Seabed Intervention Techniques Submarine Pipelines

Submarine pipelines in deepwater are mostly laid directly on the seabed. To place a pipeline securely on the seabed, the seabed should be ideally as flat and regular as possible. However, that is not the case many times due to the presence of various geophysical features on the seabed. If such geophysical features make the seabed irregular or undulating, then pipeline may face the risk of spanning and overstressing beyond the allowable limits leading to catastrophic failure. In shallower waters, pipelines may get unstable due to close exposure to waves, currents and tidal movements. Pipelines may require to be installed in a trench or buried in the seafloor to prevent the pipeline from overstressing due to such instabilities. At shore approach location, pipelines shall be buried to protect the pipeline from fishing or third-party intervention as per code requirements.


Once the submarine pipeline is laid, scouring of the seabed may take place beneath the pipeline resulting in the creation of long spans. In such situations, the pipeline may require the installation of supports at the free span location to prevent overstressing or fatigue failure of the submarine pipeline due to vortex-induced vibrations (VIV) etc. Also sometimes, the low temperature of the seawater may result in a sharp drop in the temperature of the service fluid resulting in congealing of the service fluid inside the submarine pipelines. Pipelines may be partially insulated from direct contact with the seawater by flush burying the pipeline on the seabed.

All the above situations may call for employment of specialized techniques for the seabed preparation for as-planned installation and safe operation of the pipeline. These techniques are collectively referred to as “Seabed Intervention”. The process of carrying out seabed intervention is complex, expensive and adversely affects the overall project schedule by stretching the pipeline installation period. Therefore, the pipeline should ideally be routed avoiding all undesirable seabed features. However, this may not always be practically possible and despite the best efforts of the pipeline designer, the requirement of seabed intervention cannot be completely eliminated. Thus, attempts are made during the finalization of pipeline routing to minimise the quantum of seabed intervention works required.

Seabed Intervention is carried out to primarily serve one or more of the following purpose:

• Geohazard mitigation
• Bottom roughness mitigation
• Free span rectification
• Augmenting the upheaval resistance of the pipeline
• Crossing of existing cables/ pipelines
• Lateral stabilization of pipeline
• Protection of pipeline from environmental loads like waves, currents etc.
• Protection of pipeline from third-party activities

• Dredging: Dredging technique is used to displace the sediment or debris from the seabed. The sediments and debris may be either grabbed and lifted away or sucked into a hose and disposed-off at a pre-determined distance away from the work area. This technique is used mostly for shallow water depths and before laying of the pipeline i.e. where pre-trenching is applicable.
  
  Dredgers can be classified into two groups or types depending upon the method used to transport loosened material from the sea-bed to the water surface:

• Mechanical dredgers (e.g. bucket ladder dredger, back-hoe or front shovel dredger etc.)
  
  
• Hydraulic dredgers (e.g. cutter suction dredger, plain suction dredger, Dustpan dredger etc.)
  
  

• Hydraulic dredging in comparison to mechanical dredging generally results in a wider trench with gently sloping walls as the trench profile cannot be closely controlled during hydraulic dredging. All dredgers except the trailing suction hopper dredgers are stationary dredgers, i.e. they are anchored by wires or (spud) poles during dredging operation.

• (Note: A pipeline laid along an uneven seabed profile is much more susceptible to upheaval buckling than a pipeline laid along a smooth seabed profile. A pipeline route can also be smoothed by “pre-sweeping” dredging. This is sometimes done to reduce spans but is an expensive option. Most dredging operations leave the profile of the dredged base smoother than the original seabed profile and eliminate short-wavelength irregularities.)

• Ploughing: Trench plough was first developed in 1980 for the North Sea to provide a cheaper alternative to trenching of pipelines. This technique is similar to agricultural plough, using a giant version of the agricultural plough, to open up a wide trench, on top of which the pipeline rests. The pipeline is pulled along (usually by the surface vessel), and as the ploughshare passes, the pipeline settles in the trench. If a backfill plough is also employed, this reverses the process by pushing the soil back into the trench, so burying the pipeline. This technique may be used as a pre-lay or post-lay method. Ploughing can be done in greater water depths as compared to dredging. The main advantage of the trenching plough is that it can trench a large range of pipeline sizes (up to 24-inch diameter).

• The trenching rates can be very high, depending on the soil conditions. The shape of the ditch can be precisely controlled, because a mechanical excavation method is employed, allowing the ditch to be narrower and deeper. This is probably the only system that can bury pipelines in one operation, if so required. The main disadvantage of this system is that it has a limitation on the depth that can be excavated. To date, the maximum trench depth is 2 m in a single pass. An additional disadvantage is that the plough system can cause damage to other pipelines existing nearby, especially those lines not protected by a concrete coating.

