Jacket docking includes basic criteria for the determination of dynamic response and loads generated during the offshore installation; the results of the docking analysis will allow for assessment of the installation method and of the installation equipment adequacy as well as for verification of the relevant structural elements. The docking analysis regards installation of a conventional steel jacket over two pre-installed docking piles with different heights.
Docking Piles are Piles driven, at the time of subsea installation, in order to position the jacket on the sea bed over the drilled wells.
Jacket is Welded steel structure made of tubular components, designed to be installed on the sea bed and to support the offshore platform deck. The structure can be placed on the sea bed either by direct lifting or by a launching from a transportation barge followed by upending with or without crane assistance.
Launch Barge is Special purpose cargo barge designed to transport and launch a steel jack
The whole model of the body subject to the dynamics, including jacket, docking piles (modelled as springs), mooring lines, tug actions, HLV and all connections restraining, jacket and HLV.
Response Amplitude Operator (RAO) Response Amplitude Operator is function defining the amplitude of the oscillating motion of the ship in a regular seaway of unit amplitude. Separate RAO's are defined for each kind of the vessel motion. RAO can be determined either analytically or by model tests.
Stiffness characteristics is The spring with a constant equivalent to the stiffness of the body which shall be represented (e.g. crane wires, slings, HLV restoring, HLV motions etc.).
DESIGN CONSIDERATIONS
The purpose of the docking operation is to set down a steel jacket on the sea bottom, in the exact position in respect of the pre-installed drilling position; this is performed by docking piles, driven in advance into the seabed through the sleeves.
Normally, two docking piles are used to protrude at different heights over the sea bottom in order to engage in the jacket sleeves separately.
Jacket engagement with the first pile and with both piles engaged in the respective sleeves shall be checked.
The preliminary docking analysis shall be based on the assumption that the sea state during the installation period shall be basically made up by swell waves; consequently, the main load case shall consider a regular wave of a specified height. The criteria used for the analysis shall be based on the past experience in the geographic area under consideration and on the predicted response of the respective HLV.
INSTALLATION SEQUENCE
• The jacket is towed by tugs towards the Heavy Lift Vessel with the assistance of the HLV's mooring line.
• The jacket lifting slings are connected to the HLV crane hook; ballasting operations begins in order to upright the jacket.
• The uprighted jacket floats at some distance from its final position; clearance to the highest docking pile tip is provided.
• The jacket is towed and orientated with the assistance of the Heavy lift vessel mooring lines to its final position over the docking piles.
• The set down operation is carried out by means of ballasting the jacket and lowering the crane hook; the operation is controlled by means of the remote TV sacrificial cameras placed inside the docking buckets allowing verification of the following installation phases:
DOCKING ANALYSIS
A non-linear analysis in time domain shall be carried out to determine the jacket dynamic response during the set-down/docking operations.
The overall model shall consist of the jacket model with boundaries to simulate the Heavy Lift Vessel hook, tugger mooring lines and docking piles.
The period and wave direction shall be selected in such a way as to maximise the dynamic response.
The motions of the Heavy Lift Vessel will be transferred to the jacket model by means of the associated boom tip displacements.
The non linearity of the Heavy Lift Vessel restoring capacity to heave, roll and pitch motions shall be taken into account since this effect is particularly important, when wave (swells) and barge natural frequencies are close.
In case the software being used is able to process a multi body approach, the HLV will be entirely modeled. In such case the influence of RAO’s, their phase effect and the various non-linearities due to the vessel restoring will be automatically taken into account.
The jacket model is considered as a rigid body with six degrees of freedom, subjected to wave/current loads. Therefore, no internal stresses in the jacket are derived from the analysis.
Non linear equations of the motions take into account the drag forces, the buoyancy variation of the structural members in the splash zone and the characteristics of tugger lines.
These equations are integrated with a step by step procedure in time domain, based on the prediction/correction method. The jacket motions and boundaries forces shall be calculated for the time increment and time range selected. The results of the analysis shall be provided as plotted graphs showing displacements and forces acting on the jacket rigging, tugger and towing lines and docking piles.
Results of the Docking Analysis
The purpose of the docking analysis is to define the dynamic response and the loads generated during the docking operations in order to verify the installation method, installation equipment and relevant structural elements. The results of the analysis shall include the following information:
• Jacket docking sequence (ballast sequence, definition of the jacket ballast compartments);
• Jacket motions;
• Horizontal/vertical displacements of the jacket buckets (lower tips) prior to docking;
• Loads/stresses acting on the docking piles;
• Forces acting on the Heavy Lift Vessel crane and tugger lines.
