TPG 500 IDV is a new installation/decomm- issioning vessel concept, developed by Technip-Coflexip, for operations in water depths to 120 m. It is designed to remove packages of 14,000 tons in a single operation and has a total elevating capacity of 26,400 tons.
The IDV is a bottom-founded structure, derived from the TPG 500 jackup production platforms. Two of these platforms are installed and operating in the UK North Sea. The first, on BP's Harding field, has a topsides elevated weight of 17,500 tons. The second, on Total's Elgin platform, has a topsides elevated weight of 30,000 tons. A third TPG 500 platform is being designed for BP's Shah Deniz field in the Azeri sector of the Caspian, with an elevating capacity of around 23,000 tons.
Since the IDV is bottom founded, it does not require active systems to compensate for motion due to sea conditions. The height of the hull/ deck can also be varied, providing a safe fixed working platform for preparatory work before removal of a topside structure or jacket.
Additionally, the IDV can perform the following range of tasks:
- Installing topsides onto deepwater jackets, towers, or Spars (in floating mode, but using rapid jack-down rather than slower ballasting)
- Handling subsea installation
- Acting as a work/construction platform
- Removing small platforms in one operation
- Installing wind turbines
- Acting as an accommodation platform, typically 190 beds, with utility support systems.
Although the IDV concept was produced too late to qualify for ConocoPhillips' Ekofisk Booster platform removal program in the Nor-wegian North Sea, Technip-Coflexip says it would be suitable for this work. Technip-Coflexip says it proved this through an internally funded, front-end engineering study. Water depths at the Ekofisk installations are 80.5 m, and the package weights do not exceed 5,000 tons. Both are well within the IDV's design capability. The concept has since attracted interest from various North Sea operators and is being assessed for Total's Frigg field decommissioning program.
The TPG 500 IDV is a jackup platform with a U-shaped hull/deck. It incorporates the same jacking technology as the TPG 500 platforms and employs the same installation procedures. Its hull/deck is of flat plate construction, as per the TPG 500, but without the double hull bottom. Unlike the platform, space within the IDV's hull is largely unused.
The vessel has four legs, each of a triangular lattice design, as with the TPG 500 legs. But the IDV is des-igned for less onerous environmental/ fatigue criteria, hence the cost of the legs is lower. Each leg is fitted with large spud cans that can accommodate a wide range of soil conditions – even weak clays – with minimal penetration.
Legs are deployed within a jack house, designed with jacking systems on the two inner chords. Standard six-pinion and four-pinion TPG 500 jacking boxes are installed on each of these inner chords, providing total platform jacking capacity of 26,400 tons. The jack houses also provide the sliding support for the four lifting cradles, or "shuttles," which are guided on the two inner chords.
Permanent systems onboard the IDV include:
- Hydraulic power units and controls for the jacking systems
- Trim ballast and hull bilge systems
- Eight-point mooring winches
- Power generation and switchgear equipment
- Fire protection systems
- A helideck
- Navigational aids and telecommunications.
Additional modular equipment or accommodation can be fitted if required.
The IDV has two buoyant hull structures manufactured from stiffened flat plate, linked by a transverse open lattice structure. These hulls are 10 m high, and the main plate stiffeners are positioned typically at 2.8-m centers. This combination of closed buoyant and open lattice structure provides the required buoyancy, with the (buoyancy) center being at the same location as the center of gravity of the lifted structure. This minimizes the trim ballast requirement. No equipment is housed in the hull's buoyant sections beyond trim ballast and bilge piping.
The transverse beam structure is a combination of girder and box section construction, which provides the strength to cross-link the buoyant hull sections. It is designed to allow insertion or removal of an additional central section to accommodate a range of jacket and topside geometries. The jack house structures are above the main deck and aligned with the triangular hull leg wells.
Isometric view of the TPG 500 IDV.
In combination with the guides in the lower hull, the jack houses guide the legs and incorporate the hydraulically-driven jacking systems. A lattice superstructure above deck level braces the jack houses together with the hull, providing a rigid overall structure.
