As part of any Pre-FEED or FEED studies, the configuration of the subsea distribution equipment should be defined in the final specification for the controls equipment. In order to select the ideal configuration, the following factors have been reviewed:
- Installation requirements
- System requirements
- Retrieve-ability for repair or replacement
This document presents the advantages and disadvantages for two options for an Umbilical Termination Assembly (UTA) based on the above factors.
Umbilicals to be tied back to the platform for the control and chemical injection of subsea well centers. Each well center will include multiple production trees, water injection trees, and associated manifolds. The umbilicals will contain enough hydraulic tubes, chemical tubes, and electrical conductors to control subsea trees and the production manifold.
The base case plan for installation of the umbilicals is for the first end to be pulled into the platform first. This is a preference in order to minimize the risk associated with the umbilical installation vessel proximity to the platform and precision necessary for layaway from the platform.
The subsea distribution equipment configuration required for each well center will contain a minimum of 14 hydraulic stab plates and 16 electrical connectors for the trees and manifold. There will also be a methanol supply connection point for distribution to each hydraulic flying lead. With the number of required connections, spare hydraulic stab plates and the potential for redundant electrical connectors, the overall distribution equipment will be quite complex and large.
SINGLE SUBSEA DISTRIBUTION UNIT
One option for subsea controls distribution is to combine the umbilical termination, hydraulic and chemical flying lead connection plates, and electrical distribution unit into one Subsea Distribution Unit (SDU). This style of distribution unit minimizes the number of subsea connections since jumpers are not required to connect the umbilical termination to separate equipment.
Based on historical designs, a SDU including the connections required and spares will be approximately 20 feet long and 8 feet wide with a weight of 20,000 lbs. This unit would be installed onto a mudmat subsea using a downline from the installation vessel while the umbilical is being deployed.
A single SDU is best suited for first end installation. As a second end installation, the SDU would have to be loaded onto the vessel first. The estimated dimensions would prevent the ability to spool the umbilical with the SDU attached since it would have to be loaded onto the reel or carousel first.
In order to pull in the umbilical to the platform first, a SDU could be terminated to the umbilical just prior to deployment subsea after pull-in to the platform and layaway is completed. This operation is time consuming to terminate the hydraulic lines and test for leaks and includes significant risk with the umbilical hanging from the vessel. Also, this would not be an option for a steel tube umbilical unless compression type union fittings are used in place of welded connections.
A single SDU also would be difficult, expensive, and a high risk to recover to the surface for repair or reconfiguration. An umbilical installation vessel would be required which is more expensive and lower availability than a service vessel. The possibility exists for other umbilicals, flying leads, or flowlines to have been laid over the umbilical connected to the unit in need of repair. This would prevent the ability to recover the termination assembly.
UMBILICAL TERMINATION ASSEMBLY MODULES
A second option for the subsea controls distribution is to have separate modules for the umbilical termination, hydraulic and chemical flying lead distribution, and electrical distribution unit. These modules could be installed onto a single mudmat with hydraulic and electrical jumpers connected to the UTA.
An UTA increases the number of hydraulic stab plate connections by four and the number of electrical connections by at least two increasing the risk of a component failure. However, these extra connections provide the flexibility to position the umbilical termination head away from the congestion of well centers where the SDU and EDU could be placed. The extra connections also provide direct access to the umbilical tubes and electrical quads if necessary for trouble shooting problems or modifying circuits.
A sketch of the components can be seen in the figure below. The estimated dimensions for this type of UTA are:
- Umbilical Termination Head (UTH) – 4 ft x 6 ft; 5,000 lbs
- Subsea Distribution Unit (SDU) – 14 ft x 4 ft; 15,000 lbs
- Electrical Distribution Unit (EDU) – 3 ft x 8.5 ft; 5,000 lbs
- Mudmat required - 30 ft x 8.5 ft minimum
Multiple Umbilical Termination Assembly Modules
Each of the three main components will connect to the Mudmat through a stab and funnel arrangement allowing for recovery and reinstallation of the SDU and EDU for repair if required. The UTH stab could include a hinge for a stab-and-hinge over design. As a second end installation, the preference would be for the stab to be fixed during installation of the UTH onto the Mudmat since the termination head and umbilical will be lowered horizontally.
Installation of this assembly will require three phases. First, the Mudmat with the EDU and SDU attached will be installed subsea with the use of a downline. Then the UTH would then be lowered into the funnel with a downline after pull-in to the platform and layaway is completed. Finally, jumpers would be connected between the UTH and the SDU and EDU.
Having the UTH as a separate unit from the SDU and EDU provides several benefits. During the umbilical manufacturing phase, the UTH could be sent to the umbilical manufacturer for termination and testing at their facility prior to loadout onto the installation vessel. This aids in the quality assurance of the umbilical assembly since potential leaks can be located and repaired away from the installation vessel; welds for a steel tube umbilical can be performed and inspected in a controlled environment.
Another advantage is the ability to spool the umbilical onto a carousel with the UTH attached. With the UTH loaded first, the umbilical can be pulled into the platform first.
The ability to remove the jumper between the UTH and SDU also provides benefits. If a leak is detected in the umbilical system, the jumper can be removed to isolate and pressure test the umbilical tubes to ascertain if the leak is in the umbilical tube, jumper, or SDU. To recover from a leaking umbilical tube, the jumper can be reconfigured to use a spare tube in place of the failed service line.
The UTH can be a fully welded unit to maintain the highest reliability with the assumption that it is not recoverable. Two welds are required for each umbilical tube. Male couplers are used on the stab plates which do not require seal change out and are reliable connection points. The recoverable SDU will contain in excess of 200 welds and 138 couplers including a large methanol connector.