DIVERLESS SUBSEA SYSTEMS-2 FLOW LINE CONNECTIONS KEY ELEMENT IN SUBSEA PRODUCTION

March 22, 1993
Robert H. Rothberg, Johnce E. Hall Amoco Production Co. Houston Larry D. Douglas Oil Industry Engineering Inc. Houston Kerry G. Kirkland Consulting Engineer Houston William S. Manuel Manuel Designs Inc. Houston Flow line and pipeline connections are a key area selected for emphasis in Amoco Production Co.'s development of a diverless subsea production system (DSPS). These connections significantly influence the design of subsea production systems. This second of a three-part series
Robert H. Rothberg, Johnce E. HallAmoco Production Co. HoustonLarry D. DouglasOil Industry Engineering Inc. Houston

Kerry G. KirklandConsulting Engineer Houston

William S. ManuelManuel Designs Inc. Houston

Flow line and pipeline connections are a key area selected for emphasis in Amoco Production Co.'s development of a diverless subsea production system (DSPS).

These connections significantly influence the design of subsea production systems. This second of a three-part series examines available flow line and pipeline connection technology.

CONNECTIONS

Flow line connections have influenced the evolution of subsea clustered well technology. In the early days of subsea systems development, emphasis was on careful alignment of tree and manifold connectors over a small gap that was closed by a connection tool.

Horizontal offset distances of 12-24 in. were typical between the tree connector and the manifold connector before closure. Because of this, the number of wells had to be known before fabricating the template structure, prior to drilling.

Many prospects were rendered uneconomical to develop because lead times to first production were as long as 5 years.

More recently, template-mounted trees have used vertical flow line connections to the manifold, reflecting improvements in fabrication quality control and eliminating the need for a separate running tool connection operation. However, where fabrication lead time and/or uncertainty in the number of wells exists, template-mounted trees still have several disadvantages.

The next step in the subsea system evolution was to move the tree far enough away from the manifold connection so that sufficient flexibility in the flow line could be developed. This change admitted the tie-in of satellite wells, permitted predrilling, and allowed the salvage of exploratory well bores as production wells.

External pulling and alignment forces are applied to the flow line to bring it into registry with the manifold connection hardware. Distances of several hundred feet to several thousand feet are required, depending on the type of flow line material.

The most difficult flow line connection to make, yet the one potentially most attractive, is the one where the offset is on the order of the well guidebase spacing, about 15-25 ft. With this, a modular approach is possible for subsea systems sea-floor layouts.

This design encompasses all of the benefits of the large offset while adding the ability to install manifold segments and supplemental subsea wells as project timing/funds permit. It is here that Amoco has concentrated its efforts in the flow line connection technology development.

SHORT JUMPER SPOOLS

For flow line connections, the short jumper spool (Fig. 1) was developed for the diverless subsea production system based on two concerns set out during the conceptual program.

First, many of the existing industry methods for connection of rigid flow lines and pipelines involved the use of horizontal mated interfaces, which required precise alignment along with the complex machinery and high positioning forces demanded to accomplish such precision.

Second, the existing concepts were typically capable of achieving connections only from either very close proximity, or from relatively large offsets as noted in the introduction.

Based on these concerns, functional requirements were established for the development of the resulting short jumper rigid flow line connection system.

The short jumper system solves these design problems with a highly tolerant vertical hub connection and a custom fabricated pipe run, which can accommodate a relatively wide range of mating hub positions.

The short jumper spool flow fine connection system has direct application for the building block and potentially the loose cluster, and integrated template seafloor arrangements.

TOLERANCES

Variations in hub positions may be attributed to several common problems. For instance, fabrication of large structures presents several hub tolerance contributions. Where structures are intended to provide production from a large number of wells, say 20 or more, the physical size of the finished structure may promote large fabrication tolerances, because of rough cutting, welding, and field fabrication methods.

In addition, the physical distortion of the structure (and the internal piping) may be significant, because of the large spans (potentially 200-300 ft) involved and the potential lifting and installation loads.

Subsea drilling operations tend to further complicate the problem in terms of rough tolerances achieved regarding vertical well position and angular alignment with the true vertical and the vertical axis of the manifold structure.

Orientation errors in the yaw or azimuth direction of a vertical hub are also present and are often accounted for through use of single function or coaxial/concentric connection hubs.

All of these alignment concerns led to the development of the rigid short jumper flow line connection method, which uses a vertical interface and gravity to aid in the mating of the connecting hubs.

MEASUREMENT SYSTEMS

The short jumper method has a special hub measurement system that permits fabrication of field-fitted piping runs to accommodate all of the hub position tolerance concerns described previously.

The key features of the jumper system are the high-angle release jumper connector and the measurement, installation, and retrieval (MIR) tool (Fig. 2).

The high-angle release jumper connector was developed specifically for the Amoco project, as a modification to connectors used in conventional flow line connection methods.

Standard collet-type flow line connectors are capable of 1-2 of misalignment, and were felt to be too restrictive to accommodate the potential range of misalignment expected considering all of the fabrication, installation, and operational loads discussed above. After considerable analysis of the fabrication and installation process, an angular capability of about 15 was selected for a 4-in. nominal connector design. A standard 4-in. 10,000-psi collet connector was modified to conform to this requirement.

It should be noted that this alignment capability requires the connector to translate off of the mating hub at 15 (Fig. 3) rather than rotate off at an angle. This allows simultaneous mating or demating from both ends of the jumper spool, with the potential for both hubs to be misaligned in opposite directions.

