The basic concept in the float over is to transport the topsides on a vessel in one piece, position the vessel over the substructure, and lower the topsides onto the substructure while maintaining the position of the vessel.
Topside platform shall be mated onto the preinstalled jacket by a float-over method using barge.
Float-over vessel
Floatover vessel will be used to load out the topsides, transport it to site safely, perform the float over, and return. Mostly cargo barges are used for this operation for commercial and operational feasibility.The float-over vessel sized so that it will fit inside the jacket legs, and will have sufficient Longitudinal strength and stability to load out and transport heavy topsides and platforms. Very heavy large-sized topsides usually have jacket leg clear spacing of around 50 m. Assuming proper clearance between legs and transporting barge, the vessel width shall be limited to 45 m. The vessel also needs enough strength and stability to load out and transport such heavy topsides, which usually require a wider vessel.
In case of strength and stability of floatover vessel is not sufficient, forked kind of barge is used which will have opening on the middle.The topside legs are spaced so the forked arrangement of the barge stern shall be designed to transport the topside; the barge shall be positioned so that the fork encompasses the jacket and the topside is directly over the jacket for the float-over sequence to commence. The barge shall then be ballasted so that the topside load is transferred to the jacket structure. The drilling platform can be pre-installed (lifting method) onto the jacket before the float-over installation.
The use of hydraulic jacks allow the topsides to be towed to site on the transportation vessel low and raised to float-over elevation just prior to the float-over operations. This can be critical for topsides with a high vertical center of gravity, where stability during the tow is marginal or unacceptable. The hydraulic jack system also provides speedy lowering of the topsides. However, being an active mechanical system, the jacks have to be built, maintained, and controlled at a high precision.
Sand jacks are another method to rapidly separate the transport vessel from the underside of the topsides. Sand jacks are large diameter pistons that rest on a sand column to support the entire weight of the topsides. When sufficient weight has been transferred to the substructure, trap doors are opened to dump the sand and lower the piston rapidly to complete the weight transfer. Sand jacks are more cost efficient than hydraulic jacks.
Unless a hydraulic or a sand jack system is used to lower the topsides, the weight transfer usually is accomplished by ballast transfer. A rapid ballast system (RBS) that has been used recently reduces the duration of weight transfer for 20,000-ton (18,144-metric ton) topsides to about 20 minutes. An added advantage is that it is a failsafe system; once the valves are opened there is no need to control or close them until the barge separates from the topsides.
A Load-out Support Frame (LSF) shall be used for the load-out and transportation of the topside. The LSF and topside interface shall be fitted with elastomeric pads designed to withstand the transportation loads and mainly the shock impact loads during the float-over operation (separation between barge and topside at the end of mating). Jacket fenders with protection plates shall be fitted to the jacket outside legs prior to installation.
Water depth / Tides/Current/Wind
The sea level of the Sea changes along the years.To account for sea level variations, a margin of ± 1.0m around MSL will be considered for the ballasting procedure during float-over. It is recommended to start the floatover ballasting operations during falling tide, to maximize entrance clearance between jacket and topside legs during high tide, as well as the exit gap between the barge and topside sub-lower deck during low tide.
Different wave limits shall be considered for standby and entry/mating/exit stages, they vary for head/stern/quarter and beam seas. The barge will approach the jacket with stern first.
A constant wind speed of 10m/sec shall be assumed in the floatover dynamic analyses, acting in line with waves.However, for stability checks, the wind velocity will be based on transportation 10 yr-RP adjusted extremes for the months of operation.
The impact of surface current will be investigated; the 1-yr surface current speed is shall be assumed in line with wave and wind.The installation weight of the topside, as well as its COG coordinates, shall be extracted from the latest Weight Control Report.
Ballast Arrangement
The float over barge ballasting system includes many ballast water tanks,which shall be sufficient for complete operation.Not pumpable water level shall be assumed left in the water tanks which shall be accounted for free surface correction during stability calculation.
Grillage ,miscellaneous equipments.,loadout Support Frame and Seafastening shall be accounted in this operation.
Stability Check
During float-over operation, the stability shall be checked for three stages
.
Intact Stability criteria
Topside Floatover Analysis
The overall purpose of the float-over analysis is to demonstrate the following:
The following outputs will be sorted from the various stages:
A total number of 10 stages shall be investigated
Computer Modelling of Floatover Operation
Time domain analysis method is used to predict motions and forces during the floatover process. The time domain analysis can take into account the nonlinearities in the system,including the following:
frequency domain diffraction and radiation analysis are carried out to obtain the hydrodynamic coefficients, including: added mass, radiation damping, wave excitation force and mean drift force.In the frequency domain analysis, a total of 9 wave heading (0, 22.5, 45, 67.5, 90,112.5,
135, 157.5 and 180 degrees) and wave periods from 2s to 30s shall be analyzed.
Topside platform shall be mated onto the preinstalled jacket by a float-over method using barge.
