technology, this vessel is capable of lifting and transporting entire topsides ( tonnes) and jackets ( tonnes). Experience from previous offshore concrete projects led to successful engineering and construction according to schedule. REFERENCES DNV-OS-C Offshore Standard DNV-OS-C101 Design of Offshore Steel Structures, General (LRFD Method). Det Norske Veritas. Ludescher H., Haugerud S.A., Fernàndez Ruiz M. Detail design of the MPU Heavy Lifter. Proceedings for the International fib Symposium 2008, Amsterdam. 929

6