A methodology for the rigorous assessment of pipeline freespans is described, together with a description of its application to the Beryl Field network of pipelines. The methodology is in two parts, each with two stages, and comprise preliminary stress and vibration frequency checks followed by detailed strain and fatigue life checks where appropriate. Comprehensive software, automatically linked to the inspection database, has been written to allow efficient use of the methodology. The use of a new ROV based freespan rectification technique is also described. Both the assessment and rectification techniques were successfully used in Mobil's Beryl field and the SAGE pipeline in 1992.

Electrically Traced Heated Pipe in Pipe (ETH-PiP) technology has been developed to overcome some of the challenges associated with deeper and more remote offshore oil and gas production. This active heating technology applies power to achieve a production fluid temperature above the wax or hydrate appearance temperature either continuously, during normal production, or intermittently, during shutdown periods. Concerning hydrate management, the contractor Company in collaboration with Major Operators conducting experimental and modelling studies to investigate hydrate dissociation in heated flowlines through a Joint Industry Project (JIP) kicked-off in 2012. The main objective of these investigations is to demonstrate that a long, non-permeable hydrate plug can be dissociated in a safe and controlled manner with the ETH-PiP technology. Large hydrate plugs (approximately 200 kg each) are formed in an 18m ETH-PiP 6? OD prototype, using a water and gas system equipped with DTS fiber optics systems for temperature monitoring, pressure and temperature sensors, and high accuracy gas flow meters. Different heating strategies are tested to investigate the best active heating procedure for safe hydrate plug dissociation, using temperature, pressure and released gas flow rate monitoring along the entire length of the prototype. Hydrate plug dissociations are performed in open or closed volumes for various conditions during the 2nd phase of the experimental campaign, which started at the end of 2013. High pressure differentials are applied across the hydrate plugs; non-uniform longitudinal heating profiles are applied to reproduce operating conditions similar to direct electrical heating; and three-phase dissociation experiments are conducted to simulate the influence of oil present in the hydrate pores on the plug dissociation. The paper gives an overview of the experimental set-up and measuring techniques used. It describes the hydrate plug formation, location, and characterization, as well as the successful dissociation of hydrate plugs. Preliminary simulation results based on a specifically developed "in-house" simulator are presented, as well as extrapolation of the results to real subsea conditions. This test program demonstrated that large non-permeable hydrate blockages in single line field architectures could be dissociated without local pressure build-up or plug run-away using ETH-PiP technology.

The benefits of active heating of subsea pipelines in terms of operability and flow assurance have been demonstrated, in particular for deepwater and/or remote fields with complex production conditions (hydrate formation risk, wax deposit, gelling issues, short production plateau…). Electrical Trace Heating was developed to reduce significantly field developments CAPEX by simplifying subsea architectures and topside equipment, and OPEX through a low power consumption, and a drastic reduction in the use of chemicals. After the first deployment of Electrically Trace Heated Pipe in Pipe (ETH-PiP) in reel-lay on Islay Field (North Sea, 2012), a development programme was launched in 2012, supported by major Operators to upgrade and qualify ETH-PiP technology for long tie-backs and higher power for production or export lines. This paper presents the maturity of all components of the ETH-PiP system. In a second part, the paper emphasizes on tracing cables design, production and validation testing phases as a critical aspect of the overall qualification process.

Midline bulkheads are often used as a restraining mechanism in High Pressure / High Temperature (HP / HT) pipe-in-pipe (PIP) systems. Their primary function is to share the loads between the inner and outer pipes and / or to minimize the extent of damage in an operational or installation incident. They can also be used during reeled pipelay to facilitate the reel-to-reel or trip-to-trip weld tie-in. Design of a reeled bulkhead, which is categorized as a “pipeline component” under DNV-OS-F101, requires the careful adaptation of a Pressure Vessel Code (PVC), a subsea pipeline code and compliance with the additional requirements of reeling, welding and fabrication. Modern pipeline codes, such as DNV-OS-F101, are LRFD based codes in which different limit states (failure modes) have been formulated and calibrated based on a given probability of failure. The PVCs, however, do not specifically address the pipeline design and careful consideration in selecting load factors, load combinations and the analysis method are necessary. This paper summarizes a procedure that has been adopted in design of reeled midline bulkheads. The methodology can also be used in design of reeled end bulkheads. Firstly, the initial geometric dimensions of the reelable bulkhead are defined and its reelability is confirmed. Then, appropriate load combinations are identified from ASME BPVC Section VIII – Division 2. Finally, a series of Finite Element Analyses (FEA) are performed to show the fitness-for-service of the bulkhead. The importance of selecting an appropriate and justifiable “code break” is highlighted here.

The Central Area Transmission System (CATS) in the UK sector of t he North Sea del ivers natural gas t hrough a 404 km pipeline from the CATS riser plat form to the North East coast of England. During the summer of 2007 this 36 inch diameter natural gas pi peline was dam aged by a vessel anchor. The anchor lifted the pi peline from under t he seabed, dragged i t across t he seabed, bendi ng t he pi pe and locally deforming it. This event resulted in a sig nificant in spection, assessment an d repair program me before t he pi peline co uld safely retu rn to operation. This paper descri bes t he det ailed st ructural assessm ent of the damaged pipeline and the inspection and repair operations. Following inspection of the pipeline by divers, the damage was assessed using the "Pipeline Defect Assessment Manual" (PDAM). The m anual was prepar ed from research prim arily for onshore pi pelines: t his paper di scusses t he strengths and weaknesses of PDAM and key differences in defect assessment for onshore and offshore pipelines. The paper highlights several very important lessons learnt from this incident, including: • the com plex st resses devel oped i n a pipeline that is pulled and moved by an anchor; • the need for dam age assessm ent m ethods for pi pe containing hi gh com pressive st resses and ' locked-in' stresses; • the safety aspects and com plexity of inspecting a pressurised and damaged subsea pipeline. These lessons learnt ar e then translated into recommendations for t he i ndustry, and advi ce t o ot her subsea pipeline operators.