2H has worked closely with Single Buoy Moorings, Inc. (SBM) to deliver a novel transfer pipeline system for Murphy Sabah Oil Company's Kikeh field development
Assume your aim is to transfer production fluids from a spar platform moored in deep water to a floating production, storage and offloading vessel (FPSO ) located 1.6 km away. It seems somewhat perverse to take these fluids from the spar, down a kilometre or more to the ocean floor in a riser, then through a seabed pipeline and finally up to the FPSO via a second extended riser. However, this is the normal way of doing it – or it has been. Murphy Sabah Oil is bucking the trend in its Kikeh field development, situated in 1350 m of water off the coast of Malaysia.
In this case, the transfer of fluids in four fluid-transfer lines (FTL) between the two floating structures will use a gravity-actuated pipeline (GAP) system some 1300 m long and suspended approximately 200 m beneath the surface of the sea. The FTL GAP concept, which was invented and patented by SBM, involves a neutrally buoyant bundle of steel pipes supported and tensioned by tether chains and weights linking it to the two floating facilities.
Clearly, a shorter pipeline than normal is needed, which saves a significant amount of money. And, because the produced oil does not have to travel down to the very cold seabed, the insulation can be reduced without problems connected with hydrate formation and waxing arising. The third major advantage of the concept is that the loads imposed on both the spar and the FPSO are reduced, certainly compared with those involved if steel catenary risers were to be used. The FTL GAP concept is undoubtedly elegant, but it is one that requires highly specialised engineering skills to translate it into a practical and confidence-inspiring design. 2H Offshore has been working closely with SBM for almost two years to do just that.
Soon after the two companies began to look at the design together, it was realised that ensuring the Kikeh FTL GAP had an adequate fatigue life, and then clearly demonstrating this, was going to present a serious challenge. An early engineering analysis indicated that a structure based on the outline design would have a fatigue life of the order of 200 years – not long enough for such a critical application, especially when the estimate did not take account of possible fatigue damage during tow-out and installation. Yann Helle, 2H project manager, says, “From our experience, we believed that to increase the fatigue life of the FTL GAP, we needed to concentrate on the first-order fatigue induced in the structure by wave action on the FPSO.”
As a priority, Helle, his team and SBM set out to find a way of decoupling the motion of the FPSO from the FTL GAP. (The problem is not so acute at the connection between the spar and the FTL GAP because the movements of the spar are not as extreme as those of the FPSO.) The solution adopted by SBM involved installing air cans at the FPSO end of the FTL GAP to provide extra buoyancy. It was also decided to use a heavier chain between the FPSO and the FTL GAP so that the link between the two would take the form of a catenary. An enormous amount of detailed analysis by 2H was necessary to prove the effectiveness of the modification before its approval by all the relevant parties. Significantly, the predicted fatigue life of the new design is around 1300 years.
2H also provided input to the procedures for launching and towing the FTL GAP during installation in order to reduce stresses on the structure and avoid reducing its fatigue life even before it was put into service. Interestingly, the 2H engineering analysis also offered assurance that the bulkheads built into the carrier pipe at regular intervals to resist buckling propagation along the pipe would be up to the task. This was valuable, as it meant SBM was able to avoid expensive practical tests to verify this aspect of the FTL GAP’s design.
Fabrication of the Kikeh FTL GAP started at Bintulu in eastern Malaysia in mid-2006, and the structure was launched and towed out to the field in May 2007. There has been an unusually constructive relationship between SBM and 2H throughout the project. “SBM is a world leader in critical areas of FPSO design and has a tremendous grasp of the needs of the deepwater market,” says Helle. “We can take pride from having used our expertise in riser analysis and engineering to help SBM deliver this groundbreaking project – this is the first pipeline system of its kind in the world.”
Under a separate direct contract with Murphy Sabah Oil, 2H is providing a monitoring system for the FTL system. The FTL GAP will have 18 stand-alone INTEGRIpod motion loggers fitted along its length and two pressure sensors. It is satisfying to realise that as well as verifying that this FTL GAP responds in accordance with the engineering assumptions made at the design stage, the data collected will aid the design of future novel pipeline systems of this kind.