Topsides installation options for semi-submersible FPS
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The concept selection of joining the topsides to the hull of a floating production system has far reaching consequences on the shipyard work scope and the basic engineering and therefore warrants due attention in the earliest stages of field development concept selection. The concepts, benefits and challenges of float overs, heavy lifts, skidding and 20,000 ton crane TAISUN are discussed - slides of a presentation at the Deepwater Asia Congress in Shanghai, May 22 2009
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Topsides installation options for semi-submersible FPS
- 1. Topsides installation options for semi-submersible floating production systems Niels Haakman General Manager, TAISUN division Yantai Raffles Offshore Ltd
- 2. Among Floating Production Units the (deep draft) semisub is increasingly popular Source: Offshore Engineer – May 2009
- 3. For these, you have to make the important decision of topsides installation Source: Offshore Engineer – May 2009
- 4. You have to address this early in your project
- 5. You have to address this early in your project
- 6. taisun float over skidding heavy lift
- 7. Float over Hull deeply submerged Topsides loaded on barge or self floating Mating by (de)ballasting
- 8. Float over Proven since the 1980s No modifications required on topsides Topsides can be fully completed
- 9. Float over Damage stability requires many compartments Restricted access, especially offshore Complex, risky operation
- 10. Hull kept at design draft Topsides lifted from barge, usually in pieces Limited to about 12,000 tons Heavy lift
- 11. Less risky than float over Very capable external partners No modifications to the hull Heavy lift required
- 12. Installation window narrow and expensive Offshore hook up is expensive, cumbersome Heavy lift
- 13. Installation window narrow and expensive Offshore hook up is expensive, cumbersome Massive engineering Heavy lift effort required
- 14. Topsides built or jacked to height skidding
- 15. Topsides built or jacked to height Hull skidded under topsides skidding
- 16. Topsides built or jacked to height Hull skidded under topsides Topsides skidded on top of hull skidding
- 17. Topsides can be fully completed Integration can take place at the yard Full access for all staff skidding
- 18. Requires special structure Jacking very high, especially for deep draft semi’s Massive engineering effort required skidding
- 19. Lift plates TAISUN installed on deck
- 20. Lift plates TAISUN installed on deck
- 21. Lift plates TAISUN installed on deck Topsides lifted from barge in one module
- 22. Lift plates TAISUN installed on deck Topsides lifted from barge in one module Hull towed into position
- 23. Full access for all TAISUN staff
- 24. Full access for all TAISUN staff Lifting is not a design criterion
- 25. Full access for all TAISUN staff Lifting is not a design criterion Full integration can take place at yard
- 26. Lift plates require TAISUN free space on deck Maximum lifting height of 80 meters Draft limited to dock depth
- 27. Mating cost Completed topsides Yard integration Engineering impact
- 28. Are you planning your new floating production system? niels@yantai-raffles.net or visit www.20000ton.com
Notas do Editor
- Thank you for your introduction Chairman. I’m very excited to be here, because I love to have this opportunity to share with such a wonderful crowd a subject that I’m very passionate about: offshore construction. Today I will share with you the topsides installation options for semi submersible floating production systems. Why do I think you want to know more about that? First of all, the semi submersible, and especially the deep draft version is emerging as the platform of choice <click>
- For deep water oil and gas production, several Floating Production Systems have been developed by the market such as *FPSOs,*Spars, *TLPs and *semisubmersibles. Of these three, the semisubmersible is emerging as the platform of choice because The mooring, offshore hook up and final integration is simple, cheap and fast.Benign motions of newly developed deep draft semisubmersibles make the platform suitable for SCRs and dry trees. With dry trees, the Xmas tree are no longer situated on the ocean floor but instead on the platform itself making maintenance and troubleshooting much more efficient and cheaper.Contrary to the SPAR, it can be completed and commissioned at the yard.
- But before you can start the commissioning you need to join, or “mate” the topsides to the lower hull. Since the skills required to build the process facilities of the topsides is very different from the skills needed to build the structural steel of the hull, this work is usually, but not always done at two separate contractors or fabricators. This means that you have to decide how to distribute the work and where integrating will take place.
- Because the integration influences the work scope of your fabrication yards, you have to decide the method of installation before bid packages are sent out in order to prevent cost and time consuming changes in the yard’s work scope later on.
- As we shall see later in the presentation, many of the installation options require massive re-engineering of the structure of either the topsides or the lower hull and in order to safe guard your project deadline you must address these issues during the basic design stage
- Let’s look at the four options. This is just an introduction, we will look at each of these in greater detail later. We’ll look at the options in the order they were first deployed.First to appear were the float over’s. Here the hull is deeply submerged to allow a barge or self floating deck to sail over the pontoonsHeavy lift. Specialized crane vessel of several thousand or even 14,000 ton capacity are used to install the deckThese two are the “main stream” options and in most projects, these are the only two alternatives considered. However, as we shall see later, these have the highest costs and most disadvantagesC. Skidding, or jacking of the topsides. Here the topsides is jacked up and skidded over the lower hull OR the lower hull skidded under the topsides.D. The latest development is using TAISUN. TAISUN is the name of the largest and strongest crane in the world at a Guinness World Record of 20,000 tons. In this method, the topsides are lifted of a barge, and the hull is floated underneath.
