SUBSEA TECHNOLOGY: Deepwater dropped-objects recovery takes planning, excavation, visibility

In recent years, several companies have dropped multi-million dollar items of equipment, such as blowout preventer (BOP) stacks, in deepwater. PSL Group has aided in the recovery of many dropped objects in the past several years, including the following:

• BOP recovery in 700 meters water depth off Namibia, West Africa in 1995.
• BOP recovery in 1,100 meters water depth off Nigeria, West Africa in 1998.
• Crane block recovery in 560 meters water depth in the Gulf of Mexico (US) in 1999.
• BOP recovery in 1,640 meters water depth off Angola, West Africa in 2000.
• BOP recovery in 1,590 meters water depth in the West of Shetlands area in UK waters in 2000.

By its very nature, the loss of a piece of equipment in deepwater is an unplanned event. The speed with which an effective recovery plan is put in place can have significant financial implications through loss of rig day-rates and future contracted work.

Failure to recover the lost equipment can leave a rig or vessel out of commission for lengthy periods while equipment is replaced. This article shares lessons learned from successful experiences, addresses key planning issues, and cites the unique value of a device known as the JetProp for such heavy object recover. - The JetProp has sometimes spent more time providing visibility for ROV support operations on a recovery project than actually excavating.

Preparation

From experience, immediately after a large piece of equipment has been dropped, there is great pressure to assemble recovery equipment rapidly and to make some operational progress. This may lead to decisions aimed at speeding initial progress, which can prove expensive in the longer term. The following areas will be worth considering carefully at the outset for all deepwater recovery projects:

• Seabed position fix: An accurate seabed position fixing system will be required to complete an initial survey and control all subsequent recovery operations. Although setting up a suitable system may take some time and incur additional expense, it will save time and enhance the chances of successful operations throughout the project. A system will be required that can provide a navigation screen accurately showing the position of any crater, debris, remotely operated vehicle (ROV), excavation equipment, and the dropped equipment itself, when located.
• Debris map: At the outset, a survey should be completed, mapping accurately what debris can be identified on the seabed, and the exact shape, size and position of any craters or disruption to the seabed. The survey will provide initial indications of where to look for the dropped equipment. The accurate mapping of the seabed may also be important if for instance, it is necessary at later stages in the project to be able to go to the exact site of an original crater, when the whole seabed appearance has been changed by excavation operations.
• Multi-beam profiling: If possible, the ROV supporting the recovery operation should be fitted with a deepwater multi-beam profiling sonar, such as the Reson Seabat 8101. Once excavation works are underway, visibility may be poor or non-existent. A multi-beam profiling sonar will enable an ROV to frequently and rapidly confirm progress of excavation operations and to spot any part of the dropped equipment that becomes un-buried. Visual surveys are an alternative, particularly with a JetProp, providing clean water to enhance visibility, but will always be significantly slower.
• Excavation tool: Finally, a tool capable of rapid large-scale seabed excavation in deepwater will be required. The JetProp has proved particularly well suited to the task. After the successful recovery of a 360-ton BOP in 1,600 meters water depth earlier this year, the rig's manager, Russell Millar, confirmed that the JetProp had proven to be an ideal tool for the job.

Excavation

PSL (through its subsidiary Underwater Excavation Ltd) operates a range of tools generally known as JetProp, which can excavate subsea pipelines, perform wellhead de-burial, pipeline and cable lowering, free-span rectification, drill-cuttings removal, as well as equipment recovery. The system is suited to equipment recovery operations in deepwater for four reasons:

• System excavation tools have no depth limitation
• Excavation is "non-contact," providing high levels of safety to sensitive subsea equipment, and to the excavation equipment itself in areas where the seabed contains a large amount of debris.
• The system provides for rapid excavation rates in the disrupted soils
• Most rigs working in deepwater have the needed pump capacity.

The JetProp system is a robust and reliable tool. For deepwater recovery, the system is typically deployed on a drillstring, although for the crane block recovery project cited previously, it was deployed on coiled tubing from a diving support vessel.