Float-over vessel
Floatover vessel will be used to load out the topsides, transport it to site safely, perform the float over, and return. Mostly cargo barges are used for this operation for commercial and operational feasibility.The float-over vessel sized so that it will fit inside the jacket legs, and will have sufficient Longitudinal strength and stability to load out and transport heavy topsides and platforms. Very heavy large-sized topsides usually have jacket leg clear spacing of around 50 m. Assuming proper clearance between legs and transporting barge, the vessel width shall be limited to 45 m. The vessel also needs enough strength and stability to load out and transport such heavy topsides, which usually require a wider vessel.
In case of strength and stability of floatover vessel is not sufficient, forked kind of barge is used which will have opening on the middle.The topside legs are spaced so the forked arrangement of the barge stern shall be designed to transport the topside; the barge shall be positioned so that the fork encompasses the jacket and the topside is directly over the jacket for the float-over sequence to commence. The barge shall then be ballasted so that the topside load is transferred to the jacket structure. The drilling platform can be pre-installed (lifting method) onto the jacket before the float-over installation.
The use of hydraulic jacks allow the topsides to be towed to site on the transportation vessel low and raised to float-over elevation just prior to the float-over operations. This can be critical for topsides with a high vertical center of gravity, where stability during the tow is marginal or unacceptable. The hydraulic jack system also provides speedy lowering of the topsides. However, being an active mechanical system, the jacks have to be built, maintained, and controlled at a high precision.
Sand jacks are another method to rapidly separate the transport vessel from the underside of the topsides. Sand jacks are large diameter pistons that rest on a sand column to support the entire weight of the topsides. When sufficient weight has been transferred to the substructure, trap doors are opened to dump the sand and lower the piston rapidly to complete the weight transfer. Sand jacks are more cost efficient than hydraulic jacks.
Unless a hydraulic or a sand jack system is used to lower the topsides, the weight transfer usually is accomplished by ballast transfer. A rapid ballast system (RBS) that has been used recently reduces the duration of weight transfer for 20,000-ton (18,144-metric ton) topsides to about 20 minutes. An added advantage is that it is a failsafe system; once the valves are opened there is no need to control or close them until the barge separates from the topsides.
A Load-out Support Frame (LSF) shall be used for the load-out and transportation of the topside. The LSF and topside interface shall be fitted with elastomeric pads designed to withstand the transportation loads and mainly the shock impact loads during the float-over operation (separation between barge and topside at the end of mating). Jacket fenders with protection plates shall be fitted to the jacket outside legs prior to installation.
Water depth / Tides/Current/Wind
The sea level of the Sea changes along the years.To account for sea level variations, a margin of ± 1.0m around MSL will be considered for the ballasting procedure during float-over. It is recommended to start the floatover ballasting operations during falling tide, to maximize entrance clearance between jacket and topside legs during high tide, as well as the exit gap between the barge and topside sub-lower deck during low tide.
Different wave limits shall be considered for standby and entry/mating/exit stages, they vary for head/stern/quarter and beam seas. The barge will approach the jacket with stern first.
A constant wind speed of 10m/sec shall be assumed in the floatover dynamic analyses, acting in line with waves.However, for stability checks, the wind velocity will be based on transportation 10 yr-RP adjusted extremes for the months of operation.
The impact of surface current will be investigated; the 1-yr surface current speed is shall be assumed in line with wave and wind.The installation weight of the topside, as well as its COG coordinates, shall be extracted from the latest Weight Control Report.
Ballast Arrangement
The float over barge ballasting system includes many ballast water tanks,which shall be sufficient for complete operation.Not pumpable water level shall be assumed left in the water tanks which shall be accounted for free surface correction during stability calculation.
Grillage ,miscellaneous equipments.,loadout Support Frame and Seafastening shall be accounted in this operation.
Stability Check
During float-over operation, the stability shall be checked for three stages
- Barge entrance,
- Leg Mating Unit (LMU )contact with Topside,
- Barge exit.
.
Intact Stability criteria
- The minimum range of positive stability shall be at least 15degrees.
- The area under the righting arm curve shall not be less than 1.4 times the area under
- the wind heeling curve calculated up to the angle of second intercept, angle of
Topside Floatover Analysis
The overall purpose of the float-over analysis is to demonstrate the following:
- The topside structure can be safely transferred from the transportation barge onto the preinstalled jacket.
- Loads generated in the topside and the jacket during the operation remain within acceptable limits.
- Installation barge complies with all stability and longitudinal strength requirements during the various phases of the operation.
- All installation AIDS such as fenders, leg mating units (LMU), elastomeric pads,mooring lines winches and ballast pumps are suitable for the float-over operation.
- The float-over analysis, based on weather limits given in previous section, will provide loads for structural design check of the topside and the jackets, as well as loads and operating parameters for the LMUs, jacket fenders and mooring lines.
The following outputs will be sorted from the various stages:
- Entry, Alignment and Exit: barge motions, mooring lines and hawsers tensions,impact loads between barge and jacket (fenders loads).
- Load Transfer: barge motions, loads and motions at LSF/topside interface (elastomeric pads), loads and motions at topside/jacket interface (LMUs).