- Let’s look a bit deeper at the basic concept first. As we can see in the picture the hull is deeply submerged to get the required clearance between the topsides and the hull <click>Usually the topsides are loaded on a barge or vessel, but it is also possible for the topsides to be self floating, although less common as it generally easier to use barge of vessel. The reason for this is two-fold, first of all a self floating topsides needs to be water tight and secondly there needs the joining will take place under water. <click>The mating is then accomplished by either de-ballasting the hull, or ballasting down the barge. As this is a rather slow process, some projects have referred to using explosives to quickly de-couple the barge and the topsides. <click>So we have now seen the basics, and what would be benefits for you in adopting this solution <click>
- As we saw during the introduction of the 4 concepts, the float over method is a very popular method, its usage goes back to the 1980s and they been performed close to 30 times. So a solid knowledge base of this operation has been established <click>Because the topsides are being carried on a barge, there is no need for reinforcements or other modifications to the topsides. While it is true that the topsides cantilevers out over the sides of the vessel, this is also the case when the deck is supported by the four columns. What this means is that your basic design can progress without too much concern for the mating solution which reduces the risk of schedule and cost overruns <click>Lastly and contrary to heavy lift that we will discuss later, we topsides can be fully completed. We see in the picture that all the exhaust stacks, flare booms and deck cranes, among others, have been installed prior to load out. This lowers the offshore hook up and therefore saves cost. This is a huge advantage over heavy lift, because it is in the hook up and commissioning phase where cost-overruns usually appear. If all of this work has to take place offshore, the cost escalates. <click>Looking at this list, it is not surprising that float over's are on the rise again as the method of choice for topsides installation, but what would be the catch?
- 1We saw that one of the features of the float over was having the hull deeply submerged. As a result, the free board is extremely small and the roll has to be kept at an absolute minimum to prevent flooding of the columns. Damage stability rules however require that the vessel has to withstand flooding of ANY TWO compartments. What this means is that the total number of compartments need to be drastically improved, all for this one-time occurrence. The implications include additional steel work to create the compartments, additional piping to pump ballast water in and out but also more difficult maintenance throughout the life cycle of the project, all because this one-time operation <click>2 Depending on the size of the platform, some 150 staff are involved with welding, quality control and safety. When the mating operation is taking place away from the quayside, or worse, in open waters you have no choice but to refer to crew boats, helicopters, full hotel functions or a combination of the above. What this means is a complete escalation of you labor cost and schedule <click>3 Because of the close proximities of the platforms, the many vessels and operators involved and the different motion characteristics of the modules, the entire operation is very risky. Calm water is a necessity, but in most areas of the world, sheletered areas, near shore, are not usually deep enough for the hull to be ballasted down <click>.Concluding, the float over is a practical method but risky, complicated and engineering wise challenging. This is arguably why in the early 1980s, when the new generation of crane vessels arrived, the float over method all but dissappeared.
- With the introduction to the market of the vessels that are today known as Heerema’sThialf and the Saipem 7000, heavy lift became the new method-of-choice, putting an end to the use of float over’s until the mid 1990s.When using a heavy lift crane, the hull can be kept at more-or-less the design draft and it the crane that ensure the clearance between the topsides and the hull <click>The topsides is brought in by a barge, from which it is lifted by this crane. For larger units, this is usually done in more than one module, because of the size and weight limitations of a heavy lift crane <crane>That’s why the maximum weight of a module is about 12,000 tons <click> However is the module is rather wide or high, you will not be able to get to these weights. For semis or TLPs however with a weight in the range of 6000 to 8000 tons, heavy lift would be an option. So why would you consider a heavy lift? <click>
- Like with the float over, the method is well tried and tested and risk are generally considered lower than with float over <click> This does not mean that accidents don’t happen. They do, loads are dropped incidentally <click>The companies that operate this vessels have excellent staff and experience and are very capable in handling all the associated engineering for the heavy lift. So as long as you involve them early enough, they can take care of all engineering related issues <click>Directly contrary to a float over, all engineering effort is concentrated in the topsides rather than the hull. This means fabrication of the hull will be less complicated and can be done with less management focus. Also in case you wish to use an existing hull, heavy lift would be preferential over a float over <click>Unfortunately, the downsides are considerable <click>
- These vessels are in great demand, and short supply. A number of vessels in currently under construction in the range up to 5000 tons, but above that range the choice is limited. As a result the day rates are high, the schedules are tight and inflexible. <click> If you were to incur a delay in your project you are most likely to heavy to carry over some onshore work offshore. As mentioned before, the hook up and commissioning phases is usually where the cost and schedule overruns occurs and remedial action is greatly hampered by the offshore location as can be attested by the recent problems that Shell encountered with the Perdido spar. Some of the lifeboats failed to pass the test which led to the Coast Guard not considering the Spar safe for permanent staffing and the crane vessel needing to be on stand by for hotel. Would such an incident have happened at a yard, the resulting cost would be limited to replacing the life boats. <click>
