Complex retrofit at Shell Leman platform
Richard Fletcher
Trimble Solutions (UK)
Pinnacle Consulting Engineers has used Trimble’s Tekla software to assist steel structural modeling for two recent construction programs in the southern North Sea.
Shell’s Leman Alpha ICCP (impressed current cathodic protection) project in UK waters formed part of a series of upgrade programs designed to extend the lives of the Leman gas platforms. In 2016, Pinnacle was asked to design and detail, in collaboration with Shell and its contractors, two new I-tubes for installation within empty conductor guides from Leman Alpha’s cellar deck to the seabed. The I-tubes were to house large diameter cables to the seabed forming part of an ICCP system to extend corrosion protection, and thereby the serviceable life of the Leman Alpha jacket structures.
Above: Dead weight support for new I-tube caisson at Leman Alpha. (All images courtesy Pinnacle Consulting Engineers/Trimble Solutions) Below: Bell-mouth openings of J-tubes exiting caisson subsea.
The project involved installing three Retrobuoys (500-A ICCP anode sleds) on the seabed and connecting them via a power cable to the jacket structure. To protect the primary cables as they traveled up to the platform deck level, two new 610-mm dia. I-tubes were to be installed within two of the spare conductor guides, one each on the north and south side of the platform.
For the pulling operation to proceed successfully, Pinnacle needed to consider the bend restrictions of the cables at the entry point on the seafloor. This entailed creation of two bellmouth openings, 2 m (6.6 ft) apart, to allow the pullhead mechanism to pass through the tubes and winch the cable to the top.
The two internal 210-mm dia. had to be curved and twisted in order to fit within the 610-mm dia. caissons, while maintaining alignment to the location of the Retrobuoys away from the platform on the seafloor. Pinnacle also had to ensure that the pullhead could pass through the radius section at the base of the tubes. As the caissons were connected to cellar deck level (via Merlin connectors), internal tube splices had to be created above these to accommodate a 210-mm dia. female connector, while avoiding the possibility of the cable snagging as it progressed while being winched up.
This process was then repeated at a further six locations up to the platform deck level, where a trunnion frame provided a permanent fixing to the main structure. Each I-tube section ranged in length from 7.5-9.5 m (24.6-31 ft); the overall length of each tube assembly when joined was more than 60 m (197 ft).
Pinnacle creates its models using Tekla Structures software, and then sends them to the steel fabricators which add all required elements to the models. In the Leman Alpha project, the software allowed any potential problems to be addressed early in the project, to check for clashes, and ultimately, to distribute the IFC (Industry Foundation Classes) model.
The company also served as structural engineering consultant on another project to help develop and structurally engineer a safe and reliable boat landing station offshore the Netherlands, giving personnel access to and from various gas platforms. The challenges in this case included:
• Creating landing structures fixed to the existing jackets, between sea level and the platform’s spider deck
• The landing structure had to allow safe access from the chosen vessel within a 2-m (6.6-ft) tidal range and, for personnel on the landing, easy access to spider deck level via a safe and secure stair.
In addition, the structures had to be designed for installation using the platform’s crane, without the need to call in a costly jackup barge. For safety reasons, drilling or welding was not possible on the existing jacket structures, and the new landing structure had to be installed safely and correctly the first time, without any site modifications. Once the design was finalized, Pinnacle’s brief was to produce a comprehensive set of construction drawings to be issued to a fabricator in the Netherlands.
Above: Tekla model view overlaid on completed boat landing structure. Below: View on part of retro-fit boat.
The design of the primary supporting clamps was critical to the overall solution, as these needed to be installed simply and accessed from the front, incorporating all required dimensional tolerances. The lower clamps would be installed quickly, during a period when the sea level would be below the underside of the clamps. Accuracy was also important, with limited scope for built-in tolerances within the new structure.
Once the team had assessed all the design requirements and wind, wave, tidal and impact conditions, the boat landing structure was analyzed, designed, and detailed using Tekla Structures. Pinnacle then had to ensure the project could be phased to facilitate transport and installation, the goal being to minimize the number of components while keeping these within the size and weight limitations imposed for transporting and lifting onto the platform. The early and full engagement with the steel fabricator, vessel operator, installation and rope access companies, all within the structural design and detailing phase, ensured the primary clamps and frame were split, jointed, and slung to facilitate the installation contractor’s preferred method of erection.
Pinnacle was able to quickly construct a 3D model using original as-built drawings. This initial model formed the basis for early scheme designs for the structure and clamps. Next the company generated a BIMsight model (including pre-set views to show specific details) for presentations to the client and installation contractor. The clarity and detail of the images allowed key issues to be raised early, so that adjustments could be easily worked into the design. It also helped to highlight the fine tolerances required and the need for an accurate site survey.
The company was able to import the laser survey, in dxf format, into the model and align this with the key setting-out points in the model, providing the accurate positions to the inside edges of the tubular members. The collaborative design process led to a ‘right first time’ and ‘delivered on time’ pair of boat landing structures installed safely, cost effectively, and without incident, in challenging conditions, commissioned and handed over without the need for site modifications.
