(1) The incoming end must be aligned with the jacket notch;

（2） fender The collision force with the jacket must be less than the bearing limit of the jacket and fender itself;

(3) There shall be no collision between the installation leg of the topside and the jacket leg during ship entry;

(4) The longitudinal and transverse motions and yaw angles of the installation vessel must be controlled within a certain range.

When the ship entry operation is completed, the installation legs of the topside need to be aligned with the jacket legs. In general, the allowable alignment error depends on the size of the insertion point and the connecting bell mouth. After alignment, gradually reduce the spacing between the installation leg and the jacket leg until the spigot enters the bell mouth (Figure 1.2). The distance between the installation leg and the jacket leg can be reduced by increasing the ballast water of the installation vessel.

At this stage, the following points need special attention:

(1) Effectively limit the transverse and longitudinal movement of the installation vessel;

(2) Lateral impact load of jacket shall not exceed the limit load of jacket and fender;

(3) The vertical impact load on the jacket leg shall not exceed the design limit load of jacket and docking structure.

When the distance between the installation leg and the jacket leg decreases to zero, the docking process begins. At this stage, the weight of the topside will be transferred from 100% supported by the installation vessel to 100% supported by the jacket. The weight transfer of the topside can be realized by many methods, the most common is the ballast adjustment of the installation vessel and the active hydraulic support system.

When the load transfer is completed, the impact load between the topside and the support still exists. As long as these impact loads are likely to occur, the disembarkation operation cannot be carried out. Therefore, the spacing between the topside and the supports needs to be further increased.

At this stage, the following points need special attention:

(1) Effectively limit the transverse and longitudinal movement of the installation vessel to ensure small relative movement between the support and the topside;

(2) Lateral impact load of jacket shall not exceed the limit load of jacket and fender;

(3) The vertical impact load on the support shall not exceed the design limit load of the support and the support point of the topside.

When the spacing between the support and the topside is increased to the allowable value, the installation vessel can withdraw from the jacket groove. In this stage, the following operational requirements still need to be met:

(1) The lateral impact load on the jacket leg shall not exceed the design limit load of the jacket and fenders;

(2) No vertical impact load is allowed between the support and the upper module;

(3) Control the transverse movement and azimuth of the installation vessel;

(4) A certain distance must be kept between the bottom of the installation vessel and the horizontal pipe at the bottom of the jacket installation groove;

(5) The installation vessel must have sufficient freeboard.