MUDMAT DESIGN METHODOLOGY AND ASSUMPTION
Mudmat is the temporary seafloor support for jackets and subsea equipment. The function of mudmat is to provide adequate area for load distrubation to the soil. Mudmats are designed to support the weight of structures plus additional loads imposed by the environmental conditions.
Commonly used mudmats, consist of a top plate and a number of perpendicular vertical stiffeners that function as load-bearing beams. The fundamental aspect need to be consider in the design of mudmat is the vertical resistance of soil provided by the bearing capacity of the soil, resistance of the structure against sliding and over turning. Another aspect rarely being considered is the settlement of the mudmat. Settlement of the mudmat can be predicted by comparison of applied pressure and the ultimate bearing capacity of the mudmat. Mudmats are the most reliable for on-bottom stability of drilling units. The first step in design sequence requires geometrical engineer explore soil conditions at proposed location. By drilling and sampling a boring geometrical data are determined for the critical zone generally located to a 40 to 50 feet below the seafloor. The mudmat should be designed based on bearing capacity of the seafloor soils as determined by the geometrical engineer. The bearing capacity of seafloor can be predicted by using the formula,
The Jacket is placed at the target position after lifting and upending. The on-bottom load is transferred to the soil by mudmat. The on-bottom load consists of the jacket dead load,appurtenance loads and environmental loads. After upending and prior to grouting, two conditions
shall be considered for study
These three important stages are important
Pile stabbing location shall be considered for most critical condition of COG quadrant. For environmental loads, wave/current due to maximum water depth is selected for the higher base shear and overturning moment acting on the on-bottom jacket. For bearing capacity check, minimum water becomes the governing case.
Substructure Stability Check
Stillwater and Seastate conditions are considered for stability check.The soil bearing stability is compared from the acting bearing pressures allowable bearing pressure. The overturning stability and sliding resistances of jacket shall be checked to ensure it is stable during the pre-piled condition.
Bearing Pressure Check
API RP2A WSD 21st Edition shall be adopted to evaluate the mudmat bearing pressure. The maximum vertical loads and maximum overturning moments shall be used to check bearing stability. The ultimate soil bearing capacity shall be determined by geotechnical survey of the soil.
Qult = (20.00 + 1.39 B) (1 + 0.2 B/L) for B < 6.5m (Based on Skempton’s procedure)
Qult = (23.94 + 0.78 B) (1 + 0.2 B/L) for B > 6.5m (Based on Davis and Booker’s procedure)
Where:
Qult = Ultimate bearing pressure (kPa).
B = Effective Width of the bearing area (m).
L = Effective Length of the bearing area (m).
Soil bearing capacity can be checked with Davis and Booker’s equation as width (B) for current mudmat design is more than 6.5m. The bearing safety factor is calculated as the ratio of the ultimate bearing pressure to the actual bearing pressure. The required
Overturning Stability Check
The overturning stability is determined by the ratio of the restoring capacity to the applied overturning moment. Overturning Stability can be checked by the moment couple of jacket bottom weight and external environmental load moment.
Sliding Check
The sliding stability is ratio of the sliding capacity to the applied base shear. The combination of minimum jacket on-bottom weight and maximum base shear shall be used in checking sliding stability check. Minimum safety factor of 1.5 is required to comply with API RP2A WSD 21st Edition. The method outlined in API RP2A WSD 21st Edition shall be used to evaluate the mudmat sliding stability.
Mudmat Structural Integrity Check
The mudmat framings and jacket structures are checked by applying soil pressure up-ward on mudmat framing. The substructure is pinned at outer top of substructure legs. The members and joints are then checked to API-RP2A WSD.
The mudmat timber shall be designed as simple supported beam against maximum soil pressure. Maximum span of mudmat shall be considered.
Computer Modelling
The whole substructure model can be checked as in place analysis but with following changes:
Load Simulation
The Substructure model shall be studied for the on-bottom condition and the resultants loads shall be extracted for the design of the mudmat. These are the basic load cases used in the in place model except installation loads which are used in respective pre-service analysis.
The structural dead load of the members shall be distributed as member load. This dead load includes the self-weight of the primary and caisson, J-tubes and mudmat framing.
The non-simulated structural dead loads include the weights of launch cradle, anodes,mudmat , flooding and grouting system, ring stiffeners, jacket non-modeled walkway. The weight of the above appurtenances does not contribute to the stiffness of the structure. These weights are calculated and applied at the appropriate locations along the Jacket members.
Installation load includes the weight of upending padeye, upending rigging platforms, and buoyancy tank on the jacket.
The buoyancy load include the upward forces of modelled steel and non-modelled steel, i.e. anodes and mudmat timber, launch cradle timber and other miscellaneous items.The load case of each mudmat shall be factored as per design bearing pressure to simulate Stillwater as seastate condition in the load combination.
The basic load cases are generated maximum and minimum substructure weight and environmental loading for all heading degrees. This loading generated for stability check as described .The mudmat pressure resulted from bearing check shall be applied in mudmat framing (Upward Forces) to check mudmat member and joint integrity.
Mudmat is the temporary seafloor support for jackets and subsea equipment. The function of mudmat is to provide adequate area for load distrubation to the soil. Mudmats are designed to support the weight of structures plus additional loads imposed by the environmental conditions.
