As water depth increases, current forces, external pressure and longer riser system lengths require thicker, heavier components to limit the stress. In addition, larger spacing between the risers is critical to avoid riser collision, a design change which results in larger structures to support the component loads. When moving into deeper waters, an optimal riser design is a key to the project's success.

Norwegian Deepwater Program

Established in 1996, the Norwegian Deepwater Program (NDP) is a consortium of 10 oil and gas majors operating in North Sea. The NDP has identified specific challenges facing deepwater field developments and established workgroups to address these challenges. Constantine Makrygiannis, of BP, who is the project manager of the NDP's Risers & Moorings group said, "The development of recommended practices (RPs) was a focus area, with emphasis given to bringing extended knowledge into engineering companies and making the results available to the end-user in a transparent format. We believe that DNV RPs on riser fatigue and riser interference, developed as a part of the NDP sponsored work, perfectly matches the intended objectives. They are in the timeframe of deepwater business requirements."
The new RPs are recognized as being supplementary to relevant national rules and global regulations. To familiarize the industry with state-of-the-art developments in riser fatigue and riser interference, two workshops were held in Houston, Texas, and Oslo, Norway. The initiative for both workshops was taken by DNV and NDP, which was recognized by the industry which said, "The RPs are in fact a generation ahead of the market, enabling deepwater riser solutions."

Fatigue

According to Trond Stokka Meling, discipline leader responsible for risers at Statoil, they are pleased with the outcome of the NDP work, which resulted in a DNV RP for riser fatigue. "When moving into deep water, fatigue damage is often the governing limit state in the design of metallic risers," he says. "To account for uncertainties in the fatigue analysis, a safety factor of 10 is commonly applied for safety class high, a classification from jacket design which is not necessarily appropriate. Hence, the NDP started an initiative with the objective to establish safety factors for given target safety levels of a riser system. The work ended in a DNV RP for riser fatigue, covering both the traditional safety factor approach and the new enhanced risk-based fatigue approach which enables project-specific safety factors reflecting the uncertainties in the fatigue estimates of the specified riser system."

Kim Mørk, head of section for risers at DNV, said the costs and resources associated with qualifying a deepwater riser design for satisfying the fatigue criterion can be substantial in many cases. Fatigue issues are addressed by all recognized standards and codes, which call for adequate safety against fatigue failure. Fatigue assessment methods based on stress and cycle (SN) curves typically use a fatigue safety factor of 10 times a unit's service life. An optimal choice for the fatigue safety factor is governed by the uncertainty in the stress response, which is dependent on the structural application, the damage contribution type and the prevailing environmental conditions, among other issues.

Rather than assuming that the implicit uncertainty in the fatigue estimate can be handled solely from a characteristic SN-curve and a single fatigue factor, the basic principle should be to select a rational safety factor from variability in a fatigue estimate from prevailing uncertainty sources and bias related to the applied analyses model. Only a limited amount of extra analyses are required for this approach to more accurately estimate the required fatigue safety factor. Furthermore, such an approach also identifies which parameter contributes most to the fatigue damage uncertainty.

The "risk-based fatigue criterion" ensures that the risers have adequate safety levels and can be used as a guiding tool for optimizing the design changes and fabrication improvements. Benefits can also be interpreted in terms of potential cost savings and/or feasibility of a riser design. And according to Muthu Chezhian, who is the DNV project manager for the RP on riser fatigue, this novel approach can not only be applied to existing risers to identify impending integrity problems but it can also be used to assess the actual risk and reliability levels for existing risers in both shallow and deep water.

"The typical example of applications comprise deepwater risers in the Gulf of Mexico, where the uncertainty related to loop currents is not quantified and where standard safety factors may have little practical meaning," Chezhian said. Another typical application case can be the novel hybrid riser tower concepts in a recent West African field development, where there is limited practical experience to justify standard safety factors.

The fundamental risk principles can be applied in the North Sea for potential capacity enhancements of the platforms and for extending the operational lifetime of the existing platforms and risers. If the required safety levels of the risers can be compared to the available safety levels based on the RP, on a case-to-case basis, it is relatively straightforward to evaluate the remaining lifetime of the riser and also plan for capacity enhancements of the platform.

For deepwater riser concepts, vortex induced vibrations (VIV) is often a governing design factor. Though the VIV is a recognized problem, only the cross-flow VIV is often considered in the riser design and engineering analysis - despite the fact that the inline VIV can be equally important when it comes to fatigue. The major problem so far has been that there have been no practical design methodologies for inline VIV. The RP is also the first of its kind to include a simplified design procedure for the complex but important problem of inline VIV of risers.

Interference

A more accurate tool for assessing riser interference (collision) has become increasingly more important when oil and gas exploration offshore moves to deeper water. The risk of collision between the risers increases as riser length grows. Collisions may be damaging to the risers, both from a high cycle and low cycle fatigue perspective if the collisions lead to excessive plastic displacements.
The current design practice is that riser collisions are not allowed under normal or even extreme conditions. Top-tension and riser spacing are the primary parameters for design of riser arrays against collision. The costs related to increased top tension and/or riser spacing at floater termination may be very high due to possible implications on floater- and tensioner-system design. Hence, riser collision has become a critical issue for cost-effective design of deepwater exploration operations that utilize top-tensioned riser arrays. When production moves to water depths of 6,560 ft (2,000 m) or more, the requirements will increase the costs significantly. The alternative is to design for interference in infrequent conditions.

The RP on riser interference, which has been currently issued for an industrial hearing - to obtain technical feedback from the industry - was partly managed by the NDP and builds on previously conducted NDP work and the findings of the relevant JIP titled "Management of Riser Contact", which was co-coordinated by Aker Kværner Technology and Det Norske Veritas, and sponsored by five major oil companies.

Kjell Herfjord of Norsk Hydro's Research Centre states, "Accordingly, NDP put effort in preparing for design with riser clashing as a design case. Studies to observe phenomena related to riser interaction and clashing were done in model scale at Marintek, Trondheim. Drop tests on actual riser sections were done in a laboratory. The knowledge gained was implemented into a design tool and a RP for riser interaction. The interaction phenomena are not only applicable to metallic risers, but are also valid for flexible risers as well as umbilicals."

In the RP, riser collisions are allowed provided that the riser array is subjected to state-of-the art analyses and sufficient fatigue (FLS) and ultimate (ULS) capacity are documented. The RP discusses the main items regarding riser interference comprising hydrodynamic interaction, local response of risers in contact and acceptance criteria for design. The objective of the new RP is to recommend a methodology for engineering analysis and to provide rational design criteria and guidance for assessment of riser interference.

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