JACKET UPENDING
This operation describe upending of a standard steel jacket from its horizontal free floating position to the upright position and setting it down on the sea bottom; the operation is carried out by a controlled rotation achieved by means of a step by step jacket ballasting and with the assistance of a Heavy Lift Vessel crane.
Appropriate ballasting and hook load sequences is important, so that the operations may be carried out in a predictable, safe and efficient manner and the jacket transverse and longitudinal stability meets the required criteria during all stages of upending.
Design Considerations
The upending control system and the respective marine operations, should be kept as simple as possible; consequently, the number of floodable jacket compartments should be kept to a minimum. The distribution of jacket buoyancy shall be such that at the onset of the upending operations the jacket floats in a stable, usually near horizontal position with the head slightly above the sea level and therefore accessible to the installation work crew (“head up” condition).
The jacket should achieve the "pendulum stability" (i.e. centre of buoyancy above centre of gravity) as early as possible and in any case prior to gaining the fully upright position.
If possible, the jacket ballasting capacity should be such as to permit achievement of the stable upright position on the sea bottom prior to flooding the flotation tanks (if any), obtaining in such a way the required on-bottom weight. The magnitude of the on-bottom weight should be sufficient for the jacket to withstand environmental loads and remain in a stable upright position on the sea bottom prior to piling operations.
The design of the upending system should be such, that minor variations in the jacket weight, buoyancy, and of the respective CoG and CoB positions will not result in significant changes in the jacket pitch and trim.
The upending procedure should include provisions for a large degree of flexibility, so that, if necessary, the established ballast quantities to be introduced or taken from the jacket, and in some cases sequence of operations may be changed within specified limits during all stages of upending, without affecting the safety of the operation.
Such contingency may become necessary in the event of accidental flooding of jacket compartments or due to failure of components in the upending control system.
In order to avoid hook racking, only two opposite hook prongs should be used for upending operations. Consequently, each pair of slings connected to one side of the jacket head should be engaged on the same prong; should the upending study indicate that the load acting on one prong at any stage of the upending exceeds 25% of the maximum (total) allowable hook load, the crane manufacturers shall be approached in order to verify hook resistance.
Jacket Weight and Position of CoG The upending analysis shall be carried out for the following conditions:
Jacket Reserve Buoyancy
The distribution of jacket buoyancy shall be such that at the onset of upending operations the jacket floats in a stable position, usually near horizontal, with the head slightly above the sea level and with the rigging platform of the jacket accessible to the installation work crew (“head up” condition).
The reserve buoyancy of the floating jacket shall be at least 15% of the total jacket weight. In exceptional cases the buoyancy may be reduced up to 10%, subject to the additional check of the jacket stability.
Jacket Stability
The upending system shall be designed in such a way as to assure a stable jacket condition throughout all phases of upending operations; the minimum transverse and longitudinal Metacentric Height (GM) during upending should not be less than 1 m.
The effective GM shall be calculated in the case of a crane assisted upending; the calculation shall consider the hook load and the sling geometry
For intermediate cases in which the upending analysis results in a GM value smaller than 1 m., trimming and heeling righting moment curves shall be developed in order to demonstrate the capability of the jacket to return to its stable position when subjected to minor disturbances (e.g. due to the waves; vessel motions; etc); the curves shall be developed for the induced angle of heel and trim of +/- 10 degrees.
Trimming and heeling righting moment curves shall be also developed for the jacket in a free floating condition.
Jacket Bottom Clearance
The upending operations shall be designed in such a way that the minimum bottom clearance of the jacket, during all phases of the upending sequence, shall be at least 3 to 5 metres.
For jackets with a damaged buoyancy, the minimum bottom clearance criteria will be determined on a case by case basis.
Jacket Damaged Buoyancy Analysis
The upending analysis shall also consider upending and setting down of the jacket with one buoyancy compartment damaged
The analysis for damaged buoyancy cases shall be based on the max. possible weight of the jacket; the same stability criteria as for the intact cases shall be applied.
The bottom clearance criteria will be determined on a case by case basis; in general any positive clearance would be acceptable.
Jacket Model
The jacket model designed for the upending analysis shall comprise all structural members, mud-mats, appurtenances and all temporary attachments present during the upending.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.
The minor items, such as anodes; structural nodes; etc., should be added to the model in one of the following ways:
Intact Jacket Upending Analysis The upending analysis shall be carried out in a fully three dimensional manner. The analysis shall define the sequence of ballasting and hook loads required to rotate the jacket from the free floating position to the upright configuration and to set it down on the sea bottom.
The results of the analysis will provide the following information for each phase of the upending process:
hook height and the respective load;
Damaged Jacket Upending Analysis
The analysis shall define the operational contingencies to be undertaken in the case of accidental flooding of one of the jacket compartments or failure of the upending control system components.
The analysis shall include the following:
The analysis shall be made for the nominal position of the jacket CoG.
Generally, a loss of each of the jacket compartments shall be investigated, however, in case of a specific experience with similar structures, the number of ballast compartments to be subjected to the analysis shall be left to the discretion of the designer.
This operation describe upending of a standard steel jacket from its horizontal free floating position to the upright position and setting it down on the sea bottom; the operation is carried out by a controlled rotation achieved by means of a step by step jacket ballasting and with the assistance of a Heavy Lift Vessel crane.
