As development gets deeper, so does pipelay operations. OE asked some of the leaders in the field what they see as the most challenging aspects of deepwater pipelay.
EMAS AMC’s newbuild Lewek Constellation, which is having its multi-lay tower installed at Huisman’s yard, Rotterdam, in November. Image from EMAS.
Oil and gas fields are moving further offshore in remote locations such as the Gulf of Mexico and Brazil deepwater field developments, i.e. in water depths in excess of 2000m.
These new field developments have long transit times from established port facilities and logistical support centers, hence, rendering certain traditional construction vessels to perform the work over a long duration offshore.
Therefore, high-specification and self-sufficient construction vessels are more favorable, such as the Lewek Constellation (reel lay and heavy lift) and the Lewek Centurion (S-lay), performing multiple scopes of work with limited or no support from other construction vessels.
Pipeline design engineering is challenging in deepwater, as pipeline systems will often have issues in terms of wall thickness, thermal insulation requirements, high temperature and pressure requirements, and seabed bathymetry conditions, in terms of pipelines for allowable freespans, etc. Therefore, optimization of pipeline systems is essential to allow commercialization of future field developments.
However, these pipeline systems, such as pipe-in-pipe, have to be installable with pipelay vessels that can handle top tensions in excess of 700-ton. Also suppliers have to supply high-specification pipeline components that can handle high internal and external pressures and temperatures, etc. These requirements, at times, require a good control of quality in terms of the allowable design range of pipeline components for deepwater and durability.
High temperature and pressure pipeline can also be challenging in terms of allowable controlled lateral expansion of the pipeline, including pipeline walking. This can be resolved by laying the pipeline in “snake lay,” to reduce the risk of lateral buckling and the size of the expansion jumpers. This can be established with over-bending and under-bending the pipelines during pipelay through the reel-lay vessel’s straightener, i.e. inducing pipeline snake lay configuration on the seabed for controlled lateral expansion.
Risers can also be challenging to install, as they require high-specification pull-in equipment, such as large pull-in winches or chain jacks, to overcome the loads for a second end pipeline pull-in. This also requires platforms to handle high hang-off loads, which can be governing for these types of riser, i.e. flexibles, steel catenary risers or steel lazy wave risers.
Pipeline end terminations (PLETs) or Inline Ts (ILTs) can also be challenging to install, heavy duty valve assembly, corresponding connectors, anchor flanges, etc., will be required in deepwater. This normally results in heavy duty PLETs or ILTs with large mudmats that have to be installed from a pipelay vessel. The Lewek Constellation is able to install large PLETs weighing in excess of 100-ton and Centurion has versatile A-frame that can be used to install large PLETs.
Handling pipelines under high tension requires a considerable amount of planning and engineering. Therefore, installation aids, pipelay equipment, such as tensioners and corresponding tensioner pads, are configured to the appropriate pipeline being installed. Reels and hang off clamp system inserts have to be engineered to handle the nominal and dynamic loadings that will be encountered during the offshore pipelay campaign.
DNV GL - Oil & Gas
The main challenge related to pipelines in deep and ultra-deep waters is the high external pressure that may cause the pipeline to collapse. This potential failure mode is normally dealt with by increasing the pipe wall thickness. At large water depths, this will require very costly thick-walled pipes, which in turn are difficult to manufacture and also to install due to the weight.
When a pipeline is installed, it is exposed to high bending in the so-called sag bend close to the seabed. In deep water, high external overpressure may, especially in combination with this bending, increase the risk of pipe cross-section collapse. In addition to increasing the wall thickness the collapse resistance may also be increased by improving the line pipe characteristics through manufacturing procedures with improved material characteristics, resulting in a more perfectly round pipe as well as improved collapse resistance.
However, when solving the collapse challenge by increasing the wall thickness, two new challenges occur; the manufacturing challenge caused by the heavy pipe wall and the challenge relating to the holding (tensioner) capacity during installation. Due to the heavy, thick-walled pipe and long section of the pipe from the vessel to the seabed, the required tension or holding capacity restrict the number of available installation vessels.
Both these additional challenges limit the maximum pipe diameter at different water depths. Below 1000m, the wall thickness increases almost linearly with the water depth and pipe diameter. At the same time, the ability to manufacture line pipes decreases according to the pipe diameter and the tensioner capacity requirements increase. This means there are some practical limits for the maximum pipe diameter to be installed at different water depths.
In addition to the wall thickness and weight, deep water can also provide challenges with respect to seabed intervention, geo-hazards, buckle detection, pre-commissioning, surveys and inspections, hydrates as well as any repairs needed.
Despite a fluctuating oil price, the experts predict growth in the subsea market and continued opportunities for expansion into new frontiers. The challenge for any contractor such as McDermott is to be in a position to win a share of the contracts. That means having a combination of the right people, relevant assets, innovative technology and ability to work in remote locations.
As projects become more complex, contractors will need to be able to handle deepwater umbilicals, install heavy structures and handle ever increasing top tensions making future assets such as the DLV2000 essential to the industry. Without vessels such as the NO102, a truly deepwater enabler for flexibles and umbilicals, and the DB50 with its specialized deepwater lowering system, McDermott could not have completed projects such as the recent Jack and St Malo in the Gulf of Mexico.
Allseas’ pipelay vessel, the Solitaire.
Each project has its own distinctive challenges which often require unique solutions and innovative technology. One such innovation is the creation of the world’s largest forged specialty valve for the Ichthys project offshore western Australia. Six 42in valves have been designed and created for the project; each one measures approximately nine meters high and weighs more than 100-tonne.
Assets and technology alone do not make a successful project, particularly in emerging countries where new opportunities are opening up. A successful approach requires a contractor to be experienced in investing and sensitively developing the necessary onshore infrastructure, and building local participation by employing and growing local talent. By taking this approach, projects can have a real and positive transformational change.
Challenging aspects are the high wall thicknesses of pipelines and risers, which are driven by external deepwater collapse pressures.
Pipeline end termination structures and in-line structures in deepwater are complex with their valves, connector assemblies and other equipment, operated by remotely operated vehicles.
Deepwater risers are subject to environmental loadings which affect their fatigue lives, leading to strict welding criteria.