Increasing numbers of platforms will have to be removed from the North Sea and else-where over the next decade. At Ekofisk, four sets of redundant topsides may go under a first decommissioning phase, including the notorious Ekofisk Tank.
Most platforms dismantled to date have been small. New concepts are slowly emerging to tackle the larger structures, but costly crane barges remain the only proven option. However, one of the more serious alternatives is about to leave the drawing board. Construction of the Pieter Schelte, a twin-hulled catamaran, should get under way next year, in readiness for duty in the North Sea by mid-2003. Excalibur Engin-eering, which has developed and refined the concept, felt confident enough to issue a paper at this year's OTC.
The idea for the vessel, named after the Heerema group founder, evolved initially at Allseas in The Netherlands during the late 1980s. At that time Allseas viewed it more as a heavy lift vessel, capable of delivering topside loads up to 45,000 tons with all testing and commissioning pre-executed onshore. Then the Micoperi 7000 and the DB102 arrived on the scene, so the project was ditched in favor of the large diameter pipelayer Solitaire.
Last summer, Excalibur was founded and secured patent rights to the concept. The new company started a block away from the Allseas offices in Delft, The Netherlands. According to Managing Director Josso van Boxtel, there have actually been a basic and an upgraded version of the Pieter Schelte. "The original one was for topsides and the upper part of jackets only, but then Ospar brought in legislation making jacket removal obligatory too. So we developed a system to deal also with jackets."
The concept as it stands is based on two 300,000 dwt tanker hulls, which will be converted to form a twin catamaran hull with newbuild inserts, designed to fit around the structure in question. The Pieter Schelte will have a lift capacity of 48,000 tons for topsides and 25,000 tons for jackets.
The vessel will be 336 meters long, 118 meters wide, and 29 meters deep. "The original concept was for a wider vessel," van Boxtel says, "but we reduced it, because we didn't want to have to go subsequently to yards in the Far East to drydock the vessel."
According to the Excalibur authors' paper at OTC, the vessel's favorable motion behavior "guarantees the regular occurrence of sufficiently large weather windows for a reliable planning of the removal operation. Its limited motion response in survival conditions protects the structural integrity of the platform components in transit and reduces seafastening requirements to the minimum."
The 90 meters long, 52 meters wide catamaran slot has been sized to fit all jackets coming up for decommissioning in the next decade. However, provisions have been made in the connecting structure between the two hulls to allow the slot to be widened for the four very largest jackets in the world. On location offshore, the vessel will be moored by a 12-point spread of 40 ton anchors. In transit, a safe heading should be guaranteed by its two main propulsion systems, allied to azimuth bow thrusters.
Topsides lift procedure
Topside lifting devices are situated either side of the catamaran slot - they will have been aligned for the topsides prior to arrival of the vessel. Pieter Schelte's draft will also have been adapted to the height of the platform and the location of the topside lift attachments. Assuming environmental conditions are sound, the vessel will "embrace" the platform in its slot. The lift system then engages the lift attachments - the system will have been pre-tensioned in heave compensation mode (an operation expected to take two hours for topsides weighing 25,000 tons). Water ballast will be used to adjust vessel draft and loading conditions, as the operation progresses.
The system clamps itself to the legs of the module support frame to lift the topsides at the point of their natural support structure. Depending on the topsides' structural integrity (pre-determined by Excalibur), reinforcements could be added locally to ensure a safer transition.
Through a special mechanical/hydraulic arrangement, the lift system can be connected to the platform legs while the vessel remains free to move with the prevailing waves. Pendulum rods and hydraulic motion-compensating cylinders ensure that significant loads are exerted from the vessel during this hook-up phase. Pre-tension in the lift system is then raised to transfer the weight of the topsides from the jacket to the Pieter Schelte. Once the final cuts have been performed, rapid lift-off of the topsides from the jacket (within seconds) is effected using energy stored in the hydraulic system's accumulators. This cuts out the risk of re-impact between the jacket and the topsides.
Immediately afterwards, the vessel will move away from the platform. The hydraulic pressure in the topsides lift system is eased off, with the load being laid to rest in the mechanical and structural components of the topsides lift system. It is then seafastened for the voyage back to the shore. In transit, the hydraulic motion compensation system is idle.
Jacket operations
Recovery of jackets can be performed directly after removal of the topsides. A twin-frame lifting gear attaches from the vessel to the top of the jacket legs, above the water line. Lifting points are welded to the top of the jacket legs - spreader bars and attachment points will be designed to ensure the jacket top elevations' structural integrity. Conductors, casings and other loose items will be secured to and removed with the jacket.
Using conventional cutting techniques such as abrasive water jetting, the jacket will be severed in accordance with Ospar recommendations (3 meters below the mud line, or just above the pile sleeve bottle cans, leaving a stump on the seabed). Any flooded jacket legs and braces may have to be punctured to ensure that entrained water will drain once the jacket surfaces above water. The jacket will be partly lifted above water to a pre-determined position, then tilted with the (pre-tensioned) jacket lift system to a horizontal position on the Pieter Schelte's deck. This part of the operation takes several hours. Once in place, the jacket can be seafastened for the voyage to shore.
There are various scenarios for the delivery procedure, according to the authors: "The vessel will transfer the topside by lowering it onto a purpose-built barge fitted with pre-installed load spreaders. The barge can subsequently be moved to a quayside and the topside skidded onto the quay. Any accidental spill of hazardous materials can thus be contained. Jackets can be demolished onboard the vessel. Alternatively, the jacket can be cut into smaller sections and skidded ashore, or it can be skidded in its entirety onto a barge and taken ashore."
The Pieter Schelte will have to be berthed in drydock for maintenance/inspection. Excalibur has identified several yards capable of handling the vessel's 8 meters draught and 118 meters width.
Design factors
Various design groups and institutes have assisted in the conceptual engineering, including Gusto Engineering for the hull and lifting systems, Marin for motion simulation and WS Atkins with integrity studies. The latter has specific knowledge, having undertaken design for many of the platforms approaching decommissioning. "We picked a number of typical topsides and jackets installed over the years for our studies," says van Boxtel, "some smaller, some larger."
Excalibur is currently negotiating for two 275,000 dwt VLCCs for the conversion with the work likely to be awarded to a Far East yard by second half of next year. However, the lifting systems are likely to be built in Europe. "Out of the two tankers we're buying, we want to keep the two engine rooms and if possible, the accommodation also - but we would probably have to do a refit to offshore rules. There would be a crew of 250 maximum for the lifting operations."
One to two days is the anticipated weather window for the removal operation. Van Boxtel also expects assignments to be awarded as part of a total removal package, including platform surveys and the disposal onshore. Excalibur has discussed alliancing operations with potential decommissioning yards in the UK and Norway, and parties keen to enter the well shutdown market.
Conceptual work on the Pieter Schelte has still to be finalized - it could also be used for platform installations and relocations. "If you consider the large jackets currently being launched, the installation process normally introduces a high number of local forces. Our method, however, introduces no launch forces whatsoever, therefore the engineering contractor can optimize the jacket and make it much lighter. We've discussed this with several operators, and they consider it a good idea. Also, if you can bring the whole of the topsides offshore in one package, as our method does, it saves on offshore hook-up."
Construction costs for the Pieter Schelte are expected to total $400 million. Aside from Ekofisk, platforms coming up for removal from the North Sea are thought to include Frigg, Froey, Gyda and Hutton. But there are 60 other interesting possibilities outside the North Sea, van Boxtel adds, in areas such as the Mediterranean, offshore Brazil, Australia, and California.
