Marcrist International's expertise in diamond drilling and cutting was put to the test when the company was called in as a consultant to design and develop a system for subsea use, at a depth of 90 metres, to help repair a leak at an oil platform in the North Sea.
Undersea Expertise
The Harding oil field lies in the North Sea, approximately 320 kilometres north-east of Aberdeen and has been supplying oil since April 1996. The BP Harding platform includes an 84,000 tonne concrete tank, known as a Gravity Base Tank, which acts both as the foundation for the platform and as a storage facility, and is capable of holding 580,000 barrels of oil. A two kilometre long pipeline leads from the tank to a submerged loading bay, used to transfer the oil to tankers.
When workers at the platform noticed a film of oil on the surface of the sea, indicating a leak, divers were sent down to investigate. They discovered that the leak was coming from redundant installation pipework connected to one of the compartments of the storage tank and were able to put in place a temporary solution, which involved capturing the leaking oil and returning it to the platform.
As a short-term solution, the storage tank adjacent to the compartment was emptied, thus stopping the leak. However, this meant that although the platform could continue to produce oil, its storage capacity was severely reduced, necessitating more frequent tanker visits and therefore increasing transportation costs. There was also the fear that the leak might at any time deteriorate further and result in a large and uncontrolled discharge of oil into the North Sea.
Further investigation using a Remotely Operated Vehicle (ROV) revealed that the source of the leak was a sealed access hole located in the wall between the compartment (known as compartment Q) and the storage tank. This access hole had been used to carry out a final internal inspection of the Gravity Base Tank after the platform had been floated into position and had been sealed with cementitious grout. It was decided that a permanent repair was required which would last for the remaining life of the oil field and allow the Gravity Base Tank to return to normal operation. The repair was to be affected by covering the leak with a steel plate, rubber seals and injected epoxy resin. This, however, was no small task. In order to effect the repair it would mean drilling a hole through the 1100mm thick reinforced concrete roof of the tank to enable a diver to gain access into compartment Q. To achieve this would require designing, developing and fabricating all the tooling and installation equipment necessary, assembling and training personnel, acquiring suitable diving vessels and testing the whole procedure many times over.
The design for the repair was undertaken by the original designers of the tank itself, Taywood Engineering. The repair was to comprise a flat stainless steel plate attached to the wall by a ring of expanding anchor bolts. Stainless steel was chosen for its resistance to corrosion and the plate was designed to withstand the pressure differentials that would be experienced when in service. Two circumferential seals around the plate would create pressure resistant cavities that were to be filled with epoxy resin.
The task of designing and developing the installation and cutting equipment fell to the installation contractor, Stolt Offshore Ltd.
Specialist knowledge of working within a hostile environment using remotely-operated machinery was required and so Marcrist International of Doncaster was called in as a consultant because of its background of working within the nuclear industry. It was felt that subsea experience was not essential, since the company's understanding of the problems would allow its topside expertise and techniques to be adapted for underwater use. Ian Bannister, special projects manager of Marcrist International, takes up the story.
We were asked if we could design a system that would allow a 1.2 metre diameter hole to be drilled underwater, 1.2 metres deep, in reinforced concrete, for divers to gain access to the tank and install the specially constructed equipment to effect the repair," he explained. "This posed a number of problems, both with the size of the core to be removed, and how to hold it in place once it was drilled out. The core would weigh about 2.5 tonnes and, once drilled, there was the danger that it would fall into the compartment.
"It wasn't clear exactly where the reinforcing was located in the concrete either," he continued, "and it was important not to cut through certain structural elements of it. To avoid this, the area where the hole was to be cut was grit-blasted until one of the reinforcing rods was revealed. Knowing the spacing between them from the plans of the tank allowed the divers to calculate where the others lay.
"Because of the amount of reinforcing in the concrete, we knew that it would take a long time to drill through and we decided, therefore, that the best solution was to wiresaw it. This would involve drilling a hole at each corner, threading the wire through, and sawing between the holes, much like a cheesecutter. The result would be a square, rather than a round, hole but this was not an issue since it was only required for diver and equipment access."
Rather than assemble and place one drilling rig at a time, it was felt that a quicker and more accurate method would be to mount four separate drills on a frame. In this way the position of the holes could be pre-determined exactly, most of the placement work being completed on board the support vessel. It also meant that the task would be completed a lot quicker than having to move a single rig each time a hole had been drilled - an important consideration given the high cost of the support vessel and all the ancillary personnel involved.
"We made a frame to hold the four drill rigs, together with an operating panel for each rig," said Ian. "That then fitted into a docking frame which would first be lowered from the support vessel and locked in position over where the opening was to be made."
