“Permanent mooring systems for floating production units are becoming ever more substantial, and using traditional anchor-handling vessels to install them is proving increasingly challenging. Flexibility and innovation are needed to extend the capabilities of the existing fleet of anchor handlers if we are to continue to deliver projects on time and within tight budget constraints.”
This was Todd Veselis, general manager of permanent moorings at InterMoor, speaking in 2014 just after the company had safely installed the mooring system for LLOG Exploration’s Delta House semisubmersible production facility in 1350 m of water, 210 km south-east of New Orleans in the Gulf of Mexico. But let us go back to the beginning, about a year earlier.
The mooring system for the Delta House semisubmersible was already largely designed and its key components were on order when LLOG Exploration appointed InterMoor to carry out its installation. On the face of it, this was a reasonably straightforward project for InterMoor, except that on this occasion there was a rider – in reality, an opportunity. For good economic reasons, LLOG Exploration wanted to explore the use of an anchor-handling vessel, the Joshua Chouest, which it already had on long-term charter. Although basically suitable for the task, the vessel would have to operate at the upper limit of its handling capabilities, so extremely thorough planning and preparations would be necessary.
The Delta House semisubmersible is designed to produce continuously for 25 years and to survive a 100-year storm. It has 12 taut-leg mooring lines, three at each corner. The lines comprise mainly 365 m of the semisubmersible’s own chain at the top, then 2000 m (in two sections) of 240-mm-diameter polyester rope and finally a 230-m section of anchor chain at the bottom. In addition to these main elements, each line has short polyester inserts and mid-water chains, which increased the total number of connections that had to be made on deck. Each line is connected to a seabed suction pile nearly 5 m in diameter, 26 m long and weighing about 150 t. This connection is the only one for each line that has to be made subsea; an SMC Type III automatic connector joins the lower anchor chain to a 40-m forerunner chain on the pile.
InterMoor was responsible for supplying the suction piles (which it produced at its Morgan City fabrication facility in Louisiana, USA), developing the installation procedures, the installation engineering and modifications to the vessel, and the work offshore, i.e., presetting the moorings and then hooking them up to the semisubmersible. The semisubmersible’s own chains were included in the preset mooring lines to reduce the weight of the structure during its transfer to the field.
Veselis is keen to emphasise three key features of the work: the deck tooling and vessel modifications for ensuring safe operations and avoiding damage to the various mooring components; the measures for deploying the moorings without twisting or unduly loading the polyester rope sections; and the overall creativity necessary to devise procedures to make full and efficient use of the Joshua Chouest’s available deck space and equipment.
Chain diameters of 137 mm mean that each link is about 0.9 m long and 0.45 m across, and weighs almost 220 kg. The pins used in the connecting shackles are 230 mm in diameter and nearly 1 m long, and also weigh nearly 325 kg. Assembling moorings of this kind on deck required unusual care and special handling and lifting equipment. This included an A-frame with a pair of 1.8-t chain hoists, which were of great benefit to the deck crew.
Given the size of the links, the chain chutes on the Joshua Chouest needed modifying to accommodate the chain safely. Naturally, the vessel’s shark jaws were also designed to accommodate the large links; however, the need to deploy smaller work chains at various stages in the process, for example, a messenger chain from the semisubmersible during the hook-up, called for an insert for the shark jaws so that their spacing could be altered easily and quickly.
The weight of the mooring chains and the subsequent possibility of twisting or unduly loading the rope sections during deployment and damaging their protective covers led to InterMoor using a custom-designed lowering line. This was made from high-modulus polyethylene fibre, which is very strong and has little tendency to stretch. The same material was used for the slings required to deploy and recover the mooring lines from the seabed at various stages during installation and hook-up.
Leaving aside the various physical measures needed to install the mooring system from the Joshua Chouest, of which these are just some examples, the biggest influence on the success of the project was the thinking behind the development of the offshore procedures.
To give an example, careful inspection and surveying of the vessel led to the decision to deploy the ancillary lowering line from the vessel’s main winch drum and the much larger and heavier mooring line itself from one of the two on-deck storage reels (the first choice might have been one of the two main anchor-handling drums but these could not accommodate the rope and still allow for an alternative lowering option). This was counterintuitive, but the configuration of the two winches rendered this approach far less likely to stress either of the two lines. Veselis says that it is all about having everything, the chain, rope and lowering line, in the right place at the right time.
By installing the Delta House’s mooring system from the Joshua Chouest, InterMoor closed what initially appeared to be a gap between the demands of the mooring system and the vessel’s capabilities. The project, although challenging, was conducted safely and effectively, and finished on time and without major incident. Moreover, the project was highly cost-effective; there was an economic opportunity for the client and InterMoor helped to realise it. It just goes to demonstrate that, with experience, a creative approach and open minds, remarkable things can often be achieved.
The Joshua Chouest anchor-handling vessel is approximately 106 m long, has a beam of about 22 m and is about 9.5 m high. Its deadweight is 5,193 t. Two Caterpillar diesel engines develop 16,107 kW and provide the vessel with a bollard pull of 210 t. The vessel has five winch drums: a main drum for towing and also anchor handling, two anchor-handling drums and two reels normally used to store wire or rope. These last two are on the deck above the main winch house.