Another specificity of Off-Shore facilities is that their submerged parts are subject to action from the sea.
The sea will cause the submerged part (hull) to both move and to deform. The extent of the motion and deformation will obviously depend on the sea state.
Such motion and deformation of the hull will be transferred to the topsides, as explained below, an result in constraints to be taken into account in their design.
The hull will be subject to motion from the sea (waves, swell) that will cause it to oscillate. The sketch illustrates such motion (roll) around one of the hull axis.
Such motion will create acceleration forces on the topsides, which will be maximum when the hull reaches it maximum inclination and starts to roll back.
Accelaration forces will cause the topsides to deform under the inertia of the equipment it supports.
As the sea forms an uneven support to the hull, it will be subject to deformation.
Such deformation will transfer to the topsides. The transfer is minimized by providing slidding rather than fixed supports for the topsides on the hull.
Such supports, called bearing pads, allow vertical displacements, both up and down (down motion is allowed by compression of the pad, which is made of elastomer material).
Motion and deformation of the hull will lead to relative displacements between topsides and hull equipment and, to a lesser extent, between topsides equipment. Such relative displacements are not acceptable for long shaft rotating equipment whose driver, gear boax and driven equipment need to remain strictly aligned.
The driver, gear box and driven equipment of such equipment are thus all mounted on a common skid supported on to the deck structure at 3 locations only. Use of a 3 point type support will make sure the skid remains in a plane regardless of the deflection of the underlying structure.
The motions and accelerations to which Equipment will be subjected are determined in Naval Engineering’s hydrodynamic analysis as a function of the Equipment position and elevation.
Slender equipment, such as columns, may need to be re-inforced, by increasing their thickness, to sustain the forces induced by the acceleration (inertia forces induced by the motion of the top of the column).
Rotating equipment must also be checked so that their operation remains unaffected under acceleration forces. Some equipment may need to be stopped in extreme sea conditions whereas emergency equipment need to be designed to operate under all conditions.
As motion and deformation of the hull leads to differential displacements bewteen equipment, it induces stress (expansion, compression) to the pipework that connects them.
Where thermal expansion is the prime driver for the design of piping flexibility of land faciltities, the differential displacement between equipment prevails off-shore (as high temperature are not found in off-shore processes).
Let’s consider for instance a line connecting a pumps on the hull deck to a vessel one of the topsides’ decks.
The line is fixed at both ends: the pump nozzle and that of the vessel. Ordinary stress calculations, such as the ones that would be done for an On-Shore plant, would check that the line is not subject to excessive stress when it thermally expands while reaching its service temperature. Another factor must be considered for Off-Shore facilities. Indeed, under the action of the sea, the pump attached to the hull deck and the vessel on the module will move one relatively to the other.
The same situation can be found for a line connecting equipment located on different topsides modules.
Off-shore piping flexibility calculations take into account the relative horizontal and vertical displacements of the line supports.
They will result in requirements to provide flexibility to the line, by means of directional changes, purposely made loops etc. As usual, thick and large diameter lines are the most affected, as they are the less flexible.
This is another specificity of Off-Shore engineering.