The unit creates a large volume, low-pressure column of water moving vertically down to the excavation site at up to 6 meters/sec. The large diameter column of water fluidizes the seabed and carries it away. Excavation rates in excess of 100 cu meters/hour can be achieved. This can be valuable where excavation of more than 3,000 cu meters of seabed material to make a crater 17 meters deep is required, as was the case for the most recent BOP recovery.

Equipment location

A BOP stack can be as much as 14 meters tall, 4 meters square, and weigh more than 300 tons. Therefore, it is not surprising that, when dropped, it can be sometimes difficult to locate, even when connected to the riser. There are a number of reasons for this.

The piece of equipment may be buried a long way beneath the surface and may not have travelled vertically into the seabed. The depth of burial involved for the projects listed previously ranged from 7 meters up to 25 meters.

Despite the enormous impact, there will often be little sign of a crater, or more than one crater. Riser, cable, or other equipment that appears to lead toward the equipment may be doubled over one or more times in the seabed, or may not even remain attached to the equipment. Any crater, or the equipment itself, may be hidden by other debris. All these factors can lead to substantial doubt as to where the item of lost equipment actually is.

If a clear crater can be identified, a system such as the JetProp can be used to excavate down into the crater. If there is no clear evidence of a specific impact point, the JetProp may be used to excavate along the path of other debris that disappears into the seabed and may still be connected to the equipment, such as a riser or control cables.

Removing obstructions

It is often necessary on recovery projects to remove debris obstructing the excavation. Judging when and how to remove obstructing debris can be a difficult process. Time spent early in the project planning for the removal of different types of debris will be important to ensure the correct tooling is available when required and efficient techniques are adopted.

A number of situations are likely to occur:

Debris may have to be removed that poses a threat to ROV operations, or may limit ROV movements, such as sharp severed metal edges, control cables, wires or ropes that could be drawn into a thruster, or sections of riser sticking up out of the seabed.

• Debris may need to be removed that blocks access to excavation sites, such as sections of riser that may have collapsed over a BOP.
• As a crater is excavated, loose debris will often be found sticking out of the seabed, creating a potential threat to the excavation tool or the ROV.
• Debris close to the excavation site may need to be moved to prevent it from falling into the crater.
• Debris recovery may be required simply to clear the seabed or to gather evidence for follow-up investigation.

Cutting operations

All ROV cutting operations tend to be relatively time consuming, so ensuring the best selection of tools are available will be important. A common problem experienced where ROV cutting operations have been required, is that visibility can be lost where the cutting is being carried out in a crater or on the seabed.

The JetProp has sometimes spent more time providing visibility for ROV support operations on a recovery project than actually excavating. To provide visibility, the JetProp is raised 30 meters or more from the seabed and run at very low power to induce a gentle column of clean water into the work area. It isn't always perfect ,but it does provide an ROV with adequate visibility to work, where otherwise, the whole operation might be held up for hours or days waiting for visibility.

Some heavier debris can be removed using slings run in on the drillstring. Given that it may take three hours or more to run in, and the same time again running out, it will be important to closely involve the ROV crew in developing techniques and procedures that offer the best chance of first time success.

Extracting equipment

In most of the recovery projects completed when using the JetProp equipment, locating the dropped piece of equipment has been the easier part of the project. Advancing to the point of pulling the equipment from the seabed tends to pose a range of additional challenges:

