6 Ways Riser Analysis Can Improve Riser Design

Published by Stewart Technology Associates on July 18, 2016

Dynamically positioned drill ship in storm conditions.
Riser tensioners seen actively tensioning riser until 7:50 on the timer.
Riser emergency disconnected, EDS activated at the BOP at 7:50 and the top end raised about 3 meters.
Moonpool with hung-off riser shown after EDS activated.

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In the offshore oil and gas industry, risers connect drilling and production platforms to the sea floor. They transport oil and gas from the well to the platform or inject fluids into the well to facilitate drilling or production. Risers are critical to the operation of the platform, and any failure could mean a loss of production capabilities and a serious loss of profit.

Risers enable the platform to move up and down, or side to side during production and drilling operations. They also must efficiently transfer fluids to and from the sea floor. The impact of wave action, tidal action, high pressures, low temperatures, and corrosion must not affect the platform’s movement. In order to do the job properly, with minimal maintenance and downtime, an extremely efficient and reliable riser design is required.

In the past, the main way to evaluate a riser design was through building prototypes, testing them extensively in real-world conditions and redesigning the riser until it met all the requirements and specifications necessary. This could be a tedious and expensive process, and it could still yield sub-standard designs, in some cases.

Today, riser analysis takes out some of the guesswork. Using special software packages and talented engineers, riser analysis can model the forces at work on a potential riser design. It looks at waves, tides, temperature, pressure, vortex forces and vibration to determine the strong and weak points of a design, and improves them to avoid structural fatigue, leaks, inefficient fluid transfer and other common problems. This can reduce design and prototyping costs, and lead to a final product that is efficient, durable, safe and low-maintenance.

Riser analysis is now a critical tool for improving the performance of drilling and production operations, and here are a few of the ways it can help to improve riser design:

1) Determining the Right Type of Riser for the Job

The type of riser used for a particular application will depend on the type of platform, the depth of the seafloor, the depth of the well and the sub-sea conditions. Riser analysis can be used to determine the best type of riser for the application, and help perfect the design and adapt it to particular equipment. The most common types of risers include:

  • Top-tensioned riser: A rigid riser that is held in place vertically through tension, and allows both lateral and vertical movement through a flexible connection between the riser and the platform.
  • Steel centenary riser: A curved riser used to connect two platforms, or to connect the platform to the sea floor, it can withstand some motion and is used on spars, tension-leg platforms and floating production storage and offloading platforms.
  • Flexible riser: Made from flexible pipe, these risers can withstand horizontal and vertical motion.
  • Pull-tube riser: A hollow tube attached to the center of the platform, it houses a pipeline or flowline that is pulled from the seafloor with a cable, and is used primarily on fixed platforms.
  • Attached riser: Used on fixed platforms, compliant towers, and concrete gravity structures, these risers clamp to the side of the platform and are connected to the sea floor with an export pipeline or a flowline.
  • Riser tower: A combination of a steel tower and riser that is used for deepwater drilling, a buoyancy tank is used near the surface to keep it in place through tension and flexible lines connect it to the platform
  • Drilling riser: Transfers drilling mud and other fluids to the seafloor during drilling operations and is only a temporary connection.

With riser analysis, you can choose the best riser for the situation, adapt an existing design for use with your equipment, or create an entirely new or hybrid design, then test and improve the riser before it goes into production.

2) Improving Riser Structure

The overall structure of the riser must be able to withstand the waves, the tides, the high pressures and the low temperatures present below the ocean surface. Riser analysis can be used to model these forces at work on your riser design, determining how they affect individual areas and structures of the riser and how they affect maintenance requirements and the longevity of the system. Using the resulting data, the design can be improved to make the riser stronger, more stable and safer.

3) Determining Insulation Efficacy

In deep water, the temperature can drop significantly, reducing the viscosity of oil and drilling fluid and slowing down production and drilling operations. To combat this, many risers incorporate insulation to keep the fluids at a more desirable temperature so that they can flow properly. Riser analysis can be used to design and test the insulation, and find problems that may reduce the temperature and fluid flow within the riser. By improving the design of the insulation, riser analysis can help to increase production efficiency, leading to higher profits.

4) Designing and Testing Valves

Valves in the riser help control the flow of oil, hydraulic fluid, and other substances through the system, and they must operate in cold, high-pressure conditions without frequent failures or maintenance. Riser analysis can be used to model the flow of fluids through the valves and to find failure points and other problems that can reduce their life cycle or cause leaks. With the data, the valve can be reengineered to promote longevity and efficient fluid flow.

5) Designing Buoyancy Tanks

Buoyancy tanks are critical to keeping a riser stable and accessible in sub-surface conditions, and if they malfunction, oil spills or other problems could occur. Riser analysis can help to design and test the buoyancy tanks, improving their design so that they are safer and more stable, regardless of the sub-surface conditions.

6) Improving Riser Flow

Production risers must carry fuel to the surface efficiently to meet production goals and maintain the platform’s earnings. Riser analysis can be used to model how oil and drilling fluids flow through the riser, determining points of turbulence or restriction. By improving the design of the riser and eliminating these problems, production rates, and profits can be increased.

Riser analysis is a powerful tool that can make an oil platform safer, more efficient, and more productive by improving every aspect of riser design, from the seals to the valves and the overall structure.