Optimization of Topside Facilities on a Mature Field to Eliminate Effects of Water Hammer (Case Study of Amor Field)
Authors: Onuwaje Shola 
Published: July 2014

AMOR field is located in one of the OMLs in conventional offshore area of Nigeria, in about 62m water depth. The AMOR field comprises of two production platforms namely Amor production platform-1 (APP-1), Amor production platform-2 (APP-2) and two other satellites platforms namely APP-3 and APP-4. Two-phase (gas-liquid) separation of fluid from AMOR field i.e. APP-1, APP-2 and its satellite facilities (APP-3 and APP-4) is achieved on AMOR production platform (APP-2). The liquid (oil and water) is then pumped in multi-phase mode to a Floating Storage and Ofloading (FSO) vessel located 4km away, through a 24” pipeline (APP-2 oil export line), where oil and water are separated by gravitation (see Fig 1). This 24”AMOR (APP-2) liquid export pipeline to the FSO has two Emergency Shutdown valves (ESDVs) on the import hoses at the turret, downstream the Pipeline End Manifold (PLEM), going to the inlet of the FSO vessel.

The pressure surge generated by the closure of these ESDVs has the potential to cause leakages through failures or rupture in the subsea system, including the PLEM upstream the ESDVs, because of the possibility of exceeding the design pressure of 19 bar g.

In 2002, the risk of pressure surge (with differential surge pressure reaching about 24 bar g) because of, water hammer effect generated by the closure of the ESDVs and a Shutdown valve (SDV) on the FSO (where facilities like the heat exchanger is rated at 12 bar g), was identified by dynamic simulation using Pipenet software. This risk was accentuated by the absence of an over-pressure protection device except, for a Pressure Safety Valve (PSV-1) positioned at APP-2 departure, on the 24” APP-2 liquid export line to the FSO and set at 19 bar g. Mitigation for this risk was possibly left out by oversight, as this field is now mature and designed with an older specification.

Sequels to these finding, two solutions were proposed for implementation: Convert the SDV at FSO downstream the ESDVs, to a manual valve, to protect surface facilities at FSO with 12 bar g design pressure from pressure surge effects, through internal modifications procedures.Implement an instrumented solution to close valves at APP-2 before the ESDVs closure, to isolate the source of water hammer effect. The instrumented system comprises of having a logic solver on AMOR production platform (APP-2), subsea cable between APP-2 and the FSO, redundant valve closure detection on FSO, and modification of logic diagrams.

Because to reduced production forecast from the initial ≈250 kb/d to about 170 kb/d, and the significant cost (≈$5m) of implementing option (2) above, it became necessary to re-evaluate the risk of pressure surge identified earlier (since surge pressure has a somewhat direct proportionality to the mass rate of fluid flowing in a pipe, as typified by the Joukowskyequation. Thereafter, a cost effective alternative to option (2) above, is to be proposed and studied, without jeopardizing quality and safety.

The alternative solution proposed to option (2) above; involves de-rating the set point of the existing PSV-1 on the 24” APP-2 liquid export line to FSO, from 19 bar g to 12 bar g (Fig 1). This option unfortunately, eliminates the water hammer effect upstream the ESDVs but, has the potential to cause serious operational problems, that could be catastrophic, damaging to the environment and even taint our public image.

This paper therefore highlights the analyses and modifications studies performed to identify, evaluate, optimize, and mitigate the water hammer effects, generated by possible closure of ESDVs on the turret. Also an “analyses of what could go wrong operationally” or possible effect (upstream the ESDVs) of de-rating PSV-1 set point from 19 bar g to 12 bar g, as a means of eliminating water hammer effects, and then “design out of it” as a mitigating measures.