Combining flexible and rigid pipelay functions for ultra-deepwater operations

With the advent of deepwater discov eries down to 2,500 meters, new pipelay techniques must be devel oped to bridge the gap between the current pipelay capabilities and meet the challenges of the future.

May 1st, 2000
Th 0500oscofl

CSO's Deep Blue will be able to install all types of risers, flowlines, and umbilicals in water depths beyond 2,500 meters.
Click here to enlarge image

With the advent of deepwater discov eries down to 2,500 meters, new pipelay techniques must be devel oped to bridge the gap between the current pipelay capabilities and meet the challenges of the future. Many of the deepwater fields will employ a combination of rigid or flexible production lines with or without thermal insulation, flexible tails, and steel catenary risers or dynamic flexible risers.

Coflexip Stena Offshore (CSO) has responded by investing in a new vessel, named Deep Blue, which not only covers these installation aspects, but also encompasses all the requirements of deepwater field development, except drilling. Many of the new fields are being discovered in relatively remote areas, far from any onshore base or existing infrastructure, which implies high vessel mobilization costs.

CSO calculated that a single self-contained vessel offering all services required would be extremely competitive, compared with the more conventional multi-vessel combination (DSV/survey/pipelay vessel). It therefore decided to invest in a large vessel capable of installing pipelines down to 2,500 meters, heavy lifts up to 400 tons, and with onboard remotely operated vehicle (ROV) construction and survey facilities to cover all other deepwater production requirements.

It is worth noting that although many planned or existing pipelay vessels are capable of laying either rigid or flexible pipes, none offers the capability of laying both. Further more, the target of 2,500 meters, which would appear as an extension of current methods in use in the North Sea or Brazil, has, in fact necessitated development of new technologies which do not exist today "off the shelf." The challenges faced in developing a multi-purpose vessel for ultra-deepwater cannot be underestimated.

Vessel selection

Early in the conceptual development of the project it was clear that the new pipelay vessel would have to meet the following key requirements:

  • High transit speed: Deepwater developments are likely to be geographically wildly spread in areas such as West of Africa, Gulf of Mexico and Brazil. Therefore, a relatively high transit speed for ocean crossing and port to field transits is important for efficient utilization of the vessel.
  • High payload capacity: Large deepwater fields will require a vessel with high payload capacity for both rigid and flexible pipelines. Market studies suggest 8,000 tons combined payload would be required.
  • Stable laying platform: One of the primary requirements for deepwater pipelay is a stable working platform to minimize dynamic pipelay loads resulting from vessel motions. To achieve this aim, a central moonpool location was preferred to over-side or stern-lay, in combination with a roll stabilization system.
  • Dynamic positioning capability: A high spec DP system for survey/pipelay and ROV construction activities is essential in either open water or in close proximity to floating or fixed production platforms.
  • Vessel dimensions: The vessel should be capable of passage through either the Panama or Suez Canal. This is important for efficient utilization of the vessel.

Only a monohull vessel could satisfy all of the above requirements. To this end, it was decided to build a new hull of 192 meters length and 32 meters beam, offering a sufficient deck area to properly fit the pipelay equipment spread.

The hull, based on a Gusto 10000 design, is fitted with two 5.5 MW azimuth stern main propulsion thrusters and four 3 MW retractable azimuth thrusters. Total installed power generation capability is approximately 34 MW, generated in two main engine rooms located at the stern of the vessel, each containing three main generators. The power management and DP systems are designed for stationkeeping to DP Class 2 in an environment of 3 meters significant wave height, 30 knot winds, and 2 knots current. To minimize the vessel's draft, three forward and one aft fully, retractable thrusters are incorporated. In addition, two forward tunnel thrusters are retained for harbor maneuvering. To minimize vessel roll response, a passive roll stabilizing system has also been incorporated into the design.

Pipereel equipment consists of two 2,500-ton horizontal axis reels for storage of rigid pipe, two 18-meter diameter basket carousels for flexible lines or umbilical storage, and portable reels at the back of the vessel. A 19 meter long by 7.5 meter wide pipelay moonpool is deployed for pipelay as well as tooling or PLEMs (pipeline manifold).

Storage is provided for up to 8,000 tons of payload. The vessel also has four main cranes - of these, the heavy lift crane (400 tons) will be used for any lift required offshore (manifold, template) down to 2,500 meters, in association with the abandonment and recovery (A&R) winch system.

Mooring of the vessel will be accomplished by the two forward combination anchor mooring winches, and two mooring winches located at the aft end of the vessel. Deep Blue can accommodate up to 120 crew and project personnel onboard. A helideck suitable for Sikorsky 61N type aircraft is located forward of the bridge.

Pipelay system

The primary method of rigid pipeline installation is the reel lay method. The decision to select the reel method was based on a number of factors:

  • The lay speed of the reel lay method exceeds the capability of conventional lay barges. Typical speeds of up to 1 km per hour can be achieved by reel laying, which allows installations to be completed within relatively short weather windows.
  • The laying technique selected is ideal for the laying of any flexible pipelines or umbilicals.
  • Pipe-in-pipe systems are increasingly being laid using the reel lay technique.

Pipelay will be via a centrally positioned J-ramp tower structure containing the pipe straightening equipment and pipe lay tensioners.

The J-Ramp comprises a sophisticated A&R system, straightening equipment capable of coping with pipe diameters of 4-in. to 16-in. (including PIP), and two tensioners able to sustain a weight of up to 550 tons. The capacity to lay pipelines between 75 meters and 2500 meters requires the J-ramp tower to pivot 60-90° from the horizontal.