• Mechanical trenching: Mechanical trenchers are diverless trenching machines with mechanical tools and can be lowered on the seafloor while the controls and power source are onboard a surface vessel, which via an umbilical, powers the subsea machine. It is different from a mechanical dredger, in the sense that mechanical dredgers are not lowered on the sea bottom (unlike mechanical trenchers) and only their cutting tool or the bucket interacts with the seabed. Mechanical trenchers are agile and strong but generally lackadaisical due to the availability of limited power to operate the machine and its cutting tool on the sea bottom. Mechanical trenching may be done both pre-lay and post-lay of the pipeline.

• The machine moves along the seabed on powered tracks using articulated legs whilst lifts the pipeline in its cavity while working in post trenching mode. Mechanical trenchers can be equipped with powerful shovel to cast aside boulders on the seabed. It can work on undulating rugged seafloors with slopes upto 35^o.
  
  These machines can usually handle only small-diameter pipelines and preferably flexible ones. Since they provide their own traction, the machines require reasonably firm soil. They cannot trench in very soft soil or very hard clay or rock, however, they can reach to greater depths as compared to dredging. Most mechanical trenchers are rated up to 1500m, however, Saipem’s Beluga is capable of carrying out trenching operations up to a maximum water depth of 2000 m.

• Jet trenching ROV: Jetting utilizes trenching ROVs equipped with high power water nozzles for fluidizing the seabed soil so that the heavier pipeline sinks in. The jet trenching may be performed by a sledge mounted system that is pulled by a vessel or by a self-propelled submersible tracked vehicle or by a free swimming ROV. The jetting ROV is placed over the pipeline and high-pressure water jets from nozzles blast the surrounding soil away from the pipeline. Jetting disperses the local soil away from the pipeline and creates a wide trench. In most cases, the soil is not backfilled and the pipeline gets buried with time, by naturally backfilling the sediments by the action of waves and currents at the bottom. However, some of the sophisticated jetting ROV are equipped with Eductor or back fill system by which backfilling is also possible.

• This post-lay trenching technique that can be used up to a water depth of 2000 m. Jetting is done in sands and soft clays, but in hard clay or rock, this technique is not effective.

• Jet Prop : It’s a special type of high power excavation equipment which is utilised where jet trenching by ROV cannot be performed due to high water depth or seabed morphology (such as hardened clay). The jet prop is basically a jet engine which draws in large volumes of the surrounding seawater, driving it at a high force towards the seabed clearing debris and digging the trench. The 4 tonnes jet prop is suspended above the location to be excavated from a support vessel. The cutting tool uses 24 water jets set at different angles (for hard clay) or two numbers of high volume water cannon (for soft clay) to blast a trench through the seabed. The jet prop has no water depth limitation in terms of application as it can be lowered to any water depth depending upon the capacity of manoeuvrability by the support vessel and monitoring by the ROV. Also, the trenching rates achieved by jet prop is very high when compared to jet trenching ROVs. The application of jet prop is mostly governed by the seabed morphology as jet prop is not successful for soft or sandy seabed as it leads to collapse of the trench walls due to enormous energy discharge by the jets.

• Rock dumping: Rocking dumping is a type of seabed intervention for offshore pipelines which is used for several purposes such as protection of pipelines from third-party activities, for providing upheaval resistance to the pipeline, for limiting the free span length, sectioning the pipeline to prevent transfer of loads from one section to another, for pipeline crossings, for stabilisation and support. It can be used pre- and post-lay of new pipelines and also for pipelines which are in operation. It is relatively fast and cheap method if transport distance of rocks is not too far away, such as in the North Sea. However, this method of seabed intervention is disadvantageous for fisheries as rock berms may interfere with the trawling gears. For deepwater applications upto a water depth of 900 m, Flexible Fallpipe Vessels (FFPVs) are used for rock dumping. An FFPV is a self-propelled vessel that is equipped with a flexible fallpipe.

• The vessel's design allows the fallpipe to be lowered into the water beneath the vessel. Fall pipe end positions are controlled by thrusters of the vessels. Monitoring and positioning devices are deployed at end of fall pipe for accurately placing the rocks on the seabed.
  
  Until now, deepwater rock installation is the Ormen Lange southern field extension project in Norway holds the record of deepest rock dumping operations in the maximum depths of 875 m using 2.8 million metric tons (3.08 million tons) of rock. There are strong apprehensions on the utilization of this method of seabed intervention for water depths exceeding 1000 m. In deeper water (i.e. >1000m) rock dumping may result in excavation rather than deposition of rocks.