JACKET LAUNCHING
The operations considered is launching a standard steel jacket from the launch barge in order to render it afloat in an almost horizontal position.
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Design Considerations
The planned launching operations shall be verified by means of a 3-dimensional study in order to demonstrate
The following factors shall be taken into consideration during design of the launching operations:
Launch Barge Selection
The following points shall be considered during selection of the launch barge:
Barge Pre-launch Parameters
The parameters directly influencing the maximum stern immersion of the barge and load on the rocker arm pins are the draught and trim conditions of the barge and the jacket position at the onset of the launch.
Usually, the maximum stresses in both the jacket and the barge hull, occur during rotation of the jacket on the rocker arms; this is due to the unfavourable longitudinal distribution of loads arising from the concentrated local forces at the rocker arm supports.
During launch, the barge hull bending moment can easily approach the maximum allowable value; it is therefore essential that the hull bending stresses are kept within the allowable limits by means of the appropriate ballast distribution.
Jacket Launch Trajectory
The launching operation shall be designed in such a way that the jacket follows a safe trajectory with adequate bottom clearance and maintains its stability throughout the launch; furthermore, the maximum dive depth of the jacket may be limited by the resistance of the jacket elements to the hydrostatic pressure.
The jacket trajectory is governed by the initial draught and trim of the barge, the type of lubrication and/or surface of the launch runners, and the rate of buoyancy pick-up of the jacket as it enters the water.
The distribution of jacket buoyancy shall be such that on completion of the launch the jacket floats in a stable position, usually near horizontal, with the head slightly above sea level (“head up” condition) and with the rigging platform of the jacket accessible to the installation work crew.
Barge Parameters
The results of the launch analysis should confirm that the following parameters are within the allowable limits specified for the selected launch barge:
Jacket Weight and CoG Conditions
The launch analysis shall be carried out for each of the following conditions:
Friction Coefficients
The following values of the Dynamic Friction Coefficient (Cfd) between the jacket launch runners and the barge skidways (lined with teflon) shall be considered in the launching analysis: 0,03, 0,045 and 0,06.
Furthermore, the Static Friction Coefficient (Cfs) of 0.10 shall be considered in the calculations.
Jacket Reserve Buoyancy
The reserve buoyancy of the floating jacket shall be at least 15% of the total jacket weight.
Barge and Jacket Stability
The stability of the barge and jacket, during launch, will be obtained by maintaining the transverse and longitudinal metacentric heights of the system greater than 1 m.
Upon separation, the barge and the jacket shall individually regain or retain the above stability.
The above stability criteria shall be verified for the worst conditions of jacket weight and CoG displacement.
Brief periods of reduced or negative transverse stability may however be accepted, subject to the following conditions:
Jacket Bottom Clearance
The minimum bottom clearance of the jacket, during the dynamic phase of its trajectory, shall be 10% of the water depth or 5 meters, whichever is greater.
The above criteria shall be verified for the worst conditions of jacket weight and CoG displacement.
For jackets with a damaged buoyancy, the minimum bottom clearance criteria will be determined on a case by case basis.
Jacket Damage Analysis
The analysis shall be also carried out for launch of the jacket with one of its buoyancy compartment damaged (e. g. jacket leg, sleeve or buoyancy tank).
Generally, the loss of anyone of the jacket compartments shall be investigated, however, in case of a specific experience with similar structures, the number of buoyancy compartments to be included in the analysis shall be left to the discretion of the designer.
The analysis shall be based on the assumption that the damaged compartment will be flooded upon entry in water; the same stability criteria as for the intact jacket shall be applied.
The max. possible jacket weight shall be considered in the analysis.
The bottom clearance criteria will be determined on a case by case basis. In general any positive clearance would be acceptable.
In view of the fact that it is most unlikely that the barge may sustain any hull damage during launching operations, the present Criteria do not call for any analysis in this respect.
