Cost and reliability issues trigger InterMoor innovation.
When the history of the offshore oil industry is finally written, the chances are that not too many pages will be devoted to suction piles. This is perhaps understandable in the light of the many exciting technology developments that the industry has spawned. Nevertheless, their importance should not be underestimated.
Suction piles have come to form one of the mainstays of the industry as vital components of deepwater developments on generally softer ocean floors. They are easy to install, as there is no driving involved; they withstand extreme lateral and axial loads; and they are reusable. That is not to say that the technology cannot be improved, as a recent InterMoor project for Murphy Oil in the Gulf of Mexico demonstrates.
Murphy asked InterMoor to supply and install suction piles to anchor two pipeline end terminals (PLET) as part of the Thunder Hawk field development in 1800 m of water in the Mississippi Canyon Block 734. Given the seabed conditions and knowing that each pile needed to withstand a lateral load of about 250 t, InterMoor engineers determined that 5-m-diameter, 29-m-long piles would be required: medium-sized by general standards, but larger than normal for PLET piles.
The piles, each weighing 159 t, were fabricated at InterMoor's facility in Amelia, USA, and were equipped with "devil's-claw" ratchet mechanisms to tension the chain linking each pile to its PLET : 15 m across the seabed in one case and 52 m in the other.
Although this was reasonably standard, the piles incorporated two significant improvements over previous industry designs. The first relates to the vent valves on the top of the piles. These need to be open when the pile is being lowered through the sea and when it self-penetrates the seabed, and then closed before suction is applied to the pile to pull it completely down.
"Butterfly valves have traditionally been used to vent the piles" explains InterMoor project manager Tom Bauer. "But being based on bronze castings and normally around 40 in. in diameter, they are not cheap. Further, as most are supplied from the Far East, they require extended lead times. Unfortunately, we have also experienced problems with them not sealing perfectly in deeper waters."
With cost, schedule and reliability particularly high on Murphy's agenda, Bauer's team decided the time had come to develop an alternative. With valuable input from Houston-based valve specialist Premium Solutions, InterMoor devised a vent cap system with O-ring seals. Easy to fabricate, relatively inexpensive and simple to operate using a remotely operated vehicle, the valves worked perfectly on the two Thunder Hawk piles.
"Since this job, the design has really taken off," says Bauer. "We have used similar valves on the followers that we use to install our patented suction-embedded plate anchors (SEPLA), and we have also supplied Cameron with 28 of the valves for manifold foundation piles for BP's Plutão, Saturno, Vênus and Marte development, offshore Angola."
The second innovation involved very simple additions to the piles. Pad eyes were placed roughly one-third of the way down the piles’ length so they could be rigged using so-called high hip slings (1.5–3 m above the centre of gravity). This made it possible to launch the piles over the back of the Joshua Chouest anchor-handling vessel without using an A-frame. The use of the high hip slings gave the offshore team complete control of the pile as it passed over the vessel’s roller and during the crucial short period when it rotated from horizontal to vertical. The hip slings take the weight of the pile until it gets underneath the vessel roller, when the normal top slings take over.
“We have previously almost always used a boat fitted with an A-frame to get these piles into the sea safely,” says Bauer. “Rigging them with high hip slings means we can use a wider selection of vessels, which potentially results in cost and schedule advantages for our customers.”
InterMoor intends to use the technique again and to adapt it so that it can also install SEPLAs from less-sophisticated vessels. The high hip slings will be particularly valuable here as, in their absence, it is virtually impossible to retrieve the followers over the vessel roller without an A-frame.
Bauer notes that the whole Thunder Hawk operation turned out flawlessly, aided by some dead calm seas. The launch and installation of the two piles could not have been easier. “We were fortunate with the weather on this occasion,” says Bauer, “But we are confident from what we have learned this time that we will be able to repeat this kind of exercise without an A-frame under more-demanding conditions in the future.”
For Murphy, the PLET suction piles formed a small, but vital, part of a much bigger venture; any problems or delays with the piles would have had a serious knock-on effect on the whole Thunder Hawk development schedule.
“The main reason for choosing InterMoor for this part of the development was to minimise risk,” says Luc Chabot, special adviser, Murphy Oil. “The company is well known for its knowledge and understanding of suction piles. Also important to us was InterMoor’s ability to provide a highquality, end-to-end service covering design, fabrication and installation of the piles. We trusted them to understand our requirements, provide a realistic and cost-effective solution, and then implement it with the minimum of hassle.”
Chabot explains that analysis had shown that repeated heating and cooling of the flowlines was likely to lead to an unusual effect, sometimes called pipeline walking, where the pipe gradually edges its way along the seabed. In view of this, it was important to install the piles correctly and exactly in the right spot, within 0.3 m in the direction perpendicular to the projected pipeline axis, to ensure that the maximum load capacity was achieved and without applying any twisting forces to the PLETs. “That InterMoor succeeded in doing this, even though work was hindered by the close presence of our drilling rig, the departure of which was delayed, is testament to the technical and management ability of the people they had on the project,” says Chabot.
As said at the start, suction pile projects like this are never going to make the main story. Nevertheless, there is much quiet satisfaction at InterMoor about the way the work went and the contribution the company made to this important development for Murphy Oil.