- Lastly, the engineering effort required. The impact on the structure of the loads imparted on it during the lift operation CAN NOT be overstated. In the picture we see one of the two hooks, which carries half of the total load. On the other hand, the steel and equipment weight is more or less continuously distributed throughout the structure. All this load is then to the channeled to 4 points at the outer edges of the platform where the pad eyes are located. In order for the shackle to connect, these extremely highly loaded pad eyes have holes in them! In order to facilitate the channeling of the forces, 100s if not 1000s of tons of steel have to be added to the structure, all lowering the pay load of your semi.Taking these things into account, it is hardly surprising that float overs are on the rebound and of the floating production systems, the SPAR form seems to be the main market for heavy lifts as their draft makes it impossible to perform the mating near shore.So far, we have seen the two most popular methods, float overs and heavy lifts that both have considerable drawbacks. It was then no small feat for Jurong to develop the next method, skidding <click>
- The skidding has been used by Jurong a number of times, but also some Korean yards have used this concept in the construction of drilling semi submersibles and there is no compelling reason why this could not be applied to floating production units.At Jurong, the deck box is build at a height equal to the top of the columns, on top of a massive skidding frame but it is also possible to build the topsides on the ground and later jack the topsides to the required height, before placing it on the skid frame. <click>
- The topsides in the picture is Shell’s Malampaya deck, but again there is no compelling reason why this would not work for a floating production system.In Korea, a semi submersible has been mated by jacking up the topsides before skidding the pontoons underneath from the sides. <click>
- The other option, shown again above, is to have the topsides skidded over the hull. The downside here, as we can see is the massive skidding frame, needed over a great length.However, as mentioned before, this was a great achievement with many benefits<clicK
- As discussed, being able to complete the topsides before the mating and in doing so lowering the offshore hook up and commissioning work and containing the associated risks is a huge benefit and one that is possible when using the skidding technique. <click>As a matter of fact, all integration, even of systems that run between the topsides and the hull such as cooling water systems can be integrated at the yard, reducing the offshore work to the absolute minimum. <click>Finally, all the staff required for both the mating and the integration work has easy access to the worksite and no hotel facilities need to be provided. <click>All in a method that has tremendous benefits, but also one with considerable drawbacks <click>
- As you can clearly see from the pictures, massive structures are required to support the modules and these can have a weight equal to 10% of the structure they support.<click>For deep draft semi’s, hull shapes some 60 or 70 meter deep are being developed. It will be very, very challenging to jack deck boxes up to such height<click>Lastly, and again for when Jacking is employed, the concentration of the force, as with the heavy lift solution will be great, requiring substantial reinforcements in the deck and when to going to great height, buckling in the jacking columns will also become a concern.In short, the skidding method is a great breakthrough, but might be hampered by height limitations in applications for, especially, deep draft semi submersibles. <click>
- Exactly those reasons led to the development of 20,000 ton crane TAISUN, which also adds the benefits of greater freedom and flexibility in choosing the building location because modules can be barged in from virtually anywhere.To use this method, specially designed lift plates are welded to the deck of the main structure, above structural members already present in the topsides. The crane connects to this plates by a series of sliding hooks, almost 400 in total. You can have a look at this animation to understand more clearly.<click>
- After the crane is connected, the crane lifts the topsides from the barge in one entire module, up to 20,000 tons.<click>The barge is then moved out and the hull is floated underneath <click>
- After the hull is in position, the topsides are lowered on the hull and the mating is established.So what are the benefits for you when you decide on this method? <click>
- First, unlike the float over and the heavy lift option, all staff has full access to the platform. After the topsides are lowered onto the hull, two bridges are lowered from either side of the dock establishing the connection with the land. <click>
- Here is the big one. The lifting of the structure has proven, demonstrated, NOT to be a design criterion. Of course we spend considerable engineering effort to prove and establish this, but no work has to be redone, revised or adjusted. You might be skeptical about this claim, but the proof is in this picture. What you see here is our launching barge, that was never designed to be lifted and we ballasted it down with 10,000 tons of ballast water, on top of its self weight of 10,000 tons and lifted it 30 meters high in the air. This is possible because we weld those lifts plates right above the existing structure, which leads the load throughout the structure. Semi-submersibles, which are designed to forces and loadings much greater than a launching barge have proven on 4 different designs not to require any modification to the structure. <click>
- Again, full integration can take place at the yard.<click>