Riser system developed for shallow water development

Subjecting risers to wave action

Jun 1st, 1999
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The successful installation of a new mid-water arch system in Kerr McGee's Janice Field on the UK continental shelf is said to be pushing back the barriers on riser systems for shallow water subsea developments. The system, developed by Coflexip Stena Offshore (CSO), opens the way for cost-effective subsea well developments in shallow water, previously thought possible. More than £1 million was cut from the development cost of Janice as a result of the system.

Layout schematic for Kerr McGee's Janice Field in the UK shows the flexible risers deployed in relatively shallow water.
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Part of the scope of the Janice Field development was to provide a riser system able to accommodate up to 14 risers (water injection, production, umbilical) connected between the semisubmersible Janice floating production unit (FPU) and the wells 450 meters away. This presented CSO with a difficult challenge, because Janice is in only 79 meters water depth and the risers would be subjected to significant wave action.

In such shallow water, the normal solution is for individual risers to be used with a steep or Pliant Wave formation. This uses buoyancy modules with a certain number of buoys attached to each riser to give them the "S" or wave shape catenary configuration that accommodates the movement of the FPU. Each riser is on its own and enough clearance is required between each riser to avoid clashing due to the environmental conditions.

However, this scenario is usually not cost-effective, particularly when there are more than 2-4 risers. For the configuration to be considered, the Janice wells would have had to be positioned separately, which was not possible.

In order to reduce the number of risers, consideration was given to flowing production from the wells through a single large diameter riser. The risk was that if the large riser had to shut down for any reason, the whole field would be shut down. Thus, this solution was dismissed.

Mid-water arches
The layout of the Janice wells, and the foregoing meant that CSO had to make use of the mid-water arches (MWA) supporting a number of risers. In addition, CSO had to guarantee the whole system could perform for a minimum of 15 years.

Because of the layout of the wells, the first assumption was that two MWAs would be required. They were to be in the most usual configuration, a lazy-S, where the riser path is from the FPU to the top of the MWA and then to the seabed in a free catenary. Many months were invested in computer modeling the riser system's performance during the required lifetime of the field. The initial results were not good. The main reason was that the system had to accommodate a wave height of 21.3 meters for a water depth of only 77 meters, without damaging the risers.

Due to the wave force, the MWAs, positioned some 40 meters off the seabed, were moving outside the limit of the system. The risers were subject to severe over-bending at the touchdown point with the seabed and to very high tensile loads at the FPU connection. "We had to improve the system to ensure the integrity of the risers," explained Bursill. The first idea investigated was to add buoyancy modules at the touchdown points of the risers to prevent over-bending. Although technically sound, this solution was not cost effective.

Touchdown point
"Giving it further thought, we considered that the main problem was the lack of control over the bottom catenary of the risers (from MWA to touchdown point with the seabed) and we decided to secure the touchdown point of the risers to the MWA base," said Bursill.

The results of the computer modeling of this system were far better but there were still a few extreme cases where the minimum bending radius of the risers was being compromised. The solution was to add a device on the risers in the area were there was over bending, a dynamic vertebrae that would act as a bend limiter.

The vertebrae are fitted to the riser via a clamp and they follow the movement of the riser except in the extreme cases were they lock themselves and prevent over-bending. "We had developed and perfected a new riser system, the PLIANT-S, that is able to accommodate very extreme environmental conditions in shallow water," concluded Bursill. But there was one more challenge because with 21 vertebrae to be used for each riser and a mounting time of over 10 hours for each vertebrae system, the system was still not cost effective. It was clear that mounting the vertebrae offshore was out of the question.

CSO then achieved another first by packing each riser, fitted with all the accessories (bend stiffener, bend restrictor and clamps), on the installation reels in its manufacturing plant at Le Trait, Normandy, France. This minimized the offshore installation duration of the system and brought the solution within budget.

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