Complex deepwater projects require planning and foresight that have not been required for shallowwater installations in the past. The deeper the water, the more the engineer is required to think three-dimensionally. The deepwater systems require moorings that are complex and cover large areas. This is even more complicated when several construction vessels vie for the same area. Modeling provides a more comprehensive understanding of the physical dimensions necessary to complete these ultradeepwater projects within acceptable risk and cost levels. The information acquired shows potential problem areas for which contingency plans are developed and safe installations are achieved. By physical modeling, a considerable insight into the field architecture is established prior to the finalization of the entire system layout.
History
The first physical model of the type being discussed in this paper was developed in 1976 for a remote pipeline bundle in the Canadian Arctic. The bundle consisted of two insulated 6-in. lines, 4¾-in. lines, one 4-in. line, and instrument and power cables all inside an 18-in. casing. Since there was no conventional method available for installing the pipeline system, it was imperative that the engineer be able to simulate the actual site conditions and develop the methods and procedures for the installation. Obviously the only feasible method in a remote location like this is to make the bundle on an island, launch it through a slot in the ice, pull it along the bottom and connect it to a planned wellhead without the assistance of divers. The innovative part was how to make up the lines onshore in a bundle, launch, pull, and provide a new diverless tie-in remotely from the surface of the ice. In a project of this type it is important to take advantage of all that "Mother Nature" can provide in the way of gravity, flotation, bottom friction for pipe dragging chains, the pipeline's flexibility and our ability to maneuver a long section of the pipe floating above the seabed.
Figure 1 is a picture of the first model facility. From that start, we have evolved from
2-D to 3-D and now 4-D modeling of all kinds of situations and provided many practical solutions, reducing risks and saving considerable monies along the way. This has been shown to be very effective in highlighting potential problem areas for which modification to the procedures can be developed.
The most recent breakthroughs have included complete simulation of the operations for the installation vessels including their movement, along with winches and cable for installation, riser settings, pipeline abandonment and recovery. Figure 2 is the first of the fourth-generation models which depicts three work vessels and a semisubmersible, all suspended from a platform which can be raised and lowered for initial positioning of the work spreads. The simulation of a water injection line installation is shown in Figure 3. Miniature cameras mounted on the semisubmersible hull provide the type of information that the operators installing the riser in a basket sees in the control room on the semi-host vessel. The installers
have a very limited view of what is occurring below the water surface. Viewing the model before actual installation provides the contractor's personnel a better understanding of the complete process with its pitfalls and potentials for avoiding accidents. Mimicking the installation on the model during this process provides additional reference to other potential problem areas.
Another use for a model of this type is to define the extent of riser touchdown when the hub vessel moves around its "Watch Circle." It is common when operating at these depths of water to have a surface excursion radius of 350 ft (107 m), which develops a typical touchdown footprint of 100 ft width by 600 ft in length (30 m in width by 183 m in length). Along this same line is the effect on the "Field Architecture" by moves well in excess of the normal operating "Watch Circle," (Figure 4). When a move of greater than 700 ft (213 m) is developed along the transverse axis of the vessel, for whatever reason, there is a significant repositioning of the touchdown points when the vessel is restored to its original position. The touchdown, points perpendicular to the vessels longitudional axis have displacements up to 70 ft (21 m); those along the longitudional axis recovered go back to their original touch down positions.
When one considers the complexity of operating systems of this type, it behooves the engineer to understand as fully as possible the myriad of situations that can occur which will impact costs and cause loss in revenues and downtime when an adverse event occurs. Using the 3-D or 4-D model, the engineer can gain a tremendous amount of practical experience and insight not only for the period of construction but for the operational life of the system. During the actual installation of an offshore structure, continuous as-built updating is a very useful tool for personnel to visualize the progress of the project and to anticipate potential pitfalls.
Conclusions
The purpose and use of a physical model for deepwater riser and pipeline installation is to develop and determine the total system configuration and provides a tool for analyzing and understanding the following necessary elements of work.
• Step-by-step procedures for installations to confirm acceptable clearances to avoid clashing during keel hauling and riser placements.
• Floater watch circles and their impact on riser touchdowns.
• The effect, magnitude and impact on riser sag bend stress levels during the complete riser installation process.
• Riser handoff and clashing potential between other risers, mooring lines, vessels and deep draft hulls.
• Hand over to the hub and placing the risers in the baskets.
• Larger-than-normal floater displacement to determine effect on the designed and installed field architecture.
• Modeled review of installers' pre developed methods and procedures.
• Potential clashing with multiple contractors (SIMOPS).
• Determining problems - contingency planning (HAZOPS).
• Duplicate tracking during the actual installation process, mimicking the motions of the various vessels, risers and cables to highlight potential interference and clashing problems.
• Prepare for emergency abandonment during potential construction interference from "loop currents" and or approaching inclement weather hurricanes.

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