Offshore structures towed to a job site must be launched and upended (see Fig. 1). 1),2). In relatively shallow water, a structure is upended by hoisting it with a derrick crane. In deep water, however, a structure (usually large and heavy) is often self-upended without the aid of a crane, through buoyancy adjustment.
There are two important factors involved upending procedures: the stability of the structure being upended, and the time-dependent change in the attitude of the structure.
A computer program was developed to systematically analyze the stability of a structure being upended. The upending of a hydraulically unstable structure was analyzed by this program and, as a result, a safe upending procedure has been determined. procedure has been determined. A flooding analysis method has been developed for the time-dependent attitude change. This method analyzes the time-dependent change in the surface of water flooded into a structure's flood tanks. An actual jacket structure was analyzed by the method, and the analytical results are reported in this paper.
To outline the upending analysis, the flow chart of the computer program INSTAN is shown in Fig. 2. This program was developed program INSTAN is shown in Fig. 2. This program was developed to analyze various on-site operations, including launching and upending.
NFLOAT analyzes the natural floating conditions of a structure and its stable floating attitude at every stage of the upending operation. It also calculates the righting moment about the pitching and rolling axes of an offshore structure in order to evaluate its stability.
UPEND, which was originally intended to evaluate crane-aided upending procedures, includes three-dimensional analysis sub-programs: POSITION and TENSION, both of which analyze upending by controlling hoisted loads and hook positions; and ROTATION, which controls a structure so that its tilting angle is constant. Another program, which evaluates the time-varying flooding height in the tanks, is also used in conjunction with INSTAN.
When a structure has little restoring ability during upending, rotational moments due to waves, winds and currents can cause sudden rotation of the structure, resulting in an accident. To avoid such misfortunes, stability checks must be conducted.
Righting moment (MR) is determined by calculating the unbalanced moment that occurs when the sea surface tilts relative to a certain angle ( ), as expressed in Eq. (1).
where Wo = unit weight of sea waterxi = x-horizontal distance of i-memberyi = y-horizontal distance of i-member= pitching angle= rolling angle
zo = relative vertical coordinate of sea surface
dsi = area if i-member
The relative vertical coordinate of the sea surface (zo) is determined from the conditions of balance between the buoyancy and negative buoyancy of the submerged portion of a structure. The righting moment about the rolling axis, MR( ), is also calculated by Eq. (1).
The righting moment for a large heeling angle generally assumes curves such as the one shown in Fig. 3. It is clear from Fig. 3 that the stability of a structure increases as the distance between the stable balance point and the unstable point increases. Fig. 3 also indicates that the higher the peak value of righting moments, the more stable the structure.
Improving An Upending Procedure For Hydraulically Unstable Structures
Here an improved upending method will be presented - a method that begins flooding prior to the crane operation.
According to the conventional upending method, a free-loading structure, after launching, is first hoisted by crane, then partially flooded, and finally tilted to an upright position. With the improved method, flooding starts prior to crane work, thereby giving stability to the structure before the structure is finally lifted.
The difference between the two methods is the time at which the flooding operation begins, as shown in Fig. 4.
Fig. 5 compares the stability of a structure at stage 2 of each method, while Fig. 6 compares it at stage 3. These data indicate that the flooding/hoisting method offers the following advantages over the hoisting/flooding one:
. No unstable condition exists
. The magnitude of righting moments increase continuously.
A structure is, therefore, more stable when upended by the flooding/hoisting method.