As oil exploration activities move rapidly into deeper waters and harsher environments, existing technologies will reach their limits. New challenges occur when moving into deeper waters; DNV is prepared for that move.

An optimal riser design is a key to success for offshore operators to cope with more extreme conditions. DeepC is a proven solution to quickly provide such design.

DeepC is designed to predict the extreme response and fatigue damage to mooring and riser systems under wave, wind, and current loading conditions automatically taking the coupling effect and other important non-linearities into account. It is also possible to perform traditional riser design analysis (un-coupled), marine operations as well as mooring analysis in the frequency domain.

The mooring and riser systems of many recently installed platforms have been designed or verified using proven DeepC software. The computational methods used in DeepC are extensively validated against experimental results, qualitative validations and case studies.

The DeepC program allows for design of steel catenary risers, top tension risers and hybrid configurations using both the traditional de-coupled methodology and the more accurate coupled methodology. Both options can be used to compute riser motions, stresses and fatigue life as well as performing code check of the risers.

Challenges
In a riser design, the assessment traditionally adopted by the industry for the attainment of platform motions applied to the top of the riser consists in the use of decoupled methodologies. Nowadays, due to shift of oil and gas exploitation to deeper waters, more accurate methodologies have been introduced, based on coupled analysis. The coupled analysis considers the interaction between:

• Hydrodynamic behaviour of the hull
• Structural behaviour of mooring lines
• Risers submitted to environmental loads

For deep waters, the coupling effects of lines relative to platform motions can be especially significant. One expects a reduction of the amplification of platform motions compared to the platform motions obtained from de-coupled analysis. For the deep and ultra-deep water scenarios, a steel catenary riser design adopting prescribed displacements from coupled analyses will provide more realistic and optimum results as compared to a more traditional de-coupled analysis.

Using refined modelling techniques
A complete DeepC functionality is used to compute the displacements and stresses used in a code check or fatigue analysis of the risers. Normally this is done in two steps.

Step 1: The vessel motions are calculated in the coupled analysis with a coarse model of the risers. Simo and Riflex are used by DeepC to compute the responses.

Step 2: A new refined riser de-coupled analysis is performed, using the calculated vessel motions as input when running the same time series. In this case Riflex is the sole analytical engine used by DeepC. Statistical post-processing, fatigue assessment and code checking of the risers are done using features in the DeepC program. It is also possible to run step 2 with vessel motions calculated by non-coupled approach or by importing these in form of time-series from other systems.

DeepC and Riflex are particularly known for:

• Superiority in computational speed
• Stability – easy-to-find linear static condition
• Intuitive and easy-to-learn user interface
• Handling and post-processing of large amounts of data (fatigue and code checking)
• Extensively validated through research and model tests