Motion and Load Prediction of Floating Pipelaying Equipment
- H.L. Minkenberg (Netherlands Ship Model Basin) | Tan Seng Gie (Netherlands Ship Model Basin)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1980
- Document Type
- Journal Paper
- 1,281 - 1,291
- 1980. Society of Petroleum Engineers
- 4.5.4 Mooring Systems, 1.6 Drilling Operations, 4.5 Offshore Facilities and Subsea Systems, 4.1.5 Processing Equipment, 6.5.3 Waste Management, 4.3.4 Scale, 1.10 Drilling Equipment, 4.2 Pipelines, Flowlines and Risers
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In predicting the motions and loads of floating pipelaying equipment - varying from conventional barges to large, highly sophisticated semisubmersibles - several mathematical methods and model experimental techniques are available. This paper reviews the various mathematical methods and correlates the results with model test and prototype data.
The concept of a floating barge being a stable floating unit for carrying pipelaying equipment offshore was introduced in 1956. In 1978, the first offshore pipelaying barge, the L.E. Minor, was operated. The pipeline was lowered from the barge to the seafloor using long submerged floating units. By controlling the buoyancy of these units, the required sloping launchway was obtained. Tension was applied only to prevent slipping of the pipeline. The length of such stingers was two to three times the water depth. Stingers up to about 1,000 ft long with a weight up to 250 tons were used. In deeper water and less-sheltered areas, however, stingers of such length and weight became a real problem, especially because of the very large moments generated about the attachment point to the barge. By the introduction of the curves stinger in combination with large axial tensions applied to the pipeline on the barge, the sagbend could be limited so that laying in greater water depth became possible. Allowing a stress level of about 80% of the yielded stress during the laying operations resulted in considerably shorter stingers. Hence, the very large moments about the attachment point to the barge were reduced, which made work in rougher sea conditions possible. Generally, however, the laying techniques themselves hardly changed.
The hull configuration of the unit supporting the pipelaying equipment has varied considerably during the last 10 years. Ship-shaped vessels, small semisubmersibles, and large semisubmersibles with sophisticated pipelaying-controlling systems were developed and taken into operation, in addition to conventional flat-bottom barges. The choice of the type of construction is determined mainly by the following parameters1-3: (1) operational area (worldwide or some specific area - e.g., North Sea, Gulf of Mexico), (2) water depth, (3) size of pipelines (storage capacity, deck area), (4) seabed condition, (5) sea surface condition (workable days, survival capability, stability), and (6) investment. Due to high development and operating costs of pipelaying barges, it is important to consider the various possibilities in early stage. Sea surface conditions and, thus, the hydrodynamic aspects of the various concepts play an important role in the selection of the final concept. The waves, wind, and current acting on the floating pipelaying structure determine mainly whether motions and forces become too large so that the laying activities have to be stopped. The rougher the sea conditions in which it is possible to work, the more time will be available for productive pipelaying in a given season.
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