The hydraulically driven jacking systems are identical to those used on the TPG 500 platforms. Jacking systems are fitted to the two inner chords of each of the four triangular legs. Each motorized chord has 10 jacking pinions, resulting in an 80-pinion jacking system. And each of these jacking pinions has a dynamic elevating capacity of 330 tons, bringing an overall elevating capacity of 26,400 tons. Lowering capacity for this configuration is around 28,400 tons.
Jacking pinions are installed in standard four-pinion and six-pinion box housings. They are driven by a planetary gearbox coupled to a hydraulic motor. The gearbox is failsafe and does not need hydraulic power to maintain the platform elevation. Control of hydraulic fluid to the drive motor enables accurate control of the rate of elevation and lowering, and positioning of the hull is feasible to within a 3-mm tolerance. The hydraulic power packs that deliver the jacking systems' motive power have built-in redundancy, with the provision of spare hydraulic pumps, and the overall system is controlled remotely by computer.
Each of the triangular section legs consists of an X-type trussed structure. The two inner chords facing the IDV's center are motorized, with a toothed rack plate fabricated from high-strength steel. The third outer facing chord has a rack plate without teeth. Half-shell tubulars are welded onto the rack plates and provide for attachment of forged nodes. The geometry, metallurgy, and forging procedures of these nodes leads to a design with low stress concentration factors, and also allows the leg bracing to employ standard tubulars, which simplifies the welding method to be performed at the assembly site. Each leg is terminated with a conventional large area (250 sq m) spud can.
The shuttle system comprises four cradle structures, one on each of the IDV's legs. The cantilever design of these four shuttles allows them to accommodate a wide range of platforms topside and jacket loads and configurations. They rest on top of the hull on the main deck, and are raised into position by the elevation of the hull. Internal guides in the shuttles also allow them to slide up and down the inner two leg chords.
Technip-Coflexip prepared a detailed method statement for removal of the Ekofisk Booster platforms topsides and jackets. Prior to sail-away, the seabed site would be cleared of obstructions that might clash with the IDV's spud cans. Preparatory work to the IDV itself at the shore base would include attaching two articulated longitudinal skidding beams between the shuttles and welding four longitudinal skidding beams between each shuttle's "fingers." The latter arrangement would provide support for the transverse beams used to cradle the removed platform structures.
Tow-out of the IDV to the offshore site, by two 120-ton bollard-pull tugs with an anchor-handler also in attendance, should take less than two days. On arrival, the vessel is maneuvered to a location offset the platform, known as the "waiting site."
At this point, the temporary eight-point mooring system is connected. The IDV then is winched to the waiting site and subsequently spudded and jacked up, with the foundations pre-loaded through ballast and cross-jacking of the four legs. Relative heights of the legs are recorded for future reference. The IDV, now bridge-linked to the platform, is left in situ while the lifeboat and lay-down area are dismantled to provide sufficient clearance for the IDV to envelope the platform.
Next, the IDV is installed by jacking the hull into the water by raising the legs around 2 m from the seabed. The vessel is then winched until its U-shaped hull wraps fully around the platform (this operation requires a sea state of 1-m Hs). During winching, clearance between the spud cans and jacket bottom is 4.4 m, with a similar clearance between the shuttle fendering and the top of the jacket.
Once the IDV is in position relative to the platform, the legs are lowered and spudded. The vessel is then jacked up to an air gap of around 2 m and the foundations pre-loaded through a combination of ballast and cross-jacking of the legs. The ballast is dumped and the hull elevated to an intermediate height where the transverse beams can be skidded off the pontoons and under the topsides.
With the beams in place, the hull is elevated further to the required height for the remaining preparatory work, prior to topsides lift-off. This work includes locally strengthening the topsides to accommodate lifting forces, internal sea fastening to resist transportation forces, and placing vertical and horizontal shimming at the shuttles.
On completion of these preparations, the IDV is elevated until contact is made with the topsides structure and a partial load transfer of around 20% has occurred. The jacket's legs are then cut, and the IDV elevated further at a slow rate to take the entire load, and also to separate the topside from the jacket. In the next step, the shuttles are locked to the legs and the hull jacked down into the water.