In the 12-in. nominal size, an angular capability of about 7.5 was designed into the collet connector. Both hydraulic and mechanically actuated versions of the connectors have been built and tested. The MIR tool was developed to be lowered to the seafloor to accurately measure the positions of the mating hubs on two adjacent, but independent structures.

A rigid framework comprising the backbone of the MIR tool (Fig. 4) is first preset to a coarse estimate of the distance between guide posts delimiting the two upward-looking hubs to be connected. The estimate can be obtained through either known geometry or through use of a premeasurement tool.

This premeasurement tool is essentially an hydraulically powered tape measure that has been fabricated and tank tested. The MIR framework includes substantial free play at one guide post entry to accommodate errors in the initial estimate. Two measurement boxes are attached to the MIR frame, one at each anticipated hub location.

Video cameras guide the hydraulically actuated X, Y, and Z axis motion of a measurement probe extended from each measurement box. A measurement probe is lowered into each hub, where the X, Y, Z, and two axis inclination values are locked-in (recall that azimuth is not a required orientation for the vertical hubs). Then each probe is retracted to a safe position.

The MIR tool is retrieved to the surface and lowered onto a fabrication jig that is adjusted to match the measurement tool settings (Fig. 5). A pipe jumper is then constructed on the fabrication jig. The jumper includes an internal collet-handling profile that can be used in pick-up and release of the jumper by the MIR tool. A valve or other flow control device can be included in the spool fabrication.

Thermal expansion differences between seafloor measurement temperatures and surface fabrication temperatures are effectively canceled as long as the tool and the spool are made of materials with similar coefficients of thermal expansion.

The short jumper spool connection system has been fully prototype tested both in the shop and in a test tank to validate the design. The testing demonstrated that an accurately fabricated jumper spool could be repeatedly manufactured to match the hub positions of even highly misaligned hub profiles.

The MIR tooling and jumper connectors may be used in their present form with the addition of spacer frames to measure and install the jumpers between the structurally separate seafloor components.

PIPELINE CONNECTIONS

Several different concepts were considered for the pipeline connection method. These included the deflect-to-connect, second-end connection method and a number of layaway first-end connection methods.

While the deflect-to-connect approach has been proven reliable in deepwater applications, the required sweep of the free end of the pipeline may prove undesirable in some congested pipeline scenarios. Most of the layaway first-end concepts require maneuvering the surface lay vessel in close proximity to a moored drilling or floating-production-storage (FPS) vessel.

In view of the desire to place the pipeline end at some distance from the seafloor structure (to maintain some distance between the drilling and lay vessel), and the success of the short jumper spool flow line connection method, a remote "long jumper spool" approach was developed.

LONG JUMPERS

This long jumper spool method uses elements familiar to shallow water, diver-assist pipeline spool connections with the addition of diverless techniques to measure and manipulate seafloor equipment.

This long jumper pipeline connection system (Fig. 6) initially requires measurement of the distance and angular position of the pipeline end sled to the seafloor tie-in structure. The design allows measurement of up to 200 ft distances with an included horizontal azimuth angle of about 180. In addition, a vertical angle is also measured to more precisely determine the true position from one fixture to the other.

The protractor system and measurement tooling may be remotely deployed and read by the ROV. After the measurement is recorded, the pipeline pull head is disconnected and recovered to the surface.

A fabricated long jumper pipeline spool provides a horizontal connection at the pipeline end and a vertical hub looking up at the template tie-in structure. This method then uses the short jumper spool approach described above to connect the vertical run of the long jumper to the manifold hub.

An alternative long jumper configuration has two vertical hub connections, eliminating the need for the short jumper spool at the template end. This alternate configuration (Fig. 7) is well-suited for towed pipeline bundles.

The long jumper pipeline connection method has been demonstrated onshore, using a modified commercial pipeline connection tool and a simulated tie-in structure. In addition, the operation of the ROV functions on the modified connection tool have been demonstrated in a test tank.

Unlike the case of the short jumper spool flow line connection, available tank sizes preclude a full scale test.

FLEXIBLE PIPE

In addition to the rigid pipe methods evaluated, flexible pipe connections were also considered as potential satellite completion tie-in techniques. The flexible bend-over concept (Fig. 8) is a first end connection method and has a vertical hub and a simple bending shoe arrangement to allow the flexible pipe and a hydraulic connector to be lowered onto the hub profile and locked either by ROV or from the surface.

A landing/lowering cylinder arrangement prevents heave damage of the connecting hub. With continued deployment of flexible pipe, the line is laid over the bending shoe structure that controls the minimum bend radius of the pipe and allows the pipe to be landed on the seafloor as it departs the area.

A second method was developed, called the teardrop concept, that can be used for both first and second end connections.

This concept also has a vertical hub connection and involves lowering the flexible pipe vertically to mate and connect to the hub.

However, to prevent compression in the flexible pipe, a rigid gooseneck and a loop of flexible pipe are included (Fig. 9) and supported by a soft line. This loop provides a soft system that simply relieves tension on the soft line when the connection is landed.

This method could also be used during higher-than-normal sea states to accommodate the heave of the lay vessel. Once the hydraulic connector is activated by either an ROV or a surface hydraulic line, the soft line is cut or disconnected by the ROV, and the loop of flexible line continues to be laid down the ramp to the seafloor.

Editor's note: The concluding part of this series will cover ROV tooling and subsea control.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.