Hazard Identification and Risk Assessment of the Troll Pipeline Laying and Pull-in
Authors: Deegan F.J. , Moore K.J. 
Published: December 2007

This paper will describe the risk assessment exercise carried out for pull-in and tie-in of the pipelines at the Troll GBS (Gravity Base Structure). The Troll Platform GBS will be installed and then the pipelines for the export of gas have to be pulled into the bottom of the riser shaft. Once the pipelines have been pulled in and sealed they have to be tied into the already pre-installed risers in the GBS. This pull-in and tie-in operation presents many hazards associated with working in a congested space at the bottom of a 300m riser shaft. The study will describe the HAZID methodology used, the method used to rank the risk and then the method of assessing risk reduction measure on grounds of effectiveness and ease of implementation, to reduce risk to As Low As Reasonable Practicable.

Background

The Troll field is located 80 km west Northwest of Bergen and extends over the North Sea Norwegian licence blocks 31/2, 31/3, 31/5 and 31/6.

A/S Norske Shell intends to install two 36 inch pipelines (P10 and P11) for the transportation of untreated gas from an offshore production platform to an onshore processing plant at Kollsnes Oygarden. These pipelines will be laid from a conventional laybarge and will be pulled into the platform by means of a winch installed on the topsides.

The pipelines will penetrate the bottom of the riser shaft and will finally be connected to pre-installed risers in dry atmospheric conditions. To enable this, seal tubes have been installed in the GBS during the construction phase. After pull-in temporary barriers will be activated to allow pipeline to riser connection. Finally permanent barriers are installed for long term protection. Of major importance is the use of temporary and permanent water barriers to protect personnel working in the riser shaft from flooding and the total integrity of the platform.

A/S Norske Shell have concluded that due to the nature of the operation of pulling-in and tie-in of the pipelines at the platform, there are present various hazards. These hazards can be classified into two distinct areas namely hazards to personnel and potential schedule delays. Hazards to personnel safety present the potential to result in injury or fatalities, whilst schedule delays could result in the late completion of the project with a corresponding financial cost. Therefore A/S Norske Shell have commissioned DNV Technica to evaluate the hazards and risks associated with the pipeline pull-in and sealtube operations, and assess any possible risk reduction measures with the following objectives:To identify the problems and hazards which could occur with the pipeline pull-in and tie-in.To assess the risks to both the workforce of potenial safety hazards and the schedule delay from hazards which could cause a delay.Identify risk reduction measures which would reduce the potential problem areas.Provide guidance on which risk reduction measures would bring the greatest benefit.

Methodology

The application of this type of pull-in was unproven in this depth of water and with a GBS. As a result the design of the pull-in equipment had been subjected to a very detailed design review and analysis. The control of the operation is very dependent upon the procedures that are were written to enable to operation to be undertaken. The use of procedures for new or novel operations is that the procedures form the basis of a system of controls which enables the pipelines to be pulled into the GBS. Due to the novel nature of the operation and to ensure the safety and efficient implementation of such procedures a review to identify hazards and operability problems prior to construction was to be part of the review process.