The effect of Sea current on offshore structures
How sea currents can affect offshore design?
- Currents can cause large steady excursions and slow drift motions of moored platforms.
- Currents give rise to drag and lift forces on submerged structures.
- Currents can give rise to vortex induced vibrations of slender structural elements and vortex induced motions of large volume structures.
- Interaction between strong currents and waves leads to change in wave height and wave period.
- Currents can create seabed scouring around bottom mounted structures.
Types of current . sea current
There are many types of currents such as those driven by salinity, wind, temperature or the Coriolis Effect. Two of the most commonly identified are:
2.1. Surface currents (Surface Circulation): These currents are driven by winds, are fast and make up 10% of the world’s oceans. In the northern hemisphere surface currents develop clockwise spirals that eventually turn into gyres which are, large systems of circulating ocean currents. Some currents across the globe tend to move with the seasons. North and south equatorial currents are affected also by seasonal changes because of the relocation of pressure systems and wind belts. Surface currents develop anti-clockwise spirals in the southern hemisphere.
2.2. Deep ocean currents (Thermohaline Circulation): These currents also known as thermohaline circulation or ocean conveyer belts. These currents generally move in the deep ocean and are caused by temperature, the shape of the ocean floor and salinity. Salinity changes occur because saltier water sink and displaces water that is warmer and less dense. They are also slower moving and make up 90% of the world’s oceans and are generally unnoticeable at the ocean surface.
Figure 1.This visualization was created in support of the 2011 UNESCO conference in Paris, France
Figure 2.Map of the world showing the global thermohaline circulation.
- How to calculate current force and its effect on offshore structures?
3.1. Slender Members sea current
The hydrodynamic force exerted on a slender structure in a general fluid flow can be estimated by summing up sectional forces acting on each strip of the structure. In general the force vector acting on a strip can be decomposed in a normal force fN, a tangential force fT and a lift force fL being normal to both fN and fT.
Figure 3.force vectors acting on a strip
3.1.1. Normal force
The sectional force fN is different by sea state and structure situation.
- Fixed structure in waves and current
- Moving structure in still water
- Moving structure in waves and current
- Normal drag force on inclined cylinder
=fluid particle velocity [m/s]
=fluid particle acceleration [m/s2]
A=cross sectional area [m]
D =diameter or typical cross-sectional dimension [m]
ρ =mass density of fluid [kg/m]
CA =added mass coefficient [-]
CD =drag coefficient [-]
=velocity of member normal to axis [m/s]
=acceleration of member normal to axis [m/s]
Cd =hydrodynamic damping coefficient [-]
3.1.2. Tangential force on inclined cylinder
Where CDt is the tangential drag coefficient and v is the magnitude of the total velocity.
3.1.3. Lift force
A lift force fL , on a slender structure maybe due to:
- Unsymmetrical cross-section
- Wake effects
- Wall effects
- Vortex shedding.
3.1.4. Torsion moment
The inviscid moment per unit length about the longitudinal axis of a non-circular cross-section with two planes of symmetry is:
=fluid particle velocity in directions y and z [m/s]
=normal velocity of cross-section in directions y and z [m/s]
=added moment of inertia for cross-section [kg × m]
=angular acceleration of cross-section [rad/s]
=added mass coefficient in directions y and z [-]
Figure 4.Torsion moment exerted on non-circular cross-section
These forces must be considered in design of risers with buoyancy elements and jack up leg chords.
3.2. Large Volume Structures
The term large volume structure is used for offshore structures with dimensions D on the same order of magnitude as typical wave lengths λ of ocean waves exciting the structure, usually D > λ/6.
3.2.1 Steady current loads
A steady current gives rise to a steady force in the horizontal plane and a yaw moment. The moment about
a horizontal axis may also be of importance. Empirical formulae are most often used to calculate current forces and moments on offshore structures. The forces and moments are normally a function of the current velocity squared given in the general form:
Where C is an empirical current coefficient, and Uc is the current velocity.
18.104.22.168. Column based structures
Viscous current forces on offshore structures that consist of relatively slender large volume structural parts can be calculated using the strip-theory approximation. Although these structures are classified as large-volume structures relative to the incoming waves, they may be treated as slender structures for prediction of pure current loads. This applies for instance to columns and pontoons of semi-submersibles and of TLPs.
The current velocity is decomposed into one component UcN in the cross flow direction of the slender structural part and one component in the longitudinal direction. The latter component causes only shear forces and is usually neglected. The cross flow velocity component causes high Reynolds number separation and gives rise to an inline drag force.
Where Cd is the sectional drag coefficient and D is the diameter.
22.214.171.124. Ships and FPSOs sea current
For moored ship-shaped structures, it is common to represent current forces in surge, sway and yaw by empirical global current coefficients, given as a function of the current heading β:
The coefficients Ci can be estimated based on acknowledged published or in-house data for similar ships scaling to the size of the current ship. The drag force on an FPSO in the longitudinal direction is mainly due to skin friction forces and it can be expressed as:
Where S is the wetted surface. Cd The drag coefficient is a function of the Reynolds number Re and the angle β between the current and the longitudinal axis of the ship.
The transverse current force and current yaw moment on an FPSO can be calculated using the cross flow principle. The assumption is that the flow separates due to cross flow past the ship, that the longitudinal current components do not influence the transverse forces on the cross-section, and that the transverse force on a cross-section is mainly due to separated flow effects. The transverse current force on the ship then can be written as:
Where the integration is over the length of the ship. CD(x) above is the drag coefficient for flow past an infinitely long cylinder with the cross-sectional area of the ship at position x. D(x) is the sectional draught.
The viscous yaw moment due to current flow is simply obtained by integrating the moments due to sectional drag forces along the ship. It is important to note that the yaw moment has an additional inviscid part, called the Munk moment,sea current
Where Uc is the current velocity in a direction β with the x-axis and A11 and A22 are the added mass coefficients in the x- and y-directions.sea current
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