Environmental Conditions for the Launch
The following factors shall be taken in consideration in order to define the maximum seastate allowing launch of the jacket:
The limiting wind speed during the launch should be compatible with the limiting seastate. It shall be demonstrated however by the launch analysis that such wind speed will not compromise the barge and/or jacket stability during the launch and will not cause unacceptable additional stresses.
Jacket Model
The jacket model designed for launch analysis shall comprise all structural members, mud-mats, appurtenances and all temporary attachments present during the launch.
The model weight, buoyancy and position of the respective CoG and CoB centres shall be based on the latest Weight Control Report. The information contained in the Report shall be compared with the original design data prior to finalising the model characteristics; comparisons shall be made between corresponding sub-groups of weight and buoyancy, as well as between total weight and buoyancy for the jacket. The above procedure will simplify the identification of areas where discrepancies may arise.
Minor items, such as anodes; structural nodes; etc., should be added to the model in one of the following ways:
Parametric Study
The purpose of the parametric analysis is to define the optimum launch configuration for the barge, in order not to exceed its strength limits and to ensure acceptable trajectory for the jacket.
The analysis shall demonstrate the influence of the barge initial trim and draught on the magnitude of forces acting on the rocker arms, on the extent of the barge stern immersion and on the jacket dive depth.
The jacket maximum possible weight with nominal position of the CoG shall be considered in the analysis.
The result of the analysis shall consist of a set of parameters related to the barge configuration at the onset of the launching operations.
Analysis of the Jacket Trajectory
The dynamic behaviour of the jacket and barge, and in particular the dive depth and bottom clearance of the jacket will be examined considering the barge trim and draught condition identified in the preceding paragraph.
Three-dimensional launch analysis shall be carried out in order to define the barge and jacket trajectories in function of time.
The results from the analysis will provide the following information:
Barge Strength
Static strength calculations of the barge shall be performed for the following stages of the jacket launch:
The above conditions should result in the worst hogging and sagging hull stresses. The following graphs will be presented for each of the two conditions:
Structural checks of the launch barge, if required, shall demonstrate that the following parameters are within the specified allowable limits:
Jacket Structural Checks
The jacket structural elements shall be verified for the load case consisting of the maximum reaction of the barge rocker arms applied at the respective position of the jacket launch runners.
Furthermore, the jacket and its appurtenances shall be verified for hydrostatic pressure due to immersion as well as for hydrodynamic and inertia forces.
The operations considered is launching a standard steel jacket from the launch barge in order to render it afloat in an almost horizontal position.
.
Design Considerations
The planned launching operations shall be verified by means of a 3-dimensional study in order to demonstrate
- The jacket during launching follows a safe trajectory with the adequate bottom clearance and maintains sufficient stability.
- The jacket, upon completion of the launch, floats in a stable, usually near horizontal position with the head slightly above sea level and therefore accessible to the installation work crew (“head up” condition).
The following factors shall be taken into consideration during design of the launching operations:
- Ideally the jacket position on the barge should be such that the launch truss pads cover both, the main and the secondary (if any) rocker arms, or at least the main rocker arms.
- Employment of secondary rocker arms will allow for further rotation of the jacket, once the primary rocker arms have reached their maximum rotation angle. The angle of rotation of the primary rocker arms should be set in such a way that they reach the respective stoppers just before the trailing edges of the jacket launchways pass over the primary pins; the above in order to reduce loads acting on the secondary rocker arms.
Launch Barge Selection
The following points shall be considered during selection of the launch barge:
- The size of the barge should be verified against the dimensions and weight of the jacket; this shall include verification whether the required spacing of the barge skidways and rocker arms can be obtained.
- The barge should be verified for interference with the jacket during load-out and launching operations
- The barge dimensions should be verified in order to ensure that it will be permitted to navigate waterways leading from the possible fabrication sites and that it will be able to sustain loadout of the jacket, taking account of tidal variations, wharf height and local water depth restrictions.
- The limiting factors for launching operation, i. e. the allowable stern immersion, allowable load on the rocker arms and strength of the barge transversal section.
Barge Pre-launch Parameters
The parameters directly influencing the maximum stern immersion of the barge and load on the rocker arm pins are the draught and trim conditions of the barge and the jacket position at the onset of the launch.
Usually, the maximum stresses in both the jacket and the barge hull, occur during rotation of the jacket on the rocker arms; this is due to the unfavourable longitudinal distribution of loads arising from the concentrated local forces at the rocker arm supports.
During launch, the barge hull bending moment can easily approach the maximum allowable value; it is therefore essential that the hull bending stresses are kept within the allowable limits by means of the appropriate ballast distribution.
Jacket Launch Trajectory
The launching operation shall be designed in such a way that the jacket follows a safe trajectory with adequate bottom clearance and maintains its stability throughout the launch; furthermore, the maximum dive depth of the jacket may be limited by the resistance of the jacket elements to the hydrostatic pressure.
The jacket trajectory is governed by the initial draught and trim of the barge, the type of lubrication and/or surface of the launch runners, and the rate of buoyancy pick-up of the jacket as it enters the water.
The distribution of jacket buoyancy shall be such that on completion of the launch the jacket floats in a stable position, usually near horizontal, with the head slightly above sea level (“head up” condition) and with the rigging platform of the jacket accessible to the installation work crew.
Barge Parameters
The results of the launch analysis should confirm that the following parameters are within the allowable limits specified for the selected launch barge:
- Maximum immersion of the barge stern;
- Maximum bending moment acting along the barge hull;
- Loads acting on rocker arms and respective pins;
- Maximum rotation of rocker arms.
Jacket Weight and CoG Conditions
The launch analysis shall be carried out for each of the following conditions:
- Nominal jacket weight and CoG position;
- Max. possible jacket weight (nominal weight + 5%) and nominal CoG position;
- Nominal Jacket weight reduced by 5% and nominal CoG position;
- Max. possible jacket weight (nominal weight + 5%) with the CoG position shifted as shown in the table below:
Friction Coefficients
The following values of the Dynamic Friction Coefficient (Cfd) between the jacket launch runners and the barge skidways (lined with teflon) shall be considered in the launching analysis: 0,03, 0,045 and 0,06.
Furthermore, the Static Friction Coefficient (Cfs) of 0.10 shall be considered in the calculations.
Jacket Reserve Buoyancy
The reserve buoyancy of the floating jacket shall be at least 15% of the total jacket weight.
Barge and Jacket Stability
The stability of the barge and jacket, during launch, will be obtained by maintaining the transverse and longitudinal metacentric heights of the system greater than 1 m.
Upon separation, the barge and the jacket shall individually regain or retain the above stability.
The above stability criteria shall be verified for the worst conditions of jacket weight and CoG displacement.
Brief periods of reduced or negative transverse stability may however be accepted, subject to the following conditions:
- The jacket shall be fully mobile throughout the period.
- Absence of external upsetting forces of sufficient magnitude to overcome the inertia of the jacket.
- The period of reduced/negative stability shall not exceed 30 sec., after which the 1 m Metacentric Height shall be regained.
Jacket Bottom Clearance
The minimum bottom clearance of the jacket, during the dynamic phase of its trajectory, shall be 10% of the water depth or 5 meters, whichever is greater.
The above criteria shall be verified for the worst conditions of jacket weight and CoG displacement.
For jackets with a damaged buoyancy, the minimum bottom clearance criteria will be determined on a case by case basis.
Jacket Damage Analysis
The analysis shall be also carried out for launch of the jacket with one of its buoyancy compartment damaged (e. g. jacket leg, sleeve or buoyancy tank).
Generally, the loss of anyone of the jacket compartments shall be investigated, however, in case of a specific experience with similar structures, the number of buoyancy compartments to be included in the analysis shall be left to the discretion of the designer.
