Subsea system installation
Subsea system installation is the process of using offshore vessels to lay the subsea components onto the designed seabed locations with the right connectivity. Installation has a major impact on subsea system complexity, architecture, component design, and configuration for the following reasons:
- Component installation is a significant driver of complexity
- Component installation has significantly impacted dexterity and access
- Installation systems are inherently unstable (vessel motions)
- Installation systems are extremely expensive and have high daily fees
- The subsea system is remotely located and difficult to access
- The subsea environment for deep water is a high external pressure environment
- Requirements for installation sequences and flexibility
Key challenges for subsea system installation
The ocean environment is one of the main reasons that installation of subsea systems is a major driver of the cost of the overall subsea system. The cost of installation can exceed 60% of the total investment in a subsea project. Since installation cost is very high, extreme efforts are made to minimize and economize offshore installation, especially those steps requiring large and expensive installation equipment.
Working underwater has a unique set of challenges. The sea is powerful and unpredictable, both on the surface and underwater. For example, the sea and weather conditions in the North Sea can have a huge impact on available working time for very high-cost installation equipment. It is common to plan installation activities to occur only in the summer months when weather conditions are more manageable. Underwater currents can be significant as well. Loop currents in the Gulf of Mexico can exceed 8 knots and can result in months of lost work time on subsea projects.
Working underwater means that the installation equipment is remote from the installation site. As a result, the installer cannot feel or hear anything en route to or at the installation site due to impaired or nonexistent sensory input. Remote Operated Vehicles (ROVs) are used to aid vision in the subsea environment, but the best that can be achieved is a two-dimensional view, which is many times impaired by murky conditions.
High hydrostatic pressure
Working underwater means that hydrostatic pressure must be taken into account. The tools and equipment used underwater must be designed to withstand the hydrostatic head which would be as little as 45psi in 100ft to 4500psi in 10,000 ft (0.45psi/ft). In some cases, the hydrostatic head must also be overcome, such as pumping water to jet away soil.
The installation cost and complexity of the subsea system can be a significant driver of the configuration of the system. This has led to subsea systems designs and configurations evolving into two basic categories based on installation norms.
Diver assisted installation
In water depths up to about 100 m (300 ft) installation is facilitated with divers. When divers are used the cost and complexity of installation tooling can be simpler and the diver can provide a 3-dimensional view of the installation process.
ROV assisted installation
In deeper water depths, such as the Gulf of Mexico, installation is almost always aided by ROV and requires a different set of criteria to be cost-efficient. In these environments, the cost of specialized installation equipment is so high that minimizing installation time is essential. Therefore there is a drive to break up the system into more components that can be installed with smaller, cheaper equipment, such as flowline sleds leading to jumpers.
Impact of installation
Installation can have an impact on:
Installation can impact well locations. One example would be the use of a template. The template facilitates installation of the wells from a common surface location which can have a significant impact on reducing well investment. A cluster well configuration is another example of installation driving well locations. The cluster well configuration aids installation by allowing the spacing of wells to be accessible from a surface location which does not require changing the mooring location of the rig, which can save well construction cost.
There is an added advantage in that the well spacing may also allow concurrent installation activities, such as installing the manifold concurrently with well construction activity.There is a benefit to installation by the location of the flowline and umbilical a distance away from the manifold and wells, which again provides installation flexibility with likely savings in installation cost. Installation cost can be optimized by spacing components in a way that provides maximum flexibility in the timing of installation of those components. Flexibility through component spacing helps to ensure that high-cost installation vessels are not caught waiting on each other.