The management of residual oil on drilled cuttings poses a major environmental consideration for offshore drilling. Often the methods employed are dictated by regulatory regimes. Where ‘zero discharge’ is specified by regulators then ‘skip and ship’ or ‘cuttings reinjection’ become the primary technologies available. In regulatory regimes where a degree of marine discharge is permitted, the cuttings driers or offshore thermal desorption technologies are used to reduce the % amount of oil on cuttings prior to discharge to the ocean.
Conventional cuttings driers, the technology currently used in Ghana, typically give 3% or more of oil on cuttings by weight of cuttings. Though this is relatively low and tolerable in some jurisdictions, both legislators and operators have a commitment to further reduce discharges in a progressive manner.
Most of Ghana's oilfields fall in deepwater areas and cuttings re-injection methods are not being actively considered in the near term due to less mature deepwater technology and cost. The offshore treatment using offshore thermal desorption offers an alternative method to treat drilled cuttings offshore and reduce the oil concentration on cuttings to typically less than 0.5% by weight prior to marine discharge. An overview of the method is however offered in this paper.
The Skip and ship methods where cuttings are shipped onshore and treated to remove oil on cuttings before being disposed of onshore, is used in other deepwater areas. Special offshore cuttings handling equipment plus a marine and road logistics network as well as onshore treatment infrastructure are required to support such activities.
This paper will look at the relative merits of offshore cuttings treatment by thermal desorption technology compared to skip and ship methods. It will consider the merits of each method in the context of projected Ghana drilling activity, onshore infrastructure requirements and other considerations.