• Operational limits of docking activities (sea state limit).
Docking Piles are Piles driven, at the time of subsea installation, in order to position the jacket on the sea bed over the drilled wells.
Jacket is Welded steel structure made of tubular components, designed to be installed on the sea bed and to support the offshore platform deck. The structure can be placed on the sea bed either by direct lifting or by a launching from a transportation barge followed by upending with or without crane assistance.
Launch Barge is Special purpose cargo barge designed to transport and launch a steel jack
The whole model of the body subject to the dynamics, including jacket, docking piles (modelled as springs), mooring lines, tug actions, HLV and all connections restraining, jacket and HLV.
Response Amplitude Operator (RAO) Response Amplitude Operator is function defining the amplitude of the oscillating motion of the ship in a regular seaway of unit amplitude. Separate RAO's are defined for each kind of the vessel motion. RAO can be determined either analytically or by model tests.
Stiffness characteristics is The spring with a constant equivalent to the stiffness of the body which shall be represented (e.g. crane wires, slings, HLV restoring, HLV motions etc.).
DESIGN CONSIDERATIONS
The purpose of the docking operation is to set down a steel jacket on the sea bottom, in the exact position in respect of the pre-installed drilling position; this is performed by docking piles, driven in advance into the seabed through the sleeves.
Normally, two docking piles are used to protrude at different heights over the sea bottom in order to engage in the jacket sleeves separately.
Jacket engagement with the first pile and with both piles engaged in the respective sleeves shall be checked.
The preliminary docking analysis shall be based on the assumption that the sea state during the installation period shall be basically made up by swell waves; consequently, the main load case shall consider a regular wave of a specified height. The criteria used for the analysis shall be based on the past experience in the geographic area under consideration and on the predicted response of the respective HLV.
INSTALLATION SEQUENCE
• The jacket is towed by tugs towards the Heavy Lift Vessel with the assistance of the HLV's mooring line.
• The jacket lifting slings are connected to the HLV crane hook; ballasting operations begins in order to upright the jacket.
• The uprighted jacket floats at some distance from its final position; clearance to the highest docking pile tip is provided.
• The jacket is towed and orientated with the assistance of the Heavy lift vessel mooring lines to its final position over the docking piles.
• The set down operation is carried out by means of ballasting the jacket and lowering the crane hook; the operation is controlled by means of the remote TV sacrificial cameras placed inside the docking buckets allowing verification of the following installation phases:
- Engagement of the first docking pile.
- Engagement of the second docking pile.
- Final set down of the jacket.
DOCKING ANALYSIS
A non-linear analysis in time domain shall be carried out to determine the jacket dynamic response during the set-down/docking operations.
The overall model shall consist of the jacket model with boundaries to simulate the Heavy Lift Vessel hook, tugger mooring lines and docking piles.
The period and wave direction shall be selected in such a way as to maximise the dynamic response.
The motions of the Heavy Lift Vessel will be transferred to the jacket model by means of the associated boom tip displacements.
The non linearity of the Heavy Lift Vessel restoring capacity to heave, roll and pitch motions shall be taken into account since this effect is particularly important, when wave (swells) and barge natural frequencies are close.
In case the software being used is able to process a multi body approach, the HLV will be entirely modeled. In such case the influence of RAO’s, their phase effect and the various non-linearities due to the vessel restoring will be automatically taken into account.
The jacket model is considered as a rigid body with six degrees of freedom, subjected to wave/current loads. Therefore, no internal stresses in the jacket are derived from the analysis.
Non linear equations of the motions take into account the drag forces, the buoyancy variation of the structural members in the splash zone and the characteristics of tugger lines.
These equations are integrated with a step by step procedure in time domain, based on the prediction/correction method. The jacket motions and boundaries forces shall be calculated for the time increment and time range selected. The results of the analysis shall be provided as plotted graphs showing displacements and forces acting on the jacket rigging, tugger and towing lines and docking piles.
Results of the Docking Analysis
The purpose of the docking analysis is to define the dynamic response and the loads generated during the docking operations in order to verify the installation method, installation equipment and relevant structural elements. The results of the analysis shall include the following information:
• Jacket docking sequence (ballast sequence, definition of the jacket ballast compartments);
• Jacket motions;
• Horizontal/vertical displacements of the jacket buckets (lower tips) prior to docking;
• Loads/stresses acting on the docking piles;
• Forces acting on the Heavy Lift Vessel crane and tugger lines.
• Operational limits of docking activities (sea state limit).
