MacKinnon Marine’s Alex Jackson discusses the merits of the company’s new spiral pre-lay system, which seeks to bring pipe construction closer to the lay site and eliminate the need for a spool base.
Fig.1: Schematic of spooling base and reel vessel spooling (loading). Illustrations from MacKinnon Marine.
As the requirement to lay pipelines in an ever-increasing range of distant locations continues, there is a corresponding increase in pressure to improve the utilization of pipelay vessels.
The current preferred system of pipe construction at an onshore spool base, requiring long transits of a reel lay vessel to collect the pipe, is extremely expensive and inefficient in terms of utilization of the vessel, a huge capital asset with massive running costs.
Spiral pre-lay process
It can be seen that, since the reel lay speed is much faster than the pipe construction, and the time lost in transit has been greatly reduced, there is a need to accelerate pipe construction to avoid the reel lay vessel waiting or making multiple transits with short pipe stalks, thus negating the efficiency benefits offered.
Pipe construction can be carried out by a very simple barge or by a purpose built vessel. The barge is relatively low-cost and the increased output which is required can be achieved by using a number of vessels for construction. The purpose built vessel, however, is a much higher investment and its own efficiency must be maximized by enabling it to produce a number of pipe stalks simultaneously. For instance, the deck arrangements can be optimized to create an efficient and versatile pipeline factory with multiple firing lines which extend the whole length of the vessel offering multiple weld and field joint coating stations, giving efficient cycle time.
When not contracted for spiral pre-lay the purpose built vessel or the barges can operate as a conventional S-lay facility giving high overall utilization.
Fig. 2 (left): A traditional reel lay vessel.
Multi-pipe S-lay vessel
|Fig. 3: Pipe construction on vessel and S-lay.|
Pipe is stored on and below deck and manipulated by dedicated pipe-handling equipment to give an efficient lay rate. Pipes of differing diameters and stalk lengths can be laid in parallel and additional pipe received from barges via deck cranes. The inshore sheltered conditions suit pipe replenishment and continuous 24 hour operations, unaffected by weather or vessel motions.
In the formation shown above, the vessel is laying five pipes simultaneously. These pipes may be the same or of different sizes or types.
If the lay site is very close and transits consequently short, it may be necessary to construct more than five pipe stalks to keep pace with the reel lay vessel. In this case, the construction vessel can use more than one deck and a further quantity pipe stalks can be installed.
Other benefits offered by spiral pre-lay
If desired, the constructed stalk can be pressurized while on the seabed before spooling. This aids resistance to ovalization under bending and in the case of internal liners, assures the integrity. Pressurization on the seabed offers many process and safety benefits over the current methods.
The construction vessel can produce pipes of different sizes and types, as required for the project. Unlike a spool base where stalks are available in a fixed order, the vessel can then select which stalk to spool next to suit field development requirements.
Fig. 4 (left): Pre-lay of a spiral pipe by a conventional construction vessel. Fig. 5(right): Recovery of the pre-laid pipe by a conventional reel lay vessel.
Summary of key benefits
- Eliminates geographical restrictions of the spool base entirely – The spool base is where you need it.
- Minimizes transit distances for reel lay vessel – Major cost reduction and time saving.
- Avoids large capital investment cost of land clearing and pier construction.
- No need to tolerate limited stalk lengths of typically 1-2km.
- No need to carefully schedule order of pipeline manufacture
The spiral pre-lay process and concepts for the required new technologies are currently being developed by MacKinnon Marine and are protected by patent application.
Fig. 6: An ideal location for pre-lay.
Alex Jackson is MacKinnon Marine’s R&D Manager. He is a chartered engineer with a BSc in engineering from Nottingham University. He previously worked an Automotive O.E.E., in manufacturing process development and new product introduction, before starting work in pipe lay improvement and testing eight years ago.