Retrofit of DP Systems on Marine Pipelay Vessels
Authors: Nock Mendel 
Published: October 2008

Retrofit options for the reel barge "Chickasaw" and derrick/stovepipe barge "Cherokee" are presented The DP needs of construction vessels are detailed and design concepts developed A low level Chickasaw upgrade will cost $1.25-2.75 million. ACherokeeupgrade will cost $7-12 million.

Introduetion

The progressively deepening production frontier in the Gulf of Mexico offers new opportunities and challenges to the offshore construction contractor and pipelayer. To work effectively in this province, new vessels will have to be built, or existing vessels upgraded Two key vessels in the Global Pipelines PLUS fleet have been examined with respect to their station-keeping capabilities and the option available for upgrading them. The specific requirements of reel lay, stovepipe lay, crane lift and diving support are set out explicitly to allow individually tailored alternatives.

Construction Vessel Positioning Requirements

Various Stages of Pipelay

Pipelay involves successive stages of endeavor with different requirements. These stages are referred to as initiation, underway and termination. The additional procedures ofabandonment and recovery may be required while underway.

Initiation methods are typically deadman anchor, hanging or bowstring in shallow water, J-tube in moderate to mildly deep water, and stab-in or pull-in in deep water. Initiation is the stage which for short periods of time requires the most precise station keeping and heading control However, since a project would not be commenced in difficult weather, initiation is also the period of least environmental loading.

For a vertical stab-in, a watch circle with 15-foot diameter must be maintained and orientation must remain within a sector which can be accommodated by the stab-in wedge. Since there is no horizontal tension component, the vessel thruster capacity need be no greater than required for environmental loads.

For J-tube initiation the vessel must first set-up a distance away from the structure and avoid collision while passingmessenger and pull lines. This represents a mild orientation and station keeping requirement. With the vessel in position, pipe is paid out and the pull line drawn in. Position need not be tightly maintained initially, provided tension is applied smoothly. Lateral control must be sufficient to avoid overstressing the J-tube at the bell mouth. The fleet angle between the stinger and pipe should be limited to avoid overstressing the pipe or stinger laterally. Once the pipe tip reaches the bell mouth and pull into the tube is performed, the fleet angle of the pipe at the bell mouth should be more limited than when only wire rope is in the tube. Tension applied to the pipe is close to that while laying underway.

For a pull-in to a subsea template, only the capacity to pass lines is required and represents a reduced station keeping requirement. With the vessel in position, the pipe is paid out in a manner similar to that of a J-tube pull.

The pre-existing mooring systems are assumed available for shallow water work with DP used to assist in congested areas, or to speed set-up in simple initiations.