One of the vessel's more innovative features is the two 2,500-ton rigid pipe storage reels. With a single rigid pipe reel, up to 40% of the vessel's operating time can be spent in pipe loading operations. In contrast, two reels operating simultaneously can reduce pipe loading time by half (of particular advantage for pipe-in-pipe systems). In addition, the two reels can segregate pipe sizes (small pipe on one reel and larger pipe on the other). For the first time, the pipe sections for two different operators can be loaded at one time, thereby providing a more cost effective approach to small field developments through shared mobilization/transit costs.

The angled reels and the long aft deck also allows the possibility of onboard fabrication of pipelines should this be required (the pipelines could be fabricated in a temporary fabrication facility installed on the aft deck and spooled onto the reels under the lay tower). The angled reels can even allow one reel to lay pipe, while the other is being loaded from the onboard fabrication facility.

The top of the J-ramp tower can rotate to enable proper pipeline feeding from either of the two main reels. In other words, the aligner will always follow the pipe trajectory to ensure that unspooling is done without adding additional bending stress in the pipeline. Pipe departure is through a near centrally positioned moonpool to minimize dynamic loads from vessel motions.

Unlike the CSO Apache, all the pipelay tension is supported by the two 275-ton tensioners. The rigid pipe reels are used purely for storage purposes, so they do not add significantly to the pipelay catenary loads. Flexible pipes and umbilicals are stored in two 2,000-ton capacity basket carousels housed below the main working deck.

In addition, seven 300-ton portable storage reels on the aft working deck can be mobilized onboard for short length flexible tails. The flexible pipes and umbilicals are routed via a series of chutes from the storage baskets (or reels) to the top of the J-ramp tower under low tension. These then bypass the rigid pipe straightener and are fed into the tensioners.

The key components within the lay system are the two 275-ton quad track tensioners, which are based on a well proven design currently in use on a number of vessels, but with a much increased load capacity. The requirement to lay flexible and rigid pipe of different diameters and contact pressures with the same tensioners without requiring change-out of tensioner has led to further innovative solutions, one being the steel tensioner pad.

This is designed with small serrations on the contact surface which mobilizes mechanical shear, rather than relying on the friction coefficient between the pad and the product coating. It is a major advantage in deepwater applications where high loads are held by the tensioner, allowing for a more compact tensioner track design.

The main pipe clamp and flexible pipe hang-off position are located in the moonpool just below the main working deck. Both the pipe clamp and the flexible pipe hang-off are designed for a working dynamic load of 550 tons and a maximum holding capacity of 700 tons to cater for accidental loads from flooded pipe.

The pivoting clamp is capable of longitudinal and transverse motion under full operational load within the moonpool area. This feature is necessary to align the pipe for welding at all lay angles and to allow the clamp to be cleared from the area for passage of large pipeline components.

All current deepwater diverless connection systems require deployment of relatively large connectors either attached to the end of the pipeline or as flexible tails. Other attachments such as PLEMS and SSIV valves have been installed on the end of pipelines in recent years, and each year brings a requirement to install larger components at this location.

To accommodate this requirement, the new vessel is designed with the ability to deploy pipeline end fittings up to 5 meters by 5 meters by 5 meters in dimension, attached to the end of either the flexible or rigid pipelines. This was achieved by elevating the tensioner some 10 meters above the pipe clamp and working deck.

Steel catenary risers (SCRs) are also emerging as a new product for deepwater developments. CSO currently is working on qualification of reeled SCR which should bring huge benefits, in terms of timing of the installation and quality of welds (all the welding would be performed onshore in the spool base in a more controlled environment). In addition, the option exists within the current laying spread to install rigid pipe and SCRs without reeling, using the J-ramp equipment.

Prefabricated triple joint lengths (about 36 meters) of pipe or SCR will be stored on the aft deck of the vessel. These will then be transferred to the J-ramp tower and welded at main deck level. Although considerably slower than reel lay, this technique does cater to the stringent requirements of some SCRs and clad pipes which cannot be reeled or which would be beyond the 16-in. diameter limitation specified for reel lay.

Survey and construction

Two work class ROVs are installed on the vessel, both supplied by CSO subsidiary Perry Tritech. One will be deployed via a dedicated moonpool at a midship location, while the second ROV spread is a mobile unit which can be located at any position around the edge of the vessel. Generally speaking, current ROV technology that was developed for shallow water (500-1,000 meters) operations is being stretched to achieve up to 2,000 meters. This is near the limit for the conventional umbilicals and buoyancy that is used throughout the world and to move towards 3,000 meters has resulted in a change in our approach. The decision was made, therefore, to provide differing capabilities for the two ROVs to be installed onboard.

The primary installation is a 3,000-meter rated cursor-launched, high speed, heavier weather system deployed through a centralized moonpool. The system is based on Perry Tritech's Triton ST range. It will have a powered garage (with thrusters) and an extended tether to allow touchdown monitoring and intervention at considerable distances from the vessel. The garage thrusters will help overcome adverse currents and move the system closer to the distant work site (or touchdown point) and near the neutral extended tether will provide up to 700 meters or excursion from the garage.

The secondary system is, in addition to providing backup to the primary system, configured more for the heavier construction tasks associated with the installation of deepwater field infrastructures based on Perry Tritech Trition XL. It has common components with the ST systems above but importantly, it has a greater through-frame lift and payload. CSO envisages that this system will be used to perform diverless connections of rigid and flexible flowlines to subsea templates and other "heavier" construction tasks, such as flooding, pigging, and testing at depth.

More in Rigs/Vessels