AM Best has affirmed the financial strength rating of A (Excellent) and the long-term issuer credit ratings of "a+" (Excellent) of Castle Harbour Insurance Limited (Castle Harbour) and Harrington Sound Insurance Limited (Harrington). These companies are captive insurance companies for Schlumberger Limited (SLB), a global technology company in the energy industry. Both companies are domiciled in Bermuda.The outlook of these credit ratings is stable. The ratings of Castle Harbour and Harrington reflect their balance sheet strength, which AM Best assesses as strongest, as well as their strong operating performance, neutral business profile and appropriate enterprise risk management. AM Best adds, the ratings affirm that Castle Harbour and Harrington maintain the strongest level of risk-adjusted capitalisation, as measured by Best’s Capital Adequacy Ratio , in addition to displaying strong operating performance results over each of the past five years. AM Best says the ratings also take into consideration the captives’ strategic importance in providing tailored insurance coverages for the parent and its subsidiaries. While each captive carries relatively large limits within its respective designated coverages for the parent in the general liability and property lines of business, each writes a broad scope of business and has considerable geographic diversification. As captive insurers of SLB, the companies are an integral part of the parent’s ERM framework, which includes defined risk controls and optimisation of the captives’ capital. Further, AM Best recognises the financial flexibility afforded by their parent company, as well as their strategic importance across SLB.