Establishing a safe foundation is a vital part of jack-up operations. The jack-up vessel needs to be supported by an absolutely solid foundation during the entire jack-up operation at any given site.
As the vessel is jacked up, the weight is carried by the legs and onto the seabed. The seabed must be able to provide the necessary support. If it doesn’t, there is potential for disaster.
There is always some risk, even if extremely slight, associated with this and it’s important to take all the necessary steps to mitigate these risks.
Two phenomena to avoid
Punch-through is a phenomenon that occurs when a leg of a jack-up vessel breaks through the seabed. Fortunately, it is a rare occurrence in offshore wind, but there have been catastrophic occurrences within oil and gas. It’s important to do everything possible to mitigate the risk. If the risk cannot be mitigated, then jacking up should not be attempted.
Jack-ups can also be subjected to sliding, particularly if the seabed is very hard and cannot be penetrated sufficiently by the spudcans. Strong lateral forces caused by wind and sea currents can also contribute to sliding.
Avoiding these situations involves considering seabed conditions at the jack-up site and the specific characteristics of the jack-up vessel. A vessel’s characteristics might make it particularly suitable for a certain location but less suitable for another.
An important characteristic is the spudcan design, which contributes to the pressure exerted on the seabed by the legs. Some designs are more suited to specific seabed conditions than others. Detachable spudcans are very useful, as it means that different spudcans can be fitted to the vessel according to the specific characteristics of seabed.
Jetting systems are a good solution to secure a solid foundation for the vessel legs. Jetting is a procedure that fluidizes the top soil layers, enabling the jack-up legs to be placed firmly on the hard layers beneath.
Jetting systems can either be an integrated part of the spudcan, but if that is not available, the effect can be achieved using a special hose and nozzle system.
Leg penetration analysis
To determine whether it can support the vessel, the seabed needs to be investigated. Leg penetration analysis (LPA) is an essential part of the preparation process. The LPA is based on penetration tests and core sampling of the seabed. This identifies the physical characteristics of the seabed, including determining the strata layers. The LPA includes calculating the expected ability of the seabed to withstand pressure. The suitability of a jack-up can then be assessed based on these results.
However, even though the vast majority of testing produces the correct results, it is always necessary to be prepared for unforeseen events and to monitor the situation carefully when jack-up starts and during operations.
It is also advisable to look at data provided by previous jack-up operations, so you can learn from previous experience.
The size of the jack-up is not necessarily a factor. Some larger jack-ups may be able to withstand extreme weather conditions than smaller vessels, but this is not always the case. There is no direct correlation between the size of the jack-up and the depth of leg penetration or the ability to achieve a stable foundation in the seabed soil.
At a basic level, risk is mitigated by a method called predriving. Predriving involves exerting pressure on the seabed soil over a set period of time, either with or without spudcans. This is achieved by applying maximum load on the legs, one diagonal at a time. Diagonal leg pairs are repeatedly loaded until the seabed no longer yields and no further penetration of the legs is observed. The point is to prove that the seabed can withstand the pressure, and normally you exaggerate the conditions that will be expected so that the pressure is higher than the actual pressure applied during jack-up operations.
If it is determined that the seabed is not suitable for supporting a jack-up vessel, there are a number of ways to prepare the seabed, including ‘rock dumping’ and ‘swiss cheesing’. However, these are rather costly options and perhaps undesirable from an environmental perspective.
Leaving the site
Leaving the site may actually be a greater challenge than jacking up at the site. Penetration can reach 15 metres in some areas. In this case, the sheer friction and suction effect can make it very difficult to extract the legs from the seabed.
Jetting is also useful in these cases. The jets shoot water, and on some jack-ups also air, into the soil around the legs, which equalises vacuum and suction effect and reduces the surface friction on the legs, so the legs can be retracted.
If dredging and excavating, then you need often need consent from the local authorities. You are expected to leave the seabed as far as possible in its original form.
The right equipment for the job
The most sure way to achieve effective preparations is to use equipment specifically suited to the jack-up operation and the site itself. Mostly, this means using a jack-up vessel that does not require extensive seabed penetration. On some specialised jack-up vessels, the spudcans can be reconfigured to suit the site. If necessary, they can be removed if it’s not necessary to use them or they or not suitable for use at the particular site.
Other workaround solutions are sometimes necessary, but it is best if the equipment can be used with minimum reconfiguration or extra equipment.
By Mads Albér, Master Mariner, DBB Jack-Up Services
Source: offshorewindom; Image: DBB Jack-Up Services