This docking frame was to act as a template so that when the drill frame was lowered, it fitted into locating lugs, ensuring that the four drills were positioned exactly. Once the drilling was completed, the drill frame would be removed and a second frame, with four wiresaws mounted on it, lowered and located into the same lugs, again ensuring perfect alignment of the equipment.
"We also had to ensure that the hydraulic systems would operate properly 90 metres underwater and withstand the pressure," added Ian. "We designed in special seals so that there was no ingress of water and no escape of oil into the sea during the drilling and sawing operations."
The equipment was assembled at Marcrist's Doncaster headquarters and then shipped to Aberdeen. Two Marcrist personnel, George Linstead and Simon Crawshaw, were deployed to familiarise the project team with the methods and techniques to be used. The pair spent the next six months commuting between the two locations and, as well as training the project team on how to use the equipment, had to undertake extensive offshore safety training themselves. This included baling out of mock-up helicopters into large tanks of water within a given time, while wearing special survival suits.
Meanwhile, full size replicas of the top of the Gravity Base Tank and the section of the tank wall containing the accessway were constructed to enable the cutting technique developed by Marcrist, together with the repair technique, to be tried out and refined. The mock-ups were submersed in a tank at the National Hyperbaric Test Facility to enable the dive team to practise the techniques underwater.
At the same time, the protocol for the work was completed and the divers and other team personnel were trained by Marcrist in safe procedures developed for undertaking this difficult and hazardous task.
To overcome the problem of the concrete cube falling into the compartment, Marcrist had devised a system whereby the drills would be angled at three degrees, so that when the wire saws were pulled through, the final shape would be a wedge, meaning it couldn't fall into the tank and would be easy to remove.
Once it was established that the techniques would work, the equipment was transferred to the Harding oil field and the offshore implementation of the project commenced from on-board the Seaway Osprey.
The ship is a dedicated diving support vessel and is equipped with two large three-man diving bells, enabling 24-hour operation. Each diving bell is housed within a pressurised area known as a 'moonpool' - an environmental chamber where up to 18 people can live. At the end of a dive, the diving bell docks into its own moonpool and the divers are able to get out without having to go through the time-consuming procedure of de-pressurisation. They can remain in this pressurised environment, quarantined from the rest of the ship, for weeks at a time, the Seaway Osprey moving from job to job as required. This technique, known as 'saturation diving', provides the most cost-effective and time-saving method of operating.
Working at the bottom of the sea is akin to working in outer space; both environments are extremely hostile and great care and precision must be taken when carrying out any task. The divers work shifts, one diving bell being lowered to the seabed with three divers on-board. The bell is 'parked' a short distance from the actual work site and ingress and egress, wearing cumbersome diving suits, is far from easy. Once out of the bell, the divers make their way along the seabed to the location of the task to be carried out.
Each task has a clearly defined procedure which has to be followed in minute detail. The divers work from a 'script' while, on board the Seaway Osprey, the controller of operations directs events, monitoring everything that is happening on the seabed via cameras mounted on board two ROVs and talking to the divers all the time. Whatever task is being a performed, a specialist is also present, sitting alongside the controller. During the drilling and sawing operations to repair the Gravity Base Tank, a specialist from Marcrist was present to advise the divers should any problems have arisen.
Once the scheduled tasks for that dive have been completed, the divers return to the bell which is then raised back into its 'moonpool' aboard the Seaway Osprey. Meanwhile the next crew is lowered and sets about its tasks. This 'saturation diving' continues 24 hours a day until the entire job is complete. The ship, together with its diving crew, then moves on to its next assignment.
The procedure at the Harding Oil field involved first lowering the docking frame to the Gravity Base Tank and then securing it in position to act as a guide. The drill frame, containing four diamond core drills positioned in each corner, was then lowered and secured into the docking frame. The drills were positioned and oriented to drill a 110mm diameter hole in the four corners of the cube to be removed.
It took just 40 minutes to drill each hole and once this was complete the drilling frame was removed and a modified ROV used to establish 'messenger' lines between adjacent holes which were used to draw the lengths of diamond wire through the slab. The sawing frame, containing four diamond wiresaws, was then lowered from the surface vessel and secured in the docking frame.
"The divers cut opposite sides to start with so that the block was held up by the remaining two sides," explained Ian. "It took one and a half hours to wiresaw each side and once the first two sides were cut, metal wedges were inserted to hold the cube in place. The final two sides were then cut and the block hoisted up to the surface vessel."
At this point in the operation, Marcrist's role came to an end. The systems had worked as designed, the cutting operation had been completed smoothly and the concrete block had been successfully removed, allowing divers access to the interior of the tank to effect the repair.
A month later oil was allowed to flow once more into the tank and since then no defects or leaks have been detected and the tank has been returned to its original operating condition. Details of the techniques developed and the knowledge gained from the operation will be made available for use in other subsea operations in the future.