Gravity Actuated Pipe or GAP™

With oil and gas field developments moving into ever-deeper water, SBM Offshore strategically expanded their core FPSO business to include Dry Tree Unit (DTU) technology, and identified at the end of the 1990’s a need to develop a compact fluid transfer system to provide a near-surface link between two floating production systems. This type of near-surface fluid transfer system would enable the use of a DTU to be located near an FPSO in deep water.

Unstabilized crude oil transfer options in the past were either free-hanging flexible or rigid steel risers in single or multiple waves; or flexible or steel risers or riser towers via the seabed. The problems encountered with these options were: limitations in length and diameter; flow assurance concerns due to low temperatures in deeper water; strength and fatigue problems; as well as prohibitive cost of materials and installation. These problems or drawbacks made such fluid transfer systems technically unfeasible or economically unattractive. The challenge was to develop a fluid transfer system that could meet the following requirements:

• Suitable for very deep water (independent of depth);
• Suitable to span several kilometers;
• Suitable to provide adequate flow assurance;
• To increase local fabrication content as much as possible.

This challenge has resulted in the development of the Gravity Actuated Pipe or GAP™.

The GAP Principle

The GAP principle is based on a large diameter carrier pipe supporting a number of flow lines and umbilical(s). The carrier pipe serves as a structural support for the flow lines and umbilical(s). The flow lines are attached to the carrier pipe by supports allowing movement in the axial direction. The carrier pipe terminates at each end with a towhead, which channels the forces into the carrier pipe and supports flexible jumpers between the flow lines and the floating facilities. Tether chains are suspended from floaters to support each end of the carrier pipe. Clump weights or buoyancy are mounted at the ends of the carrier pipe to maintain the proper depth and horizontal tension. The action of the clump weights or buoyancy on the angled chains keeps the carrier pipe under the design tension. This tension minimizes the hog and sag deflections of the pipe bundle and keeps the system within acceptable operating limits.

A typical average suspension depth is around 200 m. The GAP sag stay (when flow lines are flooded) and the hog (when most flow lines are filled with gas or empty) is around 100 m. The sag limit is to avoid too much cooling in the deeper water and to guarantee flow assurance. In addition the flow lines can be insulated to further improve flow assurance. The hog limit is to avoid any interference with surface activities and to minimize direct wave action.

The GAP™ configuration is designed to stay within the required depth envelope for the field duration. However, if required, the configuration could be adjusted in two ways. First, by adding or cutting the trimming chains along the bundle, which will increase or decrease the bundle submerged weight. Second, by increasing or decreasing the clump weights at the towhead end, which will increase or decrease the pre-tension in the bundle and therefore the deflections. Two supporting tether chains are used at both ends to provide redundancy. These tether chains are connected to floaters and towheads using uni-joints, allowing rotation in two directions.