Heave Compensation. Any item that floats

suffers Heave (Rise and Fall) from Sea

Surface displacwmwnta (waves). Some hulls

more than others are prone to this. Drilling

is an operation where heave is a movement

that must be removed or mitigated. To do

this most floating drilling applications require

heave compensation (hence the attraction

of a fixed platform).

These Heave Compensation Diagrams show the displacement of a Semi Sub. 1 Low Max, 2 Low Mid, 3 High

Mid, 4 High Max Wave Heigh. The blue arrow heads in the Diag show the area of maximum buoyancy for a

semi sub, the pontoon/sponson area. The transverse stiffeniner having minimal effect. Two problematical

areas of interelation between the Semi Sub , Drill String and Riser Casing exist. a) the Drilling Riser Casing

(and eventually the production Riser Casing) need to be in tension to avoid buckling (they are long and thin

and would eaily buckle if not in tension.. The contact point for this interface is the casing swivel. b) the

second point of contact is the Drill String head. This also needs to be in tension to either support its weight

or provide load to the drill bit also again to prevent buckling. The diagrams show the movement of the

component parts as the ship moves. Initially the sides of the compensation bottle are in equilibrium with

sufficient force to hold the drill casing in tension. the Semi Sub marginally moves up or down from the fixed

point, the swivel. In doing this it creates a disparity in the compensation bottles, increasing the pressure on

one side of the diaphragm over the other. As the pressures are allowed to stabilize the condition returns

where sufficient force holds the drill string in equilibrium but the correlation semi sub to swivel has changed.

These displacements contiue and increase or decrease but still holding the drill casing in a constant

tension.4d_3 West Navion shows the type of equipment in this operation. The same diagram Heave Diagrams

also show compensation at b) however they use a pantgraph style of arrangenment to accomodate heave

but at this point they also have toaccomodate the fact that the drill string is in the proces of drilling down or

being recovered at the same time. Here theforce in the Drill String must be sufficient to enable the tool bit to

drill. This is accomplished by paying out the drill winch cable sufficintly to progress drilling withoutletting the

drill string buckle. cd_3 West Navion is slightly different to this since the Rig know as a Ram Rig uses two

special long cylinders to raise or lower the drill string. Other compensation systems exist, some are all

hydraulic and some work has been undertaken for linear motor style of actuaters to contain the movements

but still ensure tension of the strings.