A total number of 10 stages shall be investigated
- Standby: the barge ballast plan has been set to reach entry even keel draft, bow anchor lines have been hooked up and tug lines at the stern are connected, barge stern is 20m from the jacket. The aim is to check the barge motions at this stage, the loads in the mooring lines/tug lines.
- Docking Stage 1 - Entrance into jacket: the barge draft is still the entry draft.The aim is to check the barge motions at this stage, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Docking Stage 2 -Barge moored in final position: the barge draft is still the entry draft, barge has reached its final position with all tether lines connected to jacket legs (longitudinal and transverse). The aim is to check the barge motions at this stage (specifically at topside legs), the loads in the mooring lines/tug lines/tether lines and the loads in the
- Premating-Leg Mating Unit (LMU) Contact: the barge is ballasted such as air gap is closed between jacket legs cones and topside legs LMUs receptacles. The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Mating Stage 1-20% Load Transfer: the barge is ballasted such as 20% of topside weight has been transferred, LMU are partially compressed. The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Mating Stage 2-50% Load Transfer: the barge is ballasted such as 50% of topside weight has beentransferred, LMU should be fully compressed (steel-to-steel contact). The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the
- Mating Stage 3-70% Load Transfer: the barge is ballasted such as 70% of topside weight has been transferred, there is steel-to-steel contact at jacket/topside legs interfaces,elastomeric soft pads start to be decompressed. The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Mating Stage 4-90% Load Transfer: the barge is ballasted such as 90% of topside weight has been transferred, there is steel-to-steel contact at jacket/topside legs interfaces,elastomeric soft pads are partly decompressed. The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Mating Stage 5-100% Load Transfer: the barge is ballasted such as 100% of topside weight has been transferred; there is steel-to-steel contact at jacket/topside legs interfaces, the elastomeric soft pads are now fully decompressed. The aim is to check the barge motions, the loads at jacket/topside legs interface and LSF/topside interfaces, the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
- Post Mating and Exit-Barge withdrawal: the barge is ballasted such that sufficient gap between top of LSF and topside creates. The aim is to check the barge motions and LSF/topside interfaces (potential recontact with barge heave motions), the loads in the mooring lines/tug lines/tether lines and the loads in the fenders.
Computer Modelling of Floatover Operation
Time domain analysis method is used to predict motions and forces during the floatover process. The time domain analysis can take into account the nonlinearities in the system,including the following:
- Second order wave forces and slow drift motions of the barge,
- Nonlinear forces in the mooring lines/tether lines,
- Nonlinear forces on the jacket sway fender,
- Nonlinear forces at the LMUs and elastomeric pads/shock cells.
frequency domain diffraction and radiation analysis are carried out to obtain the hydrodynamic coefficients, including: added mass, radiation damping, wave excitation force and mean drift force.In the frequency domain analysis, a total of 9 wave heading (0, 22.5, 45, 67.5, 90,112.5,
135, 157.5 and 180 degrees) and wave periods from 2s to 30s shall be analyzed.
- Constant wind and current loads computed through barge wind and current polars.
- Reaction forces due to contact between sway fenders and jacket legs are modeled with non-linear springs accounting for fenders deflections curves and relative clearance at the various jacket legs rows.
- The connections between the topside and the LSF are modeled with equivalent linear tensile/compressive springs for lateral connection (depending on stoppers configurations and relevant gap for exit stage) and equivalent non-linear compressive springs for the vertical connection (to account for elastomeric soft pad stiffness when the vertical gap between elastomeric hard pad and topside support is below 100mm).
- Topside and barge are modeled as two 6 degree of freedom rigid bodies.
- The topside is linked to the jacket through LMUs shock absorbers, which have some stroke before steel-to-steel contact between jacket legs and topside legs.
- The vertical connections between the jacket and the topside are modeled with equivalent multi-linear compressive springs (to account for LMU “soft” rubber-type stiffness when stroke is below 500mm, and topside/jacket legs vertical stiffness after LMU full compression). The lateral connection between the jacket and the topside is modeled through equivalent non-linear compressive springs taking into account LMU horizontal pads stiffness (for the relevant steps) and jacket/topside legs lateral
- Barge bow anchor mooring lines (connected to seabed anchors) are modeled with cable elements and as catenary lines (so-called “quasi-static” method), whereas tug stern lines are modeled as equivalent tensile springs and tether lines as equivalent tensile springs accounting for stretchers and wire ropes respective stiffnesses.
- With Newman’s approximation, the second order wave drift force is calculated based only on diagonal terms. It provides an acceptable approximation when the stiffness of the mooring system is low and the water depth is not shallow.
- Convergence studies will be conducted to determine the adequate time step and simulation duration for the time-domain analysis.
- For entry, alignment and exit, minimum duration of 1800 seconds will be used, while load transfer cases will be studied with a minimum duration of 500 seconds and 0.1 second time step for all the stages. A ramp will be included at the beginning of the simulations to avoid
