Installation of steel pipelines and risers by the reel-lay method has been generating increasing interest, particularly for deepwater service. Reel-lay offers the potential for reduced installation costs, where other lay methods such as J-Lay have been the mainstay. With the reduced offshore exposure duration, and a substantial portion of the fabrication being done onshore, there is also the opportunity for higher engineered quality. In addition, there are now more installation vessels with the ability to execute deepwater pipelay projects with risers to floating hosts by the reel-lay method. There are however, important technical considerations in reeled riser design and installation. The higher bending strains imposed by the reeling process require a proper assessment of the impact of reeling damage on subsequent field fatigue performance. This paper summarizes the important considerations in reeled riser design, including the validation of reeled riser performance for dynamic service. It is concluded that if properly engineered and qualified, reeled risers are feasible and cost-effective for deepwater pipeline and flowline applications, including those for dynamic service.
Pipeline and riser costs increase with increasing water depth and pressure, distance to the shore/ tie-back facility, and insulation needs. With the trends in deepwater oil and gas development including new infrastructure developments in deepwater, and the use of satellite wells tied back to existing infrastructure, the number and mileage of deepwater pipelines and risers is on the increase.
Depending on the installation method, offshore duration and resultant weather exposure can vary widely, and along with it the potential for cost and schedule impact. Reeled installation of deepwater pipelines and risers is generating increasing interest and attention for a variety of reasons including:
potential for reduced installation costs compared to lay methods such as J-lay or even S-lay whose lay rates are linked to cycle time for completing, inspecting and coating weld joints at one or multiple stations on the barge,
reduced offshore exposure duration, and the ability to better time mobilization with an opportunistic weather window,
potential for superior weld quality due to the bulk of the fabrication being done onshore, and
the increasing number and availability of reel-lay vessels.
While there have been several steel pipelines and flowlines installed by the reel-lay method, those involving deepwater (>3000 ft), and larger diameters (12" and up) are less numerous. Also, even with the increased use of reeling, they have not been without failures. To enable the economic advantages of reeling to be fully and reliably exploited, this paper explores some of the key considerations that impact the serviceability of the reeled risers, particularly with regard to issues stemming from high strains imparted by the reeling process. The intent is to outline the key issues for analysis, evaluation and qualification, rather than to present new data. The issues have been set forth in the context of some examples of deepwater pipelines/ flowlines and steel catenary risers.