1. Sheer scale of excavation: A large item of equipment dropped from some distance above the seabed can achieve impressive depths of self-burial. A device such as the JetProp is effective at removing disrupted soils but this will often leave steep or vertical walls, and in some cases, very deep craters. The walls may be unstable and liable to collapse, threatening to bury any equipment operating down in the crater. On some recovery projects, it has been necessary to deliberately break down the walls of the crater using an excavation device, or a drill bit where tougher soils were involved. The crater then has to be re-excavated, creating a wider crater with safer walls.
2. Equipment-debris separation: Equip-ment selected for recovery must be separated from attached debris, such as flex joints, mud retention valves, and riser. This can be a crucial and difficult stage. If an ROV is involved, it is likely to be operating in the bottom of a crater with very limited room to manuever. Where the BOP has remained close to vertical, it may be possible to run in with a suitable tool on the drillstring to sever the riser. However, the most recent project involved a BOP lying at around 30 degrees from horizontal, 15 meters below the seabed, with a hard boulder layer beneath the BOP. It is important to have prepared for such circumstances, ensuring that appropriate cutting gear such as explosive charges or diamond wire cutting equipment is available for the moment it may be required.
3. Gaining access for lifting: Where it is possible to stab into the top of a vertical BOP, gaining access may be more straightforward. Where this is not the case however, some form of lifting gear must be attached. There may be a requirement for very accurate excavation. Position fixing of the excavation tool and the ability to survey results rapidly will be critical.
4. Attaching lifting gear: In most cases, recovery slings were fitted to the equipment prior to it being dropped. However, gaining access to the slings may be difficult and there can never be certainty that the slings remain undamaged. In a recent case, where a decision was made not to use the recovery slings, an inspection after recovery showed them to have been damaged. If lifting gear has to be attached to some part of a BOP or a joint or valve above it, it will be important to have planned ahead to anticipate the potential problems. There may be numerous issues such as: where to attach a sling, how to attach a sling, what will stop the sling slipping off, what type and strength of sling, how will the ROV place the lifting slings on, and so on. Getting this stage wrong can result in damage to the rig if a sling parts or falls off, and the nightmare scenario of dropping the equipment a second time takes place.
5. Setdown and re-rigging: Another issue to consider is that if the equipment is successfully pulled from the seabed, it may not be possible to put it back down without completely burying it again in the seabed. The implication is that it may be impossible to gain access to re-rig the lifting arrangement before starting to lift the equipment to the surface. An arrangement thought adequate for pulling equipment from the seabed may cause deep concern for lifting the equipment a mile or more above the seabed and through a splash zone. For the recovery of the risers lying on the seabed the most effective means of recovery may require the design and fabrication of a special hook or tool that can be latched onto the riser by the ROV. Again, planning ahead to anticipate requirements and possible problems may deliver substantial time savings.
6. Pulling equipment free of seabed: The suction effect on a large piece of buried equipment may make it very difficult to pull free from the seabed. In one case, the JetProp device had to make a large excavation around three sides of a vertical BOP stack to reduce the suction effect to a level at which the rig could pull the stack from the seab

   Recovering to surface

   The lifting of a piece of equipment from the seabed to the surface presents new issues. Equipment that has been dropped, suffered a substantial impact with the seabed, and been pulled from complete burial, may have suffered damage. Judgements will need to be made about the risk of a failure in the equipment as it is lifted.

   Where lifting slings have been used to pull the equipment from the seabed, they may already have been subject to the stresses of working the equipment free of the seabed over a period of days. They may even have suffered visible damage, but be difficult to replace if the equipment cannot be put down on the seabed without becoming completely buried again.

   Bringing the equipment through the splash zone and out of the water may also introduce new challenges. It will be important to have planned the lifting and securing process in detail to minimize the time any equipment remains in the splash zone. For example, if a severed section of riser remains connected to the top of a BOP, can it be lifted high enough within the moonpool to secure it on the beams? Where there is any doubt, contingency plans need to be put in place to avoid any hold-up at a critical moment.

   In cases of riser recovery, such equipment is likely to have suffered significant deformation making the separation of flanges difficult and possibly requiring each length of the riser to be cut up before it is lifted or before it will pass through the rotary table or access hatches. Again, planning ahead and ensuring equipment is prepared for rapidly breaking up the riser, can save substantial rig time.

   The accidental loss of a critical piece of equipment from a rig or vessel in deepwater creates financial and operational challenges and problems. Recovering the equipment quickly can minimize the damage and rapidly put the rig or vessel back in action.

   Author

   Mike Jordan is a project engineer with PSL (Progenitive Services Ltd.) Group, which is headquartered in Aberdeen, Scotland.