The analysis shall be based on the assumption that the damaged compartment will be flooded upon entry in water; the same stability criteria as for the intact jacket shall be applied.
The max. possible jacket weight shall be considered in the analysis.
The bottom clearance criteria will be determined on a case by case basis. In general any positive clearance would be acceptable.
In view of the fact that it is most unlikely that the barge may sustain any hull damage during launching operations, the present Criteria do not call for any analysis in this respect.
Environmental Conditions for the Launch
The following factors shall be taken in consideration in order to define the maximum seastate allowing launch of the jacket:
- The seastate shall not induce excessive stresses in the jacket pitch stoppers, after cutting the jacket seafastening.
- The seastate should permit a safe transfer of the work crew to the jacket rigging platform as well as operation of tugs and workboats; typically, a significant wave height of 2 meters could be acceptable for this purpose.
The limiting wind speed during the launch should be compatible with the limiting seastate. It shall be demonstrated however by the launch analysis that such wind speed will not compromise the barge and/or jacket stability during the launch and will not cause unacceptable additional stresses.
Jacket Model
The jacket model designed for launch analysis shall comprise all structural members, mud-mats, appurtenances and all temporary attachments present during the launch.
The model weight, buoyancy and position of the respective CoG and CoB centres shall be based on the latest Weight Control Report. The information contained in the Report shall be compared with the original design data prior to finalising the model characteristics; comparisons shall be made between corresponding sub-groups of weight and buoyancy, as well as between total weight and buoyancy for the jacket. The above procedure will simplify the identification of areas where discrepancies may arise.
Minor items, such as anodes; structural nodes; etc., should be added to the model in one of the following ways:
- by introduction of additional elements simulating globally the weight, as well as hydrostatic and hydrodynamic properties of such items;
- by the appropriate modification of the existing main elements of the model.
Parametric Study
The purpose of the parametric analysis is to define the optimum launch configuration for the barge, in order not to exceed its strength limits and to ensure acceptable trajectory for the jacket.
The analysis shall demonstrate the influence of the barge initial trim and draught on the magnitude of forces acting on the rocker arms, on the extent of the barge stern immersion and on the jacket dive depth.
The jacket maximum possible weight with nominal position of the CoG shall be considered in the analysis.
The result of the analysis shall consist of a set of parameters related to the barge configuration at the onset of the launching operations.
Analysis of the Jacket Trajectory
The dynamic behaviour of the jacket and barge, and in particular the dive depth and bottom clearance of the jacket will be examined considering the barge trim and draught condition identified in the preceding paragraph.
Three-dimensional launch analysis shall be carried out in order to define the barge and jacket trajectories in function of time.
The results from the analysis will provide the following information:
- Required force of the launching winch;
- Point of self launch of the jacket;
- Forces acting on the rocker arms;
- Point of separation of the jacket;
- Jacket trajectory;
- Coordinates of the jacket minimum seabed clearance;
- Barge and jacket stability during the launch.
Barge Strength
Static strength calculations of the barge shall be performed for the following stages of the jacket launch:
- Start of the jacket rotation on the rocker arms;
- Post launch condition.
The above conditions should result in the worst hogging and sagging hull stresses. The following graphs will be presented for each of the two conditions:
- Weight curve;
- Buoyancy curve;
- Shear force;
- Bending moment;
Structural checks of the launch barge, if required, shall demonstrate that the following parameters are within the specified allowable limits:
- Rocker arms load;
- Loads on skidways and barge structure (frames, bulkheads, etc.);
- Barge stern submergence;
- Barge longitudinal bending and shear stresses.
Jacket Structural Checks
The jacket structural elements shall be verified for the load case consisting of the maximum reaction of the barge rocker arms applied at the respective position of the jacket launch runners.
Furthermore, the jacket and its appurtenances shall be verified for hydrostatic pressure due to immersion as well as for hydrodynamic and inertia forces.
