Mooring Lines Design Strength
Design basis for mooring activities with different safety factors for different classes.Jacket/topsides installation positions are considered Class 2 (as close to the final platform structures).Stand-off position is considered Class1 (as by definition far from any other vessel or platform)..
Winch Brake Capacity
The maximum tension in the mooring lines shall be lower than winch brake capacity. Winches brake capacities depends on the paid-out length.
Anchor Ultimate Holding Capacity
The maximum tensions in the mooring lines shall be lower than UHC of anchor.Sensitivity on the UHC shall be checked for the stand-off position.
Anchor Uplift
No Uplift at the anchor shall occur under intact condition. This criterion is not considered for damaged conditions (one line failure).Sufficient Mooring wire lengths are used to ensure that winch brake capacity and anchor uplift are never limiting under any intact conditions, as well as enhancing anchor holding capacities.
Allowable Offset during Jacket / Topsides Lifting
During jacket and topsides lifting some lines slack, the offset in intact positions shall be limited to ±5 m as keeping reasonable control of the barge during such operation. Such horizontal motions are mostly low frequency (100s natural period). No offset limitation under line damage shall be considered as an accidental case.
Clearance during Jacket /Topsides Installation
The minimum clearance between any part of barge and existing offshore structures shall remain 3 m under any conditions.For mooring line, a minimum clearance of 10 m is usually considered between mooring line and existing facility.
Surge Limitation is defined as follow:
Clearance during Pile Installation
The minimum clearance between any part of barge and existing offshore structures shall remain 5 m under any conditions. Mooring line allowable clearances shall also be considered as 5 m.
Surge Limitation shall be defined as follow:
Anchor Positioning and Mooring Line Clearance Anchor pattern is designed in accordance to the following points:
Design basis for mooring activities with different safety factors for different classes.Jacket/topsides installation positions are considered Class 2 (as close to the final platform structures).Stand-off position is considered Class1 (as by definition far from any other vessel or platform)..
Winch Brake Capacity
The maximum tension in the mooring lines shall be lower than winch brake capacity. Winches brake capacities depends on the paid-out length.
Anchor Ultimate Holding Capacity
The maximum tensions in the mooring lines shall be lower than UHC of anchor.Sensitivity on the UHC shall be checked for the stand-off position.
Anchor Uplift
No Uplift at the anchor shall occur under intact condition. This criterion is not considered for damaged conditions (one line failure).Sufficient Mooring wire lengths are used to ensure that winch brake capacity and anchor uplift are never limiting under any intact conditions, as well as enhancing anchor holding capacities.
Allowable Offset during Jacket / Topsides Lifting
During jacket and topsides lifting some lines slack, the offset in intact positions shall be limited to ±5 m as keeping reasonable control of the barge during such operation. Such horizontal motions are mostly low frequency (100s natural period). No offset limitation under line damage shall be considered as an accidental case.
Clearance during Jacket /Topsides Installation
The minimum clearance between any part of barge and existing offshore structures shall remain 3 m under any conditions.For mooring line, a minimum clearance of 10 m is usually considered between mooring line and existing facility.
Surge Limitation is defined as follow:
- Surge is limited by the distance between stern extension frame and the jacket structure.
- The distance between the two structures shall be ensured 3 m. So surge motion with initial gap shall not exceed this limit.
- The control points can be set at few locations to study these motions and monitor these clearances.
- Sway is limited by the movements the barge leading to clash between mooring lines and jacket legs.
- Stern-most line on each side of the barge shall be studied if they are potential lines clashing with the platform.
Clearance during Pile Installation
The minimum clearance between any part of barge and existing offshore structures shall remain 5 m under any conditions. Mooring line allowable clearances shall also be considered as 5 m.
Surge Limitation shall be defined as follow:
- Surge is limited by the movement of any mooring line towards the jacket.Surge motion at corresponding fairlead shall be studied.Surge motion shall not be a limiting criteria after damaged condition if other mooring lines are far from the jacket in drilling position.
Anchor Positioning and Mooring Line Clearance Anchor pattern is designed in accordance to the following points:
- No anchors shall be placed at a distance less than 150-200 m from a pipeline of a subsea part of a structure.
- Whenever a mooing line crosses a route of a pipeline, the anchor shall be placed at least 350-400 m away from the crossing point.
- Catenary mooring lines shall be at least 10 m above subsea line (laid on seabed) in intact condition and 2 m above subsea line in case of one midline buoy failure.
