This Section specifies the overall design philosophy that shall be applied to the structural design of internal and external turret systems for permanently moored FPSO, FSO, FLNG vessels, and FSRUs.
The turret and mooring system shall be designed to maintain station and to allow the vessel to weathervane at all times in all environments up to the most extreme environmental.The intent of the turret mooring system design is to allow personnel to remain onboard through any storm that traverses the site. Unimpeded weathervaning is critical to minimizing motions and the environmental loads that the vessel imposes on the mooring system. Excessive vessel motions and/or mooring overload possibly leading to failure, with loss of station, can become threats to the safety of personnel onboard.
Turret structural design shall, as a minimum, meet the same ULS design criteria (10–2 per year probability of exceedence event) as the hull in which it is to be installed. The structural design shall be performed in working stress design format and shall use the same ULS design safety factors as the hull.
The turret shall be designed to maintain its structural integrity in response to loads and motions imposed on it, up to and including load cases that have a 10–4 per year probability of exceedance. The turret shall be sufficiently stiff so that any deformation that occurs will not prevent rotation as required to allow the vessel to weathervane.To ensure robustness of the turret mooring design, and thus allow personnel to shelter in place, industry standards, most host country regulations, and Company practice require that the design be shown to maintain its integrity in up to an abnormal environmental event with less than 10–4 per year probability of occurrence.
The turret structure shall be designed so that any failure of a mooring line under load will not damage the main turret structure, the attachment point and supporting structure, and the turret's ability to continue rotating.The turret shall be designed with sufficient fatigue resistance to preclude the need for any repairs during its design service life.For external turret systems, sufficient clearance shall be maintained between the mooring system and the hull.
The turret shall be designed for continuous on-station operation for the design service life without recourse to dry-docking, and with minimal repair and maintenance; the design should maximize inspectability and maintainability.
1) Hydrodynamic analyses and development of hull loads and motions, including the following:
Hydrodynamic Analysis and Development of Hull Loads
Vessel Heading Analysis and Global Response
One of the most important aspects of designing the turret and mooring system of a weathervaning vessel is to reduce the potential range of headings that the vessel can take in response to the metocean conditions it encounters. The goal is a limited set of design load cases that can be thoroughly investigated. There is always a range of uncertainty in the relative angles of wind, wave, current, and swell that can impact the vessel in various events. The following requirements apply for the screening analysis:
Turret Structural Load Analysis
During its service life, the turret will be subjected to a variety of operating loads in both normal and extreme environmental conditions as well as to possible accidental loads. The turret structure shall be designed to withstand all expected combinations of the following:
Turret Bearing System
The turret and mooring system shall be designed to maintain station and to allow the vessel to weathervane at all times in all environments up to the most extreme environmental.The intent of the turret mooring system design is to allow personnel to remain onboard through any storm that traverses the site. Unimpeded weathervaning is critical to minimizing motions and the environmental loads that the vessel imposes on the mooring system. Excessive vessel motions and/or mooring overload possibly leading to failure, with loss of station, can become threats to the safety of personnel onboard.
Turret structural design shall, as a minimum, meet the same ULS design criteria (10–2 per year probability of exceedence event) as the hull in which it is to be installed. The structural design shall be performed in working stress design format and shall use the same ULS design safety factors as the hull.
The turret shall be designed to maintain its structural integrity in response to loads and motions imposed on it, up to and including load cases that have a 10–4 per year probability of exceedance. The turret shall be sufficiently stiff so that any deformation that occurs will not prevent rotation as required to allow the vessel to weathervane.To ensure robustness of the turret mooring design, and thus allow personnel to shelter in place, industry standards, most host country regulations, and Company practice require that the design be shown to maintain its integrity in up to an abnormal environmental event with less than 10–4 per year probability of occurrence.
The turret structure shall be designed so that any failure of a mooring line under load will not damage the main turret structure, the attachment point and supporting structure, and the turret's ability to continue rotating.The turret shall be designed with sufficient fatigue resistance to preclude the need for any repairs during its design service life.For external turret systems, sufficient clearance shall be maintained between the mooring system and the hull.
The turret shall be designed for continuous on-station operation for the design service life without recourse to dry-docking, and with minimal repair and maintenance; the design should maximize inspectability and maintainability.
