Jackups are an essential piece of equipment used by oil and gas majors in shallow waters around the world. Here, David Forsyth – chief surveyor at the American Bureau of Shipping (ABS) – talks to Rigzone about the safety issues involved with jackups.
Rigzone: What are the key safety measures needed when moving a jackup on and off location?
Forsyth: The first thing the drilling contractor needs to know is the bottom conditions in the area where he is going to move the rig. Obviously, the foundations are very important to the jackup. So, they do a bottom survey. They usually hire someone to do that – an offshore surveying firm.
They need to know the soil conditions, although the soil conditions can be fairly well known in a well-established oilfield like the Gulf of Mexico or in the Middle East. But they need to know what kind of bottom conditions they have – whether it’s a soft, muddy bottom or a coral bottom or sand – just so they know how to properly load the rig on.
The next most-important thing is to get a good weather window for the installation period because when you have heavy seas you really can’t put the footings down on the bottom and jackup on the legs without causing a lot of problems.
Rigzone: What safety issues can occur moving onto location?
Forsyth: When you get down close to the bottom and you are in heavy seas, you tend to get slamming on the end of the legs and on the spud cans against the bottom because the hull is bouncing up and down in the ocean. You get a lot of dynamic forces there.
In one incident off the northeast coast of South America, there was a particular area that had a really heavy swell for about six months of the year, and a jackup managed to spud, but it had to wait six months to leave…
If you’re going up against a platform where you’re going to do some work [for a well workover, for example], you need to know the location of the previous rig that was there to make sure you know where the spud can holes are … If you jack up next to a spud hole, you can slide off into the old hole and have essentially the same thing as a punch through.
(A “punch through” is when one leg of a jackup breaks through the crust of the ocean floor, which can cause a large bending moment in the other legs.)
If you are going against a fixed platform you can also have issues with manoeuvring the rig close to this large, fixed platform. If you lose control of the rig, you can have contact damage.
Rigzone: What safety issues occur when transporting a rig across the sea?
Forsyth: One of the safety issues if you are going to do what we call a ‘wet tow’, which is basically just pulling the legs up and floating the hull in the water and having tug boats pull it to the next location, is that you have your general at-sea types of problems. You have to make sure you have good water-tight integrity and, you have a good weather window because stability on a jackup can be very good or not so good depending on the water depth. Sometimes with heavy weather you can jack your legs down to get a better center of gravity. But if you’re in shallow water you may not have that advantage; so wet tows over thousands of miles are not how it’s usually done.
We generally do what we call a ‘dry tow.’ We put that jackup on the back of a heavy-lift vessel … That can have some issues itself. I’ve seen leg damage especially when you are going around the Cape in southern Africa. You get into heavy weather, heavy rolling seas, so we’ve seen some damage on the legs because you’ve literally got 400 to 500 feet of leg sticking out of the jackup and when you’re in heavy rolling seas that can cause a huge moment at the top of the jackup structure where the leg meet the decks.
That’s essentially the reason ABS has a requirement that after you do a tow you do a damage survey before you put the rig on location, so you make sure that you haven’t damaged the jackup during the tow.
Rigzone: What does the industry need to do to improve jackup safety?
Forsyth: One particular thing [the industry] needs to pay particular attention to is the ageing jackup fleet. Ageing rigs do require a lot more attention. I think that a little closer analysis when coming on and off location is needed.
The equipment ages, the structure ages, and the unit just requires a lot more maintenance … There are fatigue issues, there are issues with wastage and diminution of the structure because of the saltwater atmosphere that the structure works in. But that can easily be fixed with normal maintenance.
ABS has 192 jackups that we’ve classed that are over 31 years old, and some take a little longer than others to get through our Special Survey.” [ABS subjects jackups that it classes to a comprehensive Special Survey every five years]
“Another thing that can happen to a rig when it comes on or off location is RPD or rack phase differential. This is an issue that the industry has been working on for a long time and, actually, over time has developed instruments to measure that. Rack phase differential is when you have a leg that is a forward structure – a vertical beam actually – and you have three or four chords, and if your spud can or your bottom foundation is unevenly loaded, the chords get out of phase, and the racks that your pinions ride on become out of phase. When that happens, a lot of loading is introduced into the structure. So, RPD is something that you have to pay a lot of attention to when putting a rig on location.”
Rigzone: How is ABS helping with jackup safety?
Forsyth: We do annual surveys and, then, twice in every 5-year period we do what we call a ‘Dry Dock Survey’, although most of the time this is actually done by underwater inspection in lieu of dry dock. We examine all the bottom, all the underwater parts and we do non-destruction testing on all the high-fatigue areas such as the legs. This approach was a real change we made after our Ageing Rig Study [conducted in 2003]. We identified that after a particular age, there were several points on several different designs that we needed to take a closer look at.
“Our Technology Group, which is basically the research and development side of ABS are working on a study into the dynamic forces that a jackup experiences when it is going on location … We’re working with Keppel FELS’ engineering department on this study, and we’ll share the results with everybody, including IADC [the International Association of Drilling Contractors] Committee if they are interested.”
“There’s going to be another workshop in March. I think the study will come out sometime before the end of this year.”
For more on how jackups work, take a look at Rigzone’s How It Works guide to jackups here.