Installing Subsea Leads in Rough Seas

The previous article Flying Leads-The How, published in Ocean News detailed the “deployment, handling, and mating via specific tools and methods” of single and bundled conduits (1). The intent of this follow up article is to: place emphasis on the manner by which weather conditions affect the management of single and bundled conduits; illustrate the role that topside cranes play in regards to their handling; and above all, to have this article serve as a field/reference guide for existing and future offshore personnel.

Single Conduits in Operation     

        From an installation and handling standpoint, there are two methods by which to trip a single conduit, via their respective deployment source, to operational depth. Once a single conduit has reached its operational depth, it can then be installed, allowing for a path of continuity to be bridged between two assets in open water. The following definition for single conduits, as set forth per Reaching Ultra (1)—will be referenced, in order to give the reader a better understanding of their composition and application: 

When electrical and/or fiber optic lines are run in a single conduit Flying Lead, Pressure Balanced Oil Filled (PBOF) hoses are required, giving way to Electric, Optical, and Electrical Optical (Hybrid) Flying Leads (EFL, OFL, EOFL, respectively). A key feature of PBOF hoses is that all lines are housed within a single hose, making it the sole protective barrier for these lines (See Figure 1) via its Kevlar properties, for example.

Figure 1: Installation of single conduit shown; reference connector with orange hose

         With the definition for single conduits reaffirmed, the table below highlights the manner by which single conduits make the transition from a topside to a subsea environment. 

Figure 2: Top level description of single conduit intefacing

Figure 3: Single conduit flying lead attached to external area of ROV's cage

Weather and Crane Considerations

        Of the two methods listed in the above table, flying leads via method 1 are at greater risk of becoming damaged during anomalous weather. This is due to the fact that a Remotely Operated Vehicle (ROV) has to fly out of its cage, and travel to the area where the Flying Lead is mounted on said cage. Dismounting the Flying Lead from here can be a difficult task during anomalous conditions, which can cause  both the cage and ROV to heave, resulting in the aforementioned travelling upward and downward in an unsynchronized fashion, making it difficult to dismount the lead.

        Should an ROV make an attempt at dismounting a single conduit, in this backdrop, this can damage the fiber optic and electric lines housed within the conduit, due to an ROV’s manipulators inability to accurately grasp the Flying Lead. Conversely, because method 2 places the Flying Lead on a deployment frame, once the frame is landed on the sea floor—and is unhooked from a crane— it then remains fixed there. With this completed, the lead can be unspooled with a great degree of mobility via the ROV. Furthermore, a crane is not used in conjunction with an ROV, since single conduits weigh substantially less than bundled conduits.  

Figure 4: (Left) Flying Lead in a Figure 8 position; (Right) Figure 8 deployment frame resting on sea floor—comparable frame is used for bundled conduits

Launching Bundled Conduits

        From an installation and handling vantage point, landing a bundled conduit on the mudline is achieved via three distinct methods—as listed in the table below. Moreover, bundled conduits, as is the case with single conduits—are equally used to bridge a path of communication between two separate subsea points. Here too, bundled conduits will be defined to give the reader a better understanding of their composition and application (1):

Bundled conduits [which utilize junction plates at each end]…are specifically used for the running of hydraulic and chemical lines, giving way to Hydraulic Flying Leads (HFL) and Chemical Flying Leads (CFL). Bundled conduits can also be outfitted with electrical and fiber optic lines. A jump out panel is typically utilized to connect a PBOF hose and connector to its designated point in this backdrop [Reference Figure 5].

Figure 5: Junction plate showing on top portion of image; PBOF hose is shown on bottom of image

        It must be noted that because bundled conduits run an array of lines—this greatly increases their weight—requiring the use of a crane to work in conjunction with an ROV to unspool them. Now that the definition for bundled conduits has been reaffirmed, the below table highlights the manner by which bundled conduits travel from surface to their operational depth.

Figure 6: Top level description of bundled conduit intefacing

Figure 7: Bundled conduit completely unspooled from carrousel

 Weather and Crane Considerations

        Of the methods listed in Figure 6, method 2 has the highest likelihood of being affected by adverse weather. This is due to the fact that a crane must work in synchronicity with a topside carrousel to overboard one end of a bundled lead at a time, via a chute that is typically located on a vessel’s stern—see Figure 7 (A tugger is equally employed when a crane does not reach the overboarding point). Furthermore, when bundled leads and accompanying junction plates are deployed—during adverse sea states—via a carrousel, such sea states can impact their physical integrity, especially when entering the splash zone: this is when they are the nearest to the bottom part of the chute and vessel's stern.

        It is important to note that deploying a bundled conduit in this fashion is further compounded by the fact that an ROV must interface with the unspooled lead once it has reached its final hang off point, which can be difficult, even when a heave compensated crane is used to offset the lead's upward and downward movements. In contast, bundled conduits via methods 1 and 3 can be unspooled with greater ease, once their deployment frame is unhooked from the crane, upon landing on the sea floor. However, the crane—when a bundled conduit whose weight exceeds that of an ROV’s handling capabilities—is utilized to assist in handling and installing said conduit. And in the presence of anomalous weather, a crane’s ability keep a bundled conduit in a fixed position while an ROV attempts to handle and install it, is highly difficult.

Figure 8: (Left) Flying Lead spooled on carrousel; (Right) Deck testing of lead before prior to being installed

Conclusion

        Now that Reaching Ultra has answered the “what” and “how” of Flying Leads via two previously published articles featured in Ocean News (1)(2), this article will serve as an inseparable link between the two. A link, which will further assist existing and future offshore professionals in having a full understanding of the manner by which to commandeer Flying Leads within a subsea production scheme: from the surface to the ocean’s floor.