Commonly used mudmats, consist of a top plate and a number of perpendicular vertical stiffeners that function as load-bearing beams. The fundamental aspect need to be consider in the design of mudmat is the vertical resistance of soil provided by the bearing capacity of the soil, resistance of the structure against sliding and over turning. Another aspect rarely being considered is the settlement of the mudmat. Settlement of the mudmat can be predicted by comparison of applied pressure and the ultimate bearing capacity of the mudmat. Mudmats are the most reliable for on-bottom stability of drilling units. The first step in design sequence requires geometrical engineer explore soil conditions at proposed location. By drilling and sampling a boring geometrical data are determined for the critical zone generally located to a 40 to 50 feet below the seafloor. The mudmat should be designed based on bearing capacity of the seafloor soils as determined by the geometrical engineer. The bearing capacity of seafloor can be predicted by using the formula,
The Jacket is placed at the target position after lifting and upending. The on-bottom load is transferred to the soil by mudmat. The on-bottom load consists of the jacket dead load,appurtenance loads and environmental loads. After upending and prior to grouting, two conditions
shall be considered for study
- Stillwater (Calm sea condition)- Gravity load (Including buoyancy)
- Seastate Condition – Gravity load (Including Buoyancy) with environmental load.
These three important stages are important
- 1st Stage: Jacket is fully upright and lowered onto the seabed. All skirt pile sleeves are nonflooded.4 legs (A2, A8, B2 and B8) are flooded. This is the unpiled jacket stability condition. This condition shall be checked for minimum and maximum weight case.
- 2nd Stage: Outer pile (Gripper pile) is stabbed into skirt sleeve and allow to self penetrate.
- 3rd Stage: Second outer pile is stabbed into skirt sleeve and allow to self penetrate.
Pile stabbing location shall be considered for most critical condition of COG quadrant. For environmental loads, wave/current due to maximum water depth is selected for the higher base shear and overturning moment acting on the on-bottom jacket. For bearing capacity check, minimum water becomes the governing case.
Substructure Stability Check
Stillwater and Seastate conditions are considered for stability check.The soil bearing stability is compared from the acting bearing pressures allowable bearing pressure. The overturning stability and sliding resistances of jacket shall be checked to ensure it is stable during the pre-piled condition.
Bearing Pressure Check
API RP2A WSD 21st Edition shall be adopted to evaluate the mudmat bearing pressure. The maximum vertical loads and maximum overturning moments shall be used to check bearing stability. The ultimate soil bearing capacity shall be determined by geotechnical survey of the soil.
Qult = (20.00 + 1.39 B) (1 + 0.2 B/L) for B < 6.5m (Based on Skempton’s procedure)
Qult = (23.94 + 0.78 B) (1 + 0.2 B/L) for B > 6.5m (Based on Davis and Booker’s procedure)
Where:
Qult = Ultimate bearing pressure (kPa).
B = Effective Width of the bearing area (m).
L = Effective Length of the bearing area (m).
Soil bearing capacity can be checked with Davis and Booker’s equation as width (B) for current mudmat design is more than 6.5m. The bearing safety factor is calculated as the ratio of the ultimate bearing pressure to the actual bearing pressure. The required
Overturning Stability Check
The overturning stability is determined by the ratio of the restoring capacity to the applied overturning moment. Overturning Stability can be checked by the moment couple of jacket bottom weight and external environmental load moment.
Sliding Check
The sliding stability is ratio of the sliding capacity to the applied base shear. The combination of minimum jacket on-bottom weight and maximum base shear shall be used in checking sliding stability check. Minimum safety factor of 1.5 is required to comply with API RP2A WSD 21st Edition. The method outlined in API RP2A WSD 21st Edition shall be used to evaluate the mudmat sliding stability.
Mudmat Structural Integrity Check
The mudmat framings and jacket structures are checked by applying soil pressure up-ward on mudmat framing. The substructure is pinned at outer top of substructure legs. The members and joints are then checked to API-RP2A WSD.
The mudmat timber shall be designed as simple supported beam against maximum soil pressure. Maximum span of mudmat shall be considered.
Computer Modelling
The whole substructure model can be checked as in place analysis but with following changes:
- Skirt pile sleeve were un-flooded except for driving skirt pile sleeve
- Non-corroded section is considered in splash zone.
- No marine Growth.
- Only loads that are attached to the jacket during on-bottom state shall be modeled.
- Piles have been removed for un-piled condition however for piled condition pile has been modeled after self penetration.
- No grouting on skirt sleeve.
- No boat-landing
Load Simulation
The Substructure model shall be studied for the on-bottom condition and the resultants loads shall be extracted for the design of the mudmat. These are the basic load cases used in the in place model except installation loads which are used in respective pre-service analysis.
The structural dead load of the members shall be distributed as member load. This dead load includes the self-weight of the primary and caisson, J-tubes and mudmat framing.
The non-simulated structural dead loads include the weights of launch cradle, anodes,mudmat , flooding and grouting system, ring stiffeners, jacket non-modeled walkway. The weight of the above appurtenances does not contribute to the stiffness of the structure. These weights are calculated and applied at the appropriate locations along the Jacket members.
Installation load includes the weight of upending padeye, upending rigging platforms, and buoyancy tank on the jacket.
The buoyancy load include the upward forces of modelled steel and non-modelled steel, i.e. anodes and mudmat timber, launch cradle timber and other miscellaneous items.The load case of each mudmat shall be factored as per design bearing pressure to simulate Stillwater as seastate condition in the load combination.
The basic load cases are generated maximum and minimum substructure weight and environmental loading for all heading degrees. This loading generated for stability check as described .The mudmat pressure resulted from bearing check shall be applied in mudmat framing (Upward Forces) to check mudmat member and joint integrity.
