experience. Well hole re-entry is usually performed by maneuvering the 
surface vessel and, once sonar or TV monitoring confirms alignment, 
stabbing the lower end of the pipeline into the seafloor guide funnel. 
Assembly of seafloor well heads is usually performed by maneuvering the 
surface vessel and the use of taut guidelines and guideposts. 
In the present case, a multi-point mooring system with taut lines 
to shipboard winches, as shown in Figure 2, can control the location of 
the bottom end of the pipeline for accurate positioning and stabilization 
against random motions due to surface vessel excitation (Ret 12). This 
type of system has performed successfully to 3,000-ft water depths and 
is considered to be adaptable to deeper water. 
Successful vertical position control (heave control) methods for 
pipe strings vary from manual adjustment to telescoping joints (bumper 
subs; riser slip joints) to various passive and active tensioners for 
guidelines and riser pipes and heave compensators in the pipestring 
hoisting system between the hook and the traveling block or at the crown 
block. Stabilized platforms such as column stabilized semi-submersibles 
are an appropriate solution. 
For concrete placing by pipeline, vertical motion control is pri- 
marily needed to keep the lower end of the pipe buried in the concrete 
during discharge. The required vertical motion compensation can be 
obtained by the use of telescoping slip joints in the pipestring near 
the bottom just above the seafloor discharge device. Either specifically 
built slip joints or commercially available bumper subs can be used. 
For concreting with a 3-inch ID pipeline, it is probably more economical 
to use one or more standard bumper subs (each with a 5-foot stroke) in 
series as is common practice in oil well drilling. For larger diameter 
pipelines, standard bumper subs are special order items so telescoping 
joints would probably be more economical to build than bumper subs. 
TECHNICAL CONSIDERATIONS 
General Approach 
For successful in situ seafloor concreting the key procedures are 
transporting the concrete vertically downward and discharging the fresh 
concrete at the seafloor. Thus, techniques are needed to control the 
flow of concrete through a long vertical pipeline without blockage and 
without runaway of the flow rate, and to control the discharge behavior 
of the fresh concrete at the seafloor. The concrete delivered to the 
seafloor should have the desired characteristics (consistency, time to 
initial set, strength when hardened, etc.) and be delivered in desired 
quantities in a minimum length of time. The major factors of interest 
that interact with each other are listed in Table 3. The most important 
of these factors are probably the purpose of the placement, the water 
depth and the total quantity of concrete required. 
The rigid pipeline, suspended from the surface platform, will be 
closed to atmospheric pressure at the upper end. Fresh concrete is 
pumped into the vertical pipe under positive pressure. The concrete 
flows, primarily due to the force of gravity, down the pipe and is 
discharged at the seafloor. 
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