1) Hydrodynamic analyses and development of hull loads and motions, including the following:
- How wind and current areas and coefficients are to be determined
- Roll and yaw damping formulations and basis for coefficient selection
- Objectives of any planned wave basin model tests
- List of load cases for screening analysis and a description of how governing load cases will be determined
- How low frequency yaw motions will be determined and details about how they will be used in the analysis
- For a tropical cyclone area, a proposed procedure for how the near passage of the eye of the storm is to be simulated and how transient vessel heading response will be predicted
- Fatigue analysis and line dynamics
- How frequency and/or time domain analyses will be used to generate the global mooring loads on the turret
- How hydrodynamic loads are applied, including green water and slamming
- Transfer of mooring loads to the structure, including the method for estimating the combined action of mean, low frequency, and wave frequency loads
- Load combinations to be applied to the turret to derive design load cases for global and local structure
- An assessment of the importance of turret structural dynamic response, and, if important, a method for including dynamic structural response effects in design
Hydrodynamic Analysis and Development of Hull Loads
- Roll damping shall be developed using a method that appropriately accounts for nonlinear dependence on roll amplitude, and that has been validated against model test and field data.
- Wind tunnel tests shall be performed to develop or verify wind force coefficients.
- Current force coefficients for the hull shall be based on wind tunnel or towing tank tests. If appropriate, current force coefficients may be estimated based on available data for similar hulls. In some cases (e.g., for barge-shaped hull geometries), standard wind tunnel testing may not produce current force coefficients that are sufficiently accurate.
- Wave basin tests shall be used to verify analysis predictions and system behavior, but shall not be used to develop turret design loads. Model tests should be planned and carried out in accordance with .
- Margins for uncertainty and potential growth in wind and current areas shall be maintained at appropriate levels and updated in a timely manner as the design progresses.
Vessel Heading Analysis and Global Response
One of the most important aspects of designing the turret and mooring system of a weathervaning vessel is to reduce the potential range of headings that the vessel can take in response to the metocean conditions it encounters. The goal is a limited set of design load cases that can be thoroughly investigated. There is always a range of uncertainty in the relative angles of wind, wave, current, and swell that can impact the vessel in various events. The following requirements apply for the screening analysis:
- Both mean heading and the maximum range of low frequency yaw motion shall be determined for each governing design sea state combination. The effect on yaw motion of the variation in wind speed as modeled by an appropriate wind spectrum shall be included.
- simplified mooring model may be used for screening studies of global response to determine governing load cases.
- If used, time domain analyses shall be run long enough to eliminate starting transient effects, and to capture a sufficient number of yaw cycles, to ensure a stable estimate for maximum dynamic yaw motion.
- In a tropical cyclone area, simulations of the passage of the eye of the storm over and close to the site should be carried out if suitable time histories of wind and wave conditions at the site during storm passage are specified in the project metocean design criteria. The effect of transient heading variations on response and loading shall be assessed. The objective of the simulations is to verify that the chosen set of governing load cases will bound the relative headings and motions that can be experienced during discrete storm passages.The intent is to ensure that personnel onboard will not be exposed to some unexpected extreme vessel motions due to a temporary unfavorable heading of the vessel relative to the waves during the passage of a storm.
Turret Structural Load Analysis
During its service life, the turret will be subjected to a variety of operating loads in both normal and extreme environmental conditions as well as to possible accidental loads. The turret structure shall be designed to withstand all expected combinations of the following:
- Inertial loads due to vessel motions
- Mooring loads, including damaged cases
- Riser and umbilical loads
- Imposed moon pool deformations caused by hull bending
- Turret bearing reactions
- Wave and buoyancy loads acting on the submerged portion of the turret
- Wind loads acting on exposed turret structure
- Green water and slamming loads
- Local loads due to equipment and piping systems
- Torsion loads due to bearing friction and swivel friction (bearings and seals)
- Accidental loads (vessel collision, dropped objects, fire, and blast loads)
- Motions and accelerations
- Green water
- Wave slam
- Bearings shall be modeled using gap or contact elements at the roller or sliding element interface.
- The model shall include a sufficient extent of surrounding hull structure to accurately simulate its stiffness—approximately half a tank width fore and aft and full hull breadth.
- Hull/turret interface structure shall be modeled in detail to demonstrate that global and local deflections are within the range specified by Bearing Supplier.
- The source of the assumed friction factor for any sliding bearing element shall be documented.
- Structural natural periods for the turret/bearing/hull structure shall be calculated to investigate any structural dynamics effects.
Turret Bearing System
- The turret bearing system shall be capable of transferring all loads from the turret structure on to the vessel.
- The turret bearing system shall permit full weathervaning of the platform with no restrictions regarding weather condition or vessel operation.
- The bearings for the system shall provide axial support for all vertical loads imposed in the turret by the mooring, riser system, and umbilicals. The bearing system is also required to support the radial loads imposed by the turret due to vessel longitudinal and transverse acceleration, and bending moments imposed by the mooring system.
- The turret bearing support structure shall be designed to limit deflections to within allowable values, specified by Bearing Manufacturer, under all load cases.
- The bearing assembly and its mounting shall be designed to allow inspection of the bearings while the platform is in full operation mode.
- Fatigue damage to bearings and local support structure shall be determined as part of the turret fatigue analysis.
