Boat landings for AME-2 & AWG-1
The gas platforms AME-2 and AWG-1 are situated just off the coast of Holland and have been operational for over 30 years. As part of the ongoing review of operational expenditure, to align with lower production revenues from mature wells, there was a proposal to use boats as an alternative to expensive helicopters for personnel access to these platforms.
Pinnacle were appointed as structural engineering consultants to help develop and structurally engineer a safe and reliable boat landing station, from which personnel could access and return from the gas platforms. AME-2 AWG-1.
The principal engineering challenges were to create landing structures fixed to the existing jackets, between the sea level and the platform’s ‘spider deck’. The landing structure had to allow safe access from the chosen vessel within a 2m tidal range and, once on the landing, easy access to spider deck level via a safe and secure stair. The structures also had to be designed to be installed using the platform’s crane (Max 7 tonne lift), without the use of an expensive jack-up barge. The final structure had to be both robust enough to cope with the environmental conditions of being located in the ‘green water’ wave zone, and easy to maintain for the platform’s operator in the future. As with any offshore project safety was paramount. Under no circumstances were the existing jacket structures to be drilled or welded.
The new structure had to be installed safely ‘right first time’ without needing any site modifications. There couldn’t be any areas where water could become trapped, so SHS, RHS & CHS were the preferred sections to use. Once the design was finalised we then had to produce a full set of fabrication drawings to be issued to a fabricator in Holland.
The design of the primary supporting clamps was a critical component in the overall solution as they needed to be able to be installed simply, accessed from the front, incorporating all of the required dimensional tolerances and, in the case of the lower clamps, installed within a very short period of time, when the sea level would be below the underside of the clamps.
Accuracy was critical and the ability to build in tolerances within the new structure was limited, as such it was imperative to ensure a ‘right first time’ fit. Firstly; the jackets were modelled in 3D using the ‘As Built’ drawings from the original platform installations. A laser survey taken from each platform’s spider deck to accurately determine the relative positions and alignment of the jacket members, albeit limited to only pick up the backs of the main supporting tubes. Finally; the marine growth was removed from the horizontal bumper beam and the tubular members were physically measured using large callipers. Using the data from all three sources the structural detailing model was built and refined. Upon completion of the design details, but prior to fabrication, a plywood template of the clamp was taken to the platform and physically offered up to both the bumper beam, from boat, and the spider deck steel, from the existing walkway, to confirm fit. With all of these checks in place the clamps’ detailed design only needed to incorporate 25mm of dimensional tolerance. The structures were then fabricated using a jig within the fabricator’s workshop accurately reflecting the relative positions of the two main horizontal tubular supports.
Once all of the design actions and combinations resulting from wind, wave, tidal and impact conditions were assessed the boat landing structure was analysed and designed with careful consideration given to how the structure could be phased and spliced so as to facilitate transport and installation. The goal was to minimise the number of components whilst staying within the size and weight limitations of transporting and lifting onto the platform. The clamps were designed to be easily and safely installed, comprising both lifting points and temporary deadweight rests within their detailing. Post-installation; the clamp’s bolts could each be periodically inspected and, when needed, easily replaced, with the complete bolted assembly being removable. Similarly, the handrails, treads and steps were all designed to facilitate easy lifelong inspection, maintenance and, when needed, component replacement at the same time being robust in their capability to withstand severe conditions and storms. All items were carefully detailed and designed to reduce susceptibility to corrosion. The client was fully committed to engaging with the key stakeholders early within the design process, which enabled true collaborative design. The full and early involvement of the steel fabricator, vessel operator, installation and rope access companies all within the structural design and detailing phase, ensured that the primary clamps and frame were split, jointed and slung to facilitate the chosen installation contractor’s preferred method of erection.
The outcome of this collaborative design process was a 'right first time’ and ‘delivered on time' pair of boat landing structures installed safely, extremely cost effectively, without incident, in challenging conditions, commissioned and handed over without the need for site modifications.
We were able to quickly build up a 3D model using Tekla Structures working from original as built drawings. This initial model then formed the basis for working up our early scheme designs for the structure and clamps. From this we could produce a BIMsight model (including pre-set views created to show specific details) to use at a meeting with the client and the installation company in Holland. The clarity and detail of the images presented meant our client/team engaged immediately with the proposed design. This helped to raise key issues early so adjustments could be easily worked into our design. It also helped highlight the fine tolerances required and the need for an accurate site survey.
At all the key stages of the design process we would always create a 3D pdf for the client and team. We were able to import the laser survey (dxf format) into our model and set this to line with the key setting out points in our model. This gave us the accurate positions to the inside edges of the Tubular members. Even with all the benefits of BIM modelling it still needed verbal communication with the survey technician to understand the laser survey was only taken from each platform’s spider deck and surveyed a small segment of the tube, which had been cleaned of marine growth. Their software then calculated the rest of the tube size based on the area surveyed. This answered why none of their overall tube sizes matched what we had expected, because any slight imperfection in the curve of the tube would result in a change to the outside diameter. As a result, the clamp positions on each tube had to individually site checked.
Tekla Structures enabled us to produce a full set of fabrication drawings including material take off lists as required. On completion of our model we were also required to produce 3D dwg models and stp files for the client to import into their PDMS software for their own records.