Appropriate ballasting and hook load sequences is important, so that the operations may be carried out in a predictable, safe and efficient manner and the jacket transverse and longitudinal stability meets the required criteria during all stages of upending.
Design Considerations
The upending control system and the respective marine operations, should be kept as simple as possible; consequently, the number of floodable jacket compartments should be kept to a minimum. The distribution of jacket buoyancy shall be such that at the onset of the upending operations the jacket floats in a stable, usually near horizontal position with the head slightly above the sea level and therefore accessible to the installation work crew (“head up” condition).
The jacket should achieve the "pendulum stability" (i.e. centre of buoyancy above centre of gravity) as early as possible and in any case prior to gaining the fully upright position.
If possible, the jacket ballasting capacity should be such as to permit achievement of the stable upright position on the sea bottom prior to flooding the flotation tanks (if any), obtaining in such a way the required on-bottom weight. The magnitude of the on-bottom weight should be sufficient for the jacket to withstand environmental loads and remain in a stable upright position on the sea bottom prior to piling operations.
The design of the upending system should be such, that minor variations in the jacket weight, buoyancy, and of the respective CoG and CoB positions will not result in significant changes in the jacket pitch and trim.
The upending procedure should include provisions for a large degree of flexibility, so that, if necessary, the established ballast quantities to be introduced or taken from the jacket, and in some cases sequence of operations may be changed within specified limits during all stages of upending, without affecting the safety of the operation.
Such contingency may become necessary in the event of accidental flooding of jacket compartments or due to failure of components in the upending control system.
In order to avoid hook racking, only two opposite hook prongs should be used for upending operations. Consequently, each pair of slings connected to one side of the jacket head should be engaged on the same prong; should the upending study indicate that the load acting on one prong at any stage of the upending exceeds 25% of the maximum (total) allowable hook load, the crane manufacturers shall be approached in order to verify hook resistance.
Jacket Weight and Position of CoG The upending analysis shall be carried out for 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:
- The jacket nominal weight and centre of gravity shall be taken from the latest available weight report.
- The above CoG displacements refer to the CoG position resulting from the Weight Control Report.
- The upending analysis shall be carried out for combination of the CoG displacements .
Jacket Reserve Buoyancy
The distribution of jacket buoyancy shall be such that at the onset of upending operations the jacket floats in a stable position, usually near horizontal, with the head slightly above the sea level and with the rigging platform of the jacket accessible to the installation work crew (“head up” condition).
The reserve buoyancy of the floating jacket shall be at least 15% of the total jacket weight. In exceptional cases the buoyancy may be reduced up to 10%, subject to the additional check of the jacket stability.
Jacket Stability
The upending system shall be designed in such a way as to assure a stable jacket condition throughout all phases of upending operations; the minimum transverse and longitudinal Metacentric Height (GM) during upending should not be less than 1 m.
The effective GM shall be calculated in the case of a crane assisted upending; the calculation shall consider the hook load and the sling geometry
For intermediate cases in which the upending analysis results in a GM value smaller than 1 m., trimming and heeling righting moment curves shall be developed in order to demonstrate the capability of the jacket to return to its stable position when subjected to minor disturbances (e.g. due to the waves; vessel motions; etc); the curves shall be developed for the induced angle of heel and trim of +/- 10 degrees.
Trimming and heeling righting moment curves shall be also developed for the jacket in a free floating condition.
Jacket Bottom Clearance
The upending operations shall be designed in such a way that the minimum bottom clearance of the jacket, during all phases of the upending sequence, shall be at least 3 to 5 metres.
For jackets with a damaged buoyancy, the minimum bottom clearance criteria will be determined on a case by case basis.
Jacket Damaged Buoyancy Analysis
The upending analysis shall also consider upending and setting down of the jacket with one buoyancy compartment damaged
The analysis for damaged buoyancy cases shall be based on the max. possible weight of the jacket; the same stability criteria as for the intact cases shall be applied.
The bottom clearance criteria will be determined on a case by case basis; in general any positive clearance would be acceptable.
Jacket Model
The jacket model designed for the upending analysis shall comprise all structural members, mud-mats, appurtenances and all temporary attachments present during the upending.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.
The 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 their properties;
- by the appropriate modification of the existing main elements of the model.
Intact Jacket Upending Analysis The upending analysis shall be carried out in a fully three dimensional manner. The analysis shall define the sequence of ballasting and hook loads required to rotate the jacket from the free floating position to the upright configuration and to set it down on the sea bottom.
The results of the analysis will provide the following information for each phase of the upending process:
hook height and the respective load;
- sling loads;
- jacket position;
- jacket stability (GM);
Damaged Jacket Upending Analysis
The analysis shall define the operational contingencies to be undertaken in the case of accidental flooding of one of the jacket compartments or failure of the upending control system components.
The analysis shall include the following:
- Identification (e.g. establishment of the flooded compartments) of the damage cases for which the "intact" procedure may be still safely used;
- Identification (e.g. establishment of the flooded compartments) of compartments whose damage would require a variation to the "intact" procedure;
- Definition of alternative procedures allowing for a safe upending of the damaged jacket.
The analysis shall be made for the nominal position of the jacket CoG.
Generally, a loss of each of the jacket compartments shall be investigated, however, in case of a specific experience with similar structures, the number of ballast compartments to be subjected to the analysis shall